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7 Tropospheric Chemical Cycles
TROPOSPHERIC CHEMISTRY AND BIOGEOCHEMICAL CYCLES
BY C. C. DEEWICHE
The composition of the troposphere is determined to
a large extent by reactions in the biosphere that main-
tain a quasi-stable tropospheric composition that would
not persist in the absence of biological activity. The exist-
ence of molecular oxygen and nitrogen in the atmo-
sphere is the most obvious consequence of this activity.
These substances are present as a result of slow reaction
kinetics and are not at thermodynamic equilibrium.
The concept of geochemical "cycles" of elements is
not new, but only recently has the significance of biolog-
ical activity in these cycling processes been appreciated.
Elemental cycles developed by the atmospheric chemist,
the biologist, the geochemist, and others all have differ-
ent features of importance, depending on the interests of
the reporting scientist. In the sections that follow we will
present only a brief overview of some of these cycles to
place them in perspective from the standpoint of the
atmospheric chemist.
Most of the elements considered here have at least one
volatile component of biological origin. Most of the cy-
cles reflect the alternate oxidation and reduction of com-
pounds in the energy metabolism of one or another life
form. The process most commonly recognized is that of
the photosynthetic-respiration sequence involving car-
bon and oxygen. Other reactions, such as those of nitro-
gen fixation and denitrification and the reactions of sul-
fur oxidation and reduction, are ancillary expressions of
the primary processes involving carbon. They are gen
101
erally dependent upon the carbon cycle for their opera-
tion, although the compounds of sulfur are themselves
grist for a photosynthetic energy input.
Most of these major cycles have been altered in some
of their features on the global scale by a factor of 2 or
more as the result of human activity. Fossil fuel burning,
although only 10 percent of respiration as a source of
. · _
.
atmospheric (~)2, gives an annual increment of about
0.3 percent. Industrial nitrogen fixation and the use of
legumes have about equaled "natural" nitrogen f~xa-
tion, and sulfur from fossil fuel combustion and mining
activities has about equaled the natural sources of atmo-
spheric sulfur. Other processes, such as erosion of soil
and the injection of some heavy metals like lead into the
atmosphere, probably have altered natural cycles even
more.
The closeness with which these cycles are coupled
frequently is not appreciated in attempting to predict the
consequence of their perturbation by human or other
influences. Fundamentally, this coupling has its source
in the energy demands of living organisms. The total
system is drained of all the energy extractable from any
reaction that can yield energy in significant amounts,
and so tends to move toward a median energy level,
expressed otherwise by Lovelock and Margulis in their
treatment of the Gaia hypothesis.
Many of the compartments involved in biogeochemi-
cal cycles are shown in Figure 7.1. For our purposes, we
OCR for page 102
102
FIGURE 7.1 Diagrammatic representa-
tior~ of major compartments of biogeo-
chem~cal cycles as discussed in the text.
Although movement of oceanic plates, vol-
canic activity, and many other processes
involved in these cycles are discontinuous,
they are treated as steady-state processes for
the purpose of developing mean estimates
of their significance.
Continental Plate
will not consider long-term (hundreds of thousands of
years) cycle features, such as the sedimentary cycle or
processes of subduction and subsequent volatilization
through volcanism, except as the results of these proc-
esses contribute to the annual flux of a given element.
For comparison, Figure 7.2 and Tables 7.1 and 7.2 give
the distribution of four of the primary elements of inter-
est (carbon, nitrogen, oxygen, and sulfur) between vari-
ous "compartments" or "pools" in the environment
and the estimated rates of transfer between them. It is
important to remember that models of this type are
intended as thinking tools, giving only the best estimates
of the magnitude of the fluxes and burdens. Uncertain-
ties of a factor of 2 or more are not unusual, and only in a
secondary way is this uncertainty important to the anal-
ysis of problems or to planning.
A number of features have been omitted from Figure
7.2 for the sake of clarity. For example, the large pool of
volatiles in magma is not considered except for an indi-
cation of volcanic sources where appropriate. Most of
these volcanic sources are assumed to be the return of
volatiles subducted with sediments, but some probably
are truly "juvenile, " representing an out-gassing of the
magma that has been taking place (at a diminishing
rate) since the earth was formed. The magnitude of this
juvenile source relative to the recycling of subducted
materials is controversial and not pertinent to the argu-
ments we explore here.
Several points are evident from an examination of
this table:
~ Land Plants I
| Sediments | ~
_ . . ~ . I
Land A - als |
1 1 -
Subduction Wedge
_:
-
/
-
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
| Atmosphere |
Ocean Plate
1. The major pools of the various elements are a
function of their chemistry. Most of the nitrogen is in a
partially "reduced" form in the atmosphere; most ofthe
carbon is in carbonate rocks, bicarbonate ion in the
ocean, or more reduced materials in soil sand sedi-
ments, with only a small (but important) fraction in the
atmosphere. Sulfur is divided between the sulfate of the
oceans and evaporites or in sediments, the atmosphere
containing only a small amount in transit between these
pools.
2. Biological processes are major factors in the move-
ment of elements between the various pools, but, in
general, the biosphere constitutes only a small fraction
of the total.
3. Oxidation-reduction reactions in biological sys-
tems are responsible for most of the transfer taking
place. The separation of charge of biological processes
orobablv has created a broader range of oxidation
potentials than existed before life developed on the
planet. Thus there probably are both more oxidizing
and more reducing conditions than existed before the
appearance of life. The former is part of scientific lore,
but the latter frequently is overlooked.
4. The concentration of oxygen in the atmosphere is
determined not by the rate of photosynthesis, but by the
degree to which reduced compounds (particularly those
of carbon, nitrogen, sulfur, and iron) can be kept buried.
5. The partition of compounds between various
compartments is a function of the energy balances in-
volved. Thus, for example, the concentration of CH4 in
OCR for page 103
TROPOSPHERIC CHEMICAL CYCLES
20
en
~ 15
o
a:
~ 10
o
o
o
6 15
a:
a:
° 10
5
o
103
SE R 00
LP
CARBON
SEO SE R OB 00 AC1 AM AC2 SO
COM PA RTM E N1
N ITROGEN
Sl LP OP
OP LA OA AN ANO ANH
COMPARTM ENT
OA
FIGURE 7.2 Pool sizes of interest for the elements carbon, oxy-
gen, nitrogen, and sulfur. Ordinates give the log of the pool size in
gram-atoms of the element. Logarithmic presentation is necessary
because of the wide range in pool size. Unit increment on the scale
represents a factor of 10 in pool size. Although some pool sizes are
OXYGEN
SEO AC1
SEO SER SO
OW ICE CW FEO
COM PA RTM E NT
SU LFU R
S04 PW AO AW
S I 01 ASO ASU ASH
COMPARTMENT
known with reasonable accuracy, others are accurate only to a factor
of 2 or more. Numeric values are also presented in Table 7. 1. Values
compiled from various sources including Delwiche and Likens
(1977); Garrels, Mackenzie, and Hunt (1975); and Soderlund and
Svennson (1976).
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104
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
TABLE 7.1
Pool Sizes of Interest for Carbon, Oxygen, Nitrogen, and Sulfur
Element
Compartment Carbon Oxygen Nitrogen Sulfur
SEO Oxidized sediments 50.8 152. 0.21
SER Reduced sediments 1.04 0.1 0.60
OB Ocean bicarbonate 0.032
OO Ocean organic 0.00083 6.2 E-5
AC 1 Atmospheric CO2 0.00054 0.00 ~ 1
AM Atmospheric CH4 5.2 E-6
AC2 Atmospheric CO 2.0 E-7
SO Soil organic 0.0025 0.00021 2.2 E-5
SI Soil inorganic 0.010 1.1 E-5 0.81
LP Land plants 0.00069 0.00065 5.7 E-5
OF Ocean plants 1.5 E-6 1.2 E-7
LA Land animals 1.2 E-6 1.4 E-7
OA Ocean animals l.lE-6 1.2E-7
OW Ocean water 761
ICE Ice 9.16
CW Continental water 0.055
FEO In iron oxice 6.
SO4 In sulfates 8.5
PW Sediment pore water 177
AO Atmospheric O2 0.76
AW Atmospheric water 0.0058
AN Atmospheric N2 2.8
ANO Atmospheric N2O 1.3 E-6
ANH Atmospheric NH3 2.0 E-8
OIOceaninorganic 7.1 E-5 0.41
ASO Atmospheric SO2 3.4 E- 10
ASU Atmospheric sulfate ion 1.0 E-9
ASH Atmospheric reduced S 1.9 E- 10
NOTES: Values are in units of 102° gram-atoms ofthe element. Elements of igneous rock and magma are not included
in this compilation. Where no values are given, the pool is not applicable, insignificantly small, or unknown. The code
letters used correspond with those of Figure 7. 2.
the atmosphere probably is a direct reflection of the
energy relatior~ships of microbial processes.
6. The consequences of human alteration of these
cycles are best interpreted in terms of rates. Although
the total system probably could accommodate large per-
turbations if sufF~cient time were allowed, the rate con-
stants for many of the processes considered here are of
the order of tens of thousands of years or more, and
human activities on time scales of decades or centuries
are not accommodated.
This short overview of biogeochemical cycles is in-
tended to serve as a backdrop against which to examine
atmospheric cycles of more immediate concern to this
report. Details of these chemical cycles are available
elsewhere (see bibliography at the end of each cycle
section).
BIBLIOGRAPHY
Ahrens, L. H. (1979~. Origin and Distribution of the Elements. Perga
mon, New York,537 pp.
Bremner, J. M., andA. M. Blackmer(1978~. Nitrous oxide: emis-
sion from soils during nitrification of fertilizer nitrogen. Science
199:295-296.
Broda, E. (1975~. The history of inorganic nitrogen in the bio-
sphere. J. Mol. Evol. 7:87-100.
Broda, E. (1975~. The Evolution oftheBioenergetic Process. Pergamon,
Oxford, 211 pp.
Broecker, W. S., T. Takahashi, H. M. Simpson, and T.-H. Peng
(1979~. Fate of fossil fuel carbon dioxide and the global carbon
budget. Science206:409-418.
Delwiche, C. C. (1970~. The nitrogen cycle. Sci. Amer. 223:137-
146.
Delwiche, C. C., and B. A. Bryan (1976~. Denitrification. Ann.
Rev. Microbiol. 30: 241 -262.
Delwiche, C. C., and G. E. Likens (1977) Biological Response to
Fossil Fuel Combustion Products, in Global Chemical Cycles and
Their Alterations by Man, Werner Stumm, ed. Dahlen Konferen-
zen, Berlin, pp. 73-88.
Garrels, R. M., F. T. Mackenzie, and C. Hunt (1975~. Chemical
Cycles in the Global Environment. William Kaufmann, Los
Altos, California.
Garrels, R. M., A. Lerman, and F. T. Mackenzie (1976~. Controls
of atmospheric O2 and CO2: past, present and future. Amer. Sci.
64:306-315.
Holland, H. D. (1978~. The Chemistry of the Atmosphere and Oceans.
Wiley, New York.
OCR for page 105
TROPOSPHERIC CHEMICAL CYCLES
TABLE 7.2 Selected Transfer Rates Between Compartments
I II III IV V
Process From, To Quantity Source Ratio Sink Ratio
Carbon
Photosynthesis
land ACT, LP 4036 0.0747 0.0585
ocean ACT, OF 2080 6.4 E-4 13.7
Fossil fuel
combustion SER, AC1 388 3.7 E-6 7.2 E-3
Biological CH4
production SO(?), AM 26 1.1 E-4 0.69
Atmosphere-ocean
(CO2) exchange ACT, OB 8190 0.074 2.6 E-3
Wildfire LP, AC1 126 1.8 E-3 2.3 E-3
Oxygen
Photosynthesis
land CW, AO 8072 1.5 E-3 1.1 E-4
Fossil fuel AO, OW 1160 1.5 E-5 1.5 E-8
combustion
Nitrogen
N fixation
land AN, LP 6.9 2.5 E-8 0.12
ocean AN, OF 0.724 2.6 E-9 0.060
industrial AN, SI 2.83 1.0 E-8 2.6 E-4
Denitrif~cation
land SI,AN 8.5 7.7 E-3 3.0 E-8
ocean OI, AN 2.86 4.0 E-4 1.1 E-8
Sulfur
Fossil fuel
combustion SER, ASO 2.0 3.3 E-8 58.
Wildfire LP, ASO 0.82 1.0 E-3 24.1
Biological reduction
land SI, ASH 0.12 1.5 E-9 6.3
ocean OI, ASH 0.085 2.1 E-9 4.47
Volcanic return SER, ASO 0.12 2.0 E-9 3.5
NOTES: The symbols used in Column II correspond with those of Table 7. 1. Rates are in units of teragram ( 1 E- 12
grams) atoms per year. Column IV gives the ratio of the quantity transferred to the source quantity; Column V gives
the ratio of the quantity transferred to the sink quantity.
Holser, W. T. (1977~. Catastrophic chemical events in the history of
the ocean. Nature 267:403-408.
Junge, C. E. (1972~. The cycles ofatmospheric gases naturaland
man-made. Quart. I. Roy. Meteorol. Soc. 98:711-729.
Kellogg, W. W., R. D. Cadle, E. R. Allen, A. L. Lazrus, and E. A.
Martell (1972~. The sulfur cycle: man's contributions are com-
pared to natural sources of sulfur compounds in the atmosphere
and oceans. Science 175:587-596.
Kvenvolden, K. A., ed. (1974~. Geochemistry and the Origin of Life.
Dowden, Hutchinson and Ross, 422 pp.
Li, Yuan-Hui (1972~. Geochemical mass balance among litho-
sphere, hydrosphere, and atmosphere: the Gaia hypothesis. Tel-
1us26:1-10.
105
Margulis, L., and J. E. Lovelock (1978~. The biota as ancient and
modern modulator of the earth's atmosphere. Pure Appl. Geophys.
116:239-243.
Ponnamperuma, C. (1977~. Chemical Evolution of the Early Precam-
brian. Academic, New York, 221 pp.
Soderlund, R., and B.;H. Svensson (1976~. The global nitrogen
cycle, in Nitrogen, Phosphorus and Sulphur Global Cycles, B. H.
Svensson and R. Soderlund, eds. SCOPE Report 7. Ecol. Bull.
Stockholm 22:23-72.
Sokolova, G. A., and G. I. Karavaiko (1964~. Physiology and Geo-
chemical Activity of Thio~oacilli. Translated from Russian, 1968.
283 pp.
OCR for page 106
106
WATER (HYDROLOGICAL CYCLE)
BY R. DICKINSON
Water is such an important component of the envi-
ronment that it is not surprising to realize that it is also
one of the more important atmospheric species from the
viewpoint of chemistry. Furthermore, the general
framework that is used in this section to consider the
cycles of other atmospheric trace constituents is also
appropriate for water. Its distribution in the atmosphere
is determined by the balances between sources, sinks,
and transport, as illustrated in Figure 7.3. Water occurs
in the atmosphere in three phases vapor, liquid, and
solid and all three phases interact strongly with the
other chemical cycles. The transformations between
phases need special emphasis in viewing water as an
atmospheric chemical.
SOURCES
With the exception of a small source by CH4 oxida-
tion in the stratosphere, and minuscule amounts yielded
by some tropospheric reactions, the sources for water
are entirely at the earth's surface. Water is removed
from the earth's surface because of higher vapor pres-
sures maintained at surface interfaces than within the
atmosphere. On the global average, 1 .0 m of water per
year moves from the surface to the atmosphere and falls
again as precipitation. The water vapor at wet interfaces
is maintained at the saturation vapor pressure by equi-
librium between the wet surface and its immediately
adjacent molecular boundary layers. However, trans-
port and removal processes in the atmosphere act to
me) ! :i
Su bl l motion ////
°~snOl - - Bare Sea Ice ~ -
_ a ce
reduce water vapor pressure over much of the atmo-
sphere to values below saturation. Furthermore, surface
materials are often warmer than the overlying atmo-
sphere because they absorb solar radiation. Some com-
bination of lower temperature and lower relative hu-
midity for the overlying air makes its water vapor partial
pressure and mixing ratio lower than that of the surface.
The consequent gradient in free energy drives water
from the surface. Meteorologists often approximate the
upward flux of water from the surface, Fw, by an expres-
sion of the form
Fw=CwpaV~qs-qa)'
where Cw is a bulk transfer coefficient (under some con-
ditions deductible from micrometeorological theory);
Pa = density of the air; qS = water vapor mixing ratio at
the surface (e.g., the saturation mixing ratio evaluated
at the temperature of the surface); qa = water vapor
mixing ratio in the air, evaluated at some reference level,
usually 2 m above the ground or 10 m above the ocean;
and V = magnitude of the wind at the reference level.
Over oceans, Cw = 1.4 x 10-3 with some dependence
on wind speed and wave height.
About 70 percent of the earth is covered by water and
about 75 percent of the water entering the atmosphere
comes from the ocean surface. The remaining 25 per-
cent undergoes the interesting and complex physics of
hydrological processes on land. At the simplest level, we
can distinguish between evaporation from nonphoto-
synthesizing surfaces and transpiration. Evaporation
Atmospheric
- Transport
Cloud Condensation
~_
U' ~
'
1 ~
I l 1 1 1 '1 / Snow Covered
Planetary / /~: ~ Vegetation
Boundary Layer / /~ ~ /1/ /
~ Low Shadow Shadow /~ - p;~/
Transpiration
Leaf Temperature
Leaf Evaporation
Am/ ~ Dew Formation
b. I Surface Wind
q ~ in Canopy
~POLE
O~FAN
EQUATOR
FIGURE 7.3 Features of the hydrological cycle in the atmosphere.
OCR for page 107
TROPOSPHERIC CHEMICAL CYCLES
occurs as described above, with two additional compli-
cations: (1) the remaining water on relatively dry sur-
faces will be bound by surface tension and other stronger
forces that will lower the water vapor pressure, and (2)
the rate of water removal may be limited by the maxi-
mum rate at which water can diffuse from the interior of
the soil or other object to the surface. Water transpiring
from plants passes through the stomata of leaves, which
generate enough diffusional resistance to lower signifi-
cantly the water vapor on the outside of a leaf from its
saturation value. The stomata! resistance changes with
various environmental factors, including inability of
roots to supply enough water because of soil dryness.
Water loss by vegetation through stomata is believed to
be primarily an accidental consequence of the need for
plants to move CO2 into their leaves to supply their
photosynthetic cycles. Most other gas exchange be-
tween higher plants and the atmosphere also occurs
through the stomata. Interception is another process
involving vegetation that is of concern to hydrologists.
The leaves and other plant parts become coated with
water that can reevaporate, without the water progress-
ing farther into the ground.
The saturation vapor pressure of water varies
strongly with temperature according to the Clausius-
Clapyron relationship. Thus saturation water vapor
pressures near the surface and hence evaporation are
much larger in the tropics than in high latitudes.
TRANSPORT AND DISTRIBUTION
Water vapor moves from the surface, through the
planetary boundary layer, and then into the free atmo-
sphere, where it is redistributed horizontally and verti-
cally by atmospheric wind systems until it undergoes
gas-to-droplet conversion. On a global average, a
column of air holds about 27 kg/m2 of water. Water in
vapor form has an average lifetime of about 10 days and
can move large distances (thousands of kilometers or
more) before conversion to droplets. Liquid and ice par-
ticles generally have lifetimes of several hours or less and
so are carried distances of 100 hen or less before recon-
version to the gas phase or removal by precipitation.
Because precipitation rates do not have as strong a
latitudinal dependence as evaporation rates, large-scale
atmospheric transport moves a significant fraction of the
water evaporated in tropical latitudes into middle and
high latitudes. This transport is one of the major proc-
esses for maintaining temperatures at high latitudes
warmer than implied by radiative-convective equilib-
rium within a vertical column.
Motion processes on various scales are intimately
connected to the gas-to-droplet conversions and droplet
107
removal processes of precipitation systems described be-
low.
The mixing ratio of water vapor in the troposphere
varies over 4 orders of magnitude, from a few parts per
hundred in the tropics near the surface to less than one
part per thousand over the poles at the surface and to a
few parts per million near the tropopause. This variabil-
ity is explained to zeroth order by assuming a fixed
relative humidity and noting that the mixing ratio varies
with its saturated value. The reason relative humidity is
not too variable, with sufficient averaging, is under-
stood in terms of the role of atmospheric motion sys-
tems. By continuity, at any one time about half of the
atmosphere is moving upward and is constrained to
relative humidity near 100 percent by precipitation
processes. The rest of the atmosphere is moving down-
ward and drying the air to values much lower than
saturation (e.g., near 10 percent). Combining the up-
ward and downward streams gives an average relative
humidity near 50 percent. As suggested by this discus-
sion, instantaneous water concentrations at a given at-
mospheric level in the free atmosphere and given loca-
tion have about a factor of 10 variability depending on
the instantaneous vertical motion patterns.
TRANSFORMATION AND SINKS
In terms of chemical reactions of the water molecule
itself, the most important role of water in the atmo-
sphere is as a source for OH through the reaction,
H2O + O(iD) ~ 20H.
The production of the OH radical is fundamental to
all the elemental cycles and is discussed in more detail
in each of the other cycles sections. In the form of drop-
lets, water provides the medium for numerous heteroge-
neous and homogeneous aqueous-phase reactions that
are also fundamental to all element cycles in the
troposphere.
On the microscopic scale, atmospheric water vapor is
converted to droplets or snowflakes by migration to
condensation centers, initially submicrometer cloud
condensation nuclei. Growth of the droplets or flakes
continues by further water vapor diffusion. When sizes
of several micrometers or so are reached, further droplet
growth occurs by collisional coalescence until the drops
reach sufficiently large size (~100 Em) that their fall
velocity exceeds the velocity of upward air motion.
Their fall velocity is determined by the balance between
downward gravitational acceleration and viscous
(Stokes) drag, and so increases with increasing radius.
From a macroscopic viewpoint, water vapor con-
denses because atmospheric motions have produced wa-
ter mixing ratios near their saturation values. The satu
OCR for page 108
108
ration mixing ratio decreases strongly with altitude
because of its temperature dependence. Thus water con-
densation is driven primarily by upward transport via
atmospheric motions. Conversely, sinking air tends to
be cloudfree and of low relative humidity. The latent
heat released by condensation processes can be of major
importance in maintaining or enhancing atmospheric
vertical motions. Two kinds of precipitation systems are
distinguished, according to whether the latent heat is
their primary drive or merely a positive feedback. Con-
vective precipitation systems are driven by the latent
heat they release. These generally occur on a horizontal
scale with a fine structure of the order of 1 km and a
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
large-scale organization of the order of 10 to 100 km.
Layered precipitation systems are driven by upward
motions in large-scale atmospheric wind systems forced
by other modes of atmospheric instability. Convective
precipitation can occur within layered systems.
BIBLIOGR APHY
Baumgartner, A., and E. Reichel (1975~. The World Water Balance.
Elsevier, Amsterdam.
L'vovich, M. I. (1979~. World Water Resources and Their Future.
American Geophysical Union, Washington, D.C., 415 pp.
OCR for page 109
109
OZONE
BY H. LEVY II
Ozone (03) is both an important oxidant in its own
right and a prerequisite for the production of hydro-
peroxyl and hydroxyl radicals. These radicals play key
roles in most of the elemental cycles and control the
atmospheric lifetimes of many of the short- and me-
dium-lived trace gases. Besides its chemical role, O3 is a
significant absorber of long-wave radiation. Changes in
the concentration of tropospheric O3 will not only
change the chemical lifetimes of many trace gases, but
may also affect the climate.
SOURCES
The major sources of tropospheric O3 are strato-
spheric injection and in situ photochemical production.
There is also a small indirect contribution from the com-
bustion source of NO2
The stratospheric injection of O3 has been observed
directly in the region of "tropopause folds," inferred
from radioactivity measurements, and calculated from
general circulation/transport models. These different
approaches all arrive at a cross-tropopause flux in the
range 3-12 x 10~° molecules per square centimeter per
second.
In situ photochemical production occurs both in the
polluted boundary layer and in the free troposphere as a
whole. Significant production of oxidant, in particular
03, has been clearly demonstrated in polluted urban
environments. Not only has the production been simu-
lated in smog chambers, but highly elevated O3 concen-
trations are frequently observed in areas with high con-
centrations of hydrocarbons and NOX. What is not
known at this time is the importance of this smog source
to the global troposphere. Summertime measurements
of O3 at 500 mb would suggest that this production
extends up into the middle troposphere over regions of
surface pollution. Positive correlations between fluctua-
tions in simultaneous CO and O3 vertical profiles have
also been observed in the free troposphere, particularly
over land at midlatitude in the northern hemisphere.
This has been interpreted as demonstrating that O3 has
the same source region, the polluted boundary layer, as
CO. A realistic estimate of the contribution from the
polluted boundary layer is not yet available.
These same smog reactions,
HO2 + NO ~ NO2 + OH,
and
followed by
NO2 + hi' ~ NO + 03,
should occur throughout the troposphere. Numerous
theoretical calculations have predicted column produc-
tion rates in the background troposphere of the range 1-
10 x 10~' molecules per square centimeter per second or
more. These calculations are, however, completely de-
pendent on theoretical predictions of the peroxy radical
concentrations and on predicted or measured concen-
trations of NO. At this time, there are many uncertain-
ties in both the calculations and the measurements.
Therefore, while the calculated production rates are
much higher than the stratospheric injection rates, they
are also less certain. On the other hand, they are so
much larger that they suggest an important role for
photochemical production in the troposphere.
SINKS
The two demonstrated removal paths for O3 are de-
struction at the earth's surface and in situ photochemical
destruction. A third, the fast reaction of O3 with biologi-
cally emitted organics in the surface layer, is very diff~-
cult to separate from surface deposition and may end up
being included in many measurements of surface de-
struction rates.
The surface destruction rate, frequently expressed as
a surface deposition velocity, is highly variable depend-
ingnot only on the nature of the surface, but, in the case
of vegetation, on the type of vegetation, time of year, and
even time of day. Various methods have been used to
measure deposition velocity over a number of surfaces.
These methods include: a direct measure of loss inside a
box that covers a particular surface; indirect measure-
ments based on inferring a flux from a measured vertical
gradient; and an indirect measurement using the eddy
correlation technique that calculates an eddy flux. Sur-
face deposition velocities, while highly variable, do ap-
pear to separate into two main categories: (1) Land
either bare or covered with vegetation has values of
deposition velocity (W0) that range from 2.0 cm/s for
daytime forests and cultivated crops to 0.2 cm/s over
nighttime grassland. Bare land falls in the low end ofthis
range. (2) Water, snow, and ice surfaces have values in
the range 0. 1 to 0.02 cm/s. Estimates of average global
fluxes to the surface have been made with different val-
ues for deposition velocity as a function of surface type
and different values for O3 concentration in the surface
RO2 + NO ~ NO2 + OH,
OCR for page 110
110
layer. There is considerable uncertainty in the distribu-
tion of surface types over the globe, deposition velocities
for particular surfaces, and the global distribution of O3
in the surface layer. Nonetheless, these calculations pre-
dict deposition fluxes in the same range as predictions of
. . . .
stratosp :lerlc 1nJectlon.
The other halfof O3 photochemistry is photochemical
destruction. At this time, the major removal path is --
thought to be:
O3 + he ~ O(~D) + 02,
followed by
O(iD) + H2O ~ 2 OH.
A number of other mechanisms have been suggested:
the destruction of O3 by HOX radicals; the oxidation of
NOX to nitrate and HNO3 and their resulting deposi-
tion; the reactions involving the oxidation of halogens,
particularly I, in the maritime boundary layer. Recent
analyses of some regional boundary layer data in the
equatorial Pacific strongly supports the existence of a
photochemical removal process with an effective
column removal rate of the order of 1-2 x 10~i mole-
cules per square centimeter per second. This removal
rate is much larger than the estimated surface deposition
flux and is needed to explain the extremely low O3 mix-
ing ratios (5 to 10 ppb) that were observed at that time.
Again, as for photochemical production, a realistic esti-
mate of the global importance of this process requires
accurate calculations of radical concentrations and de-
tailed knowledge of other trace gas concentrations.
DISTRIBUTION/CLIMATOLOGY
A global data- base, sufficient to produce a coarse
resolution O3 climatology, is urgently needed for the
field of tropospheric chemistry. Not only is it needed to
produce a global distribution of OH, the principal oxi-
dizing species in the various elemental cycles, but it is
needed to provide a framework for tropospheric photo-
chemistry as a whole. Due to its high variability in the
troposphere, relative standard deviations in the range of
25 to 100 percent, a realistic global data base will require
relatively high spatial and temporal resolution.
Both the Dobson network and satellite observations
provide a global field oftotal O3. Unfortunately, approx-
imately 90 percent of the total O3 resides in the strato-
sphere, and existing techniques are not able to accu-
rately extract the small fraction of the signal that
represents the troposphere. Therefore these global fields
are, at best, of qualitative use.
The best existing tropospheric data set is provided by
individual ozonesonde stations that are now measuring
or have in the past measured vertical profiles of O3 from
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
the ground to the middle stratosphere on a more or less
regular basis. Data from stations still operating are be-
ing archived by the Canadian Department of the Envi-
ronment. Unfortunately, there are a number of very
serious problems with this data set:
1. A number of different types of sensors have been
used, many of which were never accurately intercali-
brated. Serious doubts have recently been raised about
the absolute accuracy of the ozonesonde measurements
in the troposphere, particularly for the older types of
sondes that are no longer in operation or available for
intercomparisons with current devices. Previous inter-
comparisons of operational devices alone have raised
serious doubts about combining measurements from
different research groups with different devices into a
single data set.
2. Even if all the available data were useful, the
global coverage is completely inadequate. Almost all the
stations are in the northern hemisphere, and most of
them are at midlatitude. There are a few in the high
latitudes ofthe northern hemisphere, one in the tropics,
and one operating (we hope) in the southern hemisphere
at Aspendale, Australia. There are no stations in opera-
tion in any of the oceans, even at midlatitudes in the
northern hemisphere. If all the sites that are no longer
operating are included, there is minimal improvement
in the global coverage.
While a global data set does not exist, careful analysis
of either individual station data or individual networks
using a common sensor and measurement protocol has
produced many useful insights:
1. In all cases the mean proB0es of O3 increase with
height.
2. Where it has been analyzed, the variance profile
has a maximum in the upper troposphere and in the
boundary layer with a minimum in the middle tropo-
sphere. A more detailed analysis of variance in the As-
pendale, Australia, data finds in the troposphere that it
is dominated by synoptic and shorter time scales.
3. The profiles show a spring maximum with, in the
case ofthe midlatitude northern hemisphere continental
stations, a continuation of this maximum into the sum-
mer. When analyzed, the variance also appears to be
higher in the spring.
4. Both mean values and variability are greater at
midlatitudes in both hemispheres than in the tropics,
but the tropical data base is very limited. When data
from a common instrument are considered, there is still
some evidence for a midlatitude maximum, particularly
in the lower troposphere.
5. An analysis of the North American network finds
a significant east-west asymmetry on even the regional
OCR for page 111
TROPOSPHERIC CHEMICAL CYCLES
scale However, there is not enough longitudinal resolu- ISSUES
ton In the data to determine the extent of zonal asym
metry in the global O3 field.
A number of north-south transects through the mid
dle and upper troposphere are available. The one data
set that provides more than a single snapshot has severe
troubles with absolute calibration of the sensor and was
from the upper troposphere with the likelihood of air
craft incursions into the lower stratosphere at midlati
tude and high latitude. A few single snapshot transects
with relatively accurate O3 sensors in the middle tropo
sphere are also available. For this very sparse data set,
O3 iS a minimum in low latitudes, the values either level
offor decrease from midlatitude to highlatitude, and the
maxima occur at midlatitude in the hemispheric spring.
Although these few data sets are not time mean latitudi
nal fields, they may have captured certain latitudinal
features of O3. Given the 25 percent relative standard
deviations observed in profile data in the middle tropo
sphere, it is also possible that these few profiles are atypi
cal. A repeated series of flights over the same path with
an accurate and validated sensor is certainly needed.
By far, the best time series data are available from
continuous measurements of surface O3. Unfortu
nately, the boundary layer is frequently very unrepre
sentative of the troposphere as a whole. The mean val
ues may be strongly affected by local meteorology and
surface removal, as well as local photochemical produc
tion and destruction. It is not clear what, if anything,
can be inferred about the global troposphere from these
excellent time series. In remote clean regions, the data
appear to have many of the features observed in O3
profiles with the addition of significant and currently
unexplained diurnal fluctuations. They do appear to
have significantly lower variability than is observed in
the boundary layer of the profile data. In regions with
pollution sources of NOX, they show concentration max
. . ~ . .
ma extent sing trom spring into summer.
The final sources of distribution data are field gener
ated by general circulation/transport models. A recent
calculation of tropospheric O3 with only stratospheric
injection and surface removal, photochemical produc
tion and destruction having been excluded, has pro
duced a tropospheric climatology of O3 representative of
the model meteorology. To the extent that the model me
teorology is representative of atmospheric meteorology,
the model field should represent the transport contribu
tion to the real O3 field. Outside of the boundary layer in
general and continental regions with significant anthro
pogenic pollution in particular, the real O3 field may be
dominated by real atmospheric transport. This should
be particularly true of the variance in the O3 field.
Therefore, the model variance fields may be quite useful
in designing observational networks for O3.
111
.
.
The three key issues involving tropospheric O3 are as
follows:
1. Its climatology (i.e., tropospheric distribution of
mean values and higher moments);
2 . The process or combination of processes that exert
dominant control on its climatology;
3. The possible existence of long-term changes in the
mean concentration and the causes of such trends if they
do exist.
It is obvious that these three issues are intertwined. Fur-
thermore, it is obvious that the first requirement is the
development of a reliable data set. A few stations mak-
ing very accurate long-term measurements for the de-
tection of trends are needed, along with a significantly
larger number making accurate measurements for a few
years to establish at least a coarse global climatology.
Coupled with this is the continued development and
refinement of both the theory and numerical modeling
of tropospheric transport and fast photochemistry.
The earliest view of tropospheric O3 had it being
transported down from the stratosphere and being de-
stroyed at the earth's surface. Other than boundary
layer variability, which would result from the large inho-
mogeneity of the surface destruction process, the distri-
bution and variability would be controlled by meteoro-
logical processes on all scales. This view is still supported
by much of the observational data.
In the early 1970s, an active photochemistry was pro-
posed for the clean troposphere, which led to the predic-
tion of large photochemical production and destruction
rates. These predictions depended on many reactions
that have not been quantitatively confirmed in the real
troposphere and have as inputs species concentrations
that were not well known. Nonetheless, the calculated
photochemical production and destruction rates were
much larger than measurements and estimates of strato-
spheric injection and surface removal. Furthermore,
there were observations, particularly at midlatitude,
that supported a strong role for photochemistry in the
summer.
Recently, a unification of the transport and photo-
chemical theory was proposed in which photochemical
production occurred primarily in the upper troposphere
with the precursor NOX being injected from the strato-
sphere. Ozone destruction would then dominate in the
lower troposphere where NOX was very low. This O3
destruction has been observed in one set of data taken in
the tropical Pacific boundary layer. This theory depends
critically on a tropospheric NOX distribution, which
increases strongly with height.
A recent general circulation/transport model study
OCR for page 130
130
Organic fluorine gases, Fg, have been studied fairly
extensively, at least the chlorofluorocarbon class of Fg
species (because of interest in the sources of stratospheric
chlorine). The dominant Fg species by concentration is
CC12F2. Total Fg values are about 1 ,ug/m3 STP, and this
total is growing with time because of increasing anthro-
pogenic input and the long residence time of Fg species.
There are very few known occurrences of C-F bonds in
natural products, marine or terrestrial; FIT species are
entirely anthropogenic. For Fg, there are almost no data
in nonurban air. Tropospheric observations have been
limited to SF6 and to fluoride-based analyses (probably
but not necessarily of HF) in polluted air.
Fluoride in particles is likely due to fluorine-contain-
ing contaminants released in industrial processes. This
current view is based mostly on post-1977 data that have
shown lower F- levels in precipitation and particles than
were seen earlier with cruder analytical methods.
Although the question is not settled, present indications
are that observed distributions of F- in rain and parti-
cles are mostly from continental sources.
In the Element Cycle Matrices section of this docu-
ment (Appendix C), a brief summary appears for the
state of knowledge of distributions of halogens in the
troposphere.
SOURCES OF ATMOSPHERIC HALOGENS
As in our discussion of the distributions of halogens,
here we summarize briefly the sources of each halogen
element, roughly in order of decreasing available knowl-
edge.
Organic chlorine gases have natural and industrial
sources. The most prevalent species, CH3C1, is calcu-
lated to be furnished to the atmosphere at a rate of about
2 x 106 metric tons annually; less than 5 percent of this
source is industrial. Almost all other Clg species are
anthropogenic, or mostly so. Based on knowledge from
marine natural-products chemistry, it would not be sur-
prising to find natural sources of CHC13, or even CC14.
For CH3C1, it is suspected that natural sources include
marine microbial processes and biomass burning. It is
clear that there are no in situ atmospheric sources of Cod.
R-C1 molecules, where R is an organic group such as
CH3, are not synthesized in the open air of the earth's
oxidizing atmosphere.
The principal source of chlorocarbons and chloro-
fluorocarbons is from the escape of these substances
from their usages as solvents and degreasers and in
foam-blowing processes and refrigeration units, their
release from aerosol spray cans, and their use in a vari-
ety of specialized processes in electronics, medicine, and
manufacturing.
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
Very little direct and verified information is available
on sources of Clg species such as HC1. Although its
vertical and latitudinal distributions are not known,
HC1 is probably most concentrated (1 to 2 ppbv) in the
marine boundary layer, where its residence time is per-
haps 4 days. A global source of 108 metric tons of HC1
per year would be required to maintain this concentra-
tion of HC1. Independently, it has been estimated that 3
to 20 percent of the annual input of sea-salt chloride is
liberated from these particles as gaseous species, proba-
bly HC1. If so, 2-12 x 108 tons/yrofHC1 is so produced.
Also, volcanoes and combustion are thought to emit
perhaps 6 x lo6 and 3 x 106 tons/yr, respectively, glob-
ally. Although small in comparison with the global input
from volatilization of sea-salt chloride, these latter
sources could dominate regionally. All of these sources
are represented schematically in Figure 7.6, an outline
of tropospheric halogen cycles. Processes such as the
reactions of sea-salt aerosols with polluted continental
air masses could also release NO2C1, NOC1, C12, or
even gaseous NaCI. Particulate chloride in the marine
atmosphere results from sea-salt aerosol production. As
sketched in Figure 7.6, these particles become airborne
as a result of breaking waves, whitecap bubble-bursting,
and impact of precipitation drops on the sea surface.
Gas-to-particle conversion can also produce particulate
chloride. Over continents, there are volcanic and com-
bustion sources of HC! and particulate C1- .
Organic bromine sources are much less well under-
stood, especially in light of the springtime Arctic bloom
mentioned above. Neither the sources of the back-
ground or seasonally perturbed Brg levels are clear. It is
known that usage of the agricultural fumigant, CH3Br,
can inject some volatile CH3Br into the atmosphere, but
quantities are uncertain. Also, it is likely that some ofthe
observed atmospheric C2H4Br2 is from combustion of
automobile and truck fuel additives. Also, bromoform
(CHBr3) has been observed in the Arctic Ocean surface
waters. Many bromine-containing marine natural
products have been identified, and further investiga-
tions are needed. A few other anthropogenic bromocar-
bons are also of interest. Sources of inorganic bromine
gases have not been explored at all. Clearly, the tropo-
spheric oxidation of Brg species, largely by tropospheric
OH, must produce some Brg in situ. Sources of bromide
in aerosol particles and precipitation are probably an
incorporation of sea-salt bromide and scavenging of Brg
by clouds, rain, and aerosol particles.
Sources of tropospheric iodine have also been
explored only crudely. For Ig species, only CH3I has
been studied. There are indications that biogenic CH3I
from the oceans, possibly from biological methylation of
seawater I-, is an important source. The direct emission
of I' from seawater has been suggested from certain
laboratory experiments In which O3 was allowed to react
OCR for page 131
TROPOSPHERIC CHEMICAL CYCLES
8 km (26K')-H IGH LATITUDES
TR OPOSPH E R E
, and/or
Fog
Aqueous ~
\' Chemistry ~I
| Cloud ~N Ha ~ H NO3, NO2, SO2,
~ Cycl ing |
( Particulate X )
Washout/Rainout
, .. ... . - ~\ \ I
( HX (gases), RX, XO2?, XNO?
>~ CH3CI, CC12F2, CH3Br, CH31 J
RX Vapor
(CH3 1, ?)
~X2, HX
! Bubble & Sea Spray from: ~ ~ hi' (H 1, 12, ?
~, 1 Breaking Waves / \K ,:
__ 3. Precipitations
_~_ ~SEA (71.2% of total surface area of earth) _ ~.:: LAND (28.8% of total surface area of earth)
131
Dry Deposition
~\
RX from Land (?) ~HX from Land (?)
FIGURE 7.6 Schematic diagram to show processes and to exemplify key species in global tropospheric halogen cycles. X denotes F. C1, Br,
or I.
with dissolved I-. Particulate I- (and/or IO3 ~ is highly
enriched with respect to C1- in marine aerosols. Com-
pared to the seawater ratios, I-/C1- is usually 100 or
even 1000 times enhanced, especially on small particles.
Clearly, some fractionation process is at work at the air-
sea interface as particles are injected into the atmo-
sphere. The involvement of iodine-rich organic films
has been suggested. Aged aerosol particles do gather
gaseous iodine to increase the I-/C1- ratio further, and if
so, what are the sources of Ig that allow this?
Fluorine sources, especially those for Fg, are similar to
those for chlorine. In addition to the chlorofluorocar-
bons discussed above, a few pure fluorocarbons are also
of interest. One of these, CF4, is probably from alumi-
num ore processing, but possibly also from various car-
bon-electrode processes. Certain perfluoroethanes and
perfluorocylohexanes are also entering the atmosphere
now from a variety of specialized usages, often as inad-
vertent emissions. Fg species such as HE are known to be
pollutants from industrial processes such as aluminum
refining and cement production. Also, one can imagine
that gaseous HF is released from fluoride-containing
aerosol particles as these particles become drier and
acidified (HF is a weak acid compared to H2SO4 and
HOOD. Sources of F- in particles and precipitation
indude sea-salt input and industrial airborne particles.
REACTIONS AND TRANSFORMATIONS
OF HALOGENS
Considering the many and complex reactions
homogeneous gas phase, heterogeneous (gas-particle)
and homogeneous liquid phase that are possible with
halogens in the troposphere, research on them to date is
very sparse. Consequently, very little is known about the
mechanisms of halogen reactions and transformations.
By contrast, stratospheric halogen reactions are limited
to those in the gas phase, and these are known to be
important.
Organo-halogen gases, R-X, oflow molecular weighs
are generally volatile and not very soluble in water. Pho-
tochemical reactivity increases from fluorine to chlorine
to bromine to iodine, that is, as halogens replace hydro-
gen atoms in compounds; C-F bonds are stronger than
C-CT, C-Br, or C-I bonds. Perfluorocarbons and per-
chlorocarbons are generally stable in the troposphere
and are not susceptible to attack by 03, OH, or tropo-
spheric photons. Instead, they decompose only in the
stratosphere and above when attacked by vacuum ultra-
violet and electronically excited oxygen atoms. Other
R-X species exhibit a wide range of photochemical reac-
tivity. Some are photolyzed in the troposphere (e.g.,
CH3I), and most are dissociated by OH attack to form
inorganic halogen species.
OCR for page 132
132
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
Inorganic halogen gases are potentially important in chloride into the atmosphere is around 6 x 109 tons/yr,
tropospheric photochemical cycles, although no major but much ofthis chloride is airborne for a day or less. By
specific role has yet been proven. Volatile species exist contrast, the annual input of CC12F2 is only about 3 x
for each of the halogens, and a great variety of species 105 tons/yr of chlorine, but it is transported to the strato
need to be considered. For chlorine and fluorine, the sphere, and its residence time is 100 years. Accordingly,
stability of HC1 and HF greatly slows regeneration of C] the global atmospheric cycles of halogens and the sinks
and F atoms. Hydrogen donors, R-H, react readily with for atmospheric halogens are made up of terms that are
C1 and F to form HC1 and HF, so the free atoms and their difficult or meaningless to compare.
Oxides are not thought to be very prevalent. Longer The largest single sink for atmospheric halogens is
chain lengths for gas-phase catalytic processes are possi- represented by precipitation. Perhaps two-thirds of the
ble for bromine andiodine because HBr and HIareless large sea-salt particles that carry the most mass are
stable once formed. Possible roles for Ig species in removed by precipitation, and one-third by gravita
destroying O3 and affecting other tropospheric photo- tional settling. Ninety percent of the removal occurs
chemical cycles have been proposed, but great uncer- over oceans. Although these values have been deduced
tainties exist. Examples include lack of information on for chlorine, they are probably similar for bromine and
the levels of Ig concentrations, unavailability of certain iodine, whose sources are predominantly marine. Fluo
rine sinks are probably dissimilar in their distribution.
As is indicated in Figure 7.6, dry deposition removes
halogen gases and particles. Surface-active species like
HC1, HF, or HOC1 are probably most affected by dry
deposition; the least-affected species are probably the
organo-halogen gases. Finally, those portions of the
atmospheric halogen cycles that penetrate the strato
sphere, for example, perhalocarbons like CC12F2, have
their upward flows counterbalanced by downward
return flows of HC1 and HF in precipitation and dry
deposition, at least in a steady state.
chemical kinetic data, and possible interferences by het-
erogeneous processes. Until there are some field data on
specific halogen-containing inorganic species, little pro-
gress can be expected. Stratospheric investigations have
provided some guidance, especially for chlorine and
bromine, but direct tropospheric studies are needed.
Heterogeneous reactions and transformations need
attention, but have received little. There is strong evi-
dence that heterogeneous reactions are responsible in
great part for the very existence oftropospheric HCl, yet
few, if any, mechanistic studies have been performed.
There is some evidence that particulate bromine con-
centrations increase at night and decrease by day and
that gaseous bromine exhibits opposite diurnal behav-
ior, but no studies of possible mechanisms are available.
Similarly, the processes that lead to loss and/or uptake of
halogens from marine aerosols of various sizes have not
yet been investigated, nor have analogous processes in
precipitation been studied. Such investigations are
greatly hampered by a dearth of field data and of funda-
mental kinetic and photochemical data from the labora-
tory. Equilibrium-type data such as vapor pressures,
even when available, are not necessarily valid when
complicated multiphase, multiconstituent mixtures are
to be considered. Homogeneous aqueous-phase reac-
tions and transformations are potentially very impor-
tant in clouds, rain, and water-coated aerosol particles,
for example, with halogens as oxidizing agents, but vir-
tually no research has been performed on this topic.
REMOVAL PROCESSES FOR HALOGENS
A very large spectrum of time constants exists for the
residence times of various halogen-containing gases and
particles, and the global atmospheric cycles of the halo-
gens encompass both large and small reservoirs and
transfer rates. For example, the annual input of sea-salt
BIBLIOGRAPHY
Barnard, W. R., and D. K. Nordstrom (1982~. Fluoride in precipi-
tation: II. Implications for the geochemical cycling of fluorine.
Atmos. Erwiron. 16:105-1 1 1.
Berg, W. W., P. D. Sperry, K. A. Rahn, and E. S. Gladney (~1983~.
Atmospheric bromine in the Arctic. J. Geophys. Res. 88:6719-
6736.
Chameides, W. L., and D. D. Davis (1980~. Iodine: its possible role
in tropospheric photochemistry. J. Geophys. Res. 85: 7383-7398.
Cicerone, R. J. (~1981~. Halogens in the atmosphere. Rev. Geophys.
Space Phys. 19: 123- 139.
Duce, R. A., J. W. Winchester, and R. VanNahl (~1965~. Iodine,
bromine and chlorine in the Hawaiian marine atmosphere. I.
Geophys. Res. 70: 1775-1799.
Eriksson, E. (1959~. The yearly circulation of chloride and sulfur in
nature: meteorological, geochemical and pedological implica-
tions, 1. Tellusll:375-403.
Eriksson, E. (~1960~. The yearly circulation of chloride and sulfilr in
nature: meteorological, geochemical and pedological implica-
tions, 2. Tellus 12: 63 - 109.
Rasmussen, R. A., M. A. K. Khalil, R. Gunawardena, and S. D.
Hoyt (~1982~. Atmospheric methyl iodide (CH3I). J. Geophys. Res.
87:3086-3090.
Singh, H. B., L. J. Salas, and R. E. Stiles (~1983~. Methyl halides in
and over the eastern Pacific. i. Geophys. Res. 88:3684-3690.
World Meteorological Organization (~1981~. The Stratosph~e 1981.
WMO Global Ozone Research and Monitoring Project Report
No. 11. 503 pp.
OCR for page 133
133
TRACE ELEMENTS
BY R. DUCK
CURRENT ISSUES
The cycles of most trace elements in the troposphere
have received relatively little attention from atmo-
spheric chemists for several reasons. Trace elements,
i.e., all elements except C, N. O. S. H. and the halo-
gens, are present in such low concentrations that they
have little impact on the overall photochemistry of the
troposphere, its acid-base characteristics, or on climate.
Many, if not most, trace elements are present entirely in
the particulate phase and are not directly involved in
gas-particle conversion processes or other aspects of gas-
phase tropospheric chemistry. Being primarily present
in aerosol particles, their tropospheric residence times
are of the order of days to a few weeks, and there has
been relatively little effort to evaluate global-scale
changes in their distribution caused by human activity.
The tropospheric chemistry of many trace elements is
an important part of the present-day overall biogeo-
chemical cycles of these elements, but in most cases the
tropospheric part of these cycles is poorly known. For
example, the mobilization of Hg in the environment,
whether it be from natural or pollution sources, is pri-
marily through the troposphere in the gas phase, but
very few data are available on the Hg concentrations in
the remote troposphere, and even less is known about its
chemical speciation and primary sources. Phosphorus is
one of the primary nutrients in both the terrestrial and
marine biosphere, and there is some evidence that tro-
pospheric transport of P to the ocean may be significant
in certain regions. However, the understanding of the
spatial and temporal distribution of P in the tropo-
sphere, of its chemical forms and sources, and even of
whether a long-surmised gaseous species exists is
. . .
extreme y primitive.
Trace elements can most conveniently be separated
into two groups: Group A includes those elements that
almost certainly spend their entire tropospheric lifetime
on aerosol particles. Group B includes those elements
for which a vapor phase, or likelihood of a vapor phase,
exists. Group A includes such lithophilic elements as Al,
Fe, Na, Ca, Mg, Si, V, Cr. Cu. Mn, and the rare earths.
Group B includes such elements as B. Hg, Se, As, Sb,
Cd, Pb, and possibly Zn and P.
SOURCES AND TRANSPORT
Trace element distribution patterns in aerosol parti-
cles are of considerable use in determining sources,
transport paths, and deposition for the particles them-
selves. This is quite valuable since aerosol particles are
an important end product for virtually all tropospheric
cycles through heterogeneous and homogeneous reac-
tions, and they play a major role in weather and climate.
Through the use of interelement ratios, it is often possi-
ble to determine the sources for aerosol particles. For
example, Al/Sc ratios on aerosol particles similar to that
present in the earth's crust are an indication of a crustal
weathering source, whereas Na/Mg ratios similar to
that in the ocean suggest a marine source. The use of
such "reference" elements as Al or Sc for the crust, Na
or Mg for the ocean, and noncrustal V (i.e., that vana
d~um present on aerosol particles that Is not derived
from the earth's crust) for combustion of residual fuel
oil or Pb for the combustion of gasoline containing
tetraethyllead has proven quite useful. There are many
sources that have not been so easily tagged, but efforts to
determine appropriate trace element signatures are con-
tinuing. For example, B is being examined as a signa-
ture for coal burning, and As for smelter operations.
This approach is potentially useful for identifying other
specific sources of primary aerosols, including the ter-
restrial biosphere, volcanism, extraterrestrial particles,
and a number of specific pollution sources. Recent
efforts to identify regional source areas of aerosol parti-
cles through the use of a number of trace elements also
show considerable promise. Trace elements used have
included Se, Sb, As, Zn, In, noncrustal V, noncrustal
Mn, and their interelemental ratios.
There is growing evidence that anthropogenic proc-
esses, followed by long-range tropospheric transport,
can result in significant changes in the tropospheric and
oceanic concentrations of certain toxic and essential
trace elements on the near-global scale. For example,
concentrations of Pb, both in the marine troposphere
and in the surface waters of the Atlantic and Pacific
Oceans, particularly in the northern hemisphere, are
considerably elevated as a result of the burning of gaso-
line containing tetraethyllead on the continents and its
subsequent tropospheric transport over and deposition
to the oceans. The mobilization of other toxic elements
such as Hg and Se by fossil fuel combustion and As by
smelters and in herbicides and defoliants may be equiva-
lent to or greater than mobilization by natural sources.
In fact, as is the case for most cycles, one can make much
more accurate estimates of the global source strengths
from pollution sources for trace elements than from such
natural sources as volcanism, the oceans, and the bio-
sphere.
In particular, there is virtually no information on the
production of vapor-phase trace elements or direct pro-
duction of aerosol particles containing trace elements by
OCR for page 134
134
the terrestrial biosphere. The apparent increased vola-
tilization of Hg by higher plants (relative to release from
unvegetated soils of comparable concentration) has
been explained as a "detoxification" process, although
it may simply be an expression of the ion-concentrating
processes of plants combined with the reducing poten-
tials created by the charge separation processes of
metabolism. In any case, vegetation appears to have a
major role in the cycling of Hg. For a number of other
elements, including As, Sn, Se, Pb, and Sb, biological
methylation has been observed in the laboratory, and
methylated forms of many of these elements have been
observed in highly polluted areas. For a few elements,
ionic methyl compounds have been observed in uncon-
taminated regions. A further understanding of the bio-
logical production of methylated metals requires the
development of specific detection capabilities for these
species. The entire area oftrace element release from the
biosphere requires considerable effort in the future.
DISTRIBUTION
A growing data base is developing on the trace ele-
ment composition of aerosol particles in remote regions.
Reasonably good data are available over short time
periods from the boundary layer in both polar regions
and over the Atlantic and Pacific Oceans. Virtually no
data are available on the vertical distribution of trace
elements in these regions, however, and this information
is critical to evaluate sources and fluxes of the trace
elements. Information on the mass-size distribution of
trace elements on aerosol particles is of considerable
value in ascertaining sources and source processes for
these elements. Many additional data of this type are
required.
Very little is Mown about the chemical form of the
trace elements in the vapor phase, but for elements in
Group B. a vapor phase does, or is expected to, exist.
Mercury apparently exists primarily as a gas in the form
of elemental Hg, with evidence emerging for some
organic forms as well, probably methylated. Mercury is
one of the few metals whose ions can be reduced to the
metallic state at reduction potentials frequently found in
biological systems. The metal has an appreciable vapor
pressure at 25°C (~1 x 10-3 mm Hg3; thus the pres-
ence of gaseous elemental Hg in the troposphere is not
. .
surprlslng.
Although the vapor phase apparently dominates tro-
pospheric B and B(OH)3 has been suggested as the pri-
mary vapor phase, no measurements have corroborated
the presence of B(OH)3. Although there are a number of
volatile borane derivatives, their formation requires
much more reducing conditions than those apparently
achieved by microorganisms under anoxic conditions.
Measurement of specific B species in the troposphere is
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
clearly required before even a rudimentary understand-
ing ofthe B cycle is possible.
In many respects, Se parallels S in geochemical
behavior. Vapor-phase Se has been observed in remote
and urban regions, where it may account for about 25
percent of the total Se present. However, except for
dimethylselenide and dimethyldiselenide measured in
urban and near-urban areas in Belgium, the chemical
form of the vapor phase of Se is unknown.
Approximately 10 percent of the As in the marine
boundary layer is apparently present in the vapor phase.
The form of the vapor-phase As is also unknown,
although dimethyl arsinic acid has been observed in the
oxic marine environment and trimethyl arsine is known
to be produced by certain fungi.
Methylated forms of Se, Sb, Pb, and other trace ele-
ments have been observed in other compartments of the
environment (e.g., the ocean and plants). In the case of
Sb, Hg, and Pb, it is unclear whether methylation can
occur in oxic regions or only under anaerobic condi-
tions, where microorganisms are probably of considera-
ble importance. For all these trace elements in the vapor
phase, the data base from remote regions is extremely
small, in some cases being as few as 5 to 10 samples.
TRANSFORMATIONS ED SINKS
Very little information is available on transformation
reactions involving trace elements in the troposphere.
The residence time for the vapor phase of many trace
elements in the troposphere may be very short, perhaps
only minutes or hours, as is probably the case for certain
As and Pb species. From mass balance considerations,
the residence time of total (vapor plus particle) As in the
global troposphere has been estimated as ~ 10 days.
Evidence suggests that the residence time of vapor-
phase As is considerably shorter, however. For some
trace elements the vapor phase probably has a longer
residence time than the particulate phase and is the
dominant phase in the troposphere, as is likely the case
for Hg and perhaps B. The residence time for elemental
Hg may be as long as several months. Estimates of the
residence time of vapor-phase Se, B. etc., compounds
have not been made. It is likely that the removal of these
trace elements is primarily governed by precipitation
processes. For all these elements, woefully little is known
about the specific chemical species present. Information
on the chemical form of these trace elements in the vapor
phase and details of their tropospheric reaction paths
and rates are required before the importance oftransfor-
mation reactions to their tropospheric cycles can be eval-
uated.
Certain trace elements, particularly transition
metals, may be important as catalysts for reactions in
cloud and rain droplets. For example, the oxidation of
OCR for page 135
TROPOSPHERIC CHEMICAL CYCLES
SO2 to sulfate in solution is enhanced by the presence ot
Mn ions. The importance of transition metals as cata-
lysts depends on a number of factors, including abun-
dance, chemical form, stable oxidation states, bonding
properties, and solubility. Copper, manganese, and per-
haps vanadium may be important as homogeneous cat-
alysts in solution. For heterogeneous catalysis, solubility
is not important; and the metals above, as well as Fe, Ti,
and perhaps Cr. are of potential importance. The role
played by trace element catalysis in aqueous atmo-
spheric chemical reactions is largely unknown at
present, but potentially very important and should
. . .
receive 1ncreasec . attention.
BIBLIOGRAPHY
Braman, R. S., and M. A. Tompkins (1979~. Separation and
determination of nanogram amounts of inorganic tin and
methyltin compounds in the environment. Anal. Chem. 51:12-
19.
Brinckman, F. E., G. i. Olson, and W. P. Iverson (1982~. The
production and fate of volatile molecular species in the environ-
ment: metals and metalloids, in Atmospheric Chemistry, E. D.
Goldberg, ed. Springer-Verlag, Berlin, pp. 231-249.
Cunningham, W. C ., and W. H. Zoller ~ 1 98 1 ). The chemical
composition of remote area aerosols.~. Aerosol Sci. 12: 367-384.
Duce, R. A., R. Arimoto, B. I. Ray, C. K. Unni, and P. I.
Harder (1983~. Atmospheric trace elements at Enewetak atoll:
I. Concentration, sources, and temporal variability.~. Geophys.
Res. 88:5321-5342.
Fitzgerald, W. F., G. A. Gill, and A. D. Hewitt (1983~. Air/sea
exchange of mercury, in Trace Metals in Seawater, C. S. Wong,
E. Boyle, K. W. Bruland, I. D. Burton, and E. D. Goldberg,
eds. Plenum, New York, pp. 297-315.
135
Fogg, T. R., R. A. Duce, and J. L. Fasching (1983~. Sampling
and determination of boron in the atmosphere. Anal. Chem.
55:21 79-2184.
Galloway, I. N., I. D. Thornton, S. A. Norton, H. L. Volchok,
and R. A. N. McLean (1982~. Trace metals in atmospheric
deposition: a review and assessment. Aimos. Environ. 16:1677-
1700.
Graham, W. F., and R. A. Duce (1979~. Atmospheric pathways of
the phosphorus cycle. Geochim. Cosmochim. Acta 43: 1195- 1208.
Harrison, R. M. and P. H. Laxen (1978~. Natural sources of
tetraalkyllead compounds in the atmosphere. Nature 275: 738-
739.
Jiang, S., H. Robberecht, and F. Adams (1983~. Identification
and determination of alkyl selenide compounds in environ-
mental air. Atmos. Environ. 17: 1 1 1-1 14.
Lantzy, R. L., and F. T. Mackenzie (1979~. Global cycles and
assessment of man's impact. Geochim. Cosmochim. Acta 43:511-
515.
Maenhaut, W., H. Raemdonck, A. Selen, R. Van Grieken, and
I. W. Winchester (1983~. Characterization of the atmospheric
aerosol over the eastern equatorial Pacific. i. Geophys. Res.
88:5353-5364.
Mosher, B. W., and R. A. Duce (1983~. Vapor phase and particu-
late selenium in the marine atmosphere. ]. Geophys. Res.
88:6761 -6768.
Nriagu, J. O. (1979~. Global inventory of natural and anthropo-
genic emissions of trace metals to the atmosphere. Nature
279:409-411.
Rahn, K. A., and D. H. Lowenthal (1984~. Elemental tracers of
distant pollution aerosols. Science 223: 132- 1 39.
Settle, D. M., and C. C. Patterson (1982~. Magnitudes and
sources of precipitation and dry deposition fluxes of industrial
and natural leads to the North Pacific at Enewetak. ]. Geophys.
Res. 87:8857-8869.
Slemr, F., W. Seller, and G. Schuster ~ 1 98 1 ). Latitudinal distribu-
tion of mercury over the Atlantic Ocean. J. Geophys. Res.
86:1159-1166.
Walsh, P. R., R. A. Duce, and J. L. Fasching (1979~. Consider-
ation of the enrichment, sources, and flux of arsenic in the
troposphere. J. Geophys. Res. 84: 1719-1726.
OCR for page 136
136
AEROSOL PARTICLES
BY I. M. PROSPERO
An aerosol is defined as a suspension of fine liquid
and/or solid particles in a gas. In the case of interest to
us, that gas is the atmosphere. Although in the strict
sense of the definition the word aerosol refers to the
particle and gas phases as a system, the term is often
used to refer to the particle phase alone.
The subject of aerosol particles is somewhat anoma-
lous in the general context of this section on cycles since
all the other subjects focus on specific chemical species.
Indeed, in most cases, the atmospheric aerosol is the end
product of many of the chemical processes acting on the
aforementioned species. Many of these reaction prod-
ucts are relatively unreactive in the aerosol phase.
Because of this unreactivity and because of the relatively
short residence time of aerosols in the troposphere (on
the order of a week or two), the aerosol phase can be
considered to be, in effect, the sink for many gas-phase
species (see Figure 7. 7~.
Aerosol particles are treated as a separate cycle
because they can have an impact on a number of impor-
tant physical processes in the atmosphere. The relation-
ship between aerosol properties and atmospheric proc-
esses is depicted in Part 1, Figure 2.2. For example,
aerosols play an important role in the hydrological
cycle they can affect cloud microphysics, which, in
turn, can affect the types, amounts, and distribution of
FIGURE 7.7 Aerosols as an end product
of atmospheric reactions. Major reaction
pathways for gas-phase constituents are
depicted by solid lines. Interactions
between chemical families are indicated by
dashed lines. Heavy (double) arrows show
key heterogeneous pathways involving
aerosols (A) and precipitation (P) (Turco et
al., 1982).
rainfall. Clouds play a critical role in fixing the albedo of
the earth; thus, if aerosols affect the amount, type, and
distribution of clouds, then changes in aerosol concen-
tration and properties could have the effect of changing
the albedo.
Aerosol particles can affect light as it passes through
the atmosphere by the mechanisms of scattering and
absorption. The most obvious radiative consequence of
airborne particles is the appearance of haze and degra-
dation of visibility. Less obvious, but more important,
are the possible effects of these same particles on the heat
balance ofthe earth. Particles can cause a decrease in the
amount of radiation reaching the ground, can increase
or decrease the albedo, and, if the aerosol absorbs light,
can cause atmospheric heating. In order to understand
these optical effects, it is necessary to know the chemical
composition of the aerosol and its size, because these
characteristics will determine the aerosol scattering and
absorption properties. Also, the composition of aerosols
is often size dependent; thus a specific physically (or
chemically) active species could be concentrated in a
limited portion of the particle size spectrum.
Finally, there is the concern about the impact of many
pollutants on health. The vector for many ofthese harm-
ful species is the aerosol particle. However, the ability of
aerosol particles to penetrate to the lung is dependent on
A
HO2
~ HNO2 ~ p
NO ~
''ti ~ HNO3: P. A
A, p ~ ~02 - HO2NO2
I \ \ ~ ~ A' ,, ---~ NH NH3
~ \ ~ a' A, P
t He' - ~(CH3)2S
C - ;: I :~HSO3:
CH3OOH CHO) CS2 H2S H2SO4
~ cod ~
P,A A,P
OCR for page 137
TROPOSPHERIC CHEMICAL CYCLES
the aerosol size and its chemical properties as a function
~
OI SlZe.
. ,,
SOURCES
From the standpoint of production processes, aerosol
particles can be categorized as being either primary or
secondary. Primary aerosols are those directly emitted
as a solid or a liquid, while secondary aerosols are
derived from materials initially emitted as gases. Impor-
tant natural primary aerosols are the salt residue from
sea spray, wind-blown mineral dust, ash from volcanic
eruptions, and organic particles from biota (pollen,
spores, debris, etch. A major class of anthropogenic
primary aerosol is smoke particles. However, the burn-
ing of land biota is another major source of smoke; this
source can be either natural or man-made, depending
on how the fire was started.
There are a number of major sources of secondary
aerosols. As described in preceding sections, many bio-
logical systems emit gaseous species that are eventually
converted to aerosols. Finally, many of man's activities
release gases that are aerosol precursors.
Estimates of annual aerosol mass production rates are
summarized in Table 7.8. The quality of these data
leaves much to be desired. The wide range of estimates
for some species is a reflection of the poor state of knowl-
edge of their sources. Nonetheless, it is clear that knowl-
edge of the input rates from anthropogenic sources is
much better than that of the rates from natural sources.
In the category of primary particles, natural sources
(predominantly sea salt and soil dust) far outweigh
anthropogenic sources. However, it must be borne in
mind that the anthropogenic sources are concentrated in
a relatively small area and hence will be much more
significant on a local and regional scale. Obviously, the
major source of sea-salt aerosol is the ocean. Although a
fair amount of research has focused on the sea-salt aero-
sol production mechanism and especially on the rate of
production as a function of various environmental
parameters, there is still considerable debate on the
results of such work and their interpretation. The rates
of production and the physical and chemical characteris-
tics of sea-salt aerosol are important for a number of
reasons. For example, salt spray may play an important
role in transporting trace metals and organic materials
from the ocean into the atmosphere and in absorbing or
reacting with gaseous species in the marine boundary
layer. Because the oceans are so large and the mass of
sea-salt aerosol is so great, these processes must be
understood in order to understand global chemical
cycles in general.
The major sources of soil aerosols are arid regions.
Clearly, much soil dust, such as that from the Sahara, is
137
generated by purely natural processes. However, in
some cases, the distinction between natural and anthro-
pogenic sources is not so clear. For example, recent work
has shown that large quantities of soil dust are being
transported out of Asia far into the North Pacific. Much
of this material is believed to have been deflated from
agricultural regions in China in the spring after the soils
have been ploughed for planting. Also, in the United
States, the primary standard for total suspended partic-
ulate materials, as defined in the Clean Air Act, is most
widely violated in agricultural areas because of the
mobilization of soil dust by farming activity. It is clear
from Table 7.8 that there is considerable uncertainty in
the rates of mobilization of soil dust. However, it is even
more uncertain as to what fraction is derived from natu-
ral sources as a consequence of natural processes.
The range of estimates for production rates from vol-
canoes and biomass burning is extremely large. This
uncertainty is a consequence of the difficulty in obtain-
ing data on sources that are sporadic, widely dispersed,
and relatively inaccessible.
TRANSPORT
Aerosols, whether primary or secondary, can be
transported great distances in the atmosphere. As previ-
ously stated, large quantities of soil aerosols are rou-
tinely transported thousands of kilometers over the
oceans from their sources in continental regions. Simi-
larly, pollutant aerosols can be transported great dis-
tances. For example, of the acid species in aerosols over
the northeast United States and Canada, a large fraction
is derived from sources in the central United States. On
a larger scale, the trace metal composition of aerosols in
Arctic haze episodes suggests that the sulfate-rich parti-
cles are adverted primarily from sources in Europe and
Asia. These interpretations are supported by meteoro-
logical studies and trajectory computations.
Aerosol species such as sea salt and soil dust are rela-
tively inert, and their principal physical and chemical
properties remain essentially unchanged during trans-
port in the atmosphere. Because of their relatively con-
servative nature, these species can serve as tracers for
atmospheric transport and removal processes. Some
research along these lines has already begun with some
success. Given a sufficiently large data base, measure-
ments of these species could be used to validate atmo-
spheric transport models that are currently under devel-
opment. Unreactive species are most useful for such
validation because it is not necessary to make any
assumptions about in situ chemistry during transport.
An advantage of using aerosols as tropospheric tracers is
that their lifetime is of the same order as the lifetime of a
typical synoptic meteorological event, about a week.
OCR for page 138
Source
After
Peterson
and Junge
(1971),a
<5,um
After Hidy
and Brock
(1971)a
138
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
TABLE 7.8 Estimates of Global Particle Production from Natural and Man-Made Sources ~ ~ o6 tons/yr)
After Study of
Marl 's Impact on
Climate ( 1 9 7 ~ ja Other
< 20,um < 6 lamb Estimatesa
Man-Made
Direct particle production
Transportation 1.8
Stationary fuel sources 9.6
Industrial processes 12.4
Solid waste disposal 0.4
Miscellaneous 5.4
Subtotal 29.6 37-110 10-90 6-54 54
126
Particles formed from gases
Converted sulfates 200 110 130-200
Converted nitrates 35 23 30-35
Converted hydrocarbons 15 27 15-90
Subtotal 250 160 175-325 270
Total man-made 280 269 185-415 396
Natural
Direct particle production
Sea salt
Windblown dust
Volcanic emissions
Meteoric debris
500
250
25
o
1095 300 180
60-360 100-500 60-300
4
0.02-0.2
25-150 15-90
Forest fires 5 146 3-150
Subtotal 780 1610 428- 1100 1730
Particles formed from gases
Converted sulfates 335 37-365 130-200
Converted nitrates 60 600-620 140-700 160
Converted hydrocarbons 75 182-1095 75-200 154
Subtotal 470 2080 345-1100 1319
Total natural -1250 3690 773-2200 3049
Grand Total - ~1530 3959 958-2615 3445
1000-2000
70
60-360
128 + 64
200 + 100
4.2
1-10
0.02-0.2
aFor references see Bach (1976) or Prospero et al. (1983).
hValues are for particles < 6 ,um as recomputed byJaenicke ( 1980).
SOURCE: Bach, 1976.
TRANSFORMATIONS
Gases can react in the atmosphere to produce nonvol-
atile products that end up in the aerosol phase. It is clear
from Table 7.8 that for many species the quantities of
aerosol produced in this manner equal or exceed the
quantities emitted directly as particles. This is true for
sulfate and nitrate species that are currently of interest
because of their role in the formation of acid rain.
Clouds play a dominant role in the formation, modifi-
cation, and removal of aerosols. The condensation of
water vapor on particles, and the phoretic, diffusive, or
inertial capture of particles by droplets lead to the incor
poration of particles within the aqueous phase. Solution
reactions, including those with dissolved gases, become
possible, and transformations can occur. If subsequent
droplet growth leads to precipitation, the aerosol is
removed from the atmosphere. However, if the droplet
reevaporates, as it does in over 90 percent of the cases,
then the aerosol is regenerated, but its size and composi-
tion are changed. Cloud cycling is probably the major
mechanism for modifying the atmospheric aerosol in the
lower troposphere. In contrast to the processes of parti-
cle interactions and coagulation, which are reasonably
well understood, the cloud cycling aspects of aerosol-
hydrometeor-gas interaction are poorly understood.
OCR for page 139
TROPOSPHERIC CHEMICAL CYCLES
REMOVAL PROCESSES
Precipitation is the major mechanism for the removal
of aerosols from the atmosphere. For example, it has
been shown that 80 to 90 percent of the radioactive
fallout deposited on the earth's surface was brought
down in precipitation. In turn, the composition of pre-
cipitation is determined to a considerable extent by the
composition ofthe aerosol phase.
Gravitational removal is important only for relatively
large particles (i.e., those larger than about 10-pm
diameter). Sedimentation will be important for soil
aerosols close to the source area and for sea-salt aerosols.
Sedimentation is generally not important for anthropo-
genic materials. Pollution control measures have
sharply reduced the rates of emission of large particles.
On the other hand, the size of secondary aerosols is less
than 1-,um diameter, and consequently, these aerosols
have a very small settling velocity.
DISTRIBUTION
Because of the relatively short residence time of aero-
sols in the atmosphere, their distribution will be closely
linked to the distribution and activity of sources and to
the controlling meteorological phenomena. Thus, in
order to characterize any trends in the concentration
and distribution of aerosols, it will be necessary to sam-
ple frequently on a broad spatial scale that encompasses
the suspected major source regions and the dominant
meteorological systems. Some well-conceived regional
sampling programs are currently in place. However, on
a larger scale, with a few exceptions, current sampling
efforts leave much to be desired from the standpoint of
the species studied, the quality ofthe data, the frequency
of sampling, and the location of stations.
CONCLUSIONS
We can identify a number of areas that warrant fur-
ther research on aerosols:
1. The role of aerosols in geochemical transport and
anthropogenic impacts. The atmosphere is an impor-
tant mode of transport for many species. For example,
the anthropogenic emissions of sulfur, mercury, and lead
to the atmosphere already exceed the stream loads for
these elements, while the emissions of copper, arsenic,
zinc, tin, selenium, molybdenum, antimony, and silver
are within a factor of 10 of stream fluxes.
2. Gas-particle processes. The photochemical and
chemical reactions that initially transform gases into sec-
ondary reaction products are extremely complex and
139
not yet fully understood. The particles formed by these
mechanisms are mostly in the "fine particle" size range
(i.e., submicrometer). It is estimated that several hun-
dred million tons of fine particles are formed every year
as a consequence of the emission and subsequent reac-
tion of natural and anthropogenic gaseous species.
The role of organics in aerosols is poorly understood.
Yet, the concentration of particulate organic carbon in
the atmosphere is quite high. For example, in most
ocean areas, the mean value is comparable to that of
mineral aerosols and non-sea-salt sulfate and nitrate.
Gas-to-particle conversion appears to be a major mech-
anism for the production of fine-particle carbon over the
oceans and also over the continents. Unfortunately,
there are very few concurrent measurements available
for both the vapor and the particulate phases.
3. The role of aerosols in the hydrological cycle. It is
clear that clouds play a major role in the formation and
removal of aerosols. As stated earlier, any process that
acts on clouds could have an impact on weather and
climate. There is now sufficient evidence to conclude
that anthropogenic emissions, especially of sulfur and
nitrogen species, do indeed have an impact on cloud
microphysics. Given the importance of sulfur in the
hydrological cycle and bearing in mind that about half of
the global flux of sulfur has an anthropogenic origin,
there is good cause for concern that man may be altering
weather on a larger scale. Of particular interest are the
possible effects on the urban and regional scale where
the magnitude of the anthropogenic sulfur sources is
dramatically higher for example, in the eastern
United States, where it is 10 times that of natural
sources.
The assessment of the impact of man on weather or
climate is difficult for a number of reasons. One has to
do with the fact that climate is subject to variations that
are completely natural in origin. Thus, until there is a
better understanding of the mechanisms that determine
climate, it will be difficult to ascertain the ways in which
it has been changed, or might be changed, by anthropo-
genic activities. Therefore, research efforts directed at
elucidating impacts must be balanced by efforts directed
at gaining an understanding of basic processes. In the
case of aerosols, one of the basic processes of great
importance is the role of aerosols in cloud physics.
Although the impact of aerosols on the hydrological
cycle from the standpoint of precipitation quantity and
distribution cannot be quantitatively assessed with cer-
tainty, it can be stated with certainty that there has been
a very marked impact on precipitation quality, most
notably in the increased acidity of rain.
4. Radiative transfer. There are a number of differ-
ent aerosol types that are important and that could play
OCR for page 140
140
a major role in climate by altering the radiation budget
of the earth:
a. Volcanic debris. Most important in this cate-
gory are the sulfur species that are injected into the
stratosphere, where they are oxidized to sulfuric acid
droplets, which have a residence time of years.
b. Soil dust. About one-third of the surface of the
continents is arid and a potential source of soil dust.
Here the impact of man on soil mobilization is a major
concern.
c. Elemental carbon (soot). This material is
highly efficient absorber of radiation; thus it is impor-
tant that its abundance and distribution be measured.
However, scientists have a very poor idea of the global
budget of carbon because much carbon is produced in
remote regions by slash-and-burn agricultural practices
and because carbon aerosols are difficult to measure
with current analytical techniques.
5. Characterization of temporal and spatial trends.
A recurring conclusion in the assessments of the possible
impact of aerosols on climate is that there is a serious lack
of information about the composition, concentration,
and physical properties of aerosols and their temporal
and spatial variability.
Despite the undeniable evidence that anthropogenic
materials are being transported over great distances,
there is no evidence that the particles have significantly
reduced atmospheric transmission in remote regions.
For example, there is no evidence of any long-term
decrease in transmission in the data from the Manna
Loa Observatory or from the Smithsonian Astronomi-
cal Observatories. However, we do not mean to mini-
mize the possibility that such increases may be in the
process of occurring; we merely emphasize that, with
current measurement techniques and the length of the
records on hand, one cannot separate any trends, if they
exist, from the natural variability of the atmospheric
aerosol. Indeed, it is essential that the natural processes
be understood before any anthropogenic effects can be
identified.
BIBLIOGRAPHY
Bach, W. (1976~. Global air pollution and climatic change. Rev.
Geophys. Space Phys. 14:429-474.
Barry, R. G., A. D. Hecht, J. E. Kutzbach, W. D. Sellers, T.
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
Webb, and P. B. Wright (1979). Climatic change. Rev. Geophys.
Space Phys. 17:1803-1813.
Blanchard, D. C. (1984~. The production, distribution and bacte-
rial enrichment of the sea-salt aerosol, in The Air-Sea Exchange of
Gases and Particles, W. G. N. Slinn and P. Liss, eds. D. Reidel,
Boston, Mass., pp. 407-454.
Charlson, R. I., and H. Rodhe (1982~. Factors controlling the
acidity of natural rainwater. Nature 295:683-685.
Dockery, D. W., and J. D. Spengler ( 1981~. Indoor-outdoor rela-
tionships of respirable sulfates and particles. Atmos. Environ.
15:335-343.
Edmonds, R. L., ed. (1979~. Aero biology, The Ecological Systems
Approach, US/IBPSynthesis Series, Vol. 10. Dowden, Hutchinson
and Ross, Stroudsburg, Pa., 386 pp.
Friedlander, S. K. (1978~. Aerosol dynamics and gas-to-particle
conversion, in Recent Developments irz aerosol Science, D. T. Shaw,
ed. Wiley, New York, pp. 1-24.
Galloway, J. N., G. E. Likens, W. C. Keene, and J. M. Miller
(1982~. The composition of precipitation in remote areas of the
world.~. Geophys. Res. 87:8771-8786.
Hinds, W. C. (1982~. Aerosol Technology. Wiley, New York, 424 pp.
Holland, W. W., A. E. Bennett, I. R. Cameron, C. du V. Florey,
S. R. Leeder, R. S. F. Schilling, A. V. Swan, and R. E. Wailer
(1979~. Health effects of particulate pollution: reappraising the
evidence. J. Epidemiol. 110:525-659.
Jaenicke, R. (1980~. Atmospheric aerosols and global climate. J.
AerosolSci. 11:577-588.
Lodge, Jr., J. P., A. P. Waggoner, D. T. Klodt, and C. N. Crain
(19813. Non-health effects of airborne particulate matter.
Atmos. Environ. 15:431 -48-2.
Pewe, T. L., ed. (1981~. Desert dust: origin, characteristics and
effect on man. Geol. Soc. Amer. Spec. Pap. 186, 303 pp.
Podzimek, J. (1980~. Advances in marine aerosol research. J. Res.
Atmos. 14:35-61.
Prospero, J. M. (1981~. Aeolian transport to the world ocean, in
The Sea, Vol. 7, The Oceanic Lithosphere, C . Emiliani, ed. Wiley
Interscience, New York, pp. 801-874.
Prospero, I. M., V. Mohnen, R. .iaenicke, R. Charlson, A. C.
Delany, l. Moyers, W. Zoller, and K. Rahn (1983~. The atmo-
spheric aerosol system: an overview. Rev. Geophys. Space Phys.
21: 1607-1629.
Rahn, K. A. (1981~. Relative importance of North America and
Eurasia as sources of Arctic aerosol. Atmos. Er~virorz. 15:1447-
1456.
Seller, W., and P. I. Crutzen (1980~. Estimates of gross and net
fluxes of carbon between the biosphere and the atmosphere
from biomass burning. Clim. Change2:207-247.
Subcommittee on Airborne Particles (SAP) (1979~. AirborneParti-
cles. National Research Council. University Park Press, Balti-
more, Md.,343 pp.
Toon, O. B., and J. B. Pollack (1980~. Atmospheric aerosols and
climate. Amer. Sci. 68: 268-278.
Turco, R. P., O. B. Toon, R. C. Whitten, R. G. Keesee, and P.
Hamill (1982~. Importance of heterogeneous processes to
tropospheric chemistry: Studies with a one-dimensional
model, in HeterogeneousAtmospheric Chemistry. Geophysical Mon-
ograph Series, Vol. 26, R. Schryer, ed. American Geophysical
Union, Washington, D.C., pp. 231-240.
Representative terms from entire chapter:
aerosol particles
