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OCR for page 11
~ AFr rk
To date, much of the research in tropospheric chemis-
try has focused on isolated questions or on one of the
many elemental cycles, for example, carbon, nitrogen,
or sulfur. Because of the complexity of tropospheric
chemistry and the poor state of knowledge at that time,
narrowly focused studies and exploratory programs
were justified. Indeed, such studies were successful in
that they advanced the field of tropospheric chemistry to
its present status where one can begin to discern the
overall structure and interaction of the chemical systems
in the troposphere. Because of these complex interac-
tions and the dynamic nature of the chemistry and phys-
ics of the troposphere, we believe that future advances in
many areas of tropospheric chemistry can be best
achieved by fostering research within a unifying concep-
tual framework based on atmospheric chemical proc-
esses. This framework is that of geochemical and bio-
geochemical cycles.
Four major categories of processes dominate chemi-
cal cycles in the troposphere: those related to sources, to
chemical reactions and transformations, to transport,
and to removal. Within the context of tropospheric
chemistry, a chemical cycle begins when a substance is
emitted into the troposphere. Consequently, a knowl-
edge of the strength and distribution of sources is criti-
cal. Materials injected into the troposphere can undergo
chemical reactions, some of which are cyclic and some of
which produce a wide range of species that can have
chemical and physical properties very different from
those of the reactants; such transformations can effec-
tively remove the species from a specific chemical cycle.
The distribution of a species in the troposphere will be
dependent on source characteristics and on the control-
ling chemical reactions; however, distributions are also
greatly affected by a variety of transport processes that
range in scale from that of boundary layer turbulence to
that of planetary flow. Often, physical interactions oc-
cur in which the composition can influence the radiative
or physical properties of the atmosphere or the underly-
ing surface. Finally, the tropospheric cycle is terminated
by removal of the species from the reacting system, usu-
ally through deposition at the earth's surface.
In this chapter, we outline a research strategy for tro-
pospheric chemistry that encompasses these four funda-
mental processes and their roles in mediating atmo-
spheric physical processes. A major effort would also be
directed toward the development of global tropospheric
chemistry models that can satisfactorily incorporate and
describe these processes. A process-oriented discussion
of tropospheric chemistry cycles has heuristic advan-
tages over one that focuses on individual cycles in that it
incorporates in a coherent manner many interrelated
aspects of tropospheric chemistry. The disadvantage of
this approach is that important aspects of tropospheric
chemistry might be ignored if they do not fit into the
framework ofthe discussion. The fact that a specific area
11
OCR for page 12
12
of research is not mentioned in this program is not
meant to imply that such work necessarily has a low
priority. Similarly, the fact that the program calls for an
increased level of cooperation and coordination does not
mean that it must be highly regimented. Indeed,
throughout the report, we stress that this effort can suc-
ceed only if independent active research scientists partic-
ipate fully in its design, implementation, and manage
PART I A PLAN FOR ACTION
meet. Such participation is essential because the
program is an evolutionary one in which there must be
. . . . .
an ~nterc range among scientists wor ring on various
processes and a feedback between experimental scien-
tists, theoreticians, and modelers. Finally, the research
activities of independent individual scientists, unaff~i-
ated with the proposed program, will continue to be
needed for the health of this scientific field.
SOURCES
Primary sources for tropospheric substances fall into
three major categories: surface emissions, in situ forma-
tion from other species, and, in some cases, injection
from the stratosphere. In each of these categories there
are natural and anthropogenic components.
Natural surface sources are affected by a wide range
of factors such as season, temperature, nutrient level,
organic content, and pH; consequently, source
strengths are highly variable in space and time. The
terrestrial and marine biospheres are of particular im-
portance as natural sources for chemical species in all the
element cycles, but very little quantitative information
is available on these sources. Some measurements of
local fluxes from biological sources have been made for
methane (CH4) and a few reduced sulfur compounds.
Estimates of local source strengths in the ocean, as well
as in rivers and lakes, have been made for methyl chlo-
ride (CH3C1), N2O, and reduced sulfur compounds by
measuring their degree of supersaturation in surface
waters, a quantity that varies greatly. To estimate fluxes
from measured supersaturations, theoretical models of
gas exchange across the water surface are required. At
this time, it is not possible to make accurate global esti-
mates of the natural surface sources for such globally
important trace gases as CH4, N2O, ammonia (NH3),
carbonyl sulfide (COS), carbon disulf~de (CS2), hydro-
gen sulfide (H2S), dimethyl sulfide ((CHINS), and
CH3C1. It is even more difficult to determine the natural
source strengths of the many reactive hydrocarbons that
are released by vegetation. Future research must not
only develop equipment and experimental protocols
that are capable of accurately determining the local
fluxes from the biosphere, but it must also concentrate
on understanding the biological, chemical, and mete-
orological factors that control these fluxes.
For the United States and Europe, there are relatively
accurate inventories of the anthropogenic sources of
combustion products, such as nitric oxide (NO), nitro-
gen dioxide (NO2), carbon monoxide (CO), and sulfur
dioxide (SOL; considerably less accurate estimates are
available for the rest of the globe. The same is true of a
wide range of industrial emissions. Emission rates from
low-technology combustion, such as wood fires and
slash-and-burn agriculture, are much less accurately
known, even on local or regional scales. There is grow-
ing evidence that biomass burning, particularly in the
tropics, may be a significant source of many trace spe-
cies in the troposphere. In general, existing estimates of
fluxes to the troposphere from anthropogenic surface
sources are much more accurate than those available for
natural sources. We emphasize here the need for re-
search on the latter, and indeed on a subset biological
natural sources.
Trace substances are introduced above the lowest lay-
ers of the troposphere by in situ sources, both human
(e. g., aircraft and tall stacks) and natural (e. g., lightning
and volcanoes). While in situ sources are much smaller
than surface sources on a global scale, they are more
effective than surface sources because they inject water-
soluble and surface-reactive gases into the middle and
upper troposphere, where gas lifetimes are considerably
longer than near the surface. Lightning is possibly an
important source of nitric oxide and nitrogen dioxide
(both hereafter referred to as NOX>, and volcanoes can
be major episodic sources of sulfur compounds and
other materials. Aircraft and tall stacks are important in
situ sources of anthropogenic NOx and SO2. Many spe-
cies are produced in situ by various tropospheric photo-
chemical processes; these processes are considered to be
secondary sources and will be discussed in the sections
dealing with transformations.
Stratospheric sources are the result of high-energy
ultraviolet radiation that can dissociate species such as
N2O and oxygen, which are normally nonreactive in the
troposphere. Materials are transported from the strato-
sphere through the tropopause into the troposphere;
such stratospheric injections serve as a relatively uni-
form source of many species compared to emissions of
the same species from surface sources. However, cur-
rent theories, observational data, and dynamical model
calculations all show that injections into the troposphere
occur predominantly at midlatitudes and more in the
northern than the southern hemisphere. Stratospheric
injection of ozone (03), originally produced in the mid
OCR for page 13
.A FRAMEWORK
die stratosphere, is a major and perhaps dominant
source of tropospheric O3. There is considerable contro-
versy about the importance of stratospheric injection of
O3 relative to in situ photochemical production; this
controversy extends to some other substances as well.
13
Although the stratospheric injection of NOX and nitric
acid (HNO3) is much smaller than the combustion sur-
face source, it could be the dominant source for these
substances in the upper troposphere.
TRANSPORT AND DISTRIBUTION
Transport processes involve a wide range of space and
time scales. However, the discussion of the troposphere
can be greatly simplified by separating it, conceptually,
into three layers: the planetary boundary layer, the free
troposphere, and the tropopause region.
The planetary boundary layer (PBL) is the layer of
the atmosphere that interacts directly (on the order of
hours) with the earth's surface. The PBL plays a critical
role in transport and distribution processes because
most surface sources, whether natural or anthropo-
genic, emit directly into this layer. The structure of the
PBL is strongly dependent upon surface properties such
as roughness, temperature, and quantity and type of
vegetation. Typically, the daytime continental bound-
ary layer is connectively mixed as a result of solar heat-
ing at the surface. Atmospheric constituents in the PBL
are efficiently mixed throughout its depth, which can
extend up to several kilometers. Because of the cooling
of the earth's surface at night, a stably stratified bound-
ary layer (a few hundred meters in depth) is formed over
land; the internal mixing within this layer is less efficient
than that in the daytime PBL. Chemicals emitted into
the stably stratified nighttime boundary layer may be
transported long distances horizontally without exten-
sive vertical mixing. Over the ocean, the PBL is well
mixed to a height ranging from 0.5 to 2.0 km. Where the
surface layer of water is warm, e.g., in the tropics and
the summertime midlatitudes, this mixing is primarily
convective and driven by evaporation. Where the water
is colder than the air, the mixing is caused by mechanical
turbulence, and a much shallower layer is formed. In
both cases there is no strong diurnal cycle similar to that
observed over land.
The winds in the free troposphere, i.e., above the
PBL, have a strong latitudinal component that is east-
erly in the tropics and westerly in midlatitudes. Conse-
quently, materials that have a residence time in the at-
mosphere of greater than about one month will have a
distribution that reflects the spatial distribution of
sources, and there will be a stronger concentration gra-
dient in the north-south direction than around the
latitude circle. However, superimposed on the mean
east-west horizontal flow are large-scale, wave-like oscil-
lations, particularly in midlatitudes, and strong vertical
and north-south, thermally driven regional flows such
as the Indian monsoon in the tropics. Thus the distribu-
tion of relatively long-lived trace species in the atmo-
sphere will depend primarily on the location of the
source of the injected material relative to the features of
the general circulation. The distribution of shorter-lived
species will be more complicated because the transport
patterns will be determined to a considerable degree by
the nature of the local weather systems occurring at the
time.
An especially distinctive feature of the global tropo-
spheric circulation is the relatively restricted interhemi-
spheric flow across the equatorial regions of the oceans.
As a result, the transfer of materials between the north-
ern and southern hemispheres proceeds at a relatively
slow rate, leading to differences in the interhemispheric
concentration distribution of some species. These differ-
ences are especially noticeable for certain anthropogenic
materials. Their concentrations are significantly greater
in the northern hemisphere where industrialization and
energy utilization are greater than in the southern hemi-
sphere.
The tropopause separates the relatively turbulent and
well-mixed free troposphere from the relatively stable
and stratified stratosphere. Net upward transport
through the tropopause occurs predominantly in the
tropics, and net downward transport occurs mostly at
midlatitudes.
Vertical transport processes within the troposphere
range in scale from that of deep cumulus convection
through that of cyclone-scale interactions in the polar
and subtropical jets to that of the global-scale Hadley
circulation. These processes mix trace gases and parti-
cles throughout the free troposphere and provide a link-
age between the PBL and lower stratosphere. Local
convection is particularly important in tapping the PBL
and vertically mixing the free troposphere. Synoptic-
scale cyclones also mix the free troposphere vertically.
Though less intense than individual convective clouds,
they tap much larger regions of the PBL. Cyclones, in
combination with upper tropospheric jet streams, also
provide an effective mechanism for transporting materi-
als downward from the upper troposphere and lower
stratosphere. On the largest scale, thermally driven up-
ward transport in the tropics mixes the troposphere as a
whole in that region and transports trace gases from the
OCR for page 14
14
subtropical and tropical PBL to the tropical lower strato-
sphere.
As a general rule, the smaller the ratio of the input (or
removal) rate of any substance to the total mass of that
substance in the troposphere, the more uniform is its tro-
pospheric distribution. Long-lived gases- such as N2O,
COS, and chlorofluoromethanes, which have atmo-
spheric lifetimes of decades or more are well mixed
throughout the troposphere. Gases such as CH4 and
CH3C1, with somewhat shorter tropospheric lifetimes of
a few years, are generally well-mixed vertically, and
there are only small hemispheric differences. Gases with
tropospheric lifetimes of a few months or less (CO, O3,
SO2, NO, NO2, HNO3, end reactive hydrocarbons, for
A key process in all the biogeochemical cycles is the
chemical transformation oftropospheric trace gases into
species that are either nonreactive in the troposphere or
easily removed by rain or surface deposition. The oxida-
tion of CO to CO2 by the OH radical is an example of
the former, while the oxidation of NO by O3 to NO2
followed by further oxidation to HNO3 by the OH radi-
cal is an example of the latter. All the chemical transfor-
mations can be grouped into three basic classes: homo-
geneous gas-phase reactions (reaction of one gaseous
species with another), homogeneous aqueous-phase re-
actions (reaction of one dissolved species with another),
and heterogeneous reactions (reactions of species at a
phase interface).
Homogeneous Gas-Phase Transformations
Ozone plays a significant role in tropospheric gas-
phase chemistry. It reacts directly with some compounds
such as NOX and unsaturated hydrocarbons, and, more
importantly, the photodissociation of O3 leads to the
formation of OH radicals. Hydroxyl-radical-initiated
oxidation is the major pathway for the transformation of
a large variety of tropospheric compounds and deter-
mines their chemical lifetimes. The oxidation of NO2
leads directly to HNO3 vapor. In the case of CH4 and
more complex hydrocarbons, numerous reaction inter-
mediates, induding free radicals, are produced; ulti-
mately, these are either converted to stable nonreactive
products, such as CO2 and water, or removed from the
gas phase by heterogeneous processes. Chemical trans-
formations of a number of trace gases are interrelated by
reactions involving common reactive species. This leads
to a strong chemical coupling of the various element
cycles and, in many cases, to a chemistry that is cyclic in
PART I A PLAN FOR ACTION
example) are not well mixed, and their distributions
show large vertical and latitudinal gradients that are
generated by source and sink distributions, chemical
transformations, and removal processes. As the tropo-
spheric lifetime of a species decreases, transport has less
influence on the distribution on the hemispheric and
global scale. Highly reactive species, such as the hy-
droxyl (OH) and hydroperoxyl (HO2) radicals, have
very short lifetimes; thus their concentration distribu-
tions do not depend directly on transport. However,
because of reaction pathways involving other cycles,
their distributions may depend on other species whose
distributions are influenced by transport processes (e. g.,
water, 03, and CO).
TRANSFORMATION
nature. At present, there is considerable uncertainty as
to the identity and fate of compounds produced in situ in
the troposphere during the oxidation processes. Many
of the reaction steps following the initial chemical attack
of the OH radical on CH4, NH3, SO2, and the more
complex hydrocarbons are not known for certain. A
number of potential intermediate products such as alde-
hydes, peroxides, and organic nitrates have been found
in the troposphere, but conclusive laboratory confirma-
tion of the mechanisms leading to these products and
their subsequent reactions is still needed. Furthermore,
there is a need to determine the reaction rates under
conditions similar to those found in the troposphere (1
percent water, 20 percent oxygen, and atmospheric
pressure in the range 0.1 to 1.0 atm).
Homogeneous Aqueous-Phase liransformations
Homogeneous reactions in water droplets appear to
have a significant impact on the cycles of sulfur, nitro-
gen, and perhaps other elements. This chemistry takes
place in both the submicrometer aqueous aerosol parti-
cles and the larger, 2- to 40-,um droplets found in convec-
tive and stable stratiform clouds and in fog. The reac-
tions that occur in these two aqueous environments can
be quite complex because of the presence of many inter-
acting species: neutral free radicals, free radical ions,
nonfree radical ions, as well as neutral semistable spe-
cies. Representative of this type of system is the HxO'/
sulfur/halogen system shown in Figure 2. ~ . This system
illustrates the important oxidative capacity of HxO' spe-
cies (both in their ionic and neutral forms) in the aque-
ous phase.
There is still much to learn about aqueous-phase
processes in the troposphere. A major question is the
OCR for page 15
A FRAMEWORK
~ 3 -I
Gig ~
Am: :~1 1 - 1
Mclultlstep | I} ~
rso4 ~ ~
I H O I ~ ~ of_
IHO2
~3
_~
FIGURE 2.1 The proposed chemical pathways for (OH)aq and
SHOUT in a cloud droplet. S(OH ) and S(HO2 ) represent the scav-
enging sources of these species to the droplet. The open double
arrow indicates rapid chemical equilibrium, and the closed single
arrow indicates an aqueous-phase reaction. End products are
circled.
degree to which laboratory reaction rate constants apply
to the actual environment of a water droplet. In addi-
tion, there are still many key reactions for which there
The final stage of any tropospheric elemental cycle is
removal from the troposphere. The process can involve
the episodic collection of particles and gases in water
drops and ice crystals that fall as rain and snow (wet
deposition), the more continuous direct deposition of
gases and particles at the earth's surface (dry deposi-
tion), or the chemical conversion of trace gases to inert
forms.
Aerosol particles act as nuclei for the condensation of
water vapor in warm clouds and fog and for the genera-
tion of ice crystals in supercooled clouds. These nuclea-
tion mechanisms are the basis of most cloud-forming
processes. As these drops grow in size, they serve as sites
for the conversion of SO2 to sulfate and adsorb soluble
gases such as HNO3, hydrogen chloride (HC1), hydro-
gen peroxide (H2O2), and formaldehyde (CH2O). A
small percentage ofthe droplets or ice particles will grow
sufficiently large that they fall toward the ground as
snow and rain. While falling, they collect other particles
and soluble gases. Drops that evaporate before reaching
the ground release gases to the atmosphere and the resi
15
are no data. Finally, one must recognize that there are
few data on the concentrations of many critical species
within aqueous aerosol particles and cloud droplets at
different tropospheric locations and as a function of
time.
Heterogeneous Transformations
Normally, a heterogeneous reaction occurs at an in-
terface between two phases, although several interfaces
can be involved in an overall process. Most reactions
that occur on such surfaces are thought to be noncata-
lytic. These include chemical reactions in which both
phases participate as consumable reactants and physical
processes that involve either transport or growth, or
both. Adsorption or absorption are, of course, heteroge-
neous processes. In heterogeneous catalytic processes
the interracial material or a species adsorbed on it is
conserved. A special class of heterogeneous process, gas
. . .
to-part~cle conversion, converts a trace gas to a particle
or a liquid droplet suspended in the atmosphere.
Important heterogeneous reactions in tropospheric
chemistry include the conversion of gas-phase ammonia
and nitric acid to ammonium nitrate and the conversion
of gas-phase sulfur dioxide to sulfate in cloud droplets.
There is still not much known about the role of heteroge-
neous processes in many trace gas and trace element
cycles. However, it is clear that such processes are im--
portant in removal by deposition to surfaces.
REMOVAL
due forms a particle; in effect, such droplets transport
gases and particles to lower levels in the troposphere.
This also occurs for nonprecipitating clouds, although in
this case the transport may be upward. Those drops that
reach the ground are an intermittent but highly efficient
means of converting and removing soluble trace gases
and particles from the troposphere. The key issue here is
their chemical composition and deposition rate since
these provide essential information on the sources and
sinks of various species. It is difficult to develop esti-
mates of global removal rates from local data sets be-
cause of the great spatial variability of precipitation
events and of the concentration of particles and soluble
trace gases. In addition, it is difficult to measure accu-
rately species that are often present at trace levels. At this
time there are no reliable data on global precipitation
removal rates for any of the chemical cycles.
The dry deposition of particles larger than about 20-
~m diameter is largely controlled by gravity. Submi-
crometer particles behave more like a gas; their deposi-
tion is controlled by factors such as their diffusivity and
OCR for page 16
16
their rate of turbulent transport through the lowest lay-
ers of the atmosphere. After transport to the immediate
vicinity of the surface by turbulence, trace gases and
small particles are deposited on the surfaces of vegeta-
tion, soils, the ocean, and so on. For species like SO2 and
03, the flux to vegetation is frequently governed by
biological factors, such as stomata! resistance. Reactive
gases, such as HNO3 and 03, are removed rapidly by
most surfaces, although O3 is removed quite slowly from
air over wafer. Dry deposition is a much slower but more
continuous process than wet deposition. For gases with
surface sources such as NO2 and SO2, dry removal is,
however, even more difficult to evaluate globally be-
cause local concentration measurements are extremely
dependent on the distance of the sampling site from the
sources. Deposition velocities, i.e., the ratio ofthe flux of
a substance to its mean air concentration at some refer-
ence height near the surface, have been measured for O3
over a range of surfaces, and there are some similar data
for HNO3, NO2, and SO2. At the moment there exist
only highly uncertain estimates of global dry removal
rates for some ofthese trace gases and particles. A possi-
ble exception is O3.
A transformation reaction that converts a trace gas
PART I A PLAN FOR ACTION
into a form that no longer interacts in its elemental cycle
may be classed as an in situ removal reaction. Excellent
examples of such conversion reactions are the radical-
radical reaction between the OH and HO2 radicals lead-
ing to molecular oxygen and water, and the oxidation of
CH4 to CO2 and water. Aqueous and heterogeneous
reactions that convert gaseous species to dissolved
salts e. g., NH3, NO2, and SO2 to ammonium (NH4 ),
nitrate (NO3 ), and sulfate (SO4-) can also be consid-
ered in situ removal reactions. Some species such as
unneutralized HNO3 would return to the gas phase if
the droplet evaporated.
For long-lived compounds, such as N2O and certain
chlorofluoromethanes (e.g., CF2C12 and CFC13), the
principal sinks are in the stratosphere, where high-en-
ergy ultraviolet photons, excited atomic species, and
radicals are available to dissociate them. The products of
dissociation, such as NO and the chlorine atom, un-
dergo transformations and are eventually transported
back into the troposphere, where they continue to react
in elemental cycles until deposited at the surface. For
trace gases from surface sources, the longer the tropo-
spheric lifetime the greater is the fraction that will be
destroyed in the stratosphere.
PHYSICAL EFFECTS OF TRACE SUBSTANCES IN THE TROPOSPHERE
Returning to the broader picture of the integrated
chemical systems ofthe troposphere, we see that another
rationale exists for the study of tropospheric composi-
tion. Throughout the cycle of source/transport/transfor-
mation/removal, trace substances in the troposphere
have effects on important physical processes. Some spe-
cies (e. g., O3 and SO2) absorb incoming solar ultraviolet
radiation; some (e.g., elemental carbon and NO2) ab-
sorb visible light; some (e.g., CO2, N2O, certain
chlorofluoromethanes, and many others) absorb and
emit infrared radiation; some substances in particles act
as condensation or freezing nuclei in clouds and, in
doing so, may alter the cloud radiative properties. The
source/transformation/removal cycles of trace atmo-
spheric materials are depicted in Figure 2. 2; the primary
meteorological effects in the troposphere are related to
the respective categories oftrace substances: gases, aero-
sol particles, hydrated aerosol particles, and clouds.
The absorption and emission of radiation by gases is
probably the best understood of the mechanisms by
which chemical species affect physical processes in the
atmosphere; an assessment of these effects requires
mainly the measurement of the concentrations of the
relevant gases in the atmosphere. The fundamental as-
pects of the scattering and absorption of radiation by
aerosol particles are reasonably well understood, but
direct measurements must be made to determine the
magnitude of the relevant parameters and their depend-
ence on chemical processes and composition.
For example, aerosol particles can produce either a
heating or a cooling of the earth-atmosphere system, the
end result depending on the relative magnitude of the
scattering and absorption coeth~cients for visible light
and on the total optical extinction. Scattering is often
controlled by submicrometer sulfate aerosol particles,
whereas absorption is usually due to mineral compo-
nents and elemental carbon, especially the latter.
The fundamental nature of the nucleation process
and of the freezing of cloud droplets is also understood,
but a good understanding is lacking ofthe dependence of
these processes on particle composition and the conse-
quent impact on the removal of materials from the at-
mosphere. It is known that aerosol particles play a major
role in cloud processes. Some aerosol substances act as
condensation nuclei for clouds, whereas others serve as
freezing nuclei; these nucleation characteristics are
strongly influenced by the chemical composition of the
particles. In turn, as previously discussed, clouds are a
major factor in controlling atmospheric composition,
because precipitation is the dominant mechanism for
the removal of gases and particles from the atmosphere.
Again, direct measurements are needed both to estab
OCR for page 17
Representative terms from entire chapter:
trace gases
A FRAMEWORK
FIGURE 2.2 Phase transitions within
the atmospheric chemical systems and
their consequences. Rectangles denote
chemical entities or physical environ-
ments in the atmosphere. The right side
of the figure lists atmospheric processes
affected by the species or environment
identified in the rectangles. Triangles
denote processes where material flows
primarily in one direction; diamonds
represent reversible processes: a,
sources; b, sinks; c, gas-to-particle con-
version; d, sorption; e, deliquescence; f,
efflorescence; g, Raoult's equilibrium;
h, reaction in concentrated solution
droplet; i, nucleation and condensation
of water; j, evaporation; k, capture of
aerosol by cloud drops; l, reaction in
dilute solution; m, rain; n, freezing of
supercooled drop by ice nucleus; o,
melting; p, direct sublimation of ice on
ice nucleus; q, precipitation. (From
Atmospheric Chemistry: Pro bleats and Scope,
National Academy of Sciences, Wash-
ington, D. C ., 1975 ).
1
{ - Gaseous, Nonaerosol
Precursors
~ CO, CO2, CH4,
/._ Gaseous Aerosol
Precursors
- _ SO2, H2S, NO, NO2,
HC'NH3, (H201 l
._: ~
{ - Low R H Aerosol
RH
18
As discussed above, investigation of the atmospheric
chemical processes related to biogeochemical cycles in
the troposphere provides a unifying conceptual frame-
work for the study of global tropospheric chemistry.
These processes include the sources, chemical reactions
and transformations, transport, and removal ofthe vari-
ous species within any chemical cycle in the tropo-
sphere. A quantitative understanding of these funda-
mental processes will enable predictive models of the
PART I A PLAN FOR ACTION
SUMMARY
tropospheric chemical system to be developed and the
physical effects of trace substances in the troposphere to
be determined. Predictive models will allow the effects of
future perturbations ofthe global troposphere to be eval-
uated. In the chapter that follows, the specific programs
proposed in the areas of sources, transformations, trans-
port, and removal of chemical species in the troposphere
are presented in detail, as is the need for the develop-
ment of global tropospheric chemistry systems models.
