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OCR for page 19
A Proposed Program
LONG-TERM GOALS AND OBJECTIVES
Over the past decade, tropospheric chemistry
research has shown that the various chemical cycles in
the global troposphere are interactive, complex, and of
fundamental importance to the future well-being of
humanity. Many essential biochemical and geochemical
cycles are critically susceptible to perturbations to the
global troposphere. In recognition of this, we recom-
mend that the Uniter! States assume a major role in
Initiating a comprehensive Investigation of the chem-
istry of the global troposphere.
The long-term goals of this Global Troposphenc
Chemist Program should be as follows:
1. To understand the basic chemical cycles In the
troposphere through field investigations, theory
aided by numerical modeling, and laboratory studies.
2. To predict the tropospheric responses to pertur-
bations, both natural and human-induced, to these
cycles.
3. To provicle the information required for the
maintenance and effective fixture management of the
atmospheric component of the global life support sys-
tem.
Attainment of these goals will require carefully
designed and complementary research programs, the
development of which will involve close cooperation and
. . · · · -
mteractlon among mvestlgators makmg measurements
in the field, those investigating reaction rates and mech-
anisms in the laboratory, and those attempting to model
both the chemical systems and the meteorological proc-
esses affecting the chemical distributions. Many other
laboratory investigations are essential, e.g., studies of
uptake of gases by plant leaves, emission of chemicals by
living systems, and cloud-scavenging simulations. It is
important that experiments be designed jointly by field
and laboratory scientists in conjunction with modelers
and that there be a continuing exchange between the
developing theoretical aspects of tropospheric chemistry
and the evolving laboratory and field investigations, as
illustrated in Figure 3. I.
It is known that such meteorological processes as
transport and cloud and precipitation formation are
intimately related to the chemical cycles in the tropo-
sphere, and that the instrumentation and platforms
required to investigate tropospheric chemistry on the
global scale will be expensive. Global-scale tropospheric
chemical research cannot be conducted solely by indi-
vidual investigators or even by small groups of investi-
gators, although the contribution of the individual
investigator has been critical in the development of tro
pospheric chemistry and will continue to be so in the
future. Because of the complexity and diversity of the
19
OCR for page 20
20
Provides Recommendations |
for Atmospheric
Measurements
Provides Atmospheric
Measurements for Model
Input and Validation
1
Provides Mechanisms
and Rates of Reaction
and Exchange
MODEL ~LABORATORY
DEVELOF MENT l ~INVES IGATIONS
AIL I
Provides Recommendations 1
for Kinetic Studies
Provides Standa~ dization
and Calibration Methods
and Sensitive Analytical
Techniques
Provides Atmo spheric
Measurements to
Validate Laboratory
Reaction Mechanisms
and Rates
~, , .
FIELD PROGRAMS
FIGURE 3.1 Schematic diagram of the essential yet interde-
pendent functions served by field programs, laboratory measure-
ments, and mathematical models in atmospheric chemistry
research.
chemical systems and because of the myriad sampling,
analytical, and modeling tools required to study them,
these studies will require the joint efforts of a broad
spectrum of scientists: atmospheric chemists and physi-
cists, marine chemists, meteorologists, ecologists, plant
and soil biochemists, microbiologists, plant physiolo-
gists, laboratory chemists, geochemists, engineers, and
others. An effort ofthis magnitude requires the coopera-
tion and participation of universities, industry, and gov-
ernment agencies, both in the United States and in other
countries.
The four basic processes that control chemical cycles
and their interactions in the troposphere production,
transport and distribution, chemical transformation,
and removal provide a unifying framework for the
development of the Global Tropospheric Chemistry Program.
Attainment of the program goals will also require the
development of three-dimensional models of tropo-
spheric chemical processes linked to meteorological and
climatic processes, i.e., tropospheric chemistry systems
models (TCSMs); these models will provide an overall
synthesis of data obtained in the program, and they will
provide theoretical guidance for the program's contin-
ued development. For these reasons, we recommend
that the Global Tropospheric Chemists Program be
undertaken with the following specific scientific
objectives:
PART I A PLAN FOR ACTION
1. To evaluate biological sources of chemical sub-
stances in the troposphere.
2. To determine the global distribution of tropo-
spheric trace gases and aerosol particles and to assess
relevant physical properties.
3. To test photochemical theory through field and
laboratory investigations of photochemically driven
transformation processes.
4. To investigate wet and dry removal processes for
trace gases and aerosol particles.
5. To develop global tropospheric chemistry sys-
tems models (TCSMs) and the critical submodels
required for the successful application of the TCSMs.
The research efforts in biological sources, photo-
chemical transformations, and removal processes will
require the development of individual submodels. The
submodels would serve three functions: (1) to under-
stand better the individual processes being investigated,
(2) to extrapolate from individual observational sites to
the regional and global scales, and (3) to provide compo-
nents that can be used in a comprehensive three-dimen-
sional meteorological model that is coupled to global
tropospheric chemistry a TCSM. By contrast, the
research effort in global distributions and long-range
transport would serve to help validate the overall perfor-
mance of a comprehensive TCSM. A tropospheric
chemistry systems model would focus on meteorological
transport processes that are best described by atmo-
spheric general circulation models (GCMs). These
GCMs would be especially designed not only to provide
large-scale tracer transport in the free atmosphere, but
also to parameterize transport through the planetary
boundarylayer and by cloud processes. A TCSM would
also require physically based cloud submodels, a good
description of land surfaces, and adequate treatment of
the solar radiation that drives tropospheric photochem-
~stry.
Based on the five scientific objectives above, details of
the specific scientific investigations proposed for the
Global Tropospheric Chemistry Program are developed in the
remainder of this chapter. These investigations include
the Biological Sources of Atmospheric Chemicals Study; the
Global Distributions arldt Lorlg-Range Transport Study; the
Photochemical Trar~sformatior~s Study; the Cor~versiorz and
Removal Study; and a program for the development of
global Tropospheric Chemistry Systems Models (TCSMs).
The discussions of these specific investigations are fol-
lowed by an evaluation of instrument and platform
requirements for their successful completion and a brief
review of the requirement for strong international par-
ticipation and cooperation in the Global Tropospheric
Chemistry Program.
OCR for page 21
A PROPOSED PROGRAM
BIOLOGICAL SOURCES OF ATMOSPHERIC CHEMICALS
Before an understanding of the natural or perturbed
troposphere can be claimed, the flow of chemicals
through it must be traced. This flow begins with entry of
the chemical species into the troposphere. Although the
earth's atmosphere is certainly an oxidizing environ-
ment, the actual processes of chemical and physical
transformation, transport, and eventual removal
depend on the chemical form and other intrinsic proper-
ties of the substance in question. Accordingly, the initial
physical state and chemical properties of the substance
at its source will affect its subsequent tropospheric fate.
There are many research questions, basic and ap-
plied, that require knowledge of the intensity, size, and
variability of sources of tropospheric chemical species;
e. g., what factors control the ambient concentration of a
certain chemical, or why does the concentration in-
crease or decrease in time, or how will human activity
alter the source in question? As specific examples, one
may ask why the global concentration of CH4 is increas-
ing secularly and why it varies with season, why the
observed distribution of CO varies with latitude and
season, and why there is any gaseous HC1 at all in the
troposphere?
For these reasons, we recommend that a major re-
search effort be undertaken to evaluate biological
sources of chemical substances In the global tropo-
sphere. The objectives of this Biological Sources of At-
mosphenc Chemicals Study would be (1) to evaluate the
chemical fluxes to the troposphere from critical bio-
log~cal environments (biomes) and (2) to determine
the factors that control these fluxes.
The experimental study of the sources of atmospheric
chemicals is relatively new because the nature of many
of these sources has been recognized only recently and
appropriate analytical instruments are just now being
developed. This is especially true for biological sources.
The dominant role of biological systems as sources of
tropospheric trace substances is becoming widely appre-
ciated; a more detailed discussion ofthe evidence for this
appears in Part II, Chapters 5 through 7.
In identifying topics and regions for research in the
Biological Sources of Atmospheric Chemicals Study, we have
employed several criteria; these are reviewed in Part II,
Chapter 5, in the section by Cicerone et al. We began by
focusing on key chemicals known to have important
roles in atmospheric chemistry. For some of these, exist-
ing data have already shown the global importance of
certain blames as sources. In other cases, general princi-
ples from chemistry and biology suggest that certain
blames should be important. Further, we reviewed
characteristics of various blames and arrived at criteria
to estimate their hemispheric or global importance as
21
sources. These considerations led to the field and labora-
tory investigations of biological sources of tropospheric
chemicals that are proposed here.
Much of the early field research in the Biological Sources
of Atmospheric Chemicals Study must be exploratory in na-
ture. We suggest a research strategy of preliminary in-
vestigations of the various sources using relatively small
research groups. When the nature and importance of
specific sources are better defined, more detailed and
refined field and laboratory investigations can follow.
This strategy will lead to an understanding of the un-
derlying physical, chemical, and biological factors reg-
ulating production of the compounds of interest. In
situations where sources are evident but no direct mech-
anisms are apparent, investigation of indirect paths
must be undertaken. Such investigations would require
plausible mechanisms for the relevant kinetics and for
application of the kinetics under field conditions, and
coordinated measurements to verify and test proposed
mechanisms. One might expect this process to involve
successive iterations among field measurements, labo-
ratory measurements and theory, and ultimately to re-
quire studies designed to identify processes important
for biochemical and microbiological production of pri-
mary chemical species isoprene, for example.
Certain important abiological sources (e.g., indus-
trial emissions, volcanoes, and lightning) are not dis-
cussed here. Also, some field studies of CO2 exchange
are proposed. Even though CO2 is not important in
tropospheric chemistry through its reactions, CO2 ex-
change rates can provide important information on the
mechanisms and rates of exchange of other gases.
Investigations of Specific Sources
Tundra, Taiga, and Freshwater Marshes
The size of the areas covered by tundra, taiga, and
freshwater marshes and some of their unique properties
suggest that these regions may be important contribu-
tors of CH4, N2O, NO, and other volatile species to the
troposphere. On the ocean border, volatile organic
halides and reduced sulfur species are also of interest.
Site selection and analytical techniques for these in-
vestigations are influenced by the remoteness of most
representative locations. For these reasons and because
so few data are available now, the initial phase of a
research program probably is best accomplished by
small ad hoc expeditions to acquire the basic informa-
tion and exploratory data necessary to design more sys-
tematic and extensive studies.
OCR for page 22
22
PART I A PLAN FOR ACTION
TABLE 3.1 Measurement Needs in Various Source Regions
H2S
N2O NOx CH4 DMS/RSH NMHC RX
Seaboard tundra X X X X X X
Taiga X X X X
Marshland X X X X
NOTE: RX represents organic halogen species, RSH represents mercaptans, and NMHC represents nonme-
thane hydrocarbons. Determination of emission rates of CO and CO2 is also needed; CO2 fluxes could provide
valuable mechanistic information.
Sites to be investigated should be logistically favor-
able, uncontaminated, and ecologically representative.
The present understanding of these source regions
suggests emphasis on the measurements indicated by an
"X" in Table 3.1. Climate characteristics and local
conditions will probably constrain the sampling periods
narrowly, as could the availability of nearby laboratories
and analytical capabilities, but data are needed from
various seasons, particularly during transition times.
Tropical Forests
Tropical forests represent a biome of considerable im-
portance for tropospheric chemistry. They include a sig-
nificant fraction of the total carbon content of the global
terrestrial biosphere. They may be important sources
for CH4 and N2O, gases whose global concentrations
have increased significantly over the past several de-
cades, and tropical forests may play a significant role in
the global budgets of CO, nonmethane hydrocarbons,
NOx, and various forms of volatile sulfur. Studies of
tropical forests will also lead to an increased understand-
ing of the role of microbially mediated reactions, which
should be markedly faster in the high-temperature,
high-humidity tropical regions.
An orderly strategy for the study of tropical forest
blames might begin with careful measurements of the
composition of the local troposphere to identify species
whose concentrations are elevated with respect to typical
ambient tropospheric background levels. An investiga-
tion of a region such as the Amazon Basin might be
particularly instructive. The prevailing winds are typi-
cally from the east, and the concentration of key species
might be expected to increase with time as air masses
penetrate deeper into the basin. Measurements deter-
mining the spatial and temporal change in gas concen-
trations, in combination with careful meteorological
analysis, could be used to derive an empirical estimate
for the total net flux of selected species emanating from
the local biosphere. These data, in turn, could provide
an estimate of the significance of specific individual
sources within the basin.
The extensive measurement program should be com
plemented by intensive studies of selected microenvi-
ronments within the larger biome. For example, if the
extensive program should establish an important dis-
tributed source for CH4, the nature of the source should
be clarified through more intensive investigations. Ex-
ploratory studies of rivers, flood plains, and other envi-
ronments should aid in the identification of important
source regions.
Studies of tropospheric chemistry in tropical forests
should improve the understanding of the tropical biome
as an integrated physical, chemical, and biological sys-
tem. To this end, data relating to the transformation and
redistribution of essential nutrients, such as nitrogen
and sulfur, are particularly relevant and should be avail-
able as a by-product of the proposed research.
.
Biomass Burbling
Some of the most easily recognized sources of tropo-
spheric chemicals can be described as point sources.
Dramatic in appearance and possibly in their actual
impact, these are exemplified by biomass burning,
lightning, volcanoes, animal feedlots, and industrial or
urban combustion and waste plumes. Both the nature
and the magnitude of these sources make them impor-
tant for global or hemispheric tropospheric chemistry.
High-temperature processes that synthesize otherwise
unnatural substances and other activities that process
large amounts of raw materials, e.g., biomass burning
and the refining of metals and petroleum, are particu-
larly potent. We emphasize here the need for coordi-
nated field investigations of biomass burning as a source
of atmospheric chemicals, although much research is
also needed to quantify other point sources.
Biomass burning produces a variety of gases and par-
ticles. The less reactive gases and the smaller particles
can affect the global troposphere. Regional effects are
likely from the more reactive gases and the larger parti-
cles.
We recommend a two-phased field investigation of
biomass burning. The objectives for the early phase
would be (1) to extend available methods for malting
quantitative measurements of the emissions of CO2,
OCR for page 23
A PROPOSED PROGRAM
CO, N2O, CH4, NOx, nonmethane hydrocarbons,
trace elements, and particles of various sizes from bio-
mass burning, and (2) to obtain exploratory data (as
quantitative as possible) for emission rates of COS, vari-
ous cyanide compounds (RCN mostly hydrogen cya-
nide (HCN) and methyl cyanide (CH3CN)), and
CH3C1. Also, as in the later phase, the measurements
must be made with an awareness of soil and vegetation
types and conditions, and the amounts of charcoal pro-
duced and areas burned must be measured because of
the possible importance of charcoal in the global carbon
cycle. Methods to determine charcoal production and
the sizes of the affected areas need refinement to permit
global assessments. An initial phase might focus on the
North American continent to minimize logistical prob-
lems and to take advantage of related expertise available
through forest meteorologists and forest research pro-
grams. There are, for example, prescribed, controlled
forest fires set for research purposes in the southeastern
and northwestern United States. Forest fire research
laboratories could be of considerable use in exploratory
programs. The goals of the early phase could be accom-
plished more easily at such locations, or in Alaska, than
in the tropics (where much biomass burning occurs).
Some sites should be amenable to the use of instru-
mented towers and to long-path spectroscopic absorp-
tion methods.
In the later phase, field measurements would concen-
trate on tropical fires. These are extensive and frequent,
and they would present greater logistical difficulties. On
the basis of the results and experience already gained, it
should be possible to quantify emissions of CO2, N2O,
CH4, CO, and the more stable nonmethane hydrocar-
bons from these large fires' and to obtain fairly reliable
estimates of emissions of COS, RON, CH3C1, trace
metals, particles, and nitrogen oxides.
In all phases, ground sampling of gaseous and partic-
ulate species should be undertaken. Aircraft should be
used extensively, however, because there is rapid up-
ward motion of the emissions, large areas must oe cov-
ered, and sampling at representative ground locations
could be difficult.
Coastal Wetland and Estuar?ne Enviror~m~ts
Coastal wetland and estuarine environments are hot
spots for the production and emission to the troposphere
of many chemically reduced gaseous species of carbon,
nitrogen, and sulfur. Most natural anoxic sediments
that are in close contact with the troposphere are found
in these environments. For gases such as N2O, CH4,
CO, COS, CS2, (CHESS, H2S, and perhaps a few oth-
ers, it is reasonable to propose initiating immediately a
field research program emphasizing basic processes of
23
gas production and exchange with the tropospheric
boundary layer. For other species such as NO, NH3,
and volatile metals, existing technology is inadequate
for quantitative biosphere-atmosphere exchange stud-
ies, even in relatively intense source areas. For almost all
reduced gas species, measurement technology is cur-
rently inadequate for flux studies of weak sources and
sinks.
Studies of gas fluxes in coastal wetland and estuarine
environments might focus initially on subtropical and
tropical salt marsh and mangrove habitats, because
these typically contain extensive anoxic sediments and
are characterized by high organic inputs, warm temper-
atures, and high levels of microbial activity factors
that enhance the production of reduced gas species. A
number of characteristics (e.g., vegetation, exposed
sediment surfaces, and overlying water) deserve sepa-
rate study. On the basis of existing data, one can expect
gas fluxes from these environments to be highly variable
in both time and space on scales of minutes to hours and
meters to kilometers, respectively. Critical forcing varia-
bles with regular periodicity (e.g., tides, sunlight, plant
physiological status, and sediment temperature) inter-
act with episodic events such as severe weather to influ-
ence both production and exchange rates.
Both exploratory and intensive field studies of trace
gas fluxes are needed in coastal wetland and estuarine
environments. Exploratory measurements of fluxes
from tropical mangrove and estuarine environments in
South America, Africa, and Asia are needed to assess
the relative magnitude of emissions from these sources
compared to those from more accessible sites in the
southeastern United States. Intensive, process-oriented
studies of carbon, nitrogen, and sulfur emissions from
salt marsh and mangrove habitats could be initiated in
areas such as the southeastern U.S. Atlantic coastal re-
gion, Mississippi River delta, and Florida Everglades.
Together, these studies would improve the understand-
ing of geographical variability in biogenic gas emissions
to the troposphere and the processes that control these
fluxes.
A program of exploratory gas flux determinations
requires research teams focused specifically on such
studies, and every effort should be made to determine
gas fluxes at locations where complementary meteoro-
logical and ecological or biogeochemical characteriza-
tions are available. Each gas flux determination should
be accompanied by measurements of sediment and/or
water column parameters such as water content, tem-
perature, organic content, pH, and Eh (the oxidation-
reduction potential). Surveys should include measure-
ments at each site over vegetation, exposed sediment
surfaces, and water.
Intensive studies would require more continuous ef
OCR for page 24
24
forts of research teams at sites where fluxes are deter-
mined in conjunction with biological and geochemical
studies of sedimentary biogeochemical processes.
Agricultural Biomes
Agricultural areas are enormous potential sources of
tropospheric chemicals. For example, rice growing, be-
cause of its large global extent and waterlogged anoxic
soils, appears to be a major source of atmospheric CH4.
Similarly, nitrogen-intensive grain growing (corn, cot-
ton, wheat, legumes, and some rice) can emit enough
volatile nitrogen compounds to influence large regions
(if NH3 or NOx) and the globe (if N2O). Because of the
large areas under cultivation, the growing usage of
chemicals, and the inherently large turnover rates of
nutrient elements with volatile compounds, we propose
a program of extensive field measurements. The focus of
these efforts would be on rice paddies and on heavily
fertilized crops such as those in the U.S. corn-soybean
belts.
In Part II, Chapter 5, in the section by Cicerone et al.,
we discuss the potential of emissions from rice paddies to
exert a strong influence on regional and global tropo-
spheric chemistry. Particular attention should be paid to
volatile species containing nutrient elements in reduced
valence states, e.g., CH4, N2O, NH3, NO, methylated
metals, isoprene, and possibly CO.
For CH4, detailed and systematic field measurements
are required because of the apparent importance of rice
paddies and because CH4 emission rates from rice pad-
dies depend on several factors. To determine the neces-
sary CH4 fluxes, at least two parallel efforts would be
required. The first would simply extend the current data
base by using available techniques. For the second, it
would be necessary to complete the development and
employ a state-of-the-art meteorologically based flux-
measurement technique using turbulence-correlation
or gradient measurements. Methane fluxes must be de-
termined over complete growing seasons, as functions of
nitrogen fertilization rate and in organic-rich rice pad-
dies. Soil temperature, pH, and Eh should also be mea-
sured.
Most of the data for other trace gases could be ob-
tained while the CH4 flux investigations were being
made, largely because the initially required investiga-
tions would be more exploratory than systematic. If the
results indicated significant emissions, then more exten-
sive and systematic studies would ensue, similar to those
outlined above for CH4. For NH3 and NO a similar
sequence of investigations would be required. For NH3,
adequate chemical trapping methods are available, but
NO is more problematic; a field-compatible technique is
needed.
PART I A PLAN FOR ACTION
Potentially large emissions from various heavily fer-
tilized agricultural biomes require investigation. Mod-
ern agriculture often relies on intensive management of
resources, both physical and financial. Bulk quantities
of nitrogen, phosphorus, and sulfur are applied com-
monly, as are large quantities of trace elements. Special-
ized chemicals and biochemicals, including enzymes
and brominated organics, are also used to regulate proc-
esses and to control insects and pests. To assess the role of
these intensively managed agricultural blames as
sources of atmospheric trace chemicals, some relevant
research is under way in the agriculture research com-
munity. We propose complementary research on (1)
measurements of the emissions of species such as N2O,
CH4, and CO that could have global and hemispheric
tropospheric effects, and (2) f~eld measurements of
shorter-lived gases that have been impractical up to
now. Topic (2) could include systematic measurements
of NH3 emissions from nitrogen-fertilized plots and ex-
ploratory studies to detect emission of nitrogen oxides,
volatile phosphorus, and metals. Gaseous emissions of
NH3, along with those from animal feedlots and natural
sources, could lead to NH4, (NH4~2SO4, (NH4)HSO4,
and NH4NO3 in aerosol particles in remote areas of the
globe. Further, NH3 lost from fertilized fields could also
be a key buffer of precipitation acidity. Measurements
similar to those proposed for rice paddies would address
these questions.
1 ~
.,
Operz Ocearts
The oceans are a large-area, low-intensity source of
reduced sulfur compounds to the lower troposphere.
Preliminary studies have identified a number of sulfur
compounds in seawater that are presumed to be bio-
genic, including H2S, COS, CS2, dimethyl sulfide
(DMS), dimethyl disulfide (DMDS), and dimethyl sul-
foxide (DMSO). Dimethyl sulfide appears to be the
most abundant species, contributing an estimated flux
of approximately 40 Tg S/yr to the marine boundary
layer. Once in the marine boundary layer, DMS is prob-
ably oxidized by photochemical processes to produce
SO2, with intermediates such as DMSO and methane
sulfonic acid (CH3SO3H). Qualitatively, the concentra-
tion of reduced sulfur compounds in surface seawater is
correlated with indicators of algal biomass.
A research effort to investigate the sulfur cycle in
productive areas of the world's ocean should be initiated
to elucidate sources of reduced sulfur in the ocean and
their role in the global sulfur cycle and budget. Particu-
larly important would be studies of in situ biogenic pro-
duction of sulfur species in the water column, fluxes
across the sea-air interface, and chemical processes in
the marine boundary layer determining transport and
OCR for page 25
A PROPOSED PROGRAM
fate. Most of the research could be conducted from a
major research vessel with periodic aircraft overflights to
measure the vertical distribution of sulfur species in the
troposphere and estimate rates of exchange between the
boundary layer and free troposphere. Sites for such
studies might include continental shelfwaters and a ma-
jor upwelling area.
The nitrogen cycle in the sea is strongly modulated by
biological processes. Evaluation of productive oceans as
a source of atmospheric N2O is particularly relevant to
an improved understanding of global carbon, nitrogen,
and phosphorus cycling. Capabilities should be devel-
oped to explore the possibility of significant NO emis-
sions and to measure sea-air N2O fluxes more directly. A
program of continued oceanographic studies of N2O
production and distribution should be pursued while
technology is being developed for measuring low-level
N2O and NO fluxes from surface and aircraft platforms.
Many atmospheric halogen compounds appear to
have significant oceanic sources (see Part II, Chapter 7~.
The most pressing requirement is to estimate the oce-
anic source strengths of CH3C1, CH3I, CH3Br, and pos-
sibly other organo-bromide compounds. Estimates of
the magnitude and distribution of these sources are
needed before even the most rudimentary understand-
ing of tropospheric halogen budgets and the role of the
oceanic sources in stratospheric chemistry can be
claimed. The only available experimental data on oce-
anic fluxes of methyl halides (RX) are based on mea-
sured supersaturation of surface waters. Much more
extensive measurements such as these are strongly sug-
gested.
The productive oceans may be significant, albeit sec-
ondary, sources of CO and CH4 to the global tropo-
sphere. Again, as in the examples above, the production
of these species is related to biological processes, is
patchy in distribution, and often the sea-air fluxes are
below the detection limits of existing measurement sys-
tems.
As we prepare this document, oceanographers are
revising estimates of open-ocean biological productivity,
upward by perhaps an order of magnitude. As major
oceanographic research programs of oceanic productiv-
ity are developed, every effort should tee made to include
simultaneous studies of trace gas (e.g., DMS, N2O,
CO, CH4, and RX) production and emissions to the
troposphere.
Temperate Forests
Temperate forests occupy a substantial fraction of the
global land area. Their annual production of dry matter
represents approximately 10 percent of the total global
production by the biosphere. The forests vary greatly in
25
species composition from almost pure stands of conifers
to the mixed hardwoods of temperate North America
and Europe, and they also vary in soil type, moisture,
and climate. For these reasons, they have a broad range
of properties of interest to the tropospheric chemist.
Volatile emissions from forests certainly occur, but
there is little quantitative information on these emis-
sions. Various high-molecular-weight aromatic and
aliphatic hydrocarbons and CH4 are potentially of inter-
est as forest emissions. The direct emission of significant
quantities of aerosol particles by trees has been sug-
gested, but little quantitative information is available.
To obtain the minimum necessary information on
these emissions, sampling should be conducted at repre-
sentative sites for the major forest types. It is desirable to
design a program that is flexible both in time and space,
starting with exploratory survey analyses in selected
representative locations and expanding the number of
sites and intensity of analysis as results dictate.
Wherever possible, sites should be selected where
other data are available from ongoing ecological, physi-
ological, and other studies on principal forest types (con-
ifers, deciduous hardwood, etc.~.
The current state of knowledge of volatile emissions
and reabsorption by forest ecosystems is so limited that
many exploratory studies are required to guide the di-
rection of more intensive research. During early studies,
analysis at several sites, times of day, and seasons for
CH4, CO, nonmethane hydrocarbons, and NH3 should
be undertaken. Measurement methods to provide gra-
dient information will be needed.
Savannas and Temperate Grasslands
Significant emissions of biogenic gases may occur
from temperate grasslands and from savannas (tropical
and subtropical). Both direct and indirect indicators
point to the need to obtain estimates of emission rates for
several gases. Savannas cover about 4 to 5 percent of the
earth's surface, and they display high cycling rates for
nutrients, i.e., high rates of gross and net primary pro-
ductivity. Although they store less material in their
shrubs and grasses than is found in the wood of tropical
forests, the biological material of savannas has a shorter
lifetime and higher nitrogen/carbon and sulfur/carbon
ratios than hardwood. From the rapid turnover rates,
the chemical composition of the material, and current
data that suggest a large role for termites and herbivo-
rous insects, one may conclude that significant volatile
emissions are quite likely from certain dry tropical ar-
eas. In particular, there is potential for large emissions of
CH4, CO, CO2, many nonmethane hydrocarbons,
N2O, and possibly methylated metals. Initial measure-
ments should focus on sites and processes that concen
