\ I/
United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-88/046 Feb. 1989
&EPA Project Summary
Climate Change and Its
Interactions with Air Chemistry:
Perspectives and Research
Needs
J. E. Penner, P. S. Connell, D. J. Wuebbles, and C. C. Covey
The full report outlines and
estimates where possible the
interactions between climate change
and atmospheric chemistry that need
investigation on both local and/or
regional and global scales. This
problem is enormously complex and
is not simply one of estimating
temperature change and running
chemical models already in use. The
changing climate influences many
different factors such as pre-
cipitation, atmospheric transport,
changes in budgets of species with
biological sources, changes in UV
light because of stratospheric ozone
depletion, changes in deposition
rates, etc. The single most significant
finding in this study is that very little
is known about the interactions of
the above cited effects with either
climate or air pollution. There is
however significant evidence to
indicate that atmospheric emissions
and concentrations of radiatively and
chemically important trace gases
such as CO&CH* N2O, CFjClz, CFCIa,
and CO are increasing. These
increases are largely derived from
human related activities. Current
analyses suggest that it is im-
probable that present trends towards
increasing concentrations will be
arrested or reversed in the near
future.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully docu-
mented In a separate report of the
same title (see Project Report
ordering Information at back).
Introduction
The record of COa increase, estab-
lished by a continuous record of
observations at Manna Loa, Hawaii, since
1958, together with the more recent
identification of increasing trends in CH4,
NaO, and the chlorofluorocarbons,
establishes beyond a doubt the influence
that man can have on the global
atmospheric composition. Furthermore,
the most sophisticated models that we
now possess all agree that the
continuance of these trends will lead to a
significant change in climate (see for
example, Ramanathan et al., 1987). This
change in climate must feed back to alter
the composition of the atmosphere,
because many of the sources important
for determining the composition are
sensitive to a change in climate.
Furthermore, chemical rates and inter-
actions themselves are sensitive to
climate change.
The full report attempts to outline and
estimate where possible the interactions
between climate change and atmospheric
chemistry that need investigation on both
local and/or regional and global scales.
The problem is enormously complex and
is not simply one of estimating tem-
perature change and running chemical
models we already possess to get an
answer. The ways that a changing
climate influences atmospheric chemistry
include not only temperature and
-------
precipitation changes, but changes to
atmospheric transport processes,
changes to the budgets of species with
biological sources (which respond to
temperature and moisture changes),
changes to light levels due to
stratospheric ozone depletion which
would alter photolysis rates, changes to
vegetative cover which would alter
deposition rates, changes in the rate of
export of pollutants from the urban/
regional environment to the global one,
etc. Furthermore, changes in
atmospheric chemical composition will
lead to climate change.
Figure 1 gives an overview of the
climate/chemistry system and the inter-
actions that it will be necessary to
understand in order to predict the future
state of our planet. The responses of the
various subsystems shown there are
coupled to each other and to the realized
climate change itself. For example,
climate change has the potential to alter
water vapor concentrations. Changes in
the concentration of water vapor would
profoundly alter many species concen-
trations by impacting atmospheric OH
concentrations. Because OH acts as a
scavenger for many of the pollutants
released by man, changes in this species
will alter concentrations of many other
trace species. Changes to H20 will also
alter tropospheric 03 concentrations.
Because Oa absorbs infrared radiation,
these changes will feedback to alter the
climate. As another example, climate
change also has the potential to alter the
biological sources and sinks of certain
gases. Because biological processes
supply many of the species of radiative
importance in the troposphere, changes
in these sources will feed back to alter
the climate response.
Changes in global chemistry will be
intimately tied to changes in regional
chemistry and changes on one scale will
affect changes on others. Just as the
globally averaged temperature response
to a given change in composition will be
manifested by different changes in
various different regions, the "average"
composition change will be different in
different regions. Determination of how
this all ties together to feed back on the
climate change is a formidable problem
indeed. The full report begins with the
step: a discussion of possible
compositional changes on both regional
and global scales from the variety of
changes that are expected to be
introduced by climate change, and
potential climatic changes resulting from
known and suspected composition^
change. ^
Trends in Global Atmospheric
Composition and Climate
Earth's climate is the result of the
interaction of solar and longwave
radiation with the surface of the planet
and the gaseous constituents in its
atmosphere. It happens that both the
surface properties that determine the
amount of reflected radiation and the
constituents of the atmosphere that
absorb and interact with radiation are
subject to change by man. In order of
importance, the atmospheric species that
are most significant for radiative transfer
are HaO, C02, 03, CH4, N20 and
potentially CFC-11 and CFC-12
(Ramanathan et al., 1987). Of these, all
but HgO and 03 have established
abundance trends thought to be directly
traceable to emissions of human
industrial and agricultural activity
(Wuebbles and Edmonds, 1988). H2O
and 03 may also be increasing, but trend
detection is made difficult because of
inadequate monitoring and/or spatial and
temporal heterogeneity of distributions.
Table 1 summarizes current knowledge
Direct Effects
»• Feedback Loops-
1
L
Equilibrium Values
Trends and Budgets
Trace Gas Atmospheric
Abundance
,1
¥
Shortwave (VIS/UV) 1
Absorption
Longwave (In) Absorption
1 and Emission H
Atmospheric Radiation
Transfer
J
Climate Feedbacks
Climate System
| Industrial Production 1
| Energy and Combustion \ \
Agriculture, Land Use M
and A nimal Husbandry \
Natural-Biogenic
and Geologic
Trace Gas
Source
Emissions
H
J
Atmospheric
Lifetimes
Wet and Dry
Surface Deposition
Stratospheric Oxidation \
and Photolysis |
Tropospheric
Oxidation (OH) J
Atmospheric
Trace Gas Loss
Processes
Ozone Column
and Profile
Stratospheric (NOY, CIO
Tropospheric (HO* .../
| Atmospheric
Photochemical Processes
.«- -j and Intermediates
J
1
,-J
t
Figure 1. Climate/chemistry system.
-------
of global-scale observed abundance
trends and the importance of these
trends for chemistry/climate interactions.
The full report discusses the evidence for
these trends and some of the
uncertainties associated with prediction
of future trends.
In addition to the established trends in
CH4, NgO, and the chtorocarbons
listed in Table 1. a suite of other species
have also been suspected of change.
Trends in these species are much more
difficult to establish, because their
lifetimes are generally short (1 day to
several months), leading to spatial and
temporal variations that have precluded,
so far, the global characterization of their
abundances. These species include the
nitrogen oxides, carbon monoxide, non-
methane hydrocarbons, and tropospheric
aerosols. Of these, nitrogen oxides,
carbon monoxide, and non-methane
hydrocarbons have significant fossil fuel
derived sources while tropospheric
aerosols have a known graphitic
component that is also related to
combustion processes. Thus, trends can
raft/0 1. Established and Suspected Trace Species Trends
Estimated Annual
Trace Constituent Increase Importance for Climate
CO*
CH4
CFCI9
0*30.3
Tropospheric Oj
Stratospheric Oj
CO
wo.
0.4%
f.f%
0.2%
5.0%
5.0%
5.0%
?2.0%
fO.0%
0.8%*
1%
OCS
90,
t
••
Absorbs infrared radiation;
affects stratospheric Oj
Absorbs infrared radiation;
affects tropospheric Oj and
OH; affects stratospheric HgO
and Oj
Absorbs infrared radiation;
affects stratospheric Oj
Absorbs infrared radiation;
affects stratospheric 03
Absorbs infrared radiation;
affects stratospheric Oy
Absorbs infrared radiation;
affects stratospheric 03
Absorbs infrared radiation;
affects stratospheric 03
Absorbs infrared radiation;
affects stratospheric 03
Absorbs uv and infrared
radiation
Absorbs uv and infrared
radiation
Involved in tropospheric Oj and
OH cycles
Involved in O) and OH cycles
and precursor of acidic nitrates
Involved in tropospheric Oj and
OH cydes
Produces CCM which can alter
cloud albedo; forms SOg
forms aerosol in stratosphere
which alters albedo
May be major source of OCS in
the troposphere
mtajor precursor of acid rain
Hajor natural source of SQg
tcavenqar for many
-------
be expected, as have been observed for
the fossil fuel derived trend in NaO. The
magnitude of the trend expected for each
specie will depend on its lifetime, of
course. Data to support good estimates,
however, do not exist, due to the high
variability of both sources and sinks in
space and time.
The importance of nitrogen oxides,
carbon monoxide and non-methane
hydrocarbons derive from their ability to
alter the global tropospheric chemistry of
OH and 03. Ozone directly affects
climate through its solar and infrared
absorption, while OH moderates the
concentrations of CH4, CHsCCIs, and
other hydrogen-containing halocarbons
that are infrared absorbers. OH also
directly affects the photochemical cycling
or removal of the nitrogen oxides, CO,
and the non-methane hydrocarbons.
Changes in ©3 and OH will enhance the
predicted climate change over and above
that expected from the species that have
established abundance trends.
As a consequence of the uncertainties
in budget, scenario, and climate
response aspects, climate change pre-
dictions are more appropriately made in
terms of broad likely ranges rather than
specific values. (Similarly, the impacts on
the regional topics discussed in the full
report can be construed as sensitivities
over a range of possible input and
boundary condition values.) As an
example of studies of climate-chemical
interactions projecting climate change
into the next century. Ramanathan et al.
(1987) consider three scenarios generally
characterized by liberal, conservative and
intermediate views of the extension of
currently observed abundance trends.
The range of global equilibrium
temperature change for the 50-year
period ending in 2030 is 0.8 to 4.1K,
incorporating uncertainties in both
scenario and climate sensitivity. The
range of realized temperature change
over this period is about 0.5 to 1.2K,
including uncertainty in the rate of ocean
thermal diffusivity. These are substantial
fractions of the 11K glacial-interglacial
temperature contrast deduced from
antarctic ice core data (Barnola et al.,
1987; Genthon et al., 1987). In order to
narrow the range of uncertainty in the
predicted climate change, we need to
better understand the global chemical
changes that are occurring as well as
how regional chemistry and regional
emissions impact "global" chemistry.
The next two sections show how the
expected climate changes will affect
many aspects of both global and regional
chemistry, making a full assessment of
all the feedbacks a challenging problem
for the future.
Urban/Regional Chemistry and
Climate Change
The temperature increases associated
with the trends in trace species
discussed above are global average
temperature changes. Not all regional,
however, will experience the same
temperature increase; furthermore,
changes other than just a temperature
increase can be expected. These
changes could include, for example,
changes in cloud types and amounts, t
changes in meteorological conditions in a
given region, and changes in background
free tropospheric concentrations of a
variety of species. This variety of
changes will alter the chemical processes
that take place on urban and regional
scales and that lead to oxidant formation
and acid deposition. It is important to try
to understand the effects of these
changes on regional chemistry, because
we have already implemented emission
control policies that may not be
appropriate for a climatically different
future. New policies that are implemented
should account for the possibility that
climate change will alter the formation of
oxidant and acid deposition.
Determining how climate change would
alter regional chemical processes is a
difficult problem, because many types of
change need to be accounted for. Here,
we can only list potential climate changes
and sketch the resulting changes to
oxidant formation and acid deposition.
Actual regional weather, of course,
occurs in an episodic fashion and
regional models are run consistent with
episodic events. Climate change, on the
other hand, is generally construed to be a
statistical average change. It is not yet
possible to know how the changes we A
discuss here in terms of a local or \
regional response might integrate over
time to delineate a more "statistical" and
useful answer.
Table 2 gives a list of climate and
global chemistry associated changes that
Table 2. Climate Change Parameters Important for Regional Chemistry
1. A change in the average maximum or minimum temperature and/or changes in their spatial distribution and duration leading to a change
in reaction rate coefficients and the solubility of gases in cloud water solution.
2. A change in stratospheric O3 leading to a change in photolysis rate coefficients.
3. A change in the frequency and daily pattern of cloud cover an types of cloud formed leading to a change in photolysis rate coefficients
and heterogeneous rates of conversion of SO2.
4. A change in the frequency and intensity of stagnation episodes or a change in the depth of the planetary boundary layer and its diurnal
cycle leading to more or less mixing of polluted air with background air.
5. A change in background boundary layer and/or free tropospheric concentrations of water vapor, hydrocarbons. NOX, and O3 (due to
changes in mixing processes or sources such as lightning for NOX) leading to more or less dilution of polluted air in the boundary layer
with background air and altering the chemical transformation rates in both the boundary layer and the free troposphere.
6. A change in the vegetative and/or soil emissions of hydrocarbons and NOX which are sensitive to temperature andior light levels leading
to changes in their concentrations.
7. A change in deposition rates to vegetative surfaces whose stomatal resistance is a function of temperature, light intensity and other
factors leading to changes in concentrations.
8. A change in energy usage or technology leading to a change in energy-related emissions and their concentrations.
9. A change in secondary aerosol formation leading to changes in photolysis rates, the planetary albedo, and heterogeneous reaction
rates.
10. A change in circulation and/or precipitation patterns leading to a change in the abundance of pollutants deposited locally versus
exported off the continent
-------
could impact urban and regional oxidant
formation and acid deposition along with
some notes about the chemical and
physical processes that these
changes could alter. In the full report the
effect of some of these changes on the
maximum ozone concentration generated
in a simple box model of urban smog is
estimated (see Table 3). The calculations
demonstrate that several of the expected
effects might individually increase
maximum ozone concentrations by as
much as 20%. For the other changes
listed in Table 2, it was not possible to
estimate their effects, because there was
no adequate knowledge of the forcing
function from climate simulations, and
because regional models need to be
applied. Very few calculations for any of
these effects were available from
previous analyses. The importance of
this finding is difficult to judge because
the overall contribution of gas phase
conversion rates as compared to
heterogeneous conversion rates of SO?
to S04= in a relative statistical sense is
not yet established for the episode they
modeled.
For acid deposition, even fewer
quantitative estimates of the effect of any
of these climate-related changes is
availab.e At a workshop held in February
1988, Walcek and Chang (1988)
estimated that increases in temperature
could significantly increase the gas
phase conversion rates of SOa to 804 =
in their model. The importance of this
finding is difficult to judge because the
overall contribution of gas phase
conversion rates as compared to
heterogeneous conversion rates of SOa
to S04= in a statistical sense is not yet
established. For the episode they
modeled, the heterogeneous rates of
conversion were far more important.
Hales (1988) considered the effect of a
temperature increase at constant relative
humidity on the SOa conversion rates in
a cloud/chemistry model of a cyclonic
storm. The deposition rate of SO* - was
increased significantly, because precip-
itation amounts increased. In addition,
the pH of cloud water increased,
increasing the role of 03 at the expense
of HaOa in converting SOa to 804 = .
Thus, in an altered climate, local acid
precipitation may increase and the
chemistry of 03 formation may become
far more important to acid formation.
Both these sets of researchers felt that
the effects of climate change could be
very significant.
Very little else is known about how
acid deposition might change as a result
of climate change. However, because
over 30% of sulfur that is now emitted
over land areas in North America is
exported off the coast (Galloway et al.,
1984), we infer that regional deposition
patterns could change significantly, if
circulation patterns change.
Global Chemistry and Climate
Interactions
The above discussion has shown that
climate change has the potential to alter
regional chemistry. In addition, climate-
driven changes can also alter global
chemistry. Some of these changes feed
back to alter the climate as well. Two
important ones are considered in this
section: climate-driven changes to
tropospheric OH concentrations and
climate-driven changes to tropospheric
Op. Tropospheric OH is important to
climate because it acts as the primary
chemical scavenger of a variety of gases
(some of which are infrared absorbers
and helping to create the climate
change). Tropospheric ozone is important
because it is, itself a strong absorber of
infrared radiation.
A variety of species interact with OH
and the odd hydrogen family of species
to determine its abundance. Many of the
species on whose concentration OH
depends on will be altered by climate
change; for example, HaO concentration
will probably increase in general, if
relative humidity tends to remain fixed as
temperatures rise. Similarly, ©3 may
increase in response to direct as well as
climate-driven increases in NOX,
NMHCs, and CH4 emissions. Also
changes in the emissions of NOX,
NMHCs and CH4 will directly affect OH
concentrations. Changes to stratospheric
03 could also alter tropospheric OH by
increasing tropospheric photolysis rates.
Finally.temperature increases will alter
temperature-dependent photochemical
reaction rates that are important in the
OH budget. Table 4 summarizes the
estimated response of OH to changes in
Table 3. Estimated Change in Urban Ozone
Climate-Driven
Perturbation
Temperature increase
from 25° to 30°C
Increase biogenic
Estimated 0^
Change*
+20%
3-JO%
Notes
Could be larger in regions
emissions of NMHC
consistent with a 5"C
temperature increase
25% decrease in column
Og abundance
Increase cloudiness
leading to a 10%
decrease in photolysis
rates
Increase boundary layer
depth by 2
+ 10%
-7% to +47%
-10%
-50%
where biogenic emissions
are more important
This work and Liu and
Trainer (1987)
Geryetal. (1987)
Estimated using a simple box model of urban photochemistry with initial
HCINOX = 7. See Atherton and Penner (1988) for details
-------
Table 4. Estimated Change in Global OH
Climate-Driven
Perturbation
10-30% increase in H?0
2 x O3 in troposphere
Change in OH
few percent
few percent
~ 10% over ocean
Reference
LLNL1-D model
Thompson et al. (1988)
Thompson et al. (1988)
Increase biogenic NMHC
eissions
10% increase in NOX
Simultaneous increase in
CO.CH4, NMHC
Simultaneous increase in
CO,CH4, NOX, NMHC
> 10% over continents
Current emissions
decrease OH by a factor
of 2 near surface; no
estimate for effect of
increased emissions
+ 5%
Decrease in OH
Increase in OH
Trainer et al. (1987)
Thompson et al. (1988)
Isaksen and Hov (1987)
Isaksen and Hov (1987)
tropospheric specJes concentrations,
temperatures, and photolysis rate con-
stants. However, prediction of future
changes in global-average tropospheric
OH abundances depends on accounting
for the simultaneous action of the many
coupled OHx-controlling processes.
Projected increases in CO, CH4 and
NMHCs are likely to lead to continued
decrease in total tropospheric OH
abundance, as is shown by Isaksen and
Hov (1987) in a two-dimensional model
of coupled perturbations to the
troposphere. This average conclusion
includes, however, significant regional
diversity, in which some areas of the
troposphere may be characterized by
OH increase. For example, Isaksen and
Hov (1987) find that OH is increased
slightly in their model when increases in
the emissions of NOX were added to
those of CO, CH4, and the NMHCs.
Global tropospheric ozone abundance
is a balance between transport of roughly
5 x 1010 molecules cnv2 s'1 from the
stratosphere (Levy et al., 1985), surface
deposition to land and vegetation
estimated to be of similar magnitude
(e.g., Fabian and Pruchniewicz (1977)),
and photochemical sources and sinks
whose magnitudes are estimated to be
comparable to or larger than the
stratospheric source (NRC, 1984).
Because tropospheric ozone abundances
can be controlled by global scale
transport, regional scale transport, and
photochemistry, the distribution and
lifetime of tropospheric ozone is
significantly heterogeneous in time and
space. It is clear that in some regions of
the troposphere, with urban smog the
extreme example, photochemical pro-
cesses dominate the ozone budget and
produce a net source of ozone. In the
upper troposphere, transport from the
stratosphere dominates. In still other
areas, where NOX abundances are very
low, photochemical processes lead to a
net destruction of ozone.
The photochemical production of
ozone occurs primarily when peroxy
radicals generated in the OH-initiated
oxidation of CO, CH4, and higher
hydrocarbons (NMHC) oxidize NO to
N02, which leads to ozone formation
after N02 photolysis. Destruction occurs
if the region is NO-poor and the peroxy
radical reacts with ozone instead. Ozone
loss also occurs when 0(1D), which is
generated by 03 photolysis, reacts with
i-^O. This is also the primary source of
tropospheric OH. The OH and 03
chemistry cycles are thereby thoroughly
intertwined, implying that the same
mechanisms of climate change that
could alter tropospheric OH will also
create changes in the ozone budget and
cycle both directly, by altering
photochemistry of 03, and indirectly, by
altering OH and peroxy radical
concentrations that are involved in 03
photochemistry. In addition, of course,
the amount of 03 transported into the
troposphere from the stratosphere could
change as a result of stratospheric 03
depletion or due to changes in circulation
between the two atmospheric regions. A
number of calculations indicate that
tropospheric ozone can be expected to
increase via, basically, the same
mechanisms that are responsible for the
formation of photochemical smog. For
example, in the two-dimensional model
of Isaksen and Hov (1987), when CH4
concentrations were increased by 1.5%
yr1, QS concentrations increased by
.45% yr1. Larger 03 increases were
predicted for simultaneous increases i
CH4, CO, NOX, and NMHCs. No estimate
however, has accounted for the
heterogeneity of photochemical sources
and sinks expected in different regions.
Modeling Capabilities and
Limitations
In order to fully and meaningfully
address the issues involved in the
interaction of air chemistry and climate, it
will be necessary to somehow link
climate models, global chemistry models
and urban/regional models. Figure 2
shows, in a general fashion, the
processes that need to be represented
and that will require the linking of the
three types of models. Full lines indicate
changes and effects that can, at least in
some fashion, be estimated with today's
models. Dashed lines indicate processes
that interconnect global models of
climate, global models of chemistry.and
regional chemical models, but about
which very little is known. To create a
fully interconnected system would
require models we do not now possess.
The spatial scales and time scales
represented in these three types of
models are totally different, and it is
at all clear how to join them together (se
Figure 3).
-------
Regional
Pollutants
Oxidant (Oi)
Acid Deposition
(HN03. S03")
Trends in
COi. CHt.
, CFCs
Changes in Climate:
• Temperature
• Precipitation
• Water Vapor
• Winds
• Radiation Fields
\
A
Global
Concentrations
OH
03
Budgets
Trends in:
/VO,
CO
CH4, NMHCs
Figure 2. Processes that interconnect climate, global chemistry, and regional chemistry.
Global Chemistry Models
• fxplicit Chemistry
• 1-D or 2-Dimensional
• Resolution: Entire Hemisphere or
10O's of kilometers of latitude
• Seasonal Statistics
Climate Models
• No Chemistry
• 3-Dimensional
9 Resolution ~ WO's Kilometers
• Seasonal Statistics
Regional Chemistry Models
• Parameterized HC Chemistry
• 3-Dimensional
• Resolution ~ 5-10 Kilometers
• Episodic Event Simulations
Figure 3. Characteristics of current models.
-------
Climate change is typically studied
with general circulation models. These
predict precipitation patterns, wind
patterns, global cloudiness, and
temperature change statistics with a
resolution of about several hundred
kilometers. Because many important
processes take place on smaller scales,
these must be parameterized in the
models, leading to inconsistent results on
the scales important to regional
chemistry.
Regional and urban chemistry and
deposition models need meteorological
and climate change information on much
smaller scales. Typically, an urban
oxidant model would occupy much less
than one grid square of a general
circulation model. Regional models
require a detailed description of the
meteorology appropriate to at most
several GCM grid points. A challenge for
the future is to design a means for
bridging across these spatial scales
(assuming the GCM predictions for
climate change on a regional basis
become more robust (Grotch, 1988)).
Some initial work in this direction has
begun (Giorgi, 1988) but these initial
efforts will require significant ongoing
support to understand the level of
confidence that might be placed in their
predictions. A further challenge is to try
to bridge across the evident differences
in time scales. GCM calculations attempt
to predict statistics of the predicted
climate averaged over a reasonably long
period - i.e., one week or more. Regional
and urban models have only been
applied to particular episodes - with at
most a several day time period. A
method for interpreting the results of the
episodic predictions, within the context of
the climate change statistics must be
devised.
The connections between global
chemistry models and regional and
urban models are even more tenuous.
Three-dimensional chemistry models
capable of treating the horizontally
inhomogeneous gas concentrations
evident in the global troposphere are
only now beginning to be developed.
Sub-grid scale parameterizations that
currently plague the GCM simulations
will likely have impacts for the chemistry
models as well. Further, sub-grid scale
parameterizations of chemistry are likely
to be important in these models because
chemical processes that alter concen-
tration distributions on urban and regional
scales would need to be treated in some
realistic fashion. A case in point is the
transformation of nitrogen oxides within
the urban domain to less chemically
active organic nitrate forms (Atherton and
Penner, 1988). It will likely be important
to develop schemes to make the
chemistry on these larger scales
consistent with the chemical trans-
formations that we know take place on
smaller scales.
Finally, regardless of our ability (or
lack thereof) to physically link these
modeling systems, our basic knowledge
of the global tropospheric chemical
system needs refinement and tuning.
Our knowledge of biogeochemical cycles
is crude at best.Therefore, our ability to
confidently predict trace gas trends is
crude, and certainly our ability to predict
the impact of climate change on
biologically emitted source gases is
nearly non-existent. Hence, substantial
effort is called for to truly understand the
interactions of our planet's chemistry and
climate systems.
Conclusions and Research
Needs
Some major findings of the analyses
reviewed or developed in the full report
are:
1. There is clear evidence that
atmospheric emissions and concen-
trations of radiatively and chemically
important trace gases, such as COa,
CH4> N20, CF2CI2, CFCI3, and CO
are increasing, and have been
increasing for a long time. These
increases derive largely from human
related activities. Current analyses
suggest that it is improbable that
present trends towards increasing
concentrations will be arrested or
reversed in the near future. Pre-
dictions of future scenarios for their
growth are limited by uncertainties in
trace gas budgets and in forecasting
economic growth, energy use and
other factors.
2. The direct effects from the radiative
forcing from these gases on climate
are not in question. However, there
are many uncertainties associated
with the climate feedback processes
that will determine the eventual
change in temperature and other
climatic variables. Climate models
indicate that global surface temper-
atures for a doubling of C02, the
radiative equivalence of which could
occur by mid-21 st century, is in the
range of 1.5 to 4.5K, with general
circulation models giving results inthe
upper end end of this range.
3. General circulation models are in
general agreement regarding the
effects of climate change on a globa
basis, but are in substantial
disagreement on the effects ove
specific regions. Representations o
clouds, the planetary boundary layer
and surface processes all contribute tc
the uncertainties in determining the
derived climate change. Currently, the
results from GCMs should not be
regarded as reliable indicators o
regional effects from climate change.
4. The effects of climate change or
urban and regional scale chemistry
could be quite significant but very little
information currently exists on the
sensitivity of air quality models tc
climatic parameters. The few studies
available suggest that oxidan
formation may be sensitive to changes
in temperature, in stratospheric ozone
in cloud cover, in boundary layei
depth, in background concentrations
and in induced emissions responses
No information exists on whether the
frequency of stagnation episodes
would be affected by climate change.
5. Likewise, little is known about how
acid deposition would be affected by
climate change. Types of climate
change that could affect the
conversion rate of S02 to sulfate anc
the acid deposition include changes 4
temperature, stratospheric ozone
background concentrations, circulatior
patterns, frequency and types o
clouds formed, and precipitation pat
terns.
6. The interactions we discussec
between climate change and globa
tropospheric chemistry primarily
centered around perturbations to the
distributions of ozone and the hydroxy
radical. Changes in ozone, driven by
increasing concentrations of CO anc
NOX for example, can have a direc
impact on climate. At the same time
climate change can influence ozone
concentrations. Hydroxyl radical is the
primary chemical scavenger of sucf
radiatively and chemically importan
gases as CH4, NMHC, and CO
therefore, changes in OH concen
trations, whether due to direc
emissions of CH4 or CO, or due tc
climate induced changes in temper
ature of HgO, can have a significan
impact on the lifetimes and transpor
of radiatively active gases.
7. Studies of global tropospheric
chemistry are currently limited due
unavailability of three-dimensio
global scale models that can ad
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quately account for the spatial vari-
ations in trace gas emissions and
photochemistry.
8. Urban and regional emissions,
particularly of NOX and NMHC, may
affect global scale tropospheric
chemistry and thus affect climate.
However, the chemical forms of these
species by the time they reach the
global scale are not well known, nor
represented adequately in current
global models.
9. In general , the current modeling tools
available, both in terms of climate
models and air quality models, are
insufficient to delineate meaningful
diagnostic or prognostic analyses of
all of the changes in climate
parameters of interest.
The findings from this study lead
naturally to the development of a list of
important research needs. Some have
been alluded to above. These are more
fully delineated in Chapter 6 of the full
report.
References
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J. E. Penner, P. S. Connell, D. J. Wuebbles, and C. C. Covey are
Livermore National Laboratory, Livermore, CA 94550.
J. Bufalini is the EPA Project Officer (see below).
The complete report, entitled, "Climate Change and Its Interactions with Air
Chemistry: Perspectives and Research Needs," (Order No. PB 89-126 601/AS;
Cost: $15.95, subject to change) will be available only from:
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Research Triangle Park, NC 27711
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