2. Atmospheric Chemistry and the Earth System
3. Observed and Expected Changes in the Chemical Composition of the Atmosphere 4. Causes of Atmospheric changes
3.2 Recent changes in atmospheric composition
4.2 Biomass burning
4.3 Land-use changes
4.4 Climate changes
5.2 Impacts on ecosystems
5.3 Corrosion
5.4 Health effects
Direct measurements of the chemical composition of the atmosphere during the past 30-50 years have clearly demonstrated that the concentration of several key components exhibit systematic trends. Indirect information from ice cores, lake sediments, etc. has shown that large changes also have occurred during the agro-industrial era, i.e., the past 200 years. There are several fundamental questions related to these observed changes:
We begin this chapter by introducing a number of key environmental issues related to the observed changes in atmospheric composition and their impact on ecosystems and human society. These issues have been a major driving force in atmospheric chemistry research during the past few decades.
1.1 Greenhouse gases and climate
Increasing concentrations of CO2, CH4, N2O and several other gases with absorption bands in the infrared part of the spectrum contribute to the so called greenhouse effect leading to a warming of the Earth's surface and the troposphere (and a cooling of the stratosphere and mesosphere). This process, first quantitatively studied by Arrhenius (1896), has provided an additional radiative energy input into the troposphere/surface part of the climate system of about 2.4 W/m 2 since the beginning of the agro-industrial era (IPCC, 1996; see Figure 1.1).
1.2 Aerosols and climate
An increasing load of aerosol particles in the troposphere (sulfates, organics, black carbon etc.) caused by emission from both industrial processes and biomass burning, has decreased the amount of incoming solar radiation reaching the Earth's surface by 0.5 to 2 W/m2 . This cooling can have counteracted a substantial part of the warming due to greenhouse gases. The main reason for the large uncertainty in this quantitative estimate is our poor understanding and lack of data on how the increasing aerosol mass concentration has affected the number concentration of cloud condensation nuclei (CCN) and thereby the reflectivity of the clouds (IPCC, 1996; see 10 Figure 1.2).
1.3 Acidification and eutrophication
Increasing deposition of acidifying sulfur and nitrogen compounds has led to widespread damage to terrestrial and limnic ecosystems in some parts of the world e.g. northern Europe and northeastern North America near and downwind of large industrial emission regions, following deposition on poorly buffered soils (refs.) 1.4 Enhanced tropospheric ozone
Observations at the Earth's surface and at mountain sites in Europe suggest that tropospheric ozone (O3) has increased by a factor 4 or more during the past century. This increase is believed to be mainly due to man- made emissions of NOx, hydrocarbons and CO, which interact with solar radiation to produce O3. The elevated concentration of ozone causes concern because of its impact on climate (O3 is a greenhouse gas!) and its adverse effects on plants and human health (ref.). 1.5 Depletion of stratospheric ozone
One of the most dramatic changes in atmospheric composition observed during the past decades is the depletion of ozone in the stratosphere. This global (extra-tropical) phenomenon is most pronounced at high southern latitudes during the spring months. There is now a general consensus that the ozone depletion is caused by chlorine- and bromine-containing gases of industrial origin. This leads to an increasing flux of UV radiation into the troposphere with potential impacts on human health, ecosystems and the photochemistry of the troposphere (see Figure 1.5). Furthermore, stratospheric ozone depletion provides a negative net contribution to the tropospheric energy balance (x W/m2).
1.6 Transport of toxic substances (metals, organic compounds, radionuclides)
Elevated levels of toxic substances (gaseous or bound to particles) in the atmosphere represent an issue of a different character than the previous five questions. These toxic compounds for example, Hg, Cd, PAH, PCB, Cs- 137 occur in very low concentrations and are not believed to be of any importance for the balance of the major atmospheric components. The atmosphere serves, however, as the major transport medium for these types of toxic compounds and an estimate of their effects on human health and ecosystem requires that processes of emission, transport, transformation and deposition be understood (see Figure 1.6).
1.7 Transport of nutrients
[To be completed] The most abundant gases in the Earth's atmosphere are molecular nitrogen (N2) and oxygen (O2), in contrast to what is observed in the atmosphere of our neighboring planets Mars and Venus. A particular property of the Earth's atmosphere is that many of its chemical compounds, including N2 and O2, are constantly taken up and regenerated by biological processes (microbial activity in soils, photosynthesis and respiration, foliage emissions, etc.). Reduced chemical compounds that are released to the atmosphere by terrestrial and oceanic ecosystems and as anthropogenic emissions are often oxidized in the atmosphere, and the resulting products are removed by deposition to the Earth's surface. Subsequent assimilation and reduction in the biosphere closes the atmosphere-biosphere cycle. For example, nitrous oxide (N2O), which is a product of nitrification/denitrification processes in soils and waters, is destroyed primarily by photolysis in the stratosphere and is oxidized to produce nitric oxide (NO). NO and Its oxidation product NO2 are converted into nitric acid (HNO3) which is removed from the atmosphere by precipitation or deposited on the surface. Methane (CH4), produced in oxygen- deficient environments, is oxidized in the atmosphere to water vapor and carbon monoxide (CO) and further into carbon dioxide (CO2). Similar degradation mechanisms affect nonmethane hydrocarbons, including those (such as isoprene and monoterpenes) which are produced by vegetation. Atmospheric CO2 is constantly exchanged with the ocean and continental ecosystems at rates that depend on the water temperature and on the seasonal evolution of the terrestrial biosphere, respectively. Sulfur, when produced naturally, is released from the ocean to the atmosphere, primarily as dimethyl sulfide (CH3) S (or DMS). These compounds are photolyzed or oxidized into sulfur dioxide (SO2) which is further converted into sulfuric acid (H2SO4) and sulfate aerosol particles. Water (H2O) is an important greenhouse gas and also plays a key role in the Earth system since it circulates between the ocean, the atmosphere and the biosphere and is constantly switching between the vapor, the liquid and the solid phases. It is also an important chemical compound of the atmosphere, since it is the major source of hydroxyl (OH) and hydroperoxy (HO2) radicals, and provides sites (droplets and ice particles) for chemical processes (that are not possible in the gas phase) to occur in the condensed phase or at the interface. 3.1Variability of atmospheric composition in the past: Evidence of feedbacks?
At locations where air is trapped in glacial ice, quantitative records of atmospheric composition or deposition are available on time scales of hundreds of thousands of years (i.e. multiple glacial periods) for only a few of the many substances that are important on the global change scene (CO2, CH4, N2O, sulfate, nitrate, soil dust, sea salt). Records recovered in the southern hemisphere at Vostok, Antarctica and in the northern hemisphere at Summit, Greenland show a natural variability in the biosphere/atmosphere system (Figure 3.1.1). This variability marks the response to major fluctuations in climate seemingly correlated with the cycle of solar energy input related to the orientation of the Earth to the Sun (Milankovich forcing) but also amplified by physical and biogeochemical feedbacks. For example, during past glaciations atmospheric CO2 and CH4 decreased while soil dust increased thereby reinforcing what the changes apparently initiated by Milankovich forcing. The nature of biogeochemical feedbacks is key to understanding global change. Mankind is conducting a chemical perturbation experiment by altering many biogeochemical cycles. We can ask questions, such as: How important were biogeochemical cycle feedbacks in the flip from one stable climate mode to another? How strong are these feedbacks relative to other purely physical feedbacks such as the polar-ice albedo one? Have biological feedbacks contributed to the unprecedented rate of greenhouse gas and aerosol change observed today? It is interesting to note that the periodic climate variations associated with Milankovich forcing are enhanced by positive feedbacks through greenhouse gases and soil dust, so that climate does not seem to be stabilized by biospheric responses.
3.2 Recent changes in atmospheric composition
Although considerable progress has been made in the past decade in understanding the biogeochemical cycles of greenhouse gases, oxidants and aerosols large gaps in knowledge still remain. Furthermore, uncertainties in the climate feedback response of these processes are also large. The rapid changes in atmospheric composition observed over the past 100 years have been driven by emissions from fossil fuel combustion, industry, biomass burning, and agriculture. Table 1 gives present-day global inventories of anthropogenic emissions for some environmentally important gases discussed in the previous sections, and compares to the magnitudes of natural sources (biosphere, oceans, volcanoes, lightning). Anthropogenic emissions are important, and often dominant, contributors to the abundances of many atmospheric species. Also included in the Table are forecast ranges of anthropogenic emissions for Year 2100 presented by IPCC [1999] for different possible socioeconomic scenarios. These forecasts suggest that human perturbation to atmospheric composition will generally increase over the next century. In addition, future climate change may affect the natural emissions of radiatively active gases and aerosols, resulting in complicated chemistry-aerosol-climate feedbacks.
Whereas sulfur emissions are now declining in these particular regions, emissions are increasing at a rapid rate in some other regions (China, India, Southeast Asia) raising concern about more widespread acidification problems in the future.
Fixed nitrogen (NO3 , NH4+ ) is a limiting nutrient in many ecosystems. As a consequence, the increased nitrogen deposition in and around many industrial parts of the world has caused eutrophication of both terrestrial and marine ecosystems with consequences for plant and animal species composition and also for the atmospheric carbon balance (see Figure 1.3). (ref.)
Photolysis of tropospheric O3 is the primary source of the hydroxyl radical (OH), an extremely strong oxidant. Oxidation by OH is the main sink for a large number of environmentally important atmospheric species including toxic gases such as CO, greenhouse gases such as CH4, and gases responsible for stratospheric O3 depletion such as CH3Br. In this manner, tropospheric O3 plays a critical role in determining the oxidizing and cleansing efficiency of the atmosphere. The rise in O3 and NOx since pre-industrial times has increased the source of OH, but this increase has been compensated by more rapid removal of OH due to the rise in reduced gases (particularly CO and CH4). Atmospheric models predict little change in the global mean OH concentration since pre-industrial times (less than 10% in most models), reflecting these compensating factors. The 1978 1994 record of the methylchloroform proxy shows no significant trend in global mean OH concentrations during that period (0.0 ± 0.2% yr-1 ) (Prinn et al., 1995).
These issues have received growing attention not only from the members of the scientific community, but also from decision makers in the government and industry. With the change in society's attitude regarding the environment, the relation between atmospheric chemistry research and environmental policy design has been growing substantially over the last decades and actions to protect the global environment have been taken. Major challenges, however, remain. Although substantial advances have been made in understanding fundamental processes in the chemical system of the atmosphere, our predictive capability remains limited in spite of its importance for informed decision making. New and challenging problems at the chemistry-climate and chemistry-biology interfaces are emerging and will require much attention in the future. The availability of space instrumentation in the near future to observe chemical species in the troposphere will provide unprecedented information on the global distribution and on the evolution of key biogenic and anthropogenic compounds.
2. Atmospheric Chemistry and the Earth System
The circulation of chemical elements in the Earth system is often described in terms of global biogeochemical cycles, and one of the challenges for the scientific community is to quantify the global budget (burden and residence time in each component of the geosphere and transfer rates between them) of key chemical elements or compounds. These cycles produce important feedback mechanisms in the Earth system. They have often been dramatically perturbed by human activities including agricultural practices, industrialization and urbanization. Changes in land-use including massive biomass burning, particularly in the tropics, and energy consumption, mostly fossil fuel burning, have been the sources of substantial changes in the chemical composition of the atmosphere and in the deposition rate of acids and other toxic products on the Earth's surface. An important task for the 15 scientific community is to "close" the biogeochemical cycles and to "balance" the chemical budgets in the Earth system.
The global distribution of chemical compounds in the atmosphere depends not only on the exchange rates between the surface and the atmospheric boundary layer. It is also determined by the rate at which chemical compounds are injected from the boundary layer to the free- troposphere and are transported by the large-scale atmospheric circulation. Transport by deep convective cells and frontal systems as well as exchanges through the tropopause are important processes that are not well understood. Chemical transformations through gas-phase reactions or multi-phase processes proceed at rates that need to be measured in the laboratory, while photolysis rates are a function of the solar actinic flux that penetrates into the atmosphere, and hence of absorption and scattering processes by atmospheric molecules, clouds and aerosol particles.
Important for the chemistry of the troposphere are the chemical reactions that lead to the formation of ozone. As noted by Crutzen (1972) and Chameides and Walker (1973), the conversion of nitric oxide NO to nitrogen dioxide (NO2) by peroxy radicals (HO2, CH3O2, etc.) leads to the formation of ozone during daytime. For example,
NO2 + hn -> NO + O (1b)
O + O2 + M -> O3+ M (1c)
or CH4 + OH O2 CH3O2 + H2O (3)
Tropospheric ozone is destroyed by photolysis in the ultraviolet when the resulting electronically excited oxygen atom O(1 D) reacts with water vapor
O(1 D) + H2O -> 2 OH (5)
O3 + HO2 -> O2 + 2 OH (6b)
HO2 + HO2 -> H2O2 + O2
OH + NO2 -> HNO3
HSO3 + O2 -> HO2+ SO3
SO3 + H2O -> H2SO4
SO2(aq) <-> HSO3 + H+
HSO3 + H2O2(aq) + H+ -> SO4= + 2H+
A key species for many processes involved in the chemistry of the troposphere is the OH radical. Because of its high reactivity with most chemical constituents including pollutants, OH is often dubbed the "detergent" of the atmosphere, because it determines the self-cleansing ability (or the oxidizing efficiency) of the troposphere. The diagram shown in Figure 2.1 presents a schematic representation of the most important chemical processes occurring in the troposphere. It stresses the reactions that control the formation and destruction of ozone and hydroxyl. It highlights the nonlinear behavior of the tropospheric chemical system.
As a result, the response of the tropospheric chemical system to human- induced perturbations could be very complex, and exhibit different regimes with abrupt transitions. For example, the existence of thresholds (such as temperature threshold associated with the formation of particles) contributes to the non-linearity of the chemical system. The behavior of the atmosphere and future changes in response to human-induced forcings are therefore difficult to predict and could offer surprises.
3. Observed and Expected Changes in the Chemical Composition of the Atmosphere
The concentration of greenhouse gases has increased in the atmosphere since the pre-industrial period to reach levels unprecedented in the last 2 million years (Figure 3.2.1 CO2, CH4, CFCs, N2O last 200 years). Storage in the oceans and on land of anthropogenic CO2 emissions is the he concentrations are uncertain because limited knowledge about the response of the terrestrial biosphere to climate changes and vice versa. The future CH4 level is uncertain because of unknowns in the temperature dependence of wetland sources and permafrost melting, and in atmospheric OH radical concentrations (the major sink). The evolution of future N2O levels in the atmosphere are very uncertain because of poorly defined sources and sinks. The inputs of large and growing amounts of nitrogen fertilizers in agriculture poses the question as to where the nitrogen is going and how much N2O may be released in future from accumulation of long-lived environmental subsystems (soils, waters).
Tropospheric ozone is an important greenhouse gas. It has a lifetime in the troposphere that varies from about a week in the summer to months in the dark polar winter. Increases in its occurrence in the troposphere since the pre-industrial period are evident over Europe and East Asia. They are associated with large increases in gaseous precursors (Figure 3.2.2). Elsewhere in the remote northern troposphere decreases seem to be largely in step with recent decreases in stratospheric ozone (Figure 3.2.3). The bottom line is that there are not enough observations to assess present and past changes in tropospheric ozone. The behavior of tropospheric ozone is complex and strongly associated with our understanding of the atmospheric oxidation efficiency (OH concentrations). An important question is to assess the relative contribution of stratospheric and in situ photochemical sources to tropospheric ozone.
Historical records of the composition of reactive oxidants and their precursors in the atmosphere are scarce and limited mostly to high latitude regions. No instrumental record longer than 15 years exists. Long term records of H2O2 and formaldehyde (Figure 3.2.4) in glacial ice from Summit, Greenland, although promising, are of regional importance and furthermore difficult to interpret because of variable atmospheric scavenging processes and post- depositional chemistry taking place in the surface snowpack. The latter alters the original deposition signal and can even be a source of compounds such as formaldehyde (Sumner and Shepson, 1999), nitrogen oxides (Honrath et al., 1999) and carbon monoxide to the atmosphere. Nevertheless, these records indicate an enhanced presence of these oxidants in the modern northern polar middle troposphere compared to the past. The task is to determine the role played by the surface snowpack in lower atmospheric composition and in influencing the transfer function of a chemical constituent between atmosphere and glacial firn and ice.
Since the pre-industrial era, sulfates from fossil fuel burning and smelting as well as nitrates from fossil fuel burning (such as automotive emissions) have increased in glacial snow and ice. Figure 3.2.5 compares two historical records at Agassiz glacier in the winter and at Summit, Greenland (~3 km altitude that sees middle and tropospheric air).
On a somewhat shorter time scale (two decades), instrumental records of the composition of the atmosphere and precipitation exist in the acid rain regions of the world, particularly, eastern North America, Europe and the downwind Arctic. They show a decline in SO2 but no significant change in NOx associated with regional pollution. It appears that the effects of the introduction of catalysts by automobile engines has been largely offset by increasing numbers of automobiles in these regions. In contrast, SO2 emissions from the former Soviet Union which did not decrease substantively in the 1980s, have fallen in the 1990s because of hard economic times rather than a willful intervention by man. Sulfur emissions and attendant concentrations of sulfates in the atmosphere have increased in Southeast Asia and are expected to do so for the decades ahead.
4. Causes of Atmospheric Changes
Pg C yr-1 |
Tg CH4 yr-1 |
Tg N yr-1 |
Tg S yr-1 |
Tg N yr-1 |
Tg CO yr-1 |
|
| Natural Anthropogenic Fossil Fuel and Industry Biomass burning Agriculture Landfills/sewage Total, present Total, 2100** |
60 6.8 1.6* 7.6 8-29 |
160 100 40 170 65 375 300-900 |
9 1.3 0.5 3.9 5.7 5-16 |
18 65 2 67 27-60 |
11 22 12 1 35 34-110 |
200 450 500 950 950-2500 |
4.2 Biomass burning
Biomass burning includes deforestation, agricultural fires, wood fuel use, and natural fires. Most of the world's biomass burning takes place in the tropics and is highly seasonal, peaking at the end of the dry season (January- April in the northern tropics, August-October in the southern tropics). The largest contribution to this tropical source is agricultural burning of savannas; deforestation is also an important source in some regions. Natural fires are important mainly in boreal forests at high northern latitudes, where they peak during the summer months. 4.3 Land-use changes
Major land-use changes over the past 100 years have included urbanization driven by population pressure, increase in cultivated land in the tropics, reforestation of formerly cultivated land in northern mid-latitudes regions such as the eastern United States, and desertification in some areas such as northern Africa. Tropical deforestation presently makes a significant contribution to the rise in CO2 (Table 4.1), though this contribution is expected to decrease over the next century due to decimation of the primary forests and may actually be overcome by the CO2 sink from reforestation at northern midlatitudes (IPCC, 1999). 4.4 Climate Changes
Changes in the climate system could substantially affect the chemical composition of the atmosphere. Such changes are related to interannual variability in the dynamics of the atmosphere, to quasi-periodic oscillations in the atmosphere/ocean system (such as the El Niño oscillation in the tropical Pacific), and to longer timescale climatic trends. Long-term trends are believed to be generated by external forcing including human-induced emissions of greenhouse gases. In this section we focus on three major categories of impact of changes in atmospheric composition: climate change, effects on ecosystems and human health. Of course, these categories are not always well separated. For example, a change in climate will have secondary effects on ecosystems and health. Nevertheless, we believe that this structure provides a useful framework for a discussion on impacts of changes in atmospheric composition. 5.1 Climate change
The issue of climate change in general, and the impact on climate of human-induced processes in particular, has recently been reviewed in detail by IPCC (IPCC, 1990; 1992; 1994; 1996). In our discussion we adopt the terminology used by the Intergovernmental Panel on Climate Change (IPCC) and use the term "climate forcing" to denote the change in the net irradiance (in W/m2 ) at the tropopause (after allowing for stratospheric temperatures to re-adjust to radiative equilibrium, but with the surface and tropospheric temperature and moisture held fixed), created by changes imposed on the climate system by natural (solar radiation changes, volcanic emissions of SO2 and particulate matter) or human-induced processes (greenhouse gases, aerosols, changes in surface albedo). The climate forcing concept provides a very useful means of comparing quantitatively the importance of the different factors for change. It avoids the difficult problem of how the climate system will actually respond (in terms of temperature, precipitation, winds, etc.) to the imposed changes. This latter problem, which involves an intricate web of interactions and feedbacks within the atmosphere and between the atmosphere and the underlying surface, has to be addressed using complex models of the climate system. In particular, the difficulty of assessing the climate forcing and the climate response from inhomogeneous radiative agents such as aerosols and ozone needs to be stressed. The climate response issue will not be further addressed in the present report. For this the reader is referred to reviews such as IPCC (1996) and (ref.). To be added: 5.2 Impacts on ecosystems
Ecosystems are affected by atmospheric changes in several different ways:
5.3 Corrosion
Acid deposition and high concentrations of gaseous pollutant (SO2, NO2 and O3) are known to cause damage to buildings and cultural and historical monuments mainly in urban and suburban areas. The economic loss due to such man-made corrosion in Europe has been estimated to be of the order of 1010 Euro per year (?). Mainly because of reductions in SO2 concentrations the corrosion rates are now decreasing substantially in Europe and North America. However, in many other areas, especially in Eastern Europe and some developing countries, corrosion problems remain severe or become more acute. There are indications that in warm climates NO2 and O3 alone (even without SO2) may cause serious corrosion problems.
5.4 Health effects
Air pollutants have long been recognized as a health hazard. Well known examples include the smog episodes in London - the one in December 1952 is estimated to have caused 4000 excess deaths - and the photochemical pollution (O3, PAN, etc.) in Los Angeles which cause lung damage, eye irritation and other effects. During recent years more subtle effects, some of them believed to occur not only in urban and suburban areas but also over larger regions have been identified. Here we list some of the most important health issues related to man-made air pollution.
In contrast with the situation prevailing in meteorology where a continuous monitoring effort provides detailed and global observations on the physical state of the atmosphere, only limited observational data are available on the global distribution, seasonal evolution and trends of chemical compounds in the troposphere. A major effort of the atmospheric chemistry community has therefore focused on the measurement of chemical compounds (gases and aerosols) in the troposphere and stratosphere. Many projects, often sponsored by IGAC, have used different types of instrumentation (e.g., in situ or remote sensing techniques) on different platforms (aircraft, balloon, spacecraft, ground-based). Several field campaigns performed over a limited time period (see Table 1.6a) have been organized to investigate specific photochemical processes at different locations. Other observational projects have provided information on large-scale distributions of specific chemical constituents (Table 6b). Periodic soundings of the atmosphere (balloon-borne, lidar) have revealed the structure of the vertical distribution for a limited number of atmospheric species at a few locations. Thus, in spite of efforts by the international scientific community, a satisfactory global climatology of tropospheric compounds is far from reality and a strategy to gather more information on the spatial and temporal distribution of the major species needs to be established.
Biomass burning emits the same general suite of species to the atmosphere as fossil fuel combustion but often in very different proportions; biomass burning is less efficient than fossil fuel combustion and the combustion temperatures are lower. The CO/CO2 molar emission ratio from biomass burning is 5-10%, much higher than for fossil fuel combustion, and the global source of CO from biomass burning is comparable to that from fossil fuel combustion. By contrast, the NOx/CO2 emission ratio from biomass burning is less than that from fossil fuel combustion, because temperatures in biomass fires are not sufficiently high for thermal NOx formation from atmospheric N2 (they are however sufficiently high for oxidation of biomass nitrogen to NOx). The sulfur source from biomass burning is very small compared to the fossil fuel source (Table 4.1) due to the low abundance of sulfur in vegetation.
There is little knowledge of historical trends in biomass burning, and IPCC (1999) does not venture to forecast trends for the next century. Greenland ice core records of biomass burning tracers over the past 1000 years identify several historical periods of enhanced biomass burning at high northern latitudes but indicate a decrease since the 1930s. Fire records maintained by governmental agencies in North America and Europe over the past 50 years indicate a general increase in the number of fires, due to human negligence, but not much change in area burned because of improved firefighting ability. In the tropics, where most of global biomass burning takes place, there is essentially no information on historical trends. One would expect a rise in biomass burning from increasing agriculture and deforestation over the past century, but this effect could have been offset by better fire control in response to the growing rural population. Reconstructed surface O3 records for South America in the late 19th century do not show the characteristic spring maximum seen in present-day observations, suggesting that the biomass burning source might have been weaker. Satellite observations of tropical tropospheric ozone since 1979 suggest a slight increase in biomass burning over the past 20 years, but continuous records of CO measurements at tropical sites over the past decade show no significant increases.
Agriculture is thought to be the principal cause for the rises of CH4 and N2O over the past 100 years (Table 4.1). Methane is emitted by anaerobes in ruminants and rice paddies (which are effectively human-generated wetlands). Nitrogen fertilizer application to crops stimulates microbial emission of N2O; it also stimulates emission of NOx, but the resulting source is small compared to that from fossil fuel combustion (Table 4.1).
Long-term time series of atmospheric dust over the Atlantic indicate a secular increase associated presumably with desertification of northern Africa. Dust is a major contributor to aerosol optical depth, scattering solar radiation and absorbing terrestrial radiation. In addition, deposition of dust to the ocean supplies limiting nutrients (iron and other metals) to increase the efficiency of CO2 uptake. Increasing desertification over the next 100 years is expected as a result of agricultural practices and irrigation pressure in the tropics and subtropics. The resulting increase in atmospheric dust could have important implications for climate change, both directly by radiative forcing and indirectly by fertilization of the oceans.
Although the mechanisms governing the Earth's climate are not fully understood, the effects of climate changes on the chemical composition of the atmosphere need to be identified because they provide potential positive or negative feedback mechanisms in the climate system. Such feedback mechanisms are illustrated below.
Warming of the Earth resulting from enhanced concentrations of carbon dioxide, methane, and other greenhouse gases is expected to enhance surface evaporation, and hence the tropospheric abundance of water vapor. Water not only amplifies greenhouse warming; it also leads to the formation of the hydroxyl radical (OH) and hence affects the lifetime of several chemical compounds (including greenhouse gases such as methane) in the atmosphere. The ozone density in the troposphere is also expected to change with potential effects on the related climate forcing.
Climate change could also result in more intense convective activity. As a consequence, the formation rate of nitrogen oxides by lightning and hence the concentration of ozone in the free troposphere would probably increase, producing an amplification of radiative forcing.
Surface emissions of several biogenic gases (including nitrogen oxides by soils, isoprene and other terpenes by vegetation, dimethyl sulfide by the ocean) would increase on a warmer planet. These compounds are precursors of either tropospheric ozone or of aerosols, and hence could indirectly impact the Earth's climate.
Changes in the soil moisture associated with changes in the precipitation patterns would affect the intensity and the geographical locations of savanna and forest fires, with significant consequences on biomass burning emissions, and hence on the concentration of several chemical compounds, including carbon monoxide, nitrogen oxides and ozone.
Finally, changes in precipitation rates associated with climate change would modify the rate at which soluble gases and aerosol particles are removed from the atmosphere, with consequences on the residence time of several compounds (including pollutants) in the atmosphere and potential feedbacks on the climate system.
Providing an estimate of the combined chemical effects associated with climate changes on a variety of time scales is a difficult problem. Although preliminary studies have suggested that such issue is potentially significant, the response to this question will ultimately be provided by coupled climate/chemistry/biology models.
5. Impact of Changes
Because changes in the chemical composition of the atmosphere are the main drivers of anthropogenic climate change, the scientific community involved in atmospheric chemistry has been much engaged in the climate change issue and has established strong links with the more physically oriented climate researchers. There has also been some links with ecologists who study the effects on ecosystems of increased exposure to various atmospheric gases and aerosol, and to UV radiation. The connection to the community of scientists studying the effects of atmospheric changes on human health has been considerably weaker.
In Figure 5.1.1 we show a comparison of the global average climate forcing during the past century due to different processes (IPCC, 1996 modified by Shine, 1998). Solid bars represent best estimates and vertical lines uncertainty ranges. The dominant positive forcing (heating tendency) results from the increased concentration of rather well-mixed greenhouse gases (CO2, N2O, CH4, CFCs, etc.). Tropospheric and stratospheric ozone changes are treated separately mainly because this greenhouse gas is not well mixed in the atmosphere and the observed changes have distinct spatial (horizontal and vertical) patterns.
The largest negative forcing (cooling tendency) is due to aerosol particles of human origin. Because of the limited atmospheric lifetime of aerosol particles (several days) their forcing patterns are less uniform than that of the greenhouse gases. As an example, Figure 5.1.2 shows an estimate of the geographical distribution of the climate forcing (both direct and indirect) due to sulfate aerosols of human origin. Therefore, the global forcing estimates shown in Figure 5.1.1 are not strictly comparable to the forcings of the greenhouse gases. The outstanding message of the aerosol forcing estimates is the very large uncertainty associated especially with the so called indirect forcing, i.e. the impact on cloud amount and cloud albedo of changes in the population of aerosol particles (cf. Chapter 4). If the upper (most negative) end of the uncertainty range applies, the negative forcing due to aerosols may have balanced essentially all of the positive forcing due to greenhouse gases. On the other hand, if the aerosol forcing is in the small end of the range, greenhouse warming should have been the dominating effect during the past century. It is obvious that research aiming at reducing the uncertainty of aerosol forcing is of utmost importance for the climate change issue. For example, it is likely, but not proven, that climate warming may have been delayed by the increasing aerosol load. Because the residence time of aerosol is much shorter than that of greenhouse gases (and hence aerosols will not accumulate), the warming effect of the greenhouse gases may dominate (in relative terms) over long periods of time.
The impacts may include anything from elimination of sensitive species and altered productivity to subtle shifts in the relative abundance of plant and animal species.
Whereas direct damage due to elevated concentrations of SO2 and NO2 has been documented mainly in or around urban areas and close to large pollution sources, damage due to elevated O3 levels is known to occur regularly during the summer season in extended regions in northern Europe, North America and some parts of the Middle East and Asia. The damage due to O3 cause visible leaf injury on sensitive plant species and cause a decrease in the yields of important crops, in particular wheat and soy beans. In such polluted regions, O3 concentrations are also high enough to impact the growth of some forest tree species (e.g., beach). These effects could have implications for forest production on broad geographical and time scales. There is an urgent need to increase our understanding of the current and potential future extent of O3 damage, especially in those parts of the world (e.g., China and India) where emissions of O3 precursors (NOx, HC, CO) are rising. The possibility that harmful O3 levels may occur in connection with biomass burning in tropical and subtropical regions should also be investigated.
Increased levels of UV radiation at the Earth's surface, caused by stratospheric ozone depletion, may damage terrestrial organisms including plants and microbes, but these organisms also have protective and repair processes. The major concern regarding effects on ecosystems seems to be focused on marine phytoplankton, especially in polar areas where ozone- related UV increases are the greatest. Macroalgae, seagrasses, sea urchins and corals have also been found to be sensitive to UV radiation. For some of such populations even current levels of UV may be a limiting factor (UNEP, 1998).
Large emissions of SO2 and NOx from industrial processes and traffic, and the subsequent deposition of these compounds and their associated acids have led to widespread damage to sensitive ecosystems especially in northern Europe and parts of northeastern North America. This type of impact has been well documented for freshwater ecosystems. Acid deposition reduces the alkalinity of lakes and streams. In waters with low buffering capacity, the pH can be reduced to levels that cause acute and chronic impacts associated with increased aluminum levels that accompany the lowered pH. The effects of acidification on soils and terrestrial ecosystems are more complex. Sulfur deposition has increased concentrations of absorbed sulfate in soils and caused a depletion of base cations, especially magnesium and potassium, leading to a nutrient deficiency. In parts of northern Europe, the acidity of forest soils has increased considerably (pH has decreased by 0.5-1.0 units) during the past 30- 7 60 years, at least partly as a result of acidification. This has led to reduced root distribution of forest trees. An additional stress to forests is caused by the increased mobilization of metals (e.g., Al, Cd). The combined effect of the various pollutant related stresses to forest ecosystems is not yet well characterized.
Although the sulfur emissions in Europe and North America are now declining as a result of policy actions sulfate deposition is still well above the so-called critical load above which acidification damage to sensitive ecosystem will occur. This means that soils in many areas continue to deteriorate as base cations are leached out of the soils.
In other regions of the world e.g., southern and eastern Asia sulfur emissions are growing rapidly causing concern about future acidification problems. Widespread distribution of alkaline soil dust in the atmospheres of some parts of these regions northern parts of China and a large part of India has contributed to keep rainwater pH above 5 or 6. However, this buffering effect is likely to be exhausted if emissions of SO2 and NOx continue to grow (Figure 5.1.1)
Deposition of nitrogen compounds (NO3- , NH4+ ) has led to the fertilization of many terrestrial ecosystems and, following nitrification of NH4+ , to increased nitrate leaching to groundwater and runoff. Even more important may have been the use of nitrogen fertilizers. This fertilization effect has caused changes in the functioning and stability of many sensitive ecosystems (e.g., heathlands and bogs) in the industrialized parts of the world. In very polluted areas (e.g., The Netherlands) the drinking water standard for nitrate has been exceeded. On the positive side, the addition of nitrogen in soils may have led to increased biomass and hence to net uptake of CO2 from the atmosphere.
Elevated deposition of nitrogen compounds, as well as high runoff of such compounds from adjacent land areas, have contributed to eutrophication of lakes and coastal waters in polluted regions. Episodes of high wet deposition of nitrogen are also suspected to affect more remote marine ecosystems occasionally.
Table 5.4.1 Estimated acute health effects following exposure to a 3-day episode of PM10 levels increased by 25 µg/m3 in a population of 1 million.
6. Methods to address scientific issues
Laboratory investigations have provided measurements of reaction rate constants and absorption cross sections for several molecules and have provided the fundamental knowledge for understanding atmospheric photochemistry. Much progress has been made regarding gas-phase reactions; a lot of uncertainties remain, however, in our understanding of multiphase processes. Models have been used to assess the consequences of newly measured chemical kinetics data, to analyze observations made during field campaigns, to determine the sensitivity of calculated concentrations to external forcings, to derive (for example, through inverse modeling) the magnitude and location of the surface emissions that explain these observations, and to simulate the evolution of the chemical composition in response to natural or human-induced perturbations. Progress has often resulted from a combined use of several of these tools.
The following three chapters address in a synthetic way some of the important questions presented in the present chapter and discussed over the last 10 years by the international atmospheric chemistry community. The foci are primarily on the importance of the biosphere in the control of the chemical composition of the atmosphere, the role of chemical transformations of gas- phase compounds (with emphasis on tropospheric ozone and other photooxidants), and the physical and chemical processes governing aerosol formation and their evolution and removal from the atmosphere. The questions will be addressed with a global or ubiquitous perspective, although regional specificities will often be discussed. Potential impact of anthropogenic perturbations on climate (radiative forcing), ecosystems (deposition) and health (air quality) will also be considered.
References
Last modified: Wed Apr 26 09:22:33 CEST 2000