Atmospheric Chemistry, and the Biosphere
M. O. Andreae
Max Planck Institute for Chemistry
P.O. Box 3060
D-55020 Mainz, Germany
N.B. Edited from http://www.mpch-mainz.mpg.de/~biogeo/feedbacks.htm
Burning Biomass and Ozone
Human activities are changing the composition of the atmosphere not only directly through the emission of trace gases and aerosols, but also indirectly through perturbations in the physical, chemical, and ecological characteristics of the Earth System, which in turn influence the rates of production and loss of atmospheric constituents.
The impact of direct anthropogenic emissions on the atmosphere is often relatively easy to assess, especially if they are tied to major industrial activities, where accurate and detailed records are kept for economic reasons. Classical examples are the release of chlorofluorocarbons and the emission of CO2 from fossil fuel combustion. There are, however, also cases where it is much more difficult to obtain accurate emission estimates. An example is biomass burning, for which no economic incentive for record keeping exists, and which takes on many forms, each with a different emission profile.
A more complex case exists where human activities release a precursor compound, which is transformed in the atmosphere to a climatically active substance. This can be illustrated using the example of SO2, from which sulfate aerosol can be formed. The actual amount of radiatively active sulfate aerosol produced, however, is determined by a complex interplay of atmospheric transport processes, chemical processes in the gas phase, and interactions with other aerosol species.
Some of the most important anthropogenic modifications of the atmosphere, however, are the indirect results of human-caused changes in the functioning of the Earth System. For example, when land use and agricultural practices change, the emissions of trace gases such as N2O, NO, and CH4 change in highly complex ways, which are extremely difficult to assess at the scales of interest. An even higher level of complexity is encountered, when human activities affect the atmospheric levels of some species, which in turn changes the chemical functioning of the atmosphere, and consequently the production rates and lifetimes of aerosol and greenhouse gases. An example for such a mechanism may be the large-scale change of trace gas inputs into the vast photochemical reactor of the tropical troposphere, where most of the photooxidation of long-lived trace gases takes place.
Finally, we must consider feedback loops, where global change begets global change. Climate change, caused by upsetting the Earth’s radiative balance, results in different circulation patterns, changes in water availability at the surface, water vapor content of the atmosphere, etc., all of which modify the atmospheric budgets of trace gases and aerosols. Thinning of the stratospheric ozone layer yields a higher UV flux into the troposphere, thereby accelerating photooxidation processes.
Understanding the complex interactions between tropospheric chemistry and global change presents a formidable scientific challenge, which can only be addressed by close cooperation between scientific disciplines, tight interaction between observation and modeling, and broad international cooperation.
In the public mind, "global change" has become almost synonymous with "global warming" or "climate change", a narrow reduction of the original meaning. While there is no doubt that the possibility of climate change is of great concern to the Earth’s population, we must not forget that we are living in a period when almost all components of the Earth system are undergoing change. The chemical composition of the atmosphere is being perturbed at a vast scale by human activities. The terrestrial biota are modified by land use change, biomass burning, deforestation, and species extinction. Marine life is impacted by overfishing, eutrophication and pollution. There is a tendency to see these issues as independent environmental problems, each grabbing the public’s attention for some time, and each demanding a specific solution.
This approach obscures the fact that all these phenomena are occurring simultaneously, and within the same "Earth system". As a result, they interact with one another, reinforcing or damping each other, or changing each other's temporal evolution.
It is especially important to examine the Earth system for possible feedbacks, which amplify the effect of perturbations. It is already well established, that increasing temperatures result in changes in ice albedo, atmospheric water vapor content and cloudiness, which in turn act to increase temperature. If additional positive feedbacks exist, they would add to known feedbacks, and, due to the extremely non-linear behavior at higher gains, could have disproportionately large effects (Lashof et al., 1997).
In this paper, I will explore some of these interactions between human
activities, atmospheric chemistry, climate, and ecology, using selected examples
or case studies. I will proceed from the (relatively) simple to the more
complex, keeping in mind that exploring any of the issues addressed here in its
full depth and complexity is well beyond the frame of an overview paper such as
...It may be worthwhile, however, to already emphasized one point here, to which
we will come back to several times in the following sections: the importance of
the tropics in understanding global change. The tropics are the part of the
globe with the most rapidly growing population, the most dramatic industrial
expansion, and the most rapid and pervasive change in land use and land cover.
The tropics contain also the largest standing stocks of terrestrial vegetation,
and have the highest rates of photosynthesis and respiration (Houghton and
Skole, 1990; Raich and Potter, 1995). It is therefore likely that changes in
tropical land use will have a profound impact on the global carbon cycle in
future decades (Houghton et al., 1998).
In the previous section, we have already pointed out that gases that are not themselves greenhouse gases may have a climatic effect because they change the rates of production or destruction of greenhouse gases. In this sense, we can attribute a climate forcing and greenhouse warming potential to gases such as CO, which has no significant radiative effect of its own. This is because adding CO to the atmosphere increases the lifetime and abundance of methane, results in the production of ozone, and, following oxidation, adds some CO2 to the atmosphere. When these effects were simulated in a photochemical model, the cumulative radiative forcing due to CO emissions exceeded at shorter times scales (<15 years) that due to anthropogenic N2O, one of the important greenhouse gases (Daniel and Solomon, 1998).
Tropospheric ozone, a gas that has no direct emission sources, is the third most important greenhouse gas after CO2 and CH4 (Houghton et al., 1996; Portmann et al., 1997; Roelofs et al., 1997; Shine and Forster, 1999; van Dorland et al., 1997). Since ozone has a chemical lifetime in the troposphere that is of the same order as the timescales of many atmospheric transport processes (days to weeks), its temporal and spatial distribution is highly inhomogeneous. In the absence of vertically resolved and globally representative data sets on O3 concentrations, the climatic effect due to this gas must therefore be estimated based on model calculations. The chemical precursors of ozone are hydrocarbons (including methane and NMHC), CO, and the oxides of nitrogen, NOx. The latter play an especially important role in the ozone budget, since their abundance determines if the photochemical oxidation of hydrocarbons and CO results in net O3 production or destruction (Crutzen, 1995b; National Research Council (U.S.) Committee on Tropospheric Ozone Formation and Measurement, 1991).
In many regions of the Earth, especially on the continents, biogenic NMHC emissions are relatively abundant (Fehsenfeld et al., 1992; Guenther et al., 1995), and, in the absence of strong NOx emissions, their photooxidation results in net O3 destruction. When NOx emissions in these regions increase due to development, or because deforestation lets NOx from soil microbial production escape more readily into the troposphere, the system can switch to net O3 production, strongly enhancing ozone levels (Keller et al., 1991). This is especially critical in the tropics where O3 can be entrained into the intertropical convergence zone (ITCZ) and transported by deep convection into the upper troposphere, where it has the strongest climatic effect. Modeling studies suggest that input of pollutants into convective regions may have strong effects on O3 levels in the free troposphere (Ellis et al., 1996).
Figure 4: Sources of nitrogen oxides (NOx) to the troposphere (data from Wang et al., (1998))
Figure 4 shows the sources of nitrogen oxides to the troposphere. Of
particular importance to the tropical atmosphere are the emissions from biomass
burning, most of which takes place in the tropics (Andreae, 1993), and the
production of NOx by lightning, which is also abundant in the deep
convective thunderstorms of the ITCZ. Because vegetation fires can occur only
when the vegetation is dry enough to burn, they are most abundant in the dry
season, when the trade wind inversion with its large-scale subsidence prevails
over the part of the tropics in question. Because this inversion prevents
convection to heights of more than a few kilometers, it was initially thought
that the linkage between dry conditions and subsidence more or less precluded
the transport of pyrogenic ozone precursors to the middle and upper troposphere.
Recent work has shown, however, that large amount of smoke can get swept by
low-level circulation, e.g., the trade winds, towards convergent regions over
the continents or the ITCZ, and there become subject to deep convection (Andreae
et al., 1999; Chatfield et al., 1996; Thompson et al., 1996). This transport
pattern can explain the abundance of fire-related O3 and
O3-precursors observed in the middle and upper troposphere by remote
sensing and in-situ measurements (Browell et al., 1996; Connors et al., 1996;
Olson et al., 1996). Figure 5 shows the distribution of O3 over the
tropical South Atlantic during September-October 1992 in comparison with results
from earlier studies (DECAFE-88 in the Congo (Andreae et al., 1992); Tropical
Atlantic (Kirchhoff et al., 1991)) and the ozone climatology over the Pacific
Ocean. These results show dramatically the impact O3 from biomass
burning can have on the entire tropospheric column.
Figure 5: Impact of tropical biomass burning on the vertical distribution of ozone over the Equatorial ocean regions.
Whether this impact will grow in the future depends both on climate change and on human factors. The amount of fuel available at a given place for burning is a function of ecological factors, e.g., soil fertility, precipitation, and temperature. It also depends on land use, i.e., if the area has been burned previously, is used for grazing or agriculture, and so on. If climatic variations become more extreme, as climate models have suggested, we can expect a more frequent occurrence of drought years following very wet years. This would result in large amounts of fuel ready to burn in the fire season. Furthermore, in a warmer climate, fire frequency is likely to increase, which would reduce biomass carbon storage by changing the age class structure of vegetation, as well as causing increased emissions of ozone precursors.
Human activities are of central importance to the frequency and severity of
biomass fires. If large parts of the humid tropics are further deforested, they
will transition from a biome essentially free of fires (the tropical rainforest)
to biomes with much more frequent fires (grazing lands, agricultural lands, and
wastelands). With a higher human population density, the frequency of ignition
will go up as well. And finally, the amount of biomass burned for cooking and
domestic heating, already a major source of emissions in tropical countries,
will increase further.
In the preceding sections, we have examined the linkages and connections between human perturbations of the Earth System and its chemical, physical, and ecological characteristics. We have seen that it is usually not adequate to consider just the emission of trace gases and aerosols, but that it is essential to consider the complex interconnections between any given perturbation and the overall Earth System.
There are a few cases, where the impact of anthropogenic emissions on the atmosphere is relatively easy to assess, for example those where the sources of a substance are industrial and its sinks are chemical reactions with first-order kinetics. But in most instances, the emission and removal of climatically active gases and aerosols depends on a multiplicity of human activities and ecological factors, including climate itself.
When land use and agricultural practices change, the emissions of trace gases such as N2O, NO, and CH4 change in highly complex ways, which are extremely difficult to assess at the scales of interest. When land-use change reaches such vast extent as in the deforestation of the tropics, it may even cause changes in the climate system, including the hydrological cycle. As a result of these chemical and physical perturbations, the chemical functioning of the atmosphere will be modified, and consequently the production rates and lifetimes of aerosols and greenhouse gases. The most obvious example for such a mechanism is the large-scale change of trace gas inputs into the tropical troposphere, the vast photochemical reactor where most of the photooxidation of long-lived trace gases takes place. Because of the long times scales involved in ecological change, biogeochemical cycles and climate have a memory of past land use change, and, conversely, present land use change may have long-term consequences reaching far into the future.
In some cases, the human perturbation consists of the release of a precursor compound (e.g., SO2), which is transformed in the atmosphere to a climatically active substance. In this example, the actual amount of radiatively active sulfate aerosol produced is determined by a complex interplay of atmospheric transport processes, chemical processes in the gas phase, and interactions with other aerosol species. In other cases, such as the production of organic aerosols from biogenic VOCs or sulfate aerosols from DMS, aerosol yields can be modified by anthropogenic changes in atmospheric photooxidation processes.
At longer timescales, we must consider feedback loops where climate change results in different circulation patterns, changes in water availability at the surface, water vapor content of the atmosphere, etc. These factors in turn modify the sources, sinks and atmospheric budgets of trace gases and aerosols, again affecting climate. A dramatic example for this kind of interaction is the coupling between changes in stratospheric temperatures and ozone depletion, which has shown up over the Arctic during the last decade.
Understanding the complex interactions between tropospheric chemistry and
global change presents a formidable scientific challenge. Exciting progress has
been made in this area especially over the last decade by intensified
cooperation between scientific disciplines, close interaction between
observation and modeling, and broad international cooperation. We must, however,
over our excitement with new conceptual insights into the complexity of the
Earth System’s workings not lose sight of the fact that the observational data
base for testing our concepts and models remains still rather sparse. High
priority must therefore be given to developing new tools and programs for the
investigation of our changing planet.
Andreae, M.O. (1993) The influence of tropical biomass burning on climate and the atmospheric environment. In R.S. Oremland, Ed. Biogeochemistry of Global Change: Radiatively Active Trace Gases, p. 113-150. Chapman & Hall, New York, NY.
Andreae, M.O. (1995) Climatic effects of changing atmospheric aerosol levels. In A. Henderson-Sellers, Ed. World Survey of Climatology. Vol. 16: Future Climates of the World, p. 341-392. Elsevier, Amsterdam.
Andreae, M.O., Artaxo, P., Fischer, H., Fortuin, J., Gregoire, J., Hoor, P., Kormann, R., Krejci, R., Lange, L., Lelieveld, J., Longo, K., Peters, W., Reus, M.d., Scheeren, B., Silva Dias, M.d., StrÝm, J., and Williams, J. (1999) Transport of biomass burning smoke to the upper troposphere by deep convection in the equatorial region. Nature.
Andreae, M.O., Chapuis, A., Cros, B., Fontan, J., Helas, G., Justice, C., Kaufman, Y.J., Minga, A., and Nganga, D. (1992) Ozone and Aitken nuclei over equatorial Africa: Airborne observations during DECAFE 88. J. Geophys. Res., 97, 6137-6148.
Andreae, M.O., and Crutzen, P.J. (1997) Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry. Science, 276, 1052-1056.
[...] References truncated, see online copy for full text