All greenhouse gases except CO2 and H2O are removed from the atmosphere primarily by chemical processes within the atmosphere. Greenhouse gases containing one or more H atoms (e.g., CH4, HFCs and HCFCs), as well as other pollutants, are removed primarily by the reaction with hydroxyl radicals (OH). This removal takes place in the troposphere, the lowermost part of the atmosphere, ranging from the surface up to 7 to 16 km depending on latitude and season and containing 80% of the mass of the atmosphere. The greenhouse gases N2O, PFCs, SF6, CFCs and halons do not react with OH in the troposphere. These gases are destroyed in the stratosphere or above, mainly by solar ultraviolet radiation (UV) at short wavelengths (<240 nm), and are long-lived. Because of the time required to transport these gases to the region of chemical loss, they have a minimum lifetime of about 20 years. CO2 is practically inert in the atmosphere and does not directly influence the chemistry, but it has a small in situ source from the oxidation of CH4, CO and VOC.
Tropospheric OH abundances depend on abundances of NOx, CH4, CO, VOC, O3 and H2O plus the amount of solar UV (>300 nm) that reaches the troposphere. As a consequence, OH varies widely with geographical location, time of day, and season. Likewise the local loss rates of all those gases reacting with OH also vary. Because of its dependence on CH4 and other pollutants, tropospheric OH is expected to have changed since the pre-industrial era and to change again for future emission scenarios. For some of these gases other removal processes, such as photolysis or surface uptake, are also important; and the total sink of the gas is obtained by integrating over all such processes. The chemistry of tropospheric O3 is closely tied to that of OH, and its abundance also varies with changing precursor emissions. The chemistry of the troposphere is also directly influenced by the stratospheric burden of O3, climatic changes in temperature (T) and humidity (H2O), as well as by interactions between tropospheric aerosols and trace gases. Such couplings provide a "feedback" between the climate change induced by increasing greenhouse gases and the concentration of these gases. Another feedback, internal to the chemistry, is the impact of CH4 on OH and hence its own loss. These feedbacks are expected to be important for tropospheric O3 and OH. Such chemistry-chemistry or climate-chemistry coupling has been listed under "indirect effects" in the SAR (Prather et al., 1995; Schimel et al., 1996).
This chapter uses 3-D chemistry-transport models (CTMs) to integrate the varying chemical processes over global conditions, to estimate their significance, and to translate the emission scenarios into abundance changes in the greenhouse gases CH4, HFCs, and O3. An extensive modelling exercise called OxComp (tropospheric oxidant model comparison) - involving model comparisons, sensitivity studies, and investigation of the IPCC SRES scenarios was organised to support this report.
Stratospheric circulation and distribution of O3 control the transport of the long-lived greenhouse gases to regions of photochemical loss as well as the penetration of solar UV into the atmosphere. At the same time, many of these gases (e.g., N2O and CFCs) supply ozone-depleting radicals (e.g., nitric oxide (NO) and Cl) to the stratosphere, providing a feedback between the gas and its loss rate. Another consequence of the observed stratospheric ozone depletion is that tropospheric photochemical activity is expected to have increased, altering tropospheric OH and O3. Climate change in the 21st century, including the radiative cooling of the stratosphere by increased levels of CO2, is expected to alter stratospheric circulation and O3, and, hence, the global mean loss rates of the long-lived gases. Some of these effects are discussed in WMO (1999) and are briefly considered here.
The biosphere's response to global change will impact the atmospheric composition of the 21st century. The anticipated changes in climate (e.g., temperature, precipitation) and in chemistry will alter ecosystems and thus the "natural", background emissions of trace gases. There is accumulating evidence that increased N deposition (the result of NOx and ammonia (NH3) emissions) and elevated surface O3 abundances have opposite influences on plant CO2 uptake: O3 (>40 ppb) inhibits CO2 uptake; while N deposition enhances it up to a threshold, above which the effects are detrimental. In addition, the increased N availability from atmospheric deposition and direct fertilisation accelerates the emission of N-containing trace gases (NO, N2O and NH3) and CH4, as well as altering species diversity and biospheric functioning. These complex interactions represent a chemistry-biosphere feedback that may alter greenhouse forcing.
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