Over the past decade of assessments, stratospheric O3 chemistry has been closely linked with aerosols, and global models in the recent WMO assessments have included some treatment of the stratospheric sulphate layer and polar stratospheric clouds. In the troposphere, studies have identified mechanisms that couple gas-phase and aerosol chemistry (Jacob, 2000). Many aerosols are photochemically formed from trace gases, and at rates that depend on the oxidative state of the atmosphere. Such processes are often included in global aerosol models (see Chapter 5). The feedback of the aerosols on the trace gas chemistry includes a wide range of processes: conversion of NOx to nitrates, removal of HOx, altering the UV flux and hence photodissociation rates (e.g., Dickerson et al., 1997; Jacobson, 1998), and catalysing more exotic reactions leading to release of NOx or halogen radicals. These processes are highly sensitive to the properties of the aerosol and the local chemical environment, and their importance on a global scale is not yet established. Only the first example above of aerosol chemistry is generally included in many of the CTMs represented here; however, the surface area of wet aerosols (that converts NOx to HNO3 via the intermediate species NO3 and N2O5) is usually specified and not interactively calculated. More laboratory and field research is needed to define the processes so that implementation in global scale models can evaluate their quantitative impact on these calculations of greenhouse gases.
The observed depletion of stratospheric ozone over the past three decades, which can be attributed in large part but not in total to the rise in stratospheric chlorine levels, has been reviewed extensively in WMO (1999). This depletion has lead to increases in tropo-spheric UV and hence forces tropospheric OH abundances upward (Bekki et al., 1994). The total effect of such a change is not simple and involves the coupled stratosphere-troposphere chemical system; for example, ozone depletion may also have reduced the influx of O3 from the stratosphere, which would reduce tropospheric O3 (Karlsdottir et al., 2000) and tend to reverse the OH trend. Such chemical feedbacks are reviewed as "climate-chemistry" feedbacks in WMO 1999 (Granier and Shine, 1999). There is insufficient understanding or quantitative consensus on these effects to be included in this assessment. While chlorine-driven O3 depletion becomes much less of an issue in the latter half of the 21st century, the projected increases in CO2, CH4, and N2O may cause even larger changes in stratospheric O3. The lack of coupled CTMs that include stratospheric changes adds uncertainty to these projections.
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