For atmospheric chemists, global change is not a theoretical possibility but an experimental fact. Global measurement networks continue to record dramatic, worldwide rises in long-lived atmospheric gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), various chlorofluorocarbons (CFCs), and other halocarbons such as Cl4, CH3CI3, and CF3Br [WMO, 1986, 1991; IPCC, 1990]. While trends are much more difficult to document for shorter-lived compounds, a strong case can also be made that species such as tropospheric ozone (O3) and sulfate/carbonaceous aerosol have also increased dramatically over much of the Northern Hemisphere during the last century [NRC, 1991; Lefohn et al., 1992; Charlson et al., 1992]. Furthermore, dramatic decreases in polar and mid-latitude lower stratospheric ozone levels are now apparent [WMO, 1991; Gleason et al., 1993; Bojkov et al, 1993].
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