A history of chemically and radiatively important gases in air deduced from ALE/GAGE/AGAGE

We describe in detail the instrumentation and calibrations used in the Atmo spheric Lifetime Experiment (ALE), the Global Atmospheric Cases Experiment (GAGE), and the Advanced Global Atmospheric Gases Experiment (AGAGE) and pr esent a history of the majority of the anthropogenic ozone-depleting and cl imate-forcing gases in air based on these experiments. Beginning in 1978, t hese three successive automated high-frequency in situ experiments have doc umented the long-term behavior of the measured concentrations of these gase s over the past 20 years, and show both the evolution of latitudinal gradie nts and the high-frequency variability due to sources and circulation. We p rovide estimates of the long-term trends in total chlorine contained in lon g-lived halocarbons involved in ozone depletion. We summarize interpretatio ns of these measurements using inverse methods to determine trace gas lifet imes and emissions. Finally, we provide a combined observational and modele d reconstruction of the evolution of chlorocarbons by latitude in the atmos phere over the past 60 years which can be used as boundary conditions for i nterpreting trapped air in glaciers and oceanic measurements of chlorocarbo n tracers of the deep oceanic circulation. Some specific conclusions are as follows: (1) International compliance with the Montreal Protocol is so far resulting in chlorofluorocarbon and chlorocarbon mole fractions comparable to target levels; (2) mole fractions of total chlorine contained in long-l ived halocarbons (CCl2F2, CCl3F, CH3CCl3, CCl4, CHClF2, CCl2FCClF2, CH3Cl, CH2Cl2, CHCl3, CCl2=CCl2) in the lower troposphere reached maximum values o f about 3.6 ppb in 1993 and are beginning to slowly decrease in the global lower atmosphere; (3) the chlorofluorocarbons have atmospheric lifetimes co nsistent with destruction in the stratosphere being their principal removal mechanism; (4) multiannual variations in chlorofluorocarbon and chlorocarb on emissions deduced from ALE/GAGE/AGAGE data are consistent approximately with variations estimated independently from industrial production and sale s data where available (CCl2F2 (CFC-12) and CCl2FCClF2 (CFC-113) show the g reatest discrepancies); (5) the mole fractions of the hydrochlorofluorocarb ons and hydrofluorocarbons, which are replacing the regulated halocarbons, are rising very rapidly in the atmosphere, but with the exception of the mu ch longer manufactured CHClF2 (HCFC-22), they are not yet at levels suffici ent to contribute significantly to atmospheric chlorine loading. These repl acement species could in the future provide independent estimates of the gl obal weighted-average OH concentration provided their industrial emissions are accurately documented; (6) in the future, analysis of pollution events measured using high-frequency in situ measurements of chlorofluorocarbons a nd their replacements may enable emission estimates at the regional level, which, together with industrial end-use data, are of sufficient accuracy to he capable of identifying regional noncompliance with the Montreal Protoco l.