Abstract. We describe in detail the instrumentation and calibrations used in the Atmospheric Lifetime Experiment (ALE), the Global Atmospheric Gases Experiment (GAGE), and the Advanced Global Atmospheric Gases Experiment (AGAGE) and present a history of the majority of the anthropogenic ozone-depleting and climate-forcing gases in air based on these experiments. Beginning in 1978, these three successive automated high-frequency in situ experiments have documented the long-term behavior of the measured concentrations of these gases over the past 20 years, and show both the evolution of latitudinal gradients and the high-frequency variability due to sources and circulation. We provide estimates of the long-term trends in total chlorine contained in long-lived halocarbons involved in ozone depletion. We summarize interpretations of these measurements using inverse methods to determine trace gas lifetimes and emissions. Finally, we provide a combined observational and modeled reconstruction of the evolution of chlorocarbons by latitude in the atmosphere over the past 60 years which can be used as boundary conditions for interpreting trapped air in glaciers and oceanic measurements of chlorocarbon tracers of the deep oceanic circulation. Some specific conclusions are as follows: (1 are not yet at levels sufficient to contribute significantly to atmospheric chlorine loading. These replacement species could in the future provide independent estimates of the global 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 and their replacements may enable emission estimates at the regional level, which, together with industrial end-use data, are of sufficient accuracy to be capable of identifying regional noncompliance with the Montreal Protocol. IntroductionCurrent concerns about the atmospheric levels of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), A third phase, the Advanced Global Atmospheric Gases Experiment (AGAGE), began over the 1993-1996 time period. AGAGE, which continues to the present, has two instrumental components. First, a highly improved gas chromatographic system measures five biogenic/anthropogenic gases [CH4, N20 , CHC13, CO, and hydrogen ( 2. The second objective is to accurately document the global distributions and temporal behavior of the biogenic/ anthropogenic gases N20 , CH4, CO, H2, CH3C1, CH3Br , and CHC13 over the globe. N20 and CH 4 are important in both the chemistry and radiative budget of the atmosphere, and changes in N20 and CH 4 may also be regarded as sensitive signals of current change in the global biosphere. CO is the major sink for OH, and both CO and CH3C1 are important indicators for regional biomass burning. Together CH3C1 and CHC13 contribute about 20% of the stratospheric chlorine content, and CH3Br contributes about 50% of bromine content [Solomon et al., 1995].3. The third objective is to optimall...

The hydroxyl radical (OH) is the dominant oxidizing chemical in the atmosphere. It destroys most air pollutants and many gases involved in ozone depletion and the greenhouse effect. Global measurements of 1,1,1-trichloroethane (CH3CCl3, methyl chloroform) provide an accurate method for determining the global and hemispheric behavior of OH. Measurements show that CH3CCl3 levels rose steadily from 1978 to reach a maximum in 1992 and then decreased rapidly to levels in 2000 that were lower than the levels when measurements began in 1978. Analysis of these observations shows that global OH levels were growing between 1978 and 1988, but the growth rate was decreasing at a rate of 0.23 +/- 0.18% year(-2), so that OH levels began declining after 1988. Overall, the global average OH trend between 1978 and 2000 was -0.64 +/- 0.60% year(-1). These variations imply important and unexpected gaps in current understanding of the capability of the atmosphere to cleanse itself.

The growth in global methane (CH 4 ) concentration, which had been ongoing since the industrial revolution, stalled around the year 2000 before resuming globally in 2007. We evaluate the role of the hydroxyl radical (OH), the major CH 4 sink, in the recent CH 4 growth. We also examine the influence of systematic uncertainties in OH concentrations on CH 4 emissions inferred from atmospheric observations. We use observations of 1,1,1-trichloroethane (CH 3 CCl 3 ), which is lost primarily through reaction with OH, to estimate OH levels as well as CH 3 CCl 3 emissions, which have uncertainty that previously limited the accuracy of OH estimates. We find a 64-70% probability that a decline in OH has contributed to the post-2007 methane rise. Our median solution suggests that CH 4 emissions increased relatively steadily during the late 1990s and early 2000s, after which growth was more modest. This solution obviates the need for a sudden statistically significant change in total CH 4 emissions around the year 2007 to explain the atmospheric observations and can explain some of the decline in the atmospheric 13 CH 4 / 12 CH 4 ratio and the recent growth in C 2 H 6 . Our approach indicates that significant OH-related uncertainties in the CH 4 budget remain, and we find that it is not possible to implicate, with a high degree of confidence, rapid global CH 4 emissions changes as the primary driver of recent trends when our inferred OH trends and these uncertainties are considered., the second most important partially anthropogenic greenhouse gas, is observed to vary markedly in its year to year growth rate (Fig. 1). The causes of these variations have been the subject of much controversy and uncertainty, primarily because there is a wide range of poorly quantified sources and because its sinks are ill-constrained (1). Of particular recent interest are the cause of the "pause" in CH4 growth between 1999 and 2007 and the renewed growth from 2007 onward (2-7). It is important that we understand these changes if we are to better project future CH4 changes and effectively mitigate enhanced radiative forcing caused by anthropogenic methane emissions.The major sources of CH4 include wetlands (natural and agricultural), fossil fuel extraction and distribution, enteric fermentation in ruminant animals, and solid and liquid waste. Our understanding of the sources of CH4 comes from two approaches: "bottom up," in which inventories or process models are used to predict fluxes, or "top down," in which fluxes are inferred from observations assimilated into atmospheric chemical transport models. Bottom-up methods suffer from uncertainties and potential biases in the available activity data or emissions factors or the extrapolation to large scales of a relatively small number of observations. Furthermore, there is no constraint on the global total emissions from bottom-up techniques. The topdown approach is limited by incomplete or imperfect observations and our understanding of atmospheric transport and chemical sinks. For CH4, these di...

Abstract. Emission inventories for major reactive tropospheric CI species (particulate CI, HC1, C1NO2, CH3CI, CHCI3, CH3CCI3, C2C14, C2HC13, CH2C12, and CHCIF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea-salt production and dechlorination, biomass burning, industrial emissions, fossil-fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid-displacement reactions involving Sov)+O3, S(Iv)+ H202, and H2SO4 and HNO3 scavenging account for minor fractions of sea-salt dechlorination globally. Other important chemical pathways involving sea-salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3CI from known sources account for about half of the modeled sink, suggesting fluxes from known sources were unde:estimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3CI emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2C12 (25%), C2HC13 (10%), and C2C14 (5%) are emitted from the surface ocean; the combined contributions of C2C14 and C2HC13 from all natural sources may be substantially higher than the estimated oceanic flux.

Abstract. Since the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments came into effect, growth rates of the major ozone depleting substances (ODS), particularly CFC-11, -12 and -113 and CH3CCl3, have declined markedly, paving the way for global stratospheric ozone recovery. Emissions have now fallen to relatively low levels, therefore the rate at which this recovery occurs will depend largely on the atmospheric lifetime of these compounds. The first ODS measurements began in the early 1970s along with the first lifetime estimates calculated by considering their atmospheric trends. We now have global mole fraction records spanning multiple decades, prompting this lifetime re-evaluation. Using surface measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the National Oceanic and Atmospheric Administration Global Monitoring Division (NOAA GMD) from 1978 to 2011, we estimated the lifetime of CFC-11, CFC-12, CFC-113 and CH3CCl3 using a multi-species inverse method. A steady-state lifetime of 45 yr for CFC-11, currently recommended in the most recent World Meteorological Organisation (WMO) Scientific Assessments of Ozone Depletion, lies towards the lower uncertainty bound of our estimates, which are 544861 yr (1-sigma uncertainty) when AGAGE data were used and 524561 yr when the NOAA network data were used. Our derived lifetime for CFC-113 is significantly higher than the WMO estimates of 85 yr, being 10999121 (AGAGE) and 10997124 (NOAA). New estimates of the steady-state lifetimes of CFC-12 and CH3CCl3 are consistent with the current WMO recommendations, being 11195132 and 11295136 yr (CFC-12, AGAGE and NOAA respectively) and 5.044.925.20 and 5.044.875.23 yr (CH3CCl3, AGAGE and NOAA respectively).