Advanced Global Atmospheric Gases Experiment Research Articles

Halocarbon emissions estimated from Advanced Global Atmospheric Gases Experiment measured pollution events at Trinidad Head, California

The emissions of halogenated gases from the West Coast region of the United States are estimated from measurements from 1995 to 2003 at the Advanced Global Atmospheric Gases Experiment site at Trinidad Head, California. The emissions estimation procedure uses pollution events combined with population densities integrated along back trajectories, and the estimates are constrained by independent estimates of CH4 and N2O emissions from the U.S. West Coast region. The best fit, average emissions of CH4 and N2O and the average chloroform emissions in California, Oregon, and Washington combined from 1996 to 2002 are 44, 3.7, and 0.07 kg person−1 yr−1, respectively. The emissions per person of CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CCl2FCClF2), and methyl chloroform (CH3CCl3) from California in 1996–1998 are calculated to be factors of approximately 2.2, 1.3, 0.7, and 1.6, respectively, less (more for CFC‐113) than those reported for the northeastern United States by Barnes et al. (2003). The emission per person of all these gases in the U.S. West Coast region decreased from 1998 to 1999 by a factor of 2 or more, but from 1999 to 2002 the estimated emissions of all four gases have remained fairly constant and are 0.016, 0.048, 0.002, and 0.006 kg person−1 yr−1, respectively. The methyl chloroform estimates suggest a delay of up to 1 year in the decline of the emissions from 1996 to 1998, but otherwise, and in 1999–2000, in contrast to the Millet and Goldstein (2004) results, they are in agreement with the average methyl chloroform emissions per person for the United States based on the UNEP country by country consumption figures (A. McCulloch, private communication, 2004). Averaging the Trinidad Head and the Barnes et al. (2003) per person estimates and multiplying by the U.S. population suggests average methyl chloroform emissions in the United States of 18 Gg yr−1 in 1996 to 1998. In 2001–2002, if the ratio of the emissions per person in these two regions was the same as in 1996–1998, we estimate U.S. emissions of 2.2 Gg yr−1, which is one half of the Millet and Goldstein (2004) estimate.

Improved continuous in situ measurements of C1–C3 PFCs, HFCs, HCFCs, CFCs and SF6 in Europe and Australia

Improved monitoring of non-CO2 greenhouse gases in air samples is presented, achieved using a new analytical system based on preconcentration, gas-chromatography and mass spectrometry. In addition to the major HFCs, HCFCs and CFCs, the new observations include the first in situ time series of the C1–C3 PFCs (CF4, C2F6 and C3F8) and the more volatile of the HFCs (CHF3, CH2F2, CH3CF3) alongside SF6, all of which are now monitored routinely as part of the Advanced Global Atmospheric Gases Experiment (AGAGE). Observed trends in newly monitored species are shown, obtained from 1–2 years continuous in situ air analyses at remote monitoring sites at Mace Head (Ireland) and Cape Grim (Australia). Observed deviations in the air background for these gas species are linked to modelled trajectories of air masses arriving at the monitoring stations to indicate potential source regions for emissions in Europe and Australia. In addition, preliminary estimates of 2004 mixing ratio growth rates of compounds are deduced from the observations, which highlight the importance of continuous atmospheric monitoring for verification of consumption-based emission estimates of non-CO2 greenhouse gases.

Rapid growth of hydrofluorocarbon 134a and hydrochlorofluorocarbons 141b, 142b, and 22 from Advanced Global Atmospheric Gases Experiment (AGAGE) observations at Cape Grim, Tasmania, and Mace Head, Ireland

An update of in situ Advanced Global Atmospheric Gases Experiment (AGAGE) hydrofluorocarbon (HFC)/hydrochlorofluorocarbon (HCFC) measurements made at Mace Head, Ireland, and Cape Grim, Tasmania, from 1998 to 2002 are reported. HCFC‐142b, HCFC‐141b, HCFC‐22 and HFC‐134a show continued rapid growth in the atmosphere at mean rates of 1.1, 1.6, 6.0, and 3.4 ppt/year, respectively. Emissions inferred from measurements are compared to recent estimates from consumption data. Minor updates to the industry estimates of emissions are reported together with a discussion of how to best determine OH concentrations from these trace gas measurements. In addition, AGAGE measurements and derived emissions are compared to those deduced from NOAA‐Climate Monitoring and Diagnostics Laboratory flask measurements (which are mostly made at different locations). European emission estimates obtained from Mace Head pollution events using the Nuclear Accident Model (NAME) dispersion model and the best fit algorithm (known as simulated annealing) are presented as 3‐year rolling average emissions over Europe for the period 1999–2001. Finally, the measurements of HCFC‐141b, HCFC‐142b, and HCFC‐22 discussed in this paper have been combined with the Atmospheric Lifetime Experiment (ALE)/Global Atmospheric Gases Experiment (GAGE)/AGAGE measurements of CFC‐11, CFC‐12, CFC‐113, CCl4, and CH3CCl3 to produce the evolution of tropospheric chlorine loading.

Atmospheric Methyl Iodide at Cape Grim, Tasmania, from AGAGE Observations

Atmospheric mixing ratios of methyl iodide (CH3I) and other methyl halides have been measured at Cape Grim, Tasmania (41°S, 145°E), since early 1998 as part of the Advanced Global Atmospheric Gases Experiment (AGAGE). This paper analyses about 1700 ambient air CH3I measurements from the 14-month period (March 1998–April 1999). Mixing ratios peaked during the summer, despite faster photolytic loss, suggesting local oceanic emissions were about 2.2–3.6 times stronger in summer than in winter. Back trajectories show that CH3I levels are strongly dependent on air mass origin, with highest mixing ratios in air from the Tasman Sea/Bass Strait region and lowest levels in air originating from the Southern Ocean at higher latitudes. CH3I mixing ratios were not well correlated with other methyl halides in unpolluted marine air. The large variations with season and air mass origin suggest that high frequency, continuous data from key locations will make a significant contribution to the understanding of sources and sinks of this important short-lived atmospheric species.

Critical evaluation of emissions of potential new gases for OH estimation

Reactions with OH in the troposphere provide the dominant removal path for a wide variety of greenhouse and ozone‐depleting gases. Accurate determination of global and regional tropospheric OH concentrations is therefore very important and can be achieved by measuring gases that react with OH and whose emissions are well known. CH3CCl3 has been used for this purpose. Prior studies have shown that three of the new chlorofluorocarbon (CFC) substitutes, HFC‐134a (CH2FCF3), HCFC‐141b (CH3CCl2F), and HCFC‐142b (CH3CClF2), could be used potentially to derive accurate global‐average OH concentrations in the future provided that the industrial emissions of these gases can be reliably estimated. As a test of their currently available emission estimates, we optimally determine the monthly emissions for these three gases using global measurements from the NOAA‐CMDL and Advanced Global Atmospheric Gases Experiment (AGAGE) networks. Our study uses two distinct OH scenarios (constant OH, quadratically varying OH) derived from CH3CCl3 observations. We conclude that current published industrial estimates [Alternative Fluorocarbon Environmental Assessment Study (AFEAS), 2002] of HCFC‐142b and HCFC‐141b need to be increased by 18 ± 15% over the 1992–2000 period and 10−13+15% over the 1993–2000 period, respectively, while the emissions of HFC‐134a (from 1993 to 2000) are only 4 ± 10% more than those yielding the best agreement with atmospheric observations. Estimates of global‐average OH concentrations using measurements and AFEAS [2002] emissions differ statistically from the average OH derived from CH3CCl3 for HCFC‐141b and HCFC‐142b but not for HFC‐134a. On the other hand, OH trends deduced from all three HCFC/HFC gases are implausibly large implying problems with the AFEAS [2002] estimates of the temporal variations in their emissions. As the viability of CH3CCl3 for estimating OH declines in the future, additional evaluations of the emissions and OH kinetics of HCFC‐141b, HCFC‐142b, and HFC‐134a therefore have high priority.

In situ chloroform measurements at Advanced Global Atmospheric Gases Experiment atmospheric research stations from 1994 to 1998

Measurements of atmospheric chloroform (CHCl3) by in situ gas chromatography using electron capture detection are reported from the Advanced Global Atmospheric Gases Experiment (AGAGE) network of atmospheric research stations. They are some of the most comprehensive in situ, high‐frequency measurements to be reported for CHCl3 and provide valuable information not only on clean “baseline” mixing ratios but also on local and regional sources. Emissions from these sources cause substantial periodic increases in CHCl3 concentrations above their baseline levels, which can be used to identify source strengths. This is particularly the case for measurements made at Mace Head, Ireland. Furthermore, these local sources of CHCl3 emissions are significant in relation to current estimates of global emissions and illustrate that the understanding of competing sources and sinks of CHCl3 is still fragmentary. These observations also show that CHCl3 has a very pronounced seasonal cycle with a summer minimum and winter maximum presumably resulting from enhanced destruction by OH in the summer. The amplitude of the cycle is dependent on sampling location. Over the 57 months of in situ measurements a global average baseline concentration of 8.9±0.1 ppt was determined with no appreciable trend in the baseline detected.

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 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) 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‐lived halocarbons (CCl2F2, CCl3F, CH3CCl3, CCl4, CHClF2, CCl2FCClF2, CH3Cl, CH2Cl2, CHCl3, CCl2=CCl2) in the lower troposphere reached maximum values of about 3.6 ppb in 1993 and are beginning to slowly decrease in the global lower atmosphere; (3) the chlorofluorocarbons have atmospheric lifetimes consistent with destruction in the stratosphere being their principal removal mechanism; (4) multiannual variations in chlorofluorocarbon and chlorocarbon emissions deduced from ALE/GAGE/AGAGE data are consistent approximately with variations estimated independently from industrial production and sales data where available (CCl2F2 (CFC‐12) and CCl2FCClF2 (CFC‐113) show the greatest discrepancies); (5) the mole fractions of the hydrochlorofluorocarbons and hydrofluorocarbons, which are replacing the regulated halocarbons, are rising very rapidly in the atmosphere, but with the exception of the much longer manufactured CHClF2 (HCFC‐22), they 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.