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Simmonds, PG, Derwent RG, Manning AJ, Fraser PJ, Krummel PB, O'Doherty S, Prinn RG, Cunnold DM, Miller BR, Wang HJ, Ryall DB, Porter LW, Weiss RF, Salameh PK.  2004.  AGAGE observations of methyl bromide and methyl chloride at Mace Head, Ireland, and Cape Grim, Tasmania, 1998-2001. Journal of Atmospheric Chemistry. 47:243-269.   10.1023/B:JOCH.0000021136.52340.9c   AbstractWebsite

In situ AGAGE GC-MS measurements of methyl bromide (CH3Br) and methyl chloride (CH3Cl) at Mace Head, Ireland and Cape Grim, Tasmania (1998-2001) reveal a complex pattern of sources. At Mace Head both gases have well-defined seasonal cycles with similar average annual decreases of 3.0% yr(-1) (CH3Br) and 2.6% yr(-1) (CH3Cl), and mean northern hemisphere baseline mole fractions of 10.37 +/- 0.05 ppt and 535.7 +/- 2.2 ppt, respectively. We have used a Lagrangian dispersion model and local meteorological data to segregate the Mace Head observations into different source regions, and interpret the results in terms of the known sources and sinks of these two key halocarbons. At Cape Grim CH3Br and CH3Cl also show annual decreases in their baseline mixing ratios of 2.5% yr(-1) and 1.5% yr(-1), respectively. Mean baseline mole fractions were 7.94 +/- 0.03 ppt (CH3Br) and 541.3 +/- 1.1 ppt (CH3Cl). Although CH3Cl has a strong seasonal cycle there is no well-defined seasonal cycle in the Cape Grim CH3Br record. The fact that both gases are steadily decreasing in the atmosphere at both locations implies that a change has occurred which is affecting a common, major source of both gases (possibly biomass burning) and/or their major sink process (destruction by hydroxyl radical).

Prinn, RG, Huang J, Weiss RF, Cunnold DM, Fraser PJ, Simmonds PG, McCulloch A, Harth C, Salameh P, O'Doherty S, Wang RHJ, Porter L, Miller BR.  2001.  Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science. 292:1882-1888.   10.1126/science.1058673   AbstractWebsite

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.

Simmonds, PG, Cunnold DM, Weiss RF, Prinn RG, Fraser PJ, McCulloch A, Alyea FN, O'Doherty S.  1998.  Global trends and emission estimates of CCl4 from in situ background observations from July 1978 to June 1996. Journal of Geophysical Research-Atmospheres. 103:16017-16027.   10.1029/98jd01022   AbstractWebsite

Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (ALE/GAGE/AGAGE) measurements of CCl4 at five remote surface locations from 1978 to 1996 are reported. The Scripps Institution of Oceanography (SIO) 1993 absolute calibration scale is used, reducing the concentrations by a factor of 0.77 compared to previous ALE/GAGE reports. Atmospheric concentrations of CCl4 reached a peak in 1989-1990 of 104.4 +/-. 3.1 parts per trillion (ppt) and have since been decreasing 0.7 +/-. 0.1 ppt yr(-1). Assuming an atmospheric lifetime of 42 +/- 12 years, the emissions averaged 94(+22)(-11) x 10(6) kg from 1979 to 1988 and 49(+26)(-13) x 10(6) kg from 1991 to 1995. The reduction in the emissions in 1989-1990 coincided with a substantial decrease in the global production of the chlorofluorocarbons (CFCs). The total emission of CCl4 from countries that report annual production is estimated to have declined from 11% in 1972 to 4% in 1995 of the CCl4 needed to produce the CFC amounts reported. This implies that nonreporting countries released substantial amounts of CCl4 into the atmosphere in the 1980s and that their releases have exceeded those from the reporting countries since 1991.