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McNorton, J, Chipperfield MP, Gloor M, Wilson C, Feng WH, Hayman GD, Rigby M, Krummel PB, O'Doherty S, Prinn RG, Weiss RF, Young D, Dlugokencky E, Montzka SA.  2016.  Role of OH variability in the stalling of the global atmospheric CH4 growth rate from 1999 to 2006. Atmospheric Chemistry and Physics. 16:7943-7956.   10.5194/acp-16-7943-2016   AbstractWebsite

The growth in atmospheric methane (CH4) concentrations over the past 2 decades has shown large variability on a timescale of several years. Prior to 1999 the globally averaged CH4 concentration was increasing at a rate of 6.0aEuro-ppbaEuro-yr(-1), but during a stagnation period from 1999 to 2006 this growth rate slowed to 0.6aEuro-ppbaEuro-yr(-1). From 2007 to 2009 the growth rate again increased to 4.9aEuro-ppbaEuro-yr(-1). These changes in growth rate are usually ascribed to variations in CH4 emissions. We have used a 3-D global chemical transport model, driven by meteorological reanalyses and variations in global mean hydroxyl (OH) concentrations derived from CH3CCl3 observations from two independent networks, to investigate these CH4 growth variations. The model shows that between 1999 and 2006 changes in the CH4 atmospheric loss contributed significantly to the suppression in global CH4 concentrations relative to the pre-1999 trend. The largest factor in this is relatively small variations in global mean OH on a timescale of a few years, with minor contributions of atmospheric transport of CH4 to its sink region and of atmospheric temperature. Although changes in emissions may be important during the stagnation period, these results imply a smaller variation is required to explain the observed CH4 trends. The contribution of OH variations to the renewed CH4 growth after 2007 cannot be determined with data currently available.

Rigby, M, Muhle J, Miller BR, Prinn RG, Krummel PB, Steele LP, Fraser PJ, Salameh PK, Harth CM, Weiss RF, Greally BR, O'Doherty S, Simmonds PG, Vollmer MK, Reimann S, Kim J, Kim KR, Wang HJ, Olivier JGJ, Dlugokencky EJ, Dutton GS, Hall BD, Elkins JW.  2010.  History of atmospheric SF6 from 1973 to 2008. Atmospheric Chemistry and Physics. 10:10305-10320.   10.5194/acp-10-10305-2010   AbstractWebsite

We present atmospheric sulfur hexafluoride (SF(6)) mole fractions and emissions estimates from the 1970s to 2008. Measurements were made of archived air samples starting from 1973 in the Northern Hemisphere and from 1978 in the Southern Hemisphere, using the Advanced Global Atmospheric Gases Experiment (AGAGE) gas chromatographic-mass spectrometric (GC-MS) systems. These measurements were combined with modern high-frequency GC-MS and GC-electron capture detection (ECD) data from AGAGE monitoring sites, to produce a unique 35-year atmospheric record of this potent greenhouse gas. Atmospheric mole fractions were found to have increased by more than an order of magnitude between 1973 and 2008. The 2008 growth rate was the highest recorded, at 0.29 +/- 0.02 pmol mol(-1) yr(-1). A three-dimensional chemical transport model and a minimum variance Bayesian inverse method was used to estimate annual emission rates using the measurements, with a priori estimates from the Emissions Database for Global Atmospheric Research (EDGAR, version 4). Consistent with the mole fraction growth rate maximum, global emissions during 2008 were also the highest in the 1973-2008 period, reaching 7.4 +/- 0.6 Gg yr(-1) (1-sigma uncertainties) and surpassing the previous maximum in 1995. The 2008 values follow an increase in emissions of 48 +/- 20% since 2001. A second global inversion which also incorporated National Oceanic and Atmospheric Administration (NOAA) flask measurements and in situ monitoring site data agreed well with the emissions derived using AGAGE measurements alone. By estimating continent-scale emissions using all available AGAGE and NOAA surface measurements covering the period 2004-2008, with no pollution filtering, we find that it is likely that much of the global emissions rise during this five-year period originated primarily from Asian developing countries that do not report detailed, annual emissions to the United Nations Framework Convention on Climate Change (UNFCCC). We also find it likely that SF(6) emissions reported to the UNFCCC were underestimated between at least 2004 and 2005.

Simmonds, PG, Manning AJ, Cunnold DM, McCulloch A, O'Doherty S, Derwent RG, Krummel PB, Fraser PJ, Dunse B, Porter LW, Wang RHJ, Greally BR, Miller BR, Salameh P, Weiss RF, Prinn RG.  2006.  Global trends, seasonal cycles, and European emissions of dichloromethane, trichloroethene, and tetrachloroethene from the AGAGE observations at Mace Head, Ireland, and Cape Grim, Tasmania. Journal of Geophysical Research-Atmospheres. 111   10.1029/2006jd007082   AbstractWebsite

[1] In situ observations ( every 4 hours) of dichloromethane (CH(2)Cl(2)) from April 1995 to December 2004 and trichloroethene (C(2)HCl(3)) and tetrachloroethene (C(2)Cl(4)) from September 2000 to December 2004 are reported for the Advanced Global Atmospheric Gases Experiment (AGAGE) station at Mace Head, Ireland. At a second AGAGE station at Cape Grim, Tasmania, CH(2)Cl(2) and C(2)Cl(4) data collection commenced in 1998 and 2000, respectively. C(2)HCl(3) is below the limit of detection at Cape Grim except during pollution episodes. At Mace Head CH(2)Cl(2) shows a downward trend from 1995 to 2004 of 0.7 +/- 0.2 ppt yr(-1) ( ppt: expressed as dry mole fractions in 10(12)), although from 1998 to 2004 the decrease has been only 0.3 +/- 0.1ppt yr(-1). Conversely, there has been a small but significant growth of 0.05 +/- 0.01 ppt yr(-1) in CH(2)Cl(2) at Cape Grim. The time series for C(2)HCl(3) and C(2)Cl(4) are relatively short for accurate trend analyses; however, we observe a small but significant decline in C(2)Cl(4) (0.18 +/- 0.05 ppt yr(-1)) at Mace Head. European emissions inferred from AGAGE measurements are compared to recent estimates from industry data and show general agreement for C(2)HCl(3). Emissions estimated from observations are lower than industry emission estimates for C(2)Cl(4) and much lower in the case of CH(2)Cl(2). A study of wildfires in Tasmania, uncontaminated by urban emissions, suggests that the biomass burning source of CH(2)Cl(2) may have been previously overestimated. All three solvents have distinct annual cycles, with the phases and amplitudes reflecting their different chemical reactivity with OH as the primary sink.