Abstract
Abstract. Methyl chloroform (MCF) is a man-made chlorinated solvent contributing to the destruction of stratospheric ozone and is controlled under the "Montreal Protocol on Substances that Deplete the Ozone Layer" and its amendments, which called for its phase-out in 1996 in developed countries and 2015 in developing countries. Long-term, high-frequency observations of MCF carried out at three European sites show a constant decline in the background mixing ratios of MCF. However, we observe persistent non-negligible mixing ratio enhancements of MCF in pollution episodes, suggesting unexpectedly high ongoing emissions in Europe. In order to identify the source regions and to give an estimate of the magnitude of such emissions, we have used a Bayesian inversion method and a point source analysis, based on high-frequency long-term observations at the three European sites. The inversion identified southeastern France (SEF) as a region with enhanced MCF emissions. This estimate was confirmed by the point source analysis. We performed this analysis using an 11-year data set, from January 2002 to December 2012. Overall, emissions estimated for the European study domain decreased nearly exponentially from 1.1 Gg yr−1 in 2002 to 0.32 Gg yr−1 in 2012, of which the estimated emissions from the SEF region accounted for 0.49 Gg yr−1 in 2002 and 0.20 Gg yr−1 in 2012. The European estimates are a significant fraction of the total semi-hemisphere (30–90° N) emissions, contributing a minimum of 9.8% in 2004 and a maximum of 33.7% in 2011, of which on average 50% are from the SEF region. On the global scale, the SEF region is thus responsible for a minimum of 2.6% (in 2003) and a maximum of 10.3% (in 2009) of the global MCF emissions.
Highlights
IntroductionBased on our understanding that OH is the main removal mechanism for methyl chloroform (MCF) in the troposphere, long-term global measurements of MCF, combined with emission estimates, have been used to infer both hemispheric and global average mole fractions and trends, as well as variability of the OH radical on annual timescales (see, for example, Montzka et al, 2000, 2011a; Prinn et al, 2001, 2005)
The global average background mixing ratio of methyl chloroform (MCF) is around 5 ppt, which can be compared to the maximum value of ca. 130 ppt reached in the mid-1990s (Montzka et al, 2011b)
In order to identify and quantify potential European source regions for MCF, we set up a Bayesian inversion methodology initially tested over a 2-year period (2008–2009) for which high-resolution (0.25◦ × 0.25◦) meteorological data were available, which was subsequently extended to the full 11-year record with lower resolution meteorological information
Summary
Based on our understanding that OH is the main removal mechanism for MCF in the troposphere, long-term global measurements of MCF, combined with emission estimates, have been used to infer both hemispheric and global average mole fractions and trends, as well as variability of the OH radical on annual timescales (see, for example, Montzka et al, 2000, 2011a; Prinn et al, 2001, 2005). According to Krol et al (2003), who used data from a short-term tropospheric measurement campaign, annual European emissions in 2000 were greater than 20 Gg, whereas Reimann et al (2005), based on 4 years (2000–2004) of high-frequency data from two European stations (Mace Head, Ireland, and Jungfraujoch, Switzerland), have derived considerably lower annual emissions, ranging from 0.3 to 3.4 Gg yr−1
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