Global trends, seasonal cycles, and European emissions of dichloromethane, trichloroethene, and tetrachloroethene from the AGAGE observations at Mace Head, Ireland, and Cape Grim, Tasmania

Abstract

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.