Increasing concentrations of dichloromethane, CH₂Cl₂, inferred from CARIBIC air samples collected 1998–2012

Abstract

Abstract. Dichloromethane, CH2Cl2, is a short-lived chlorocarbon of predominantly anthropogenic origin. Increasing industrial usage and associated emissions resulted in an increasing atmospheric burden throughout the 1900s. Atmospheric abundance peaked around 1990 and was followed by a decline in the early part of the 21st century. Despite the importance of ongoing monitoring and reporting of atmospheric CH2Cl2 (it is a regulated toxic air pollutant and a contributor to stratospheric ozone depletion) no time series has been discussed in detail since 2006. The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) has analysed the halocarbon content of whole air samples collected at altitudes of between ~10–12 km via a custom-built container installed on commercial passenger aircraft since 1998, providing a long-term record of CH2Cl2 observations. In this paper we present this unique CH2Cl2 time series, discussing key flight routes which have been traversed at various times over the past 15 years. Between 1998 and 2012 increases were seen in all northern hemispheric regions and at different altitudes, ranging from ~7–9 ppt in background air to ~12–15 ppt in regions with stronger emissions (equating to a 38–69% increase). Of particular interest is the rising importance of India as a source of atmospheric CH2Cl2: based on CARIBIC data we provide regional emission estimates for the Indian subcontinent and show that regional emissions have increased from 3–15 Gg yr−1 (1998–2000) to 16–25 Gg yr−1 (2008). Potential causes of the increasing atmospheric burden of CH2Cl2 are discussed. One possible source is the increased use of CH2Cl2 as a feedstock for the production of HFC-32, a chemical used predominantly as a replacement for ozone-depleting substances in a variety of applications including air conditioners and refrigeration.