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

Abstract. Atmospheric concentrations of dichloromethane, CH₂Cl₂, a regulated toxic air pollutant and minor contributor to stratospheric ozone depletion, were reported to have peaked around 1990 and to be declining in the early part of the 21st century. Recent observations suggest this trend has reversed and that CH₂Cl₂ is once again increasing in the atmosphere. Despite the importance of ongoing monitoring and reporting of atmospheric CH₂Cl₂, 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 used 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 ~ 7–10 ppt in background air to ~ 13–15 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 3–14 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.