Aircraft-Based Observations of Ozone-Depleting Substances in the Upper Troposphere and Lower Stratosphere in and Above the Asian Summer Monsoon

Abstract

Recent studies show that the Asian summer monsoon anticyclone (ASMA) transports emissions from the rapidly industrializing nations in Asia into the tropical upper troposphere. Here, we present a unique set of measurements on over 100 air samples collected on multiple flights of the M55 Geophysica high altitude research aircraft over the Mediterranean, Nepal, and Northern India during the summers of 2016 and 2017 as part of the European Union project StratoClim. These air samples were measured for 27 ozone-depleting substances (ODSs), many of which were enhanced above expected levels, including the chlorinated very short-lived substances, dichloromethane (CH2Cl2), 1,2-dichloroethane (CH2ClCH2Cl), and chloroform (CHCl3). CH2Cl2 mixing ratios in the tropopause region were 65–136 parts per trillion (ppt) in comparison to previous estimates of mixing ratios in the tropical tropopause layer of 30–44 ppt in 2013–2014. Backward trajectories, calculated with the trajectory module of the chemistry-transport model CLaMS and driven by the ERA5 reanalysis, indicate possible source regions of CH2Cl2 in South Asia. We derived total equivalent chlorine (ECl), and equivalent effective stratospheric chlorine (EESC) and found that these quantities were substantially higher than previous estimates in the literature. EESC at mean age-of-air of 3 years based on the 2016 measurements was 1,861–1,872 ppt in comparison to a previously estimated EESC of 1,646 ppt. Our findings show that the ASMA transports larger than expected mixing ratios of long-lived and very short-lived ODSs into the upper troposphere and lower stratosphere, likely leading to an impact on the stratospheric ozone layer.