Investigating the roles of the Asian monsoon, the North American monsoon, and Hurricanes for efficient transport of chlorinated short-lived species to the UTLS based on in situ observations

Chlorinated very short-lived substances (Cl-VSLS) are not controlled by the Montreal Protocol but the recent emission increase of the Cl-VSLS CH₂Cl₂ (Dichloromethane) and CHCl₃ (Chloroform) is believed to significantly increase the stratospheric chlorine loading from VSLS. Provided efficient transport of Cl-VSLS from the source region into the stratosphere further emission increases could ultimately even cause a significant delay of the predicted recovery date of the ozone layer to pre-1980 values. During the WISE (Wave-driven ISentropic Exchange) campaign in autumn 2017 excessive probing of the UTLS (upper troposphere lower stratosphere) region above Western Europe and the Atlantic Ocean was conducted from aboard the HALO (High Altitude and Long range) research aircraft. We use real-time in situ WISE measurements of CH₂Cl₂ and CHCl₃ from HAGAR-V (High Altitude Gas AnalyzeR – 5 channel version) in correlation with N₂O from UMAQS (University of Mainz Airborne QCL Spectrometer), as well as CLaMS (Chemical Lagrangian Model of the Stratosphere) global 3-dimensional simulations of air mass origin tracers and backward trajectories to identify the most efficient transport mechanisms for Cl-VSLS entering the LS region in northern hemispheric summer.

The WISE measurements reveal two distinct transport pathways into the UTLS region of particularly CH₂Cl₂-rich and CH₂Cl₂-poor air. CH₂Cl₂-rich air could be identified to be transported by the Asian summer monsoon within about 4-10 weeks from its source regions in Asia into the stratosphere above the Atlantic Ocean at around 380 K and above. CH₂Cl₂-poor air could be identified to be mainly uplifted to potential temperatures of about 365 K by the North American monsoon above the region of Central America with transport times of only 2-5 weeks. In addition, we could link backward trajectories of CH₂Cl₂-poor air in the LS region to be uplifted by the category 5 hurricane Maria in September 2017. Based on all analyzed WISE measurements, we found that almost all young (transport time < 4 months) air masses were uplifted either above Asia or above Central America, emphasizing not only the impact of the Asian summer monsoon on the stratospheric tracer distribution but also that of the North American monsoon and hurricanes.

The measurements of both CH₂Cl₂ and CHCl₃ show the lowest stratospheric mixing ratios originating in the region of Central America and enhanced mixing ratios from Asia (enhancements > 100 % and > 50 %, respectively). However, the source distribution of CHCl₃ is much less clear than that of CH₂Cl₂ and inconspicuous CH₂Cl₂ measurements can also contain enhanced CHCl₃ mixing ratios. Nevertheless, the anthropogenic impact on CHCl₃ -rich air from Asia is clearly visible in the measurements and we believe it is likely that a future increase of Asian CHCl₃ emissions could lead to similarly large stratospheric enhancements as already observed for CH₂Cl₂. Consequently, this would further increase ozone depletion from stratospheric chlorine deposition of VSLS.