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
Authors: Lauther Valentin , Wintel Johannes , Gerhardt Emil , Rau Andrea , Hoor Peter , Kluschat Björn , Vogel Bärbel , Müller Rolf , Volk C. Michael 
Published: March 2021

&lt;p&gt;Chlorinated very short-lived substances (Cl-VSLS) are not controlled by the Montreal Protocol but the recent emission increase of the Cl-VSLS CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; (Dichloromethane) and CHCl&lt;sub&gt;3&lt;/sub&gt; (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 CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; and CHCl&lt;sub&gt;3&lt;/sub&gt; from HAGAR-V (High Altitude Gas AnalyzeR &amp;#8211; 5 channel version) in correlation with N&lt;sub&gt;2&lt;/sub&gt;O 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.&lt;/p&gt;&lt;p&gt;The WISE measurements reveal two distinct transport pathways into the UTLS region of particularly CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;-rich and CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;-poor air. CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;-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. CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;-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 CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;-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 &lt; 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.&lt;/p&gt;&lt;p&gt;The measurements of both CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; and CHCl&lt;sub&gt;3&lt;/sub&gt; show the lowest stratospheric mixing ratios originating in the region of Central America and enhanced mixing ratios from Asia (enhancements &gt; 100 % and &gt; 50 %, respectively). However, the source distribution of CHCl&lt;sub&gt;3&lt;/sub&gt; is much less clear than that of CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; and inconspicuous CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; measurements can also contain enhanced CHCl&lt;sub&gt;3&lt;/sub&gt; mixing ratios. Nevertheless, the anthropogenic impact on CHCl&lt;sub&gt;3&lt;/sub&gt; -rich air from Asia is clearly visible in the measurements and we believe it is likely that a future increase of Asian CHCl&lt;sub&gt;3&lt;/sub&gt; emissions could lead to similarly large stratospheric enhancements as already observed for CH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;. Consequently, this would further increase ozone depletion from stratospheric chlorine deposition of VSLS.&lt;/p&gt;