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 &#8211; 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 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.</p

Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves

Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic in autumn 2017, supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anticyclonic side of the jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small-scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated with small-scale waves at the upper edge of the jet stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained a small amount of attention in terms of mixing in baroclinic waves