Unusual chlorine partitioning in the 2015/16 Arctic winter lowermost stratosphere: Observations and simulations

Abstract

The Arctic winter 2015/16 was characterized by cold stratospheric temperatures. Here we present a comprehensive view of the temporal evolution of chlorine in the lowermost stratosphere (LMS) over the course of this winter. We utilize two-dimensional vertical cross sections of ozone (O 3 ) and chlorine nitrate (ClONO 2 ), measured by the airborne limb-imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) during the POLSTRACC/ GW-LCYCLE II/ GWEX/ SALSA campaigns, to investigate in detail the tropopause region. Observations from three long-distance flights in Jan-5 uary, February and March 2016 are discussed. ClONO 2 volume mixing ratios up to 1100 pptv were measured at 380 K potential temperature in mesoscale structures. Similar mesoscale structures are also visible in O 3 measurements. Both trace gas measurements are applied to evaluate simulation results from the chemistry transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) and the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry). These comparisons show agreement within the expected performance of these models. Satellite measurements from Aura/MLS (Microwave 10 Limb Sounder) and SCISAT/ACE-FTS (Atmospheric Chemistry Experiment -Fourier Transform Spectrometer) provide an overview over the whole winter and information about the stratospheric situation above flight altitude. Time series of these satellite measurements reveal unusually low hydrochloric acid (HCl) and ClONO 2 at 380 K from the beginning of January to the end of February 2016, while chlorine monoxide (ClO) is strongly enhanced. In March 2016, unusually rapid chlorine deactivation into HCl is observed instead of deactivation into ClONO 2 , the more typical pathway for deactivation in the Arctic.15Chlorine deactivation observed in the satellite time series is well reproduced by CLaMS. Sensitivity simulations with CLaMS demonstrate the influence of low abundances of O 3 and reactive nitrogen (NO y ) due to ozone depletion and sedimentation of NO y -containing particles, respectively. On the basis of the different altitude and time ranges of these effects, we conclude that the substantial chlorine deactivation into HCl at 380 K arose as a result of very low ozone abundances together with low temperatures. Additionally, CLaMS estimates ozone depletion of at least 0.4 ppmv at 380 K and 1.75 ppmv at 490 K, which 20 is comparable to other extremely cold Arctic winters. We have used CLaMS trajectories to analyze the history of enhanced 1 Atmos. Chem. Phys. Discuss., https://doi.