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

Abstract

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 over the course of the studied winter. We utilize two-dimensional vertical cross sections of ozone (O3) and chlorine nitrate (ClONO2), 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 the tropopause region in detail. Observations from three long-distance flights in January, February, and March 2016 are discussed. ClONO2 volume mixing ratios up to 1100 pptv were measured at 380 K potential temperature in mesoscale structures. Similar mesoscale structures are also visible in O3 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 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 the flight altitude. Time series of these satellite measurements reveal unusually low hydrochloric acid (HCl) and ClONO2 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 ClONO2, the more typical pathway for deactivation in the Arctic. Chlorine deactivation observed in the satellite time series is well reproduced by CLaMS. Sensitivity simulations with CLaMS demonstrate the influence of low abundances of O3 and reactive nitrogen (NOy) due to ozone depletion and sedimentation of NOy-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 is comparable to other extremely cold Arctic winters. We have used CLaMS trajectories to analyze the history of enhanced ClONO2 measured by GLORIA. In February, most of the enhanced ClONO2 is traced back to chlorine deactivation that had occurred within the past few days prior to the GLORIA measurement. In March, after the final warming, air masses in which chlorine has previously been deactivated into ClONO2 have been transported in the remnants of the polar vortex towards the location of measurement for at least 11 d.

POLSTRACC: Airborne Experiment for Studying the Polar Stratosphere in a Changing Climate with the High Altitude and Long Range Research Aircraft (HALO)

“…GLORIA measurements during flight PGS-08 on 20 January 2016, spatially close to Ny-Ålesund show on average an agreement with the profile measured by the ozone sonde within 25 ppbv, even in the upper troposphere. This comparison extends the finding of Johansson et al (2018), who found GLORIA O 3 retrievals to agree with FAIRO in situ O 3 measurements within the estimated errors of the instruments. Ozone measurements above the aircraft were possible with the WALES instrument during flight PGS-05 on 21 December 2015.…”

“…High-resolution ECMWF forecasts, together with chemistry transport modeling from the CLaMS model and the Mission Support System (MSS) flight planning tool (Rautenhaus et al 2012) and satellite data from MLS and CALIPSO, were of great value in flight planning. The excellent agreement between the GLORIA infrared remote sensing data with in situ observations (Johansson et al 2018) enabled us to extend the findings sampled in situ at flight altitude throughout the lowermost stratosphere and sometimes even across the tropopause into the Arctic upper troposphere. An impressive demonstration of these capabilities has been provided for a case study of a tropopause folding event and assessment of the quality of ECMWF meteorological forecasts (Woiwode et al 2018).…”

“…Due to these different measurement geometries, atmospheric variabilities with horizontal gradients lead to differences in the observed trace gases between in situ and remote sensing measurements. Interestingly, particularly strong discrepancies are manifested in comparisons of HNO 3 , ClONO 2 , and O 3 at similar parts of the flight which indicates strongly filamented structures along the GLORIA line of sight (Johansson et al 2018).…”

“…In fact, the Cl y and ClO x concentrations derived from the in situ observations agree very well with calculations from the CLaMS model. GLORIA observations (Johansson et al 2018(Johansson et al , 2019 provided simultaneous observations on the vertical extent of the chlorine processing in the lowermost stratosphere (Figs. 15c,d).…”

“…Interestingly, both models underestimated maximum mixing ratios of ClONO 2 for the late winter flight PGS-21, which indicates that the diabatic descent over the course of the winter is underestimated in the meteorological analyses, which define dynamics in both simulations. In addition, it was found that CLaMS tends to overestimate ClONO 2 at potential temperature altitudes of 325-350 K, which is suspected to result from the lower boundary condition of the model (Johansson et al 2019).…”