Denitrification, dehydration and ozone loss during the Arctic winter 2015/2016

Abstract

<p><strong>Abstract.</strong> The Arctic winter 2015/2016 was one of the coldest stratospheric winters in recent years. A stable vortex formed by early December and the early winter was exceptionally cold. Cold pool temperatures dropped below the Nitric Acid Trihydrate (NAT) existence temperature of about 195 K, thus allowing Polar Stratospheric Clouds (PSCs) to form. The low temperatures in the polar stratosphere persisted until early March allowing chlorine activation and catalytic ozone destruction. Satellite observations indicate that sedimentation of PSC particles led to denitrification as well as dehydration of stratospheric layers. Model simulations of the Arctic winter 2015/2016 nudged toward European Center for Medium-Range Weather Forecasts (ECMWF) analyses data were performed with the atmospheric chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC) for the Polar Stratosphere in a Changing Climate (POLSTRACC) campaign. POLSTRACC is a High Altitude and LOng Range Research Aircraft (HALO) mission aimed at the investigation of the structure, composition and evolution of the Arctic Upper Troposphere and Lower Stratosphere (UTLS). The chemical and physical processes involved in Arctic stratospheric ozone depletion, transport and mixing processes in the UTLS at high latitudes, polar stratospheric clouds as well as cirrus clouds are investigated. In this study an overview of the chemistry and dynamics of the Arctic winter 2015/2016 as simulated with EMAC is given. Further, chemical-dynamical processes such as denitrification, dehydration and ozone loss during the Arctic winter 2015/2016 are investigated. Comparisons to satellite observations by the Aura Microwave Limb Sounder (Aura/MLS) as well as to airborne measurements with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) performed on board of HALO during the POLSTRACC campaign show that the EMAC simulations are in fairly good agreement with observations. We derive a maximum polar stratospheric O<sub>3</sub> loss of ~ 2 ppmv or 100 DU in terms of column in mid March. The stratosphere was denitrified by about 8 ppbv HNO<sub>3</sub> and dehydrated by about 1 ppmv H<sub>2</sub>O in mid to end of February. While ozone loss was quite strong, but not as strong as in 2010/2011, denitrification and dehydration were so far the strongest observed in the Arctic stratosphere in the at least past 10 years.</p>

Nitrification of the lowermost stratosphere during the exceptionally cold Arctic winter 2015/16

“…The choice of ozone as a passive tracer is based on the assumption that ozone depletion is small in that period as the air is hardly exposed to sunlight. The model study by Khosrawi et al (2017) supports this assumption. Two aspects can affect the correlation: 1) Mixing with extra-vortex air masses not affected by nitrification would lead to an underestimation of HNO 3 introduced into the LMS by nitrification and 2) Potential ozone depletion would shift higher HNO 3 mixing ratios to lower ozone values, thus enhancing estimated nitrification.…”

Denitrification, dehydration and ozone loss during the 2015/2016 Arctic winter

“…Ice PSCs persisted in 2015/2016 over a much longer time period than in, e.g. the 2010/2011 Arctic winter, as can be seen in the EMAC results for the 2010/2011 Arctic winter shown in Khosrawi et al (2017).…”

“…PSCs composed of NAT form in EMAC as soon as temperatures drop below T NAT − 3 K, which often results in a too-early formation of NAT particles. Among other things, this has also an impact on the denitrification, as was found in another study comparing EMAC simulations for the 2009/2010 and 2010/2011 Arctic winters with Envisat/MIPAS and Aura/MLS observations (Khosrawi et al, 2017). Because NAT is calculated before STS in the model, the NAT formation occurs at the expense of STS since the available HNO 3 is first consumed by the NAT clouds (e.g.…”

“…In another study (Khosrawi et al, 2017), the EMAC simulations of HNO 3 , temperature and PSC volume density were compared for the 2009/2010 and 2010/2011 Arctic winters with satellite observations (Envisat/MIPAS and Aura/MLS). Here, we consider in addition to temperature and HNO 3 other trace gases such as O 3 , ClO (Sect.…”

Toward a Reanalysis of Stratospheric Ozone for Trend Studies: Assimilation of the Aura Microwave Limb Sounder and Ozone Mapping and Profiler Suite Limb Profiler Data

“…The year 2016 was selected for the test experiments since it featured several unusual events occurring in or affecting the stratosphere. Of particular interest are the following: The boreal winter of 2015/2016 characterized by exceptionally cold and strong polar vortex and near‐record ozone loss due to heterogeneous chemistry terminated by a relatively early final warming that began on 6 March 2016 (Khosrawi et al, ; Manney & Lawrence, ; Matthias et al, ). The unusual disruption of the quasi‐biennial oscillation (QBO; Osprey et al, , Newman et al, ) that, combined with a very strong El Niño event, had a significant impact on tracer transport in the lower stratosphere (Diallo et al, ; Tweedy et al, ). …”