Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere during the cold winter 2015/2016

Abstract

Abstract. During winter 2015/2016 the Arctic stratosphere was characterized by extraordinarily low temperatures in connection with the occurrence of extensive polar stratospheric clouds. From mid of December 2015 until mid of March 2016 the German research aircraft HALO (High Altitude and Long–Range Research Aircraft) was deployed to probe the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar Stratosphere in a Changing Climate) mission. More than twenty flights have been conducted out of Kiruna/Sweden and Oberpfaffenhofen/Germany, covering the whole winter period. Besides total reactive nitrogen (NOy), observations of nitrous oxide, nitric acid, ozone and water were used for this study. Total reactive nitrogen and its partitioning between gas- and particle phase are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. The redistribution of total reactive nitrogen was evaluated by using tracer–tracer correlations. In January air masses with extensive nitrification were encountered at altitudes between 12 and 15 km. The excess NOy amounted up to about 6 ppb. During several flights, along with gas–phase nitrification, indications for extensive occurrence of nitric acid containing particles at flight altitude were found. These observations support the assumption of sedimentation and subsequent evaporation of nitric acid containing particles leading to redistribution of total reactive nitrogen. Remnants of nitrified air masses have been observed until mid of March. Between end of February and mid of March also de-nitrified air masses have been observed in connection with high potential temperatures. Using tracer–tracer correlations, missing total reactive nitrogen was estimated to amount up to 6 ppb. This indicates the downward transport of air masses that have been denitrified during the earlier winter phase. Observations within POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from the overlying Arctic winter stratosphere. The comparison of the observations with CLaMS model simulations confirm and complete the picture arising from the present measurements. The simulations confirm, that the ensemble of all observations is representative for the vortex–wide vertical NOy-redistribution.

Investigating the radiative effect of Arctic cirrus measured in situ during the winter 2015–2016

“…The campaign was based in Kiruna (67.5 • N, 20.3 • E), Sweden, and spanned the boreal winter-spring season from 17 December 2015 to 18 March 2016 with 18 science flights and 156 flight hours. The campaign accommodated a number of different science goals, including trace gas composition in the upper troposphere and lower stratosphere (Marsing et al, 2019;Johansson et al, 2019;Braun et al, 2019;Keber et al, 2020;Ziereis et al, 2022), gravity waves (Krisch et al, 2020) and polar stratospheric clouds (Voigt et al, 2018). Therefore, the flight strategy often needed to avoid extended sections in cirrus clouds.…”

Challenge of modelling GLORIA observations of upper troposphere–lowermost stratosphere trace gas and cloud distributions at high latitudes: a case study with state-of-the-art models

“…Furthermore, the nitric acid distribution shows a local maximum at and below flight altitude from ∼ 15:00 to ∼ 17:00 UTC. As discussed by Ziereis et al (2021), during this flight nitrified air masses were observed in situ at flight altitude during the first leg and the last leg, while descended denitrified air masses were observed at flight altitude during the central part of the flight. The local maximum in the GLORIA HNO 3 distribution below flight altitude between about 15:00 and 17:00 UTC is interpreted as subsided nitri- As shown in Fig.…”

“…12c clearly shows that this species is enhanced in the simulation by ∼ 1 to 1.5 ppbv in large parts of the LMS as a consequence of nitrification by evaporated HNO 3 -containing particles that sedimented from polar stratospheric clouds. In the middle of the flight, air masses that are affected by HNO 3 depletion reach into the LMS and are the consequence of subsidence of denitrified air masses in the polar vortex (see Khosrawi et al, 2017, andZiereis et al, 2021). Above flight altitude, sequestration of HNO 3 on existing PSC particles might still play a role under sufficiently cold conditions.…”