Unlike bromine, the effect of iodine chemistry on the Arctic surface ozone budget is poorly constrained. We present ship-based measurements of halogen oxides in the high Arctic boundary layer from the sunlit period of March to October 2020 and show that iodine enhances springtime tropospheric ozone depletion. We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine.

We report the retrieval of Na concentration profiles in the mesopause region from satellite observations of the Na D‐line nightglow emission near 589 nm made by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) on the Envisat spacecraft. The retrieval assumes the Na D‐line excitation mechanism originally proposed by Chapman in 1939. The retrieval approach, including treatment of self‐absorption by Na, a retrieval uncertainty budget, and first retrieval results, is presented. The retrieved Na profiles are compared to independent satellite measurements. Good agreement in terms of peak altitude, peak concentration, and vertical column density is found. The retrievals constitute the first Na profile retrievals from satellite observations of the Na D‐line nightglow emission profile. They enable our understanding of the Na nightglow excitation mechanism to be tested.

Near-surface mercury and ozone depletion events occur in the lowest part of the atmosphere during Arctic spring. Mercury depletion is the first step in a process that transforms long-lived elemental mercury to more reactive forms within the Arctic that are deposited to the cryosphere, ocean, and other surfaces, which can ultimately get integrated into the Arctic food web. Depletion of both mercury and ozone occur due to the presence of reactive halogen radicals that are released from snow, ice, and aerosols. In this work, we added a detailed description of the Arctic atmospheric mercury cycle to our recently published version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem 4.3.3) that includes Arctic bromine and chlorine chemistry and activation/recycling on snow and aerosols. The major advantage of our modelling approach is the online calculation of bromine concentrations and emission/recycling that is required to simulate the hourly and daily variability of Arctic mercury depletion. We used this model to study coupling between reactive cycling of mercury, ozone, and bromine during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spring season in 2020 and evaluated results compared to land-based, ship-based, and remote sensing observations. The model predicts that elemental mercury oxidation is driven largely by bromine chemistry and that particulate mercury is the major form of oxidized mercury. The model predicts that the majority (74%) of oxidized mercury deposited to land-based snow is re-emitted to the atmosphere as gaseous elemental mercury, while a minor fraction (4%) of oxidized mercury that is deposited to sea ice is re-emitted during spring. Our work demonstrates that hourly differences in bromine/ozone chemistry in the atmosphere must be considered to capture the springtime Arctic mercury cycle, including its integration into the cryosphere and ocean.

Abstract. During polar spring, Ozone Depletion Events (ODEs) are often observed in combination with Bromine Explosion Events (BEEs) in Ny-Ålesund. In this study, two long term ozone data sets (2010–2021) from ozone sonde launches and in-situ ozone measurements have been evaluated between March and May of each year, to study ODEs in Ny-Ålesund. Ozone concentrations below 15 ppb were marked as ODE. We applied a composite analysis to evaluate tropospheric BrO retrieved from satellite data and the prevailing meteorological conditions during these events. During ODEs, both data sets show a blocking situation with a low pressure anomaly over the Barents Sea and anomalously high pressure in the Icelandic low area, leading to transport of cold polar air from the north to Ny-Ålesund with negative temperature and positive BrO anomalies found around Svalbard. Also higher wind speed and a higher, less stable boundary layer are noticed, supporting the assumption that ODEs often occur in combination with polar cyclones. Applying a 20 ppb ozone threshold value to tag ODEs resulted in only a slight attenuation of the BrO and meteorological anomalies compared to the 15 ppb threshold. Monthly analysis showed that BrO and meteorological anomalies are weakening from March to May. Therefore, ODEs associated with low pressure systems, high wind speeds and blowing snow more likely occur in early spring, while ODEs associated with low wind speed and stable boundary layer meteorological conditions seem to occur more often in late spring. In an annual evaluation, similar prevailing meteorological conditions were found for several years as well as in the overall result of the composite analysis. However, some years show different meteorological patterns deviating from the results of the mean analysis. Finally, an ODE case study from the beginning of April 2020 in Ny-Ålesund is presented, where ozone was depleted for two consecutive days in combination with increased BrO values. The meteorological conditions are representative of the results of the composite analysis. A low pressure system arrived from the north-east to Svalbard resulting in high wind speeds with blowing snow and transport of cold polar air from the north.

<p lang="en-US">Ozone Depletion Events (ODEs) have been observed since the late 1990s in the polar regions during spring, often in combination with Bromine Explosion Events (BEEs). In a heterogeneous, autocatalytic, chemical chain reaction cycle, inorganic bromine is released from the cryosphere into the troposphere and depletes ozone, sometimes to below detection limit. Besides low temperatures favoring the bromine explosion reactions, two different meteorological conditions are mainly observed during these events: on the one hand, low wind speeds and a stable boundary layer, where bromine can accumulate and deplete ozone, and on the other hand, high wind speeds above approximately 10 m/s with blowing snow and a higher, unstable boundary layer. The second condition often occurs in combination with polar cyclones, where bromine can be recycled aloft on snow and aerosol surfaces.</p> <p align="justify"><span lang="en-US">In this study, </span><span lang="en-US">two </span><span lang="en-US">long term ozone data set</span><span lang="en-US">s, one</span><span lang="en-US"> from </span><span lang="en-US">ozone sondes launched in </span><span lang="en-US">Ny</span><span lang="en-US">-</span><span lang="en-US">Å</span><span lang="en-US">lesund </span><span lang="en-US">and the other from in-situ measurements on</span><span lang="en-US"> Zeppelin mountain – located close to </span><span lang="en-US">Ny</span><span lang="en-US">-</span><span lang="en-US">Å</span><span lang="en-US">lesund –</span><span lang="en-US"> ha</span><span lang="en-US">ve</span><span lang="en-US"> been evaluated </span><span lang="en-US">from March until May </span><span lang="en-US">between 2010 and 2021 to detect ODEs. </span><span lang="en-US">To analyze the prevailing weather conditions during the</span><span lang="en-US">se</span><span lang="en-US"> events, ERA5 reanalysis dat</span><span lang="en-US">a</span><span lang="en-US"> has been used and </span><span lang="en-US">separated</span><span lang="en-US"> between weather conditions during ODEs and no-ODEs based on the </span><span lang="en-US">respective</span><span lang="en-US"> oz</span><span lang="en-US">one data set. </span>The evaluation of the two data sets led to very consistent results: <span lang="en-US">d</span><span lang="en-US">uring ODEs, lower pressure </span><span lang="en-US">is observed </span><span lang="en-US">east of Svalbard and higher pressure over Greenland, leading to a transport of c</span><span lang="en-US">old polar air from the north to </span><span lang="en-US">Ny</span><span lang="en-US">-</span><span lang="en-US">Å</span><span lang="en-US">lesund</span><span lang="en-US">.</span> <span lang="en-US">Also higher wind speed and a higher boundary layer are </span><span lang="en-US">noticed</span><span lang="en-US">, supporting the assumption, that ODEs often occur in combination with polar cyclon</span><span lang="en-US">e</span><span lang="en-US">s.</span></p> <p lang="en-US" align="justify">Using the same approach, the long-term tropospheric BrO data set from Bougoudis et al., 2020 in combination with S5P TROPOMI retrievals of tropospheric BrO has been used to analyze BrO patterns. During ODEs in Ny-Ålesund, the satellite data show elevated values all over the Arctic, but especially north of Svalbard.</p> <p lang="en-US"> </p> <p align="justify"><em>This work was supported by the DFG funded Transregio-project TR 172 “Arctic Amplification (AC)³“ <span lang="en-GB">in subproject C03</span>.</em></p>