Abstract. Within the framework of the International Arctic Systems for Observing the Atmosphere (IASOA), we report a modelling-based study on surface ozone across the Arctic. We use surface ozone from six sites – Summit (Greenland), Pallas (Finland), Barrow (USA), Alert (Canada), Tiksi (Russia), and Villum Research Station (VRS) at Station Nord (North Greenland, Danish realm) – and ozone-sonde data from three Canadian sites: Resolute, Eureka, and Alert. Two global chemistry models – a global chemistry transport model (parallelised-Tropospheric Offline Model of Chemistry and Transport, p-TOMCAT) and a global chemistry climate model (United Kingdom Chemistry and Aerosol, UKCA) – are used for model data comparisons. Remotely sensed data of BrO from the GOME-2 satellite instrument and ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) at Eureka, Canada, are used for model validation. The observed climatology data show that spring surface ozone at coastal sites is heavily depleted, making ozone seasonality at Arctic coastal sites distinctly different from that at inland sites. Model simulations show that surface ozone can be greatly reduced by bromine chemistry. In April, bromine chemistry can cause a net ozone loss (monthly mean) of 10–20 ppbv, with almost half attributable to open-ocean-sourced bromine and the rest to sea-ice-sourced bromine. However, the open-ocean-sourced bromine, via sea spray bromide depletion, cannot by itself produce ozone depletion events (ODEs; defined as ozone volume mixing ratios, VMRs, < 10 ppbv). In contrast, sea-ice-sourced bromine, via sea salt aerosol (SSA) production from blowing snow, can produce ODEs even without bromine from sea spray, highlighting the importance of sea ice surface in polar boundary layer chemistry. Modelled total inorganic bromine (BrY) over the Arctic sea ice is sensitive to model configuration; e.g. under the same bromine loading, BrY in the Arctic spring boundary layer in the p-TOMCAT control run (i.e. with all bromine emissions) can be 2 times that in the UKCA control run. Despite the model differences, both model control runs can successfully reproduce large bromine explosion events (BEEs) and ODEs in polar spring. Model-integrated tropospheric-column BrO generally matches GOME-2 tropospheric columns within ∼ 50 % in UKCA and a factor of 2 in p-TOMCAT. The success of the models in reproducing both ODEs and BEEs in the Arctic indicates that the relevant parameterizations implemented in the models work reasonably well, which supports the proposed mechanism of SSA production and bromide release on sea ice. Given that sea ice is a large source of SSA and halogens, changes in sea ice type and extent in a warming climate will influence Arctic boundary layer chemistry, including the oxidation of atmospheric elemental mercury. Note that this work dose not necessary rule out other possibilities that may act as a source of reactive bromine from the sea ice zone.
<p>Bromine compounds play an important role in atmospheric chemistry with respect to ozone chemistry and the resulting oxidation capacity. Large amounts of reactive bromine can be released by an autocatalytic heterogeneous mechanism called &#8220;bromine explosion&#8221;, and plumes of enhanced bromine monoxide (BrO) have been observed over polar sea ice regions by satellite measurements in spring. These enhancements of BrO columns result from increases in stratospheric or tropospheric bromine columns or both. As nadir-viewing UV-visible spectrometers have limited vertical resolution, it is not straight-forward to separate total BrO columns into tropospheric and stratospheric partial columns using satellite data.</p><p>In this study, an algorithm for tropospheric BrO retrieval from satellite measurements including TROPOMI, which provides much improved spatial resolution, was developed. The retrieval algorithm is based on the Differential Optical Absorption Spectroscopy (DOAS) technique and three different stratospheric correction methods were tested based on: output from a 3D atmospheric chemistry model, a climatology of stratospheric BrO profiles, and an empirical multiple linear regression model to separate the tropospheric partial column from the total column.</p><p>Retrieved tropospheric BrO columns from satellite measurements were compared with ground-based MAX-DOAS BrO observations at the NDACC station in Ny-&#197;lesund. The comparisons between ground-based and satellite measurements of tropospheric BrO show good agreement in both time-series and scatter plots, demonstrating the satellite retrieval algorithm is valid and applicable to study bromine release in the tropospheric layer. In particular, TROPOMI shows improved validation results for short distance collection compared to previous satellite data, which suggests the applicability of high-resolution satellite data on small-scale bromine explosion events observed during the MOSAiC campaign.</p>
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