Role of oceanic ozone deposition in explaining short-term variability of Arctic surface ozone

Abstract. Dry deposition is an important removal mechanism for tropospheric ozone (O3). Currently, O3 deposition to oceans in atmospheric chemistry and transport models (ACTMs) is generally represented using constant surface uptake resistances. This is despite the fact that considering the role of solubility, waterside turbulence and O3 reacting with ocean water reactants such as iodide and dissolved organic matter results in substantial spatiotemporal variability in O3 deposition and concentrations in marine boundary layers. We hypothesize that O3 deposition to the cold Arctic ocean, with relatively low reactivity, is also overestimated in current models with consequences for background concentrations, lifetime of O3 and long-range transport of O3. In this study, we investigate the role of the representation of oceanic O3 deposition to the simulated magnitude and spatiotemporal variability in Arctic surface O3. This study also serves as a preparatory study to understand the year-round Arctic O3 concentration and deposition flux measurements as part of the MOSAiC field campaign. Furthermore, it is also important to enhance our understanding and quantification of Arctic ocean-atmosphere exchange of O3 and other climate-active trace gases given the anticipated opening of the Arctic ocean. We have coupled the Coupled Ocean-Atmosphere Response Experiment Gas transfer algorithm (COAREG) to the mesoscale meteorology and atmospheric chemistry model Polar-WRF-Chem (WRF) and introduced a dependence of O3 deposition on ocean waterside turbulent mixing conditions and biogeochemical composition. We have also reduced the O3 deposition to sea ice and snow. Here, we evaluate the performance of WRF and the CAMS reanalysis data against hourly-averaged surface O3 observations at 25 sites (latitudes > 60º N) including the ASCOS campaign observations. This is the first time such a coupled modelling system has been evaluated against hourly observations at Pan-Arctic sites to study the sensitivity of the deposition scheme to the magnitude and short-term temporal variability in Arctic surface O3. We also analyze the impact of nudging WRF to the synoptic conditions from the ECMWF ERA5 reanalysis data on simulated Arctic meteorology and comparison of observed and simulated O3 concentrations. We show that the more mechanistic representation of O3 deposition over oceans and reduced snow/ice deposition improves simulated Arctic O3 mixing ratios both in terms of magnitude but also regarding observed temporal variability. Using the newly implemented approach, O3 deposition velocities have been simulated in the order of 0.01 cm s−1 compared to ~0.05 cm s−1 in the constant surface uptake resistance approach. The simulated spatial variability in the mechanistic approach (0.01 to 0.018 cm s−1) expresses the sensitivity to chemical enhancement with dissolved iodide whereas the temporal variability (up to ±20 % around the mean) expresses differences in waterside turbulent transport. The bias for all observational sites above 70º N reduced from −7.7 ppb to 0.3 ppb with nudging and the revision to ocean and snow/ice deposition. Our study confirms that O3 deposition to oceans and snow/ice is overestimated in current models. We recommend that a mechanistic representation of oceanic O3 deposition is used in ACTMs to improve the representation of Arctic surface O3 concentrations in terms of magnitude and short-term temporal variability. The revised ocean-atmosphere exchange representation can be further refined using the MOSAiC flux measurements as well as complementary observations such as sea ice and ocean water iodide concentrations.