Role of oceanic ozone deposition in explaining temporal variability in surface ozone at High Arctic sites

Dry deposition is an important removal mechanism for tropospheric ozone (O-3). Currently, O-3 deposition to oceans in atmospheric chemistry and transport models (ACTMs) is generally represented using constant surface uptake resistances. This occurs despite the role of solubility, waterside turbulence and O-3 reacting with ocean water reactants such as iodide resulting in substantial spatiotemporal variability in O-3 deposition and concentrations in marine boundary layers. We hypothesize that O-3 deposition to the Arctic Ocean, having a relatively low reactivity, is overestimated in current models with consequences for the tropospheric concentrations, lifetime and long-range transport of O-3. We investigate the impact of the representation of oceanic O-3 deposition to the simulated magnitude and spatiotemporal variability in Arctic surface O-3. We have integrated the Coupled Ocean-Atmosphere Response Experiment Gas transfer algorithm (COAREG) into the mesoscale meteorology and atmospheric chemistry model Polar-WRF-Chem (WRF) which introduces a dependence of O-3 deposition on physical and biogeochemical drivers of oceanic O-3 deposition. Also, we reduced the O-3 deposition to sea ice and snow. Here, we evaluate WRF and CAMS reanalysis data against hourly averaged surface O-3 observations at 25 sites (latitudes > 60 degrees N). This is the first time such a coupled modeling system has been evaluated against hourly observations at pan-Arctic sites to study the sensitivity of the magnitude and temporal variability in Arctic surface O-3 on the deposition scheme. We find that it is important to nudge WRF to the ECMWF ERA5 reanalysis data to ensure adequate meteorological conditions to evaluate surface O-3. We show that the mechanistic representation of O-3 deposition over oceans and reduced snow/ice deposition improves simulated Arctic O-3 mixing ratios both in magnitude and temporal variability compared to the constant resistance approach. Using COAREG, O-3 deposition velocities are in the order of 0.01 cm s(-1) compared to similar to 0.05 cm s(-1) in the constant resistance approach. The simulated monthly mean 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 mainly differences in waterside turbulent transport. The mean bias for six sites above 70 degrees N reduced from -3.8 to 0.3 ppb with the revision to ocean and snow/ice deposition. Our study confirms that O-3 deposition to high-latitude oceans and snow/ice is generally overestimated in ACTMs. We recommend that a mechanistic representation of oceanic O-3 deposition is preferred in ACTMs to improve the modeled Arctic surface O-3 concentrations in terms of magnitude and temporal variability.