Modelling Arctic Lower Tropospheric Ozone: processes controlling seasonal variations

Abstract. Previous assessments on modelling Arctic tropospheric ozone (O₃) have shown that most atmospheric models continue to experience difficulties in simulating tropospheric O₃ in the Arctic, particularly in capturing the seasonal variations at coastal sites, primarily attributed to the lack of representation of surface bromine chemistry in the Arctic. In this study, two independent chemical transport models (CTMs), DEHM (Danish Eulerian Hemispheric Model) and GEM-MACH (Global Environmental Multi-scale – Modelling Air quality and Chemistry), were used to simulate Arctic lower tropospheric O₃ for the year 2015 at considerably higher horizontal resolutions (25-km and 15-km, respectively) than the large-scale models in the previous assessments. Both models include bromine chemistry and a representation of snow-sourced bromine mechanism: a blowing-snow bromine source mechanism in DEHM and a snowpack bromine source mechanism in GEM-MACH. Model results were compared with a suite of observations in the Arctic, including hourly observations from surface sites and mobile platforms (buoys and ship) and ozonesonde profiles, to evaluate models’ ability to simulate Arctic lower tropospheric O₃, particularly in capturing the seasonal variations and the key processes controlling these variations.

The study found that both models behave quite similarly outside the spring period and are able to capture the observed overall surface O₃ seasonal cycle and synoptic scale variabilities, as well as the O₃ vertical profiles in the Arctic. GEM-MACH (with the snowpack bromine source mechanism) was able to simulate most of the observed springtime Ozone Depletion Events (ODEs) at the coastal and buoy sites well, while DEHM (with the blowing-snow bromine source mechanism) simulated much fewer ODEs. The study showed that the springtime O₃ depletion process plays a central role in driving the surface O₃ seasonal cycle in Central Arctic, and that the bromine-mediated ODEs, while occurring most notably within the lowest few hundred metres of air above the Arctic Ocean, can induce a 5–7 % of loss in the total pan-Arctic tropospheric O₃ burden during springtime. The model simulations also showed an overall enhancement in the pan-Arctic O₃ concentration due to northern boreal wildfire emissions in summer 2015; the enhancement is more significant at higher altitudes. Higher O₃ excess ratios (ΔO₃/ΔCO) found aloft compared to near the surface indicate greater photochemical O₃ production efficiency at higher altitudes in fire-impacted air masses. The model simulations further indicated an enhancement in NO_y in the Arctic due to wildfires; a large portion of NO_y produced from the wildfire emissions is found in the form of PAN that is transported to the Arctic, particularly at higher altitudes, potentially contributing to O₃ production there.