Source mechanisms of tropospheric bromine monoxide in Ny-Ålesund between 2017 and 2023

Arctic tropospheric bromine monoxide (BrO) plays a critical role in atmospheric chemistry, particularly during ozone depletion events and the oxidation of gaseous elemental mercury in spring. The contributions of various potential sources, such as sea ice, open ocean, and aerosols, to the production of reactive bromine remain unclear. In this study, we present long-term observations of BrO and aerosol profiles retrieved from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Ny-Ålesund, Svalbard (78.92°N, 11.93°E), covering the periods from March to May between 2017 and 2023. Retrieved tropospheric BrO partial columns are then compared with BrO observations from the GOME-2 satellite instrument and model results from a global chemistry transport model, p-TOMCAT, respectively. Aerosol extinction exhibits the strongest correlation with BrO (R=0.67 in March, 0.54 in April, 0.47 in May), indicating that airborne particles are associated with the enhancement of reactive bromine.

Five days of backward trajectories in an altitude range of 0–3 km (at 200 m intervals) were used to calculate the contact time of air masses with various surface types (sea ice, open ocean, land, and the free troposphere). Along the trajectories, whenever the air mass meets open ocean or sea ice surface (e.g. < 500 m), the corresponding bromine emission flux from sea salt aerosols generated from open ocean (Gong et al., 2003) and blowing snow (Yang et al., 2008) is calculated and accumulated. Results show that, in March, MAX-DOAS BrO is in a positive correlation with sea ice contact time (R=0.29) and bromine emission flux from blowing snow on sea ice (R=0.33), suggesting that sea-ice-sourced sea salt aerosols generated by blowing snow could represent a significant source of reactive bromine. Throughout the entire spring (March-May), the contact time with sea ice accounts for 52.41% of all bromine explosion events (BEEs) observed in Ny-Ålesund, whereas the contact time with open ocean accounts for only 2.85%. This indicates that, in comparison to sea ice, the contribution of open ocean is less significant in Ny-Ålesund. These results confirm the critical role of sea ice-related processes in the production of reactive bromine during spring.