The impacts of marine-emitted halogens on OH radicals in East Asia during summer

• Published in: Atmospheric chemistry and physics 2022-06, Vol.22 (11), p.7331-7351
• Main Authors: Fan, Shidong, Li, Ying
• Format: Article
• Language: English
• Subjects: Air; Air pollution; Air-sea interaction; Analysis; Atmosphere; Atmospheric chemistry; Boundary conditions; Bromine; Chemical reactions; Chemistry; Coastal zone; Dominant species; Emissions; Free radicals; Gases; Halocarbons; Halogens; Hydroxides; Iodine; Methods; Ocean-atmosphere interaction; Oceans; Oxidation; Photolysis; Pollutants; Radiation; Radicals (Chemistry); Sea spray; Simulation; Species extinction; Spray; VOCs; Volatile organic compounds
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-22-7331-2022

Relationships between oceanic emissions and air chemistry are intricate and still not fully understood. For regional air chemistry, a better understanding of marine halogen emission on the hydroxyl (OH) radical is crucial. The OH radical is a key species in atmospheric chemistry because it can oxidize almost all trace species in the atmosphere. In the marine atmosphere, OH levels could be significantly affected by the halogen species emitted from the ocean. However, due to the complicated interactions of halogens with OH through different pathways, it is not well understood how halogens influence OH and even what the sign of the net effect is. Therefore, in this study, we aim to quantify the impact of marine-emitted halogens (including Cl, Br, and I) through different pathways on OH in the high OH season by using the WRF-CMAQ model with process analysis and state-of-the-art halogen chemistry in East Asia and near the western Pacific. Results show a very complicated response of the OH production rate (POH) to marine halogen emissions. The monthly POH is generally decreased over the ocean by up to a maximum of about 10 %–15 % in the Philippine Sea, but it is increased in many nearshore areas by up to about 7 %–9 % in the Bohai Sea. In the coastal areas of southern China, the monthly POH could also decrease 3 %–5 %, but hourly values can decrease over 30 % in the daytime. Analysis of the individual reactions using the integrated reaction rate shows that the net change in POH is controlled by the competition of three main pathways (OH from O3 photolysis, OH from HO2 conversion, and OH from HOX, X=Cl, Br, I) through different halogen species. Sea spray aerosol (SSA) and inorganic iodine gases are the major species influencing the strengths of these three pathways and therefore have the most significant impacts on POH. Both of these two types of species decrease POH through physical processes, while generally increasing POH through chemical processes. In the ocean atmosphere, inorganic iodine gases determine the basic pattern of ΔPOH through complicated iodine chemistry, which generally positively influences POH near O3 sources while negatively influencing it when O3 experiences longer transport over the ocean. Over the continent, SSA is the controlling species, and the SSA extinction effect leads to the negative ΔPOH in southern China. Our results show that marine-emitted halogen species have notable impacts over the ocean and potential impacts on coastal atmos