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

Abstract. 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 hydroxyl (OH) radical is crucial. 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 level 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 great uncertain in the signs of net effect. 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 WRF-CMAQ model with process analysis and state-of-the-art halogen chemistry in the East Asia Seas. Results show a very complicated response of OH production rate (POH) to marine halogen emissions. The monthly POH is generally decreased over the ocean with maxima of about 10–15 % in the Philippine Sea, but is increased in many nearshore areas with maxima of about 7–9 % in the Bohai Sea. In the coastal areas of southern China, the monthly POH could also decrease 3–5 % in the Greater Bay Area, but with a daytime hourly maximum decrease over 30 %. Analysis to the individual reactions using integrated reaction rate (IRR) show that the net change of POH is controlled by the competitions of three main pathways through different halogen species. Sea spray aerosols (SSA) and inorganic iodine gases are the main species to influence 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 increase POH through chemical processes. In the ocean atmosphere, the controlling species are inorganic iodine gases and the complicated iodine chemistry determines the basic pattern of ΔPOH, while over the continent, SSA is the controlling species and the SSA extinction effect leads to the negative ΔPOH in the southern China. Our results indicate that marine-emitted halogen species have notable impacts over the ocean and have potential impact on the coastal atmospheric oxidation. The identified main (previously known or unknown) pathways and their controlling factors from different halogen species to OH radical explains the halogen-induced change of POH East Asia and also can be applied in other circumstances (e.g., different domains, regions, and emission rates).