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

[摘要] Relationships between oceanic emissions and air chemistryare intricate and still not fully understood. For regional air chemistry, abetter understanding of marine halogen emission on the hydroxyl (OH) radical iscrucial. The OH radical is a key species in atmospheric chemistry because itcan oxidize almost all trace species in the atmosphere. In the marineatmosphere, OH levels could be significantly affected by the halogen speciesemitted from the ocean. However, due to the complicated interactions ofhalogens with OH through different pathways, it is not well understood howhalogens influence OH and even what the sign of the net effect is.Therefore, in this study, we aim to quantify the impact of marine-emittedhalogens (including Cl, Br, and I) through different pathways on OH in thehigh OH season by using the WRF-CMAQ model with process analysis andstate-of-the-art halogen chemistry in East Asia and near the westernPacific. Results show a very complicated response of the OH production rate(P OH ) to marine halogen emissions. The monthly P OH isgenerally decreased over the ocean by up to a maximum of about 10 %–15 % inthe Philippine Sea, but it is increased in many nearshore areas by up to about7 %–9 % in the Bohai Sea. In the coastal areas of southern China, themonthly P OH could also decrease 3 %–5 %, but hourly values can decreaseover 30 % in the daytime. Analysis of the individual reactions using theintegrated reaction rate shows that the net change in P OH is controlledby the competition of three main pathways (OH from O 3 photolysis, OHfrom HO 2 conversion, and OH from HO X , X =Cl , Br, I) through differenthalogen species. Sea spray aerosol (SSA) and inorganic iodine gases are themajor species influencing the strengths of these three pathways andtherefore have the most significant impacts on P OH . Both of these twotypes of species decrease P OH through physical processes, whilegenerally increasing P OH through chemical processes. In the oceanatmosphere, inorganic iodine gases determine the basic pattern of Δ P OH through complicated iodine chemistry, which generally positivelyinfluences P OH near O 3 sources while negatively influencing it when O 3 experiences longer transport over the ocean. Over the continent, SSA is thecontrolling species, and the SSA extinction effect leads to the negative Δ P OH in southern China. Our results show that marine-emittedhalogen species have notable impacts over the ocean and potential impacts oncoastal atmospheric oxidation by species (SSA, inorganic iodine, andhalocarbons), processes (chemistry, radiation, and deposition), and mainpathways. The notable impacts of the marine-emitted halogen species on theatmospheric oxidation capacity have further implications for the lifetime oflong-lived species such as CH 4 in the long term and the quantity of airpollutants such as O 3 in the episodic events in East Asia and inother circumstances (e.g., different domains, regions, and emission rates).

[学科分类] 医学（综合）