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

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 (P-OH ) to marine halogen emissions. The monthly Poll 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 P-OH 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 P-OH is controlled by the competition of three main pathways (OH from O-3 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 P-OH. Both of these two types of species decrease Poll through physical processes, while generally increasing P-OH through chemical processes. In the ocean atmosphere, inorganic iodine gases determine the basic pattern of Delta P-OH through complicated iodine chemistry, which generally positively influences Poll near O-3 sources while negatively influencing it when O-3 experiences longer transport over the ocean. Over the continent, SSA is the controlling species, and the SSA extinction effect leads to the negative Delta P-OH in southern China. Our results show that marine-emitted halogen species have notable impacts over the ocean and potential impacts on coastal atmospheric oxidation by species (SSA, inorganic iodine, and halocarbons), processes (chemistry, radiation, and deposition), and main pathways. The notable impacts of the marine-emitted halogen species on the atmospheric oxidation capacity have further implications for the lifetime of long-lived species such as CH4 in the long term and the quantity of air pollutants such as O-3 in the episodic events in East Asia and in other circumstances (e.g., different domains, regions, and emission rates).

Automated summary

The relationships between oceanic emissions and air chemistry are complex and not fully understood. The impact of marine halogen emissions on the hydroxyl (OH) radical is crucial for regional air chemistry. This study aims to quantify the influence of marine-emitted halogens on OH and analyzes the pathways through which these halogens affect OH. Results show a complex response of OH production rate to marine halogen emissions, with both increases and decreases observed in different regions.