Sources and Emissions of Marine‐Derived Halocarbons Over the Coastal and Open Ocean of the Northwest Pacific

Abstract The spatial variability and sources of atmospheric methyl halides (CH 3 Cl, CH 3 Br, and CH 3 I) over the northwest Pacific and its marginal seas—the East China Sea (ECS) and South China Sea (SCS)—were comprehensively characterized from 2017 to 2020. This study represents the first large‐scale investigation of these compounds across coastal and open‐ocean environments. Atmospheric CH 3 Cl and CH 3 Br concentrations were substantially elevated in the ECS and SCS, exceeding Northern Hemisphere background levels by 12%–28%, whereas concentration enhancements over the northwest Pacific were 3%–5%. Elevated CH 3 Cl and CH 3 Br concentrations were attributed to long‐range terrestrial atmospheric transport from China, Korea, Japan, and the Philippines, as well as localized phytoplankton‐driven production in nutrient‐rich coastal waters. In contrast, CH 3 I concentrations were primarily controlled by photochemical processes mediated by dissolved organic carbon (DOC), with elevated levels being observed in the oligotrophic subtropical northwest Pacific (10°–30°N). Multiple regression analyses revealed that CH 3 Cl and CH 3 Br distributions were mainly influenced by terrestrial transport in coastal areas, whereas oceanic processes dominated their distributions in the open ocean. CH 3 I distributions primarily reflected oceanic photochemical production, with marginal terrestrial influence limited to the ECS. The production rates of CH 3 Br and CH 3 Cl in the ECS and SCS, respectively, were 2–3 times higher than those in the northwest Pacific, consistent with enhanced biological activity. Sea‐to‐air flux estimates demonstrated that the northwest Pacific and its marginal seas acted as net sources of all three methyl halides, with fluxes generally exceeding global averages by approximately 1.5–48 times. These findings underscore the importance of the study area to the atmospheric methyl halides budget.