Seasonal distributions and sources of low molecular weight dicarboxylic acids, ω-oxocarboxylic acids, pyruvic acid, α-dicarbonyls and fatty acids in ambient aerosols from subtropical Okinawa in the western Pacific Rim

Abstract

Environmental context Water-soluble dicarboxylic acids and related compounds are ubiquitous in atmospheric aerosols. They are abundantly emitted from Asian countries and transported to the Pacific Ocean. During the long-range transport, photochemical processing modifies organic aerosols. We conducted a 1-year observation of diacids and related compounds at Okinawa Island, an outflow region of the Asian Continent, to clarify their sources and photochemical aging. Abstract Ambient aerosol samples were collected for 1 year at Okinawa Island, Japan, and were analysed for water-soluble dicarboxylic acids, oxoacids, α-dicarbonyls and fatty acids to better understand biogenic v. anthropogenic sources and the formation–transformation of organic aerosols during long-range atmospheric transport. Here, we report seasonal variations of diacids and related compounds in Okinawa. We found a predominance of oxalic acid (C2) followed by malonic (C3) and succinic (C4) acid. Total diacids and oxoacids maximised in spring when air masses originated from the Asian Continent with westerly winds. In contrast, phthalic acid (Ph), a tracer of anthropogenic sources, peaked in winter. We found an increased C3/C4 ratio in summer, suggesting an enhanced photochemical aging of organic aerosols. The average ratio of total diacid-C/total carbon (TC) (5.4%) is higher than that (3.1%) from the East China Sea, suggesting that Okinawa aerosols are more aged than East Asian aerosols but less aged compared to those from the remote Pacific including tropics (8.8%). Biogenic short-chain fatty acids and azelaic acid (C9), the latter is a specific oxidation product of unsaturated fatty acids, maximised in summer, whereas higher plant-derived long-chain fatty acids maximised in spring. This study demonstrates that the ambient aerosols in Okinawa are strongly influenced by the Asian outflow in winter and spring and by biogenic organic matter in summer and spring. Enhanced contribution of oxalic acid to aerosol TC in spring suggests that Okinawa organic aerosols are mainly produced in East Asia and photochemically transformed during the transport.