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.

Sources and Formation Processes of Short-Chain Saturated Diacids (C2–C4) in Inhalable Particles (PM10) from Huangshi City, Central China

“…Higher values were found in winter (January, February, March) for C 2 (Figures 3a and 4a) and C 4 (Figures 3c and 4c), suggesting an enhanced emission potentially from coal/biomass combustion. In contrast, higher values were observed in summer (July, August, September) for C 3 diacid (Figures 3b and 4b), which is consistent with an enhanced secondary production due to higher oxidant concentrations and warmer temperatures [10,31]. The higher values for the contribution of WSOC to OC were found in summer (July, August, September) (Figure 4d), which further implies an enhanced secondary photochemical production, although it should also exist in other diacids and the related compounds except C 2 -C 4 diacids.…”

“…The mean concentration of C 4 diacid in HS (2.09 ± 0.52 ng·m −3 ) was lower than all those in other urban aerosols ( Figure 1). It has been proposed that C 4 diacid can be degraded to C 3 and C 2 diacids due to the abstraction of its hydrogen by OH radicals and subsequent decarboxylation reactions [10,31]. Therefore, such low concentrations of C 4 diacid suggest that HS aerosols should be more photochemically aged than at other urban sites.…”

“…If the physical parameters such as lower mixing heights, surface inversion layers, and infrequent precipitations were the causes for the maximum concentrations, the concentrations of diacids should indicate the same seasonal trend [7]. Thus, the different seasonal trends between C 2 or C 4 and C 3 diacid in HS aerosols should be caused by the difference in the emission sources or photochemical processing [10,31]. It had been reported that organic precursors that produce the compounds that showed maximum concentrations in winter are mainly aromatics and glyoxal (Gly), which can lead to the formation of C 2 diacid [7,13].…”

“…Analytical errors in duplicate analysis were less than 12%. The detailed analytical procedure for WSOC was identical with those reported in Kunwar and Kawamura [31] and Hoque [32]. Water-soluble inorganic ions (WSIIs) were analyzed by an ion chromatograph (Dionex Inc. Sunnyvale, CA, USA).…”

“…A known area of filter (with the diameter of 8 mm) was extracted under ultrasonication with organic-free Milli-Q water (10 mL × 3). The water extract was adjusted to pH = 8.5-9.0 and concentrated to dryness by a rotary evaporator [31]. The dried sample was then reacted with 14% boron trifluoride (BF 3 ) in n-butanol at 100 • C for 1 h. During this procedure, carboxyl functional groups were derived to butyl ester [30].…”