Abstract
Total suspended particle (TSP) samples were collected at Cape Hedo, Okinawa on a basis of 24, 12 and 3 h intervals in March to April 2007, when the Asian outflow is enhanced. The filter samples were analysed for dicarboxylic acids, oxoacids, benzoic acid and α-dicarbonyls to better understand the sources, chemical composition and photochemical ageing of organic aerosols during long-range transport. Their molecular distributions showed a predominance of oxalic acid (C2) followed by malonic (C3) and succinic (C4) acids. Day/night samples did not show clear diurnal variations, suggesting that local anthropogenic influence is not important. The highest concentrations of C2 and other diacid species were observed in April 10 when air masses were delivered from north China. Higher concentrations were observed when air masses arrived from East Asia and coastal China. The 3 h samples showed higher concentrations of oxalic acid at 16:30 or at 13:30 during April 14 and 16, whereas higher relative abundance of C2 in total diacids and higher concentration ratios (C2/C3, C2/C4, C3/C4, C2/Gly, and C2/ωC2) were observed at 7:30 during April 14–16. These results suggest that aerosols were accumulated in the atmosphere during night and photochemically modified early in the morning. Oxalic acid–C/OC did not show a prominent peak in 3-h samples. Temporal variations in the concentrations of diacids, oxoacids and α-dicarbonyls, relative abundances and diacid–C/OC ratios together with 5 day back trajectory analyses show that the aerosols from Cape Hedo are strongly influenced by long-range atmospheric transport from East Asia, especially in China. Strong correlations of C2 with nss-SO42− and NO3− in night-time suggest a secondary formation of water-soluble diacids via heterogeneous aqueous phase oxidation during long-range transport.
Highlights
Atmospheric aerosols contain high abundance of water-soluble dicarboxylic acids (Grosjean et al, 1978)
We detected a homologous series of α,ω-dicarboxylic acids (C2–C12), ω-oxocarboxylic acids, pyruvic acid, α-dicarbonyls and benzoic acid in the aerosol samples from Okinawa
Oxalic acid (C2) was found as the most abundant species followed by malonic (C3) and succinic (C4) acid, which is consistent with previous studies of coastal marine aerosols
Summary
Atmospheric aerosols contain high abundance of water-soluble dicarboxylic acids (Grosjean et al, 1978). There are few studies of diacids and related compounds in Okinawa and its surroundings (Kunwar and Kawamura, 2014a; Kundu et al, 2010; Deshmukh et al, 2016), there is no time series (e.g. 12 and 3 h) study to decipher the source and photochemical formation of organic acids The lack of such studies motivates us for collecting time-resolved samples in this region. We report molecular distributions of diacids, oxoacids, pyruvic acid, benzoic acid and α-dicarbonyls in ambient aerosols from Cape Hedo, Okinawa on a basis of daily (24 h) and diurnal intervals (day/night and 3 h) to better understand the sources of organic aerosols and in situ photochemical production of water-soluble organic species during long-range atmospheric transport. In the Asian Pacific region, the East Asian monsoon dominates in spring, delivering continental air masses from East Asian countries over Okinawa (Hatakeyama et al, 2001; Kunwar and Kawamura, 2014a, 2014b)
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