Abstract
Abstract. We calculated the organic matter to organic carbon mass ratios (OM/OC mass ratios) in PM2.5 collected from 14 Chinese cities during summer and winter of 2003 and analyzed the causes for their seasonal and spatial variability. The OM/OC mass ratios were calculated two ways. Using a mass balance method, the calculated OM/OC mass ratios averaged 1.92 ± 0.39 year-round, with no significant seasonal or spatial variation. The second calculation was based on chemical species analyses of the organic compounds extracted from the PM2.5 samples using dichloromethane/methanol and water. The calculated OM/OC mass ratio in summer was relatively high (1.75 ± 0.13) and spatially-invariant due to vigorous photochemistry and secondary organic aerosol (OA) production throughout the country. The calculated OM/OC mass ratio in winter (1.59 ± 0.18) was significantly lower than that in summer, with lower values in northern cities (1.51 ± 0.07) than in southern cities (1.65 ± 0.15). This likely reflects the wider usage of coal for heating purposes in northern China in winter, in contrast to the larger contributions from biofuel and biomass burning in southern China in winter. On average, organic matter constituted 36% and 34% of Chinese urban PM2.5 mass in summer and winter, respectively. We report, for the first time, a high regional correlation between Zn and oxalic acid in Chinese urban aerosols in summer. This is consistent with the formation of stable Zn oxalate complex in the aerosol phase previously proposed by Furukawa and Takahashi (2011). We found that many other dicarboxylic acids were also highly correlated with Zn in the summer Chinese urban aerosol samples, suggesting that they may also form stable organic complexes with Zn. Such formation may have profound implications for the atmospheric abundance and hygroscopic properties of aerosol dicarboxylic acids.
Highlights
1.92 ± 0.39 year-round, with no significant seasonal or spa- form stable organic complexes with Zn
We analyzed the organic matter (OM)/OC mass ratios in PM2.5 collected from 14 cities throughout China during winter and summer of 2003
We examined the organic species driving the spatiotemporal variability of the OM/OC mass ratios and discussed the implications for China urban organic aerosol (OA) sources
Summary
We first calculated OM/OC mass ratios using the mass balance method, where the OM mass was estimated as the difference between the total PM2.5 mass and the mass sum of measured inorganic species: OM/OC =. The 14city-mean OM/OC mass ratio was 1.94 ± 0.51 in summer, not significantly different from that in winter (1.91 ± 0.25) (paired t test, p value > 0.8). Adopting the mean estimates for the fractions of evaporated nitrate and water-insoluble transient metals, we estimated that the uncertainty in the OM/OC mass ratios calculated by the mass balance method to be 55 % in summer and 12 % in winter
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