Seasonal and spatial variability of the OM/OC mass ratios and high regional correlation between oxalic acid and zinc in Chinese urban organic aerosols

Abstract

We calculated the organic matter to organic carbon mass ratios (OM/OC mass ratios) in PM_2.5 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 PM_2.5 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 PM_2.5 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

Chemical Composition and Light Extinction Contribution of PM2.5 in Urban Beijing for a 1-Year Period

“…OM was derived from multiplying OC concentrations by a factor of 1.6 to account for unmeasured atoms, such as hydrogen, oxygen, and nitrogen in organic materials. The factor of 1.6 was employed in this study according to Xing et al (2013), which demonstrated that the calculated OM/OC mass ratio in summer was relatively high (1.75 ± 0.13) and in winter was lower (1.59 ± 0.18) in PM 2.5 collected from 14 Chinese cities.…”

Chemical Composition of PM2.5 Based on Two-Year Measurements at an Urban Site in Beijing

“…It was reported that mineral elements contribute less (0.5%) to PM 2.5 (Zhang et al, 2012b). The ratio of organic matter (OM) to organic carbon (OM: OC) could vary from 1.4 to 2.0 in Beijing (Xing et al, 2013). Provided that OM equal to twice of OC, 13%, 19% and 20% of PM 2.5 could be defined as non-C atoms organic matters.…”

Modelling organic aerosol concentrations and properties during ChArMEx summer campaigns of 2012 and 2013 in the western Mediterranean region

“…Hence, a high OM / OC ratio indicates a high degree of oxidation of the organic aerosols and probably a high degree of hygroscopicity (Jimenez et al, 2009). There is a variety of methods that have been used to calculate OM / OC ratio as reported by Xing et al (2013). In our case, the ambient OM / OC ratio is calculated by weighting the ratio (OM/OC) i of each surrogate species i by the relative mass of the surrogate:…”