Carbon isotope seasonal characteristics of fine carbonaceous aerosol in Jinzhong City, Shanxi Province, China

Abstract

Stable carbon isotope signatures were observed in organic carbon (OC) and elemental carbon (EC) fractions of fine carbonaceous aerosol (PM2.5) sampled in spring, summer, and winter (December 2013 to August 2014) in Jinzhong in Shanxi Province. The stable carbon isotope composition of EC (δ13CEC) varied from −25.93‰ to −22.20‰ during the sampling (mean value of −24.35 ± 1.11‰), with depletion in summer (−25.44 ± 0.32‰) and enrichment in winter (−23.04 ± 0.71‰). The contribution of vehicle exhaust was 65% in summer, and the contribution of fossil fuel combustion was 92% in winter based on carbon isotope mass balance model. The stable carbon isotope compositions of OC (δ13COC) over the whole sampling period ranged from −29.75‰ to −22.87‰ (mean value of −25.10 ± 1.47‰), with enrichment in winter (−24.12 ± 0.88‰) and depletion in spring (−26.40 ± 1.50‰). Compared with δ13CEC, the δ13COC in spring is depleted in 13C (Δ13COC-EC = δ13COC − δ13CEC = −1.55‰) because of the formation of secondary OC (SOC) from the photo-oxidation reactions. Summertime δ13COC (−24.52 ± 0.28‰) is more positive than δ13CEC because of the strong photochemical aging. The negative value of Δ13COC-EC in winter may be attributed to a complex set of reasons, including the unfavorable meteorological conditions (e.g., low temperature, low boundary layer height, and frequent temperature inversions), coal combustion, vehicle exhaust, gaseous fuel burning, and SOC formation from photo-oxidation reactions of volatile organic compounds (VOCs) caused by fossil fuel combustion. The seasonal variation of δ13CTC was consistent with that of δ13COC, and the annual average δ13CTC (−24.80 ± 1.12‰) implicated fossil fuels combustion. This study highlights that the isotope signatures of EC can be used for source apportionment, reveals the strong influence of atmospheric processing on the isotope signature of OC (e.g., SOC formation and photochemical aging), and suggests the potential application of isotope technology in air pollution research.