Methanol Extracted Brown Carbon in PM2.5 Over Xi’an, China: Seasonal Variation of Optical Properties and Sources Identification

Abstract

Aerosol optical properties, including absorption coefficient (babs), absorption Angstrom exponent (AAE), and mass absorption coefficient (MAC), of methanol extracts in PM2.5 samples were measured at an urban site in Xi’an, northwest China in a winter and summer season. Water soluble ions, organic carbon (OC), element carbon, eight carbon fractions, levoglucosan, and 13 polycyclic aromatic hydrocarbons were also determined to explore the sources of brown carbon (BrC) in PM2.5. babs at 365 nm wavelength showed a clear seasonal variation with much higher value in winter than summer. High levoglucosan and K+ concentrations in winter and their good correlations with babs inferred the important contribution of biomass burning to BrC. Good correlations between babs and OC1, OC2, OC3, or OP indicated the mixed sources of biomass burning and coal combustion for BrC. Relatively poor correlations were found between babs and indeno[1,2,3-cd]pyrene (IP) or Benzo[ghi]-perylene (BghiP), indicating limited contribution of vehicle exhaust to BrC. Seasonal distribution of AAE indicated that secondary organic carbon (SOC) BrC was mainly fresh SOC in winter but a mix of fresh and aged SOC in summer. MAC of BrC in winter was more than double of that in summer, suggesting that BrC from biomass burning and fresh SOC were stronger light absorbing in winter than the aged SOC in summer. A 7-wavelength Aethalometer model AE31 was used to evaluate the seasonal variations of babs for BrC (370 nm) and black carbon (BC) (880 nm). BrC and BC were both important in light absorption in winter, but BC prevailed over BrC in summer.