Possible sources of two size-resolved water-soluble organic carbon fractions at a roadway site during fall season

To examine the formation pathways of two size-resolved water-soluble organic carbon (WSOC) fractions, a total of 16 sets of size-segregated aerosol samples were collected using a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) at a roadway site in Korea from September 29 to October 29, 2010. A XAD7HP (non-ionic aliphatic acrylic polymer) resin column was used to separate the filtered extracts into hydrophilic (WSOCHPI) and hydrophobic (WSOCHPO) WSOC fractions. Also the size distributions of water-soluble inorganic species and oxalate were examined to determine the formation pathways of size-resolved WSOCHPI and WSOCHPO fractions. The size distribution of WSOCHPI showed a dominant mode at 0.55 μm, while the WSOCHPO had dominant modes at both 0.17–0.32 μm and 0.55 μm. On the basis of the size distribution characteristics, it was found that the formation pathways of both WSOCHPI and WSOCHPO were dependent on the particle size; in the condensation mode (0.17–0.32 μm), both the WSOCHPI and WSOCHPO could be produced through atmospheric processes similar to those of SO42− and oxalate, which were derived from the gas-phase oxidations of organic compounds. In the droplet mode (0.55–1.8 μm), the cloud processing of both the organic compounds and biomass burning emissions could be a major pathway for the WSOCHPI formation. However, the droplet mode WSOCHPO was likely produced through cloud processing and heterogeneous reactions or aerosol-phase reactions. In the coarse mode (>3.1 μm), the WSOCHPI formation was more likely associated with soil-related particles (Ca(NO3)2 and CaSO4) than with sea-salt particles (NaNO3 and Na2SO4). •Size-resolved measurements of two water-soluble organic carbon (WSOC) fractions.•Hydrophilic WSOC (WSOCHPI) showed dominant modes at 0.55 μm.•Hydrophobic WSOC (WSOCHPO) peaked at both 0.17–0.32 μm and 0.55 μm.•Droplet mode WSOCHPI was likely attributed to the cloud processing and biomass burning emissions.•Droplet mode WSOCHPO was likely produced through cloud processing and heterogeneous reactions.