Chemical characterization of water‐soluble organic carbon aerosols at a rural site in the Pearl River Delta, China, in the summer of 2006

Online measurements of water‐soluble organic carbon (WSOC) aerosols were made using a particle‐into‐liquid sampler (PILS) combined with a total organic carbon (TOC) analyzer at a rural site in the Pearl River Delta region, China, in July 2006. A macroporous nonionic (DAX‐8) resin was used to quantify hydrophilic and hydrophobic WSOC, which are defined as the fractions of WSOC that penetrated through and retained on the DAX‐8 column, respectively. Laboratory calibrations showed that hydrophilic WSOC (WSOC HPI ) included low‐molecular aliphatic dicarboxylic acids and carbonyls, saccharides, and amines, while hydrophobic WSOC (WSOC HPO ) included longer‐chain aliphatic dicarboxylic acids and carbonyls, aromatic acids, phenols, organic nitrates, cyclic acids, and fulvic acids. On average, total WSOC (TWSOC) accounted for 60% of OC, and WSOC HPO accounted for 60% of TWSOC. Both WSOC HPI and WSOC HPO increased with photochemical aging determined from the NO x /NO y ratio. In particular, the average WSOC HPO mass was found to increase by a factor of five within a timescale of ∼10 hours, which was substantially larger than that of WSOC HPI (by a factor of 2–3). The total increase in OC mass with photochemical aging was associated with the large increase in WSOC HPO mass. These results, combined with the laboratory calibrations, suggest that significant amounts of hydrophobic organic compounds (likely containing large carbon numbers) were produced by photochemical processing. By contrast, water‐insoluble OC (WIOC) mass did not exhibit significant changes with photochemical aging, suggesting that chemical transformation of WIOC to WSOC was not a dominant process for the production of WSOC during the study period.