Abstract
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 (WSOCHPI) included low‐molecular aliphatic dicarboxylic acids and carbonyls, saccharides, and amines, while hydrophobic WSOC (WSOCHPO) 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 WSOCHPO accounted for 60% of TWSOC. Both WSOCHPI and WSOCHPO increased with photochemical aging determined from the NOx/NOy ratio. In particular, the average WSOCHPO mass was found to increase by a factor of five within a timescale of ∼10 hours, which was substantially larger than that of WSOCHPI (by a factor of 2–3). The total increase in OC mass with photochemical aging was associated with the large increase in WSOCHPO 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.
Highlights
[2] Water-soluble organic carbon (WSOC) can significantly alter the hygroscopicity of aerosols and can be important in determining the ability of particles to act as cloud condensa
This concentration range is similar to that observed for ambient air sampled by the particle-into-liquid sampler (PILS) for at least the standards examined in this study
An average ratio of organic matter (OM) measured by the aerosol mass spectrometer (AMS) to OC during the second half of the period (OM/OC = 1.88) [Takegawa et al, 2009a] was similar to that found during the first half (OM/OC = 1.79), even though the size cut of OM was the same throughout the period. These results indicate that the observed total WSOC (TWSOC) and OC were dominantly in the PM1 mode during the measurement period
Summary
[2] Water-soluble organic carbon (WSOC) can significantly alter the hygroscopicity of aerosols and can be important in determining the ability of particles to act as cloud condensa-. An important class of hydrophobic WSOC is humic-like substances (HULIS) These are a mixture of high-molecular-weight compounds that are strong surfactants [Decesari et al, 2000], which could decrease surface tension of aerosol and fog water [Facchini et al, 2000; Kiss et al, 2005]. Previous studies on the chemical analysis of organic compounds in the PRD region have been made primarily at urban sites, in Hong Kong and Guangzhou [e.g., Zheng et al, 1997; Feng et al, 2006] Most of these studies identified solventextractable OC, including n-alkanes, polycyclic aromatic hydrocarbons, n-fatty acids, n-alkanols and dicarboxylic acids, mainly from the viewpoint of source identification. The dependence of the fractionated WSOC on photochemical aging is discussed
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