Significant impact of water-soluble organic matter on hygroscopicity of fine particles under low relative humidity condition

Uncertainties in estimating the hygroscopicity of bulk aerosols under conditions of low relative humidity (RH) or below the deliquescent RH (DRH) of aerosols remain to be significant, mainly due to the presence of water-soluble organic matter (WSOM). To quantify the contributions of WSOM to aerosol hygroscopicity and associated uncertainties, a field campaign was conducted to measure the hygroscopic growth curve (f(RH)) of bulk aerosols online, dominant chemical compositions in PM2.5 online and offline, and size distributions of the dominant chemical compositions offline during the dry and wet seasons of 2019-2020 in urban Guangzhou of south China. Based on the measured f(RH), the hygroscopicity parameter (kappa) of bulk aerosols (kappa(-f(RH))) exhibits a logarithmic increase with increasing RH until RH reaches 69 %. Beyond this threshold, kappa(-f(RH)) increases very slowly with further increase of RH, reaching 0.32 +/- 0.04 during the dry season and 0.31 +/- 0.05 during the wet season. The kappa of WSOM (kappa(-WSOM)) was further estimated to be 0.22 +/- 0.03 and 0.13 +/- 0.04 in the dry and wet seasons, respectively, when RH > 69 %. WSOM significantly affects kappa(-f(RH)) by retarding the deliquescence process of aerosols and altering the mass ratio of water-soluble inorganic salts (WSIS) to WSOM within the size range of 0.4-0.9 mu m, especially under low RH conditions (<60 %). The kappa(-f(RH)) under low RH conditions was revised based on the logarithmic regression equation between RH and the ratio of measured kappa(-f(RH)) to estimated kappa(-f(RH>69%)). f(RH) of WSIS and WSOM were then corrected using the revised kappa(-f(RH)) under low RH conditions, which showed 22-31 % lower values than those produced by the IMPROVE formulas.