Aerosol hygroscopicity derived from size-segregated chemical composition and its parameterization in the North China Plain

Hygroscopic growth of aerosol particles is of significant importance in quantifying the aerosol radiative effect in the atmosphere. In this study, hygroscopic properties of ambient particles are investigated based on particle chemical composition at a suburban site in the North China Plain during the HaChi campaign (Haze in China) in summer 2009. The size-segregated aerosol particulate mass concentration as well as the particle components such as inorganic ions, organic carbon and water-soluble organic carbon (WSOC) are identified from aerosol particle samples collected with a ten-stage impactor. An iterative algorithm is developed to evaluate the hygroscopicity parameter kappa from the measured chemical composition of particles. During the HaChi summer campaign, almost half of the mass concentration of particles between 150 nm and 1 mu m is contributed by inorganic species. Organic matter (OM) is abundant in ultrafine particles, and 77% of the particulate mass with diameter (D-p) of around 30 nm is composed of OM. A large fraction of coarse particle mass is undetermined and is assumed to be insoluble mineral dust and liquid water. The campaign's average size distribution of kappa values shows three distinct modes: a less hygroscopic mode (D-p < 150 nm) with kappa slightly above 0.2, a highly hygroscopic mode (150 nm< Dp < 1 mu m) with kappa greater than 0.3 and a nearly hydrophobic mode (D-p > 1 mu m) with kappa of about 0.1. The peak of the kappa curve appears around 450 nm with a maximum value of 0.35. The derived kappa values are consistent with results measured with a high humidity tandem differential mobility analyzer within the size range of 50-250 nm. Inorganics are the predominant species contributing to particle hygroscopicity, especially for particles between 150 nm and 1 mu m. For example, NH4NO3, H2SO4, NH4HSO4 and (NH4) 2SO4 account for nearly 90% of kappa for particles of around 900 nm. For ultrafine particles, WSOC plays a critical role in particle hygroscopicity due to the predominant mass fraction of OM in ultrafine particles. WSOC for particles of around 30 nm contribute 52% of kappa. Aerosol hygroscopicity is related to synoptic transport patterns. When southerly wind dominates, particles are more hygroscopic; when northerly wind dominates, particles are less hygroscopic. Aerosol hygroscopicity also has a diurnal variation, which can be explained by the diurnal evolution of planetary boundary layer, photochemical aging processes during daytime and enhanced black carbon emission at night. kappa is highly correlated with mass fractions of SO4 2-, NO3-and NH4+ for all sampled particles as well as with the mass fraction of WSOC for particles of less than 100 nm. A parameterization scheme for kappa is developed using mass fractions of SO42-, NO3-, NH4+ and WSOC due to their high correlations with kappa, and kappa calculated from the parameterization agrees well with kappa derived from the particle's chemical composition. Further analysis shows that the parameterization scheme is applicable to other aerosol studies in China.