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Hygroscopic growth of aerosol particles is of significant importance inquantifying the aerosol radiative effect in the atmosphere. In this study,hygroscopic properties of ambient particles are investigated based onparticle chemical composition at a suburban site in the North China Plainduring the HaChi campaign (Haze in China) in summer 2009. Thesize-segregated aerosol particulate mass concentration as well as theparticle components such as inorganic ions, organic carbon and water-solubleorganic carbon (WSOC) are identified from aerosol particle samples collectedwith a ten-stage impactor. An iterative algorithm is developed to evaluatethe hygroscopicity parameter κ from the measured chemicalcomposition of particles. During the HaChi summer campaign, almost half of the massconcentration of particles between 150 nm and 1 μm is contributed byinorganic species. Organic matter (OM) is abundant in ultrafine particles,and 77% of the particulate mass with diameter (Dp) of around 30 nm iscomposed of OM. A large fraction of coarse particle mass is undetermined andis assumed to be insoluble mineral dust and liquid water. The campaign'saverage size distribution of κ values shows three distinct modes: aless hygroscopic mode (Dp < 150 nm) with κ slightly above0.2, a highly hygroscopic mode (150 nm < Dp < 1 μm)with κ greater than 0.3 and a nearly hydrophobic mode(Dp > 1 μm) with κ of about 0.1. The peak of theκ curve appears around 450 nm with a maximum value of 0.35. Thederived κ values are consistent with results measured with a highhumidity tandem differential mobility analyzer within the size range of 50–250 nm. Inorganics are the predominant species contributing to particlehygroscopicity, especially for particles between 150 nm and 1 μm. Forexample, NH4NO3, H2SO4, NH4HSO4 and(NH4)2SO4 account for nearly 90% of κ for particlesof around 900 nm. For ultrafine particles, WSOC plays a critical role inparticle hygroscopicity due to the predominant mass fraction of OM inultrafine particles. WSOC for particlesof around 30 nm contribute 52% of κ. Aerosol hygroscopicity is related to synoptic transportpatterns. When southerly wind dominates, particles are more hygroscopic;when northerly wind dominates, particles are less hygroscopic. Aerosolhygroscopicity also has a diurnal variation, which can be explained by thediurnal evolution of planetary boundary layer, photochemical aging processesduring daytime and enhanced black carbon emission at night. κ ishighly correlated with mass fractions of SO42−, NO3− andNH4+ for all sampled particles as well as with the mass fractionof WSOC for particles of less than 100 nm. A parameterization scheme forκ is developed using mass fractions of SO42−,NO3−, NH4+ and WSOC due to their high correlations withκ, and κ calculated from the parameterization agrees wellwith κ derived from the particle's chemical composition. Furtheranalysis shows that the parameterization scheme is applicable to otheraerosol studies in China

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