Hygroscopicity Of The Ambient Submicron Particles In Urban Shanghai, Hangzhou

Aerosols are continuously interacting with the ambient water vapor of the surrounding air. Understanding the hygroscopic properties is of vital importance as the water absorbed on aerosols influences the actual size, chemical composition, and subsequently the aqueous phase chemistry of particles. Furthermore, the optical properties and the cloud condensation nuclei (CCN) activity of aerosols can be greatly altered by hygroscopic growth of particles.In this study, hygroscopicity of size-segregated ambient submicron particles was studied in Shanghai during 2009 autumn and in Hangzhou during 2009 winter, using a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA).The submicron particles exhibited most frequently a trimodal growth factor (GF) distribution under 90% relative humidity (RH) in Pudong, Shanghai, composed of the less hygroscopic group (GF<1.2), the hygroscopic group (1.2<GF<1.4) and the more hygroscopic group (GF>1.4). In contrast, bimodal distribution was predominant under 85% RH during our Yangpu campaign.The GFs for 30,50,80,100,130,150,180, and 200 nm particles were measured at varied RH in our Hangzhou campaign. The submicron particles showed a minor hygroscopic growth at 73% RH, and then grew significantly after the RH reached 82%. Bimodal distribution was also predominant under 82% RH.Deconvolution of the bimodal distribution indicated a less hygroscopic group and a more hygroscopic group, with the fraction of the more hygroscopic group increasing with the initial dry particle size and then remaining almost constant for accumulation mode particles.Our results implied the hygroscopic properties of ambient particles were mainly a function of their size and chemical composition. Ammonium sulfate and ammonium nitrate are the main hygroscopic species in submicron particles. Long range transportation, local emission especially vehicle exhaustion, day time photochemical oxidation and night time ammonium nitrate formation altered the chemical composition of particles and contributed to the diurnal variation of GFs.