Measurement report: The variation properties of aerosol hygroscopic growth related to chemical composition during new particle formation days in a coastal city of Southeast China

Abstract. The scattering of solar radiation by aerosol is significantly affected by relative humidity (RH) due to the aerosol hygroscopicity. In order to better understand the characteristics of aerosol scattering hygroscopic growth and its influencing factors during new particle formation (NPF) days, we conducted an in situ campaign from February to April 2022 in Xiamen, a coastal city in Southeast China. The aerosol scattering hygroscopic growth factor f(RH), commonly used to describe the aerosol indirectly hygroscopicity, varies greatly due to the influence of aerosol chemical composition and size. In the relatively clean atmosphere of Xiamen, NPF events occur frequently, and the variation in chemical composition during events has a substantial influence on the aerosol scattering hygroscopic growth. In this study, we investigated the features and influencing factors of f(RH) on the NPF days. The research results emphasized that f(RH) differed significantly between NPF and non-NPF days, mainly impacted by the aerosol chemical compositions, especially sulfate and nitrate. On NPF days, sulfate was the dominant contributor to f(RH), distinguishing it from the non-NPF days. Aerosol hygroscopicity–chemical composition closure demonstrated that NH4HSO4 was the main driving force (30.78 %) of the hygroscopicity parameter κf(RH) when NPF events happened, while NH4NO3 played a dominant role in κf(RH) (up to 35 %) for non-NPF days. Although the uncertainty of the organic aerosol (OA) to hygroscopicity might exist due to the varieties of chemical components and oxidation level, it was the crucial driving factor for the variation in aerosol hygroscopicity. The findings of this study would be helpful for the further understanding of the properties of aerosol hygroscopicity in the coastal area, and it would complement the hygroscopic growth factors in the models of air quality and climate change.