Measurement report: On the difference of aerosol hygroscopicity between high and low RH conditions in the North China Plain

. Atmospheric processes, including both primary emissions and secondary formation, may exert complex effects on aerosol hygroscopicity, which is of significant importance in understanding and quantifying the effect of aerosols on climate and human health. In order to explore the influence of local secondary formation processes on hygroscopicity, we investigated the hygroscopic properties of This was conducted by simultaneous measurements of aerosol hygroscopicity and chemical composition, using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM). The hygroscopicity results showed that the particles during the entire campaign were mainly externally mixed, with a more hygroscopic (MH) mode and a less hygroscopic (LH) mode. The mean hygroscopicity parameter values 35 ( κ mean ) derived from hygroscopicity measurements for particles at 60, 100, 150, and 200 nm were 0.16, 0.18, 0.16, and 0.15, respectively. During this study, we classified two distinct episodes with different RH/ T conditions, indicative of different primary emissions and secondary formation processes. It was observed that aerosols at all measured sizes were more hygroscopic under the high RH (HRH) episode than those under the low RH (LRH) episode. During the LRH, κ decreased with increasing particle size, 40 which may be explained by the enhanced domestic heating at low temperature, causing large emissions of non- or less-hygroscopic primary aerosols. This is particularly obvious for 200 nm particles, with a dominant number fraction (> 50 %) of LH mode particles. Using O : C-dependent hygroscopic parameters of secondary organic compounds ( κ SOA ), closure analysis between the HTDMA measured κ and the ACSM derived κ was carried out. The results showed that κ SOA under the LRH episode was less 45 sensitive to the changes in organic oxidation level, while κ SOA under the HRH had a relatively stronger dependency on the organic O : C. This feature suggests that the different sources and aerosol evolution processes, partly resulting from the variation in atmospheric RH/ T conditions, may lead to significant changes in aerosol chemical composition, which will further influence their corresponding physical properties. O : C-dependency. Based on these findings, we concluded that intricate atmospheric processes/emission sources could exert significant influence on the chemical composition of atmospheric aerosols, leading to synthetic variation of aerosol hygroscopicity. The variability of aerosol hygroscopicity, on the other 585 hand, may in turn benefit our understanding of the formation and evolution of complex atmospheric processes.