Measurement report: On the difference in aerosol hygroscopicity between high and low relative humidity 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 emissions and secondary formation processes on aerosol hygroscopicity, we investigated the hygroscopic properties of submicron aerosol particles at a rural site in the North China Plain (NCP) in winter 2018. This was conducted by simultaneous measurements of aerosol hygroscopicity and chemical composition, using a custom-built 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 (kappa(mean)) values 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 relative humidity (RH) and temperature (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, kappa decreased with increasing particle size, which may be explained by the enhanced domestic heating at low temperature, causing large emissions of non-hygroscopic 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 (kappa(SOA)), closure analysis between the HTDMA-measured kappa and the ACSM-derived kappa was carried out. The results showed that kappa(SOA )under the LRH episode was less sensitive to the changes in organic oxidation level, while kappa(SOA) under HRH had a relatively stronger dependency on the organic O : C ratio. This feature suggests that the different sources and aerosol evolution processes, partly resulting from the variation in atmospheric RH and T conditions, may lead to significant changes in aerosol chemical composition, which will further influence their corresponding physical properties.