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. Atmospheric processes, including both primary emissions andsecondary formation, may exert complex effects on aerosol hygroscopicity,which is of significant importance in understanding and quantifying theeffect of aerosols on climate and human health. In order to explore theinfluence of local emissions and secondary formation processes on aerosolhygroscopicity, we investigated the hygroscopic properties of submicronaerosol particles at a rural site in the North China Plain (NCP) in winter2018. This was conducted by simultaneous measurements of aerosolhygroscopicity and chemical composition, using a custom-built hygroscopictandem differential mobility analyzer (HTDMA) and a capture-vaporizertime-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM). Thehygroscopicity results showed that the particles during the entire campaignwere mainly externally mixed, with a more hygroscopic (MH) mode and a lesshygroscopic (LH) mode. The mean hygroscopicity parameter (κmean) 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. Duringthis study, we classified two distinct episodes with different relative humidity (RH) and temperature (T) conditions, indicative of different primary emissions and secondaryformation processes. It was observed that aerosols at all measured sizeswere more hygroscopic under the high-RH (HRH) episode than those under thelow-RH (LRH) episode. During the LRH, κ decreased with increasingparticle size, which may be explained by the enhanced domestic heating atlow temperature, causing large emissions of non-hygroscopic or less hygroscopic primaryaerosols. This is particularly obvious for 200 nm particles, with a dominantnumber fraction (&gt;50 %) of LH mode particles. Using O:C-dependent hygroscopic parameters of secondary organic compounds (κSOA), closure analysis between the HTDMA-measured κ and theACSM-derived κ was carried out. The results showed that κSOA under the LRH episode was less sensitive to the changes in organicoxidation level, while κSOA under HRH had a relativelystronger dependency on the organic O:C ratio. This feature suggests that thedifferent sources and aerosol evolution processes, partly resulting from thevariation in atmospheric RH and T conditions, may lead to significant changes inaerosol chemical composition, which will further influence theircorresponding physical properties.

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