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

Abstract. Atmospheric processes, including both primary emissions and secondary formation, may exert complex effects on aerosol hygroscopicity, which is of significant importance in understanding quantifying the effect aerosols climate human health. In order to explore influence local formation processes we investigated hygroscopic properties submicron particles at a rural site North China Plain (NCP) winter 2018. This was conducted by simultaneous measurements hygroscopicity chemical composition, using custom-built tandem differential mobility analyzer (HTDMA) capture-vaporizer time-of-flight speciation monitor (CV-ToF-ACSM). The results showed that during entire campaign were mainly externally mixed, with more (MH) mode less (LH) mode. mean parameter (κmean) values derived from for 60, 100, 150, 200 nm 0.16, 0.18, 0.15, respectively. During this study, classified two distinct episodes different relative humidity (RH) temperature (T) conditions, indicative processes. It observed all measured sizes under high-RH (HRH) episode than those low-RH (LRH) episode. LRH, κ decreased increasing particle size, be explained enhanced domestic heating low temperature, causing large non-hygroscopic or aerosols. particularly obvious particles, dominant number fraction (>50 %) LH particles. Using O:C-dependent parameters organic compounds (κSOA), closure analysis between HTDMA-measured ACSM-derived carried out. κSOA LRH sensitive changes oxidation level, while HRH had relatively stronger dependency O:C ratio. feature suggests sources evolution partly resulting variation atmospheric RH T lead will further their corresponding physical properties.