In this study, we investigated the chemical composition and hygroscopicity of water-soluble fraction in PM2.5 collected from a rural site of Guanzhong Basin, a highly polluted region in northwest China. Hygroscopic growth factors, g(RH), of water-soluble matter(WSM) were measured by hygroscopic tandem differential mobility analyzer(H-TDMA) with an initial dry particle diameter of 100 nm. The g(90)WSM and κWSM was in the range of 1.08~1.49(1.35{plus minus}0.10) and 0.04~0.29(0.19{plus minus}0.06) in summer, 1.24~1.45(1.36{plus minus}0.07) and 0.12~0.26(0.20{plus minus}0.04) in winter, respectively. We found that increased nitrate concentration at night in summer suppressed 60-70% of the deliquescent point, and increased g(RH) at elevated relative humidity, compared to daytime. Secondary inorganic ions were the main components in heavy haze day, and greatly contributed to the hygroscopicity of particles. In contrast, more potassium compound and WSOM existed during Chinese Spring Festival event but exhibited no deliquescence point in the process of hygroscopic growth with the elevated RH. The g(90)WSOM and κWSOM, obtained using ZSR model, were in the range of 1.06~1.52(1.25{plus minus}0.14) and 0.024~0.32(0.13{plus minus}0.09) in summer, 1.06~1.58(1.38{plus minus}0.15) and 0.02~0.38(0.22{plus minus}0.10) in winter, respectively. The mean g(90)WSOM was in the range of that of biomass burning aerosols, and a good correlation (R=0.71) was found between g(90)WSOM and levoglucosan, confirming that the aerosol’s hygroscopicity were highly influenced by biomass burning in winter. Briefly, it is revealed that the aerosol in rural regions of Guanzhong Basin is mainly influenced by biomass burning based on the hygroscopicity in winter and summer.

In this study, particle number size distribution (PNSD) and concentration of cloud condensation nuclei (NCCN) were observed at the summit of Mt. Hua during Dec. 16th 2020 - Jan. 23rd 2021. The concentration of nucleation mode particles with the growth ratio of 0.83 nm·h−1 erupted frequently from 13:00 to 18:00 local time due to the intense photochemistry. The explosive increase of the small diameter particles could not be activated into droplets, but they had the potential to adsorb or absorb the polluted gaseous to promote the growth conversion into CCN. In the growth process, relative humidity had reverse effect on diameter and concentration with temperature, while higher wind speed was beneficial for the removal of large particles. SO2 and NH3 had a synergistic effect in contributing to the increase of particle diameter and concentration for the relative larger diameter, but NH3 had inverse effect on the number concentration for the nucleation mode particles. Additionally, the influence of O3 on the distribution characteristics of particle may be regulated by temperature. The small k value that fitted by two-parameter power model suggested the larger or hygroscopic particles, while larger value for the ultrafine or hydrophobic particles. Hygroscopicity parameters (κ) showed a downward trend with the increase of supersaturation as the source direction of the air mass gradually shifted from northwest to southwest. By comparing the calculated and the measured NCCN, we inferred that the hygroscopicity parameter decreased from 0.22 to 0.13 with the supersaturation from 0.2 to 1.0% in Alpine region.