Hygroscopicity of Water-Soluble PM_2.5 in Rural Northwest China: Contrasting Contributors Between Summer and Winter

In this study, we investigated the chemical composition and hygroscopicity of water-soluble fraction in PM_2.5 collected from a rural site of Guanzhong Basin, a highly polluted region in northwest China. We found that inorganic components are the main substances promoting the hygroscopic properties of particulate matter (PH) in summer, while in winter, an increasing contribution from water-soluble organic matters (WSOM) was observed. The sulfate-nitrate-ammonium concentration is higher on heavily polluted days, and a high concentration of water-soluble organic matter was observed in non-polluting days. Hygroscopic growth factors, g(RH), with elevated relative humidity (RH) of water-soluble matter (WSM) were measured for 50, 75, 100, 125, 150 nm by hygroscopic tandem differential mobility analyzer. An initial dry particle diameter of 100 nm was chosen as the research object because the particle size has little effect on the hygroscopic growth factor at RH = 90%. The g(90%)_WSM and κ_WSM was in the range of 1.19–1.49 and 0.06–0.22 in summer, 1.24–1.49 and 0.08–0.22 in winter, respectively, which is similar to the hygroscopicity parameters of PH produced by biomass burning. The g(90%)_WSOM and κ_WSOM, obtained using E-AIM model and Zdanovskii-Stokes-Robinson approach, were in the range of 1.06–1.69 and 0.02–0.36 in summer, 1.06–1.58 and 0.02–0.28 in winter, respectively. The hygroscopicity parameters of water-soluble organic matter is close to the value of levoglucosan, an indicator of biomass burning, implying this region is highly affected by biomass burning. Overall, our results revealed that PM_2.5 in rural regions of Guanzhong Basin is mainly influenced by biomass burning.