Ozone (O3) levels in East China suffered from rapid increases during the COVID-19 period. To clarify the reason for the O3 increase, a continuous campaign was performed in a industrial city in North China Plain (NCP). Meanwhile, the machine-learning technique and the box model were employed to reveal the mechanisms of O3 increase from the perspective of meteorology and photochemical process, respectively. The result suggested that the ambient O3 level in Tangshan increased from 18.7 ± 4.63 to 45.6 ± 8.52 μg/m3 (143%) after COVID-19 lockdown, and the emission reduction and meteorology contributed to 77% and 66% of this increment, respectively. The higher Wind speed (WS) coupled with regional transport played a significant role on O3 increase (30.8 kg/s). The O3 sensitivity verified that O3 production was highly volatile organic compounds (VOC)-sensitive (Relative incremental reactivity (RIR): 0.75), while the NOx showed the negative impact on O3 production in Tangshan (RIR: -0.59). It suggested that the control of VOCs rather than NOx might be more effective in reducing O3 level in Tangshan because it was located on the VOC-limited regime. Besides, both of ozone formation potential (OFP) analysis and observation-based model (OBM) demonstrated that the alkenes (36.3 ppb) and anthropogenic oxygenated volatile organic compounds (OVOCs) (15.2 ppb) showed the higher OFP compared with other species, and their reactions released a large number of HO2 and RO2 radicals. Moreover, the concentrations of these species did not experience marked decreases after COVID-19 lockdown, which were major contributors to O3 increase during this period.

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.