Co-benefits of reducing PM_2.5 and improving visibility by COVID-19 lockdown in Wuhan

The less improvement of ambient visibility suspects the government's efforts on alleviating PM2.5 pollution. The COVID-19 lockdown reduced PM2.5 and increased visibility in Wuhan. Compared to pre-lockdown period, the PM2.5 concentration decreased by 39.0 µg m−3, dominated by NH4NO3 mass reduction (24.8 µg m−3) during lockdown period. The PM2.5 threshold corresponding to visibility of 10 km (PTV10) varied in 54–175 µg m−3 and an hourly PM2.5 of 54 µg m−3 was recommended to prevent haze occurrence. The lockdown measures elevated PTV10 by 9–58 µg m−3 as the decreases in PM2.5 mass scattering efficiency and optical hygroscopicity. The visibility increased by 107%, resulted from NH4NO3 extinction reduction. The NH4NO3 mass reduction weakened its mutual promotion with aerosol water and increased PM2.5 deliquescence humidity. Controlling TNO3 (HNO3 + NO3−) was more effective to reduce PM2.5 and improve visibility than NHx (NH3 + NH4+) unless the NHx reduction exceeded 11.7–17.5 µg m−3.

Liquan Yao, Shaofei Kong, Huang Zheng, Nan Chen, Bo Zhu, Ke Xu, Wenxiang Cao, Ying Zhang, Mingming Zheng, Yi Cheng, Yao Hu, Zexuan Zhang, Yingying Yan, Dantong Li, Tianliang Zhao, Yongqing Bai, Shihua Qi