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 PM 2.5 pollution. The COVID-19 lockdown reduced PM 2.5 and increased visibility in Wuhan. Compared to pre-lockdown period, the PM 2.5 concentration decreased by 39.0 μg m −3 , dominated by NH 4 NO 3 mass reduction (24.8 μg m −3 ) during lockdown period. The PM 2.5 threshold corresponding to visibility of 10 km (PTV 10 ) varied in 54–175 μg m −3 and an hourly PM 2.5 of 54 μg m −3 was recommended to prevent haze occurrence. The lockdown measures elevated PTV 10 by 9–58 μg m −3 as the decreases in PM 2.5 mass scattering efficiency and optical hygroscopicity. The visibility increased by 107%, resulted from NH 4 NO 3 extinction reduction. The NH 4 NO 3 mass reduction weakened its mutual promotion with aerosol water and increased PM 2.5 deliquescence humidity. Controlling TNO 3 (HNO 3 + NO 3 − ) was more effective to reduce PM 2.5 and improve visibility than NH x (NH 3 + NH 4 + ) unless the NH x reduction exceeded 11.7–17.5 μg m −3 .