Haze Occurrence Caused by High Gas-to-Particle Conversion in Moisture Air under Low Pollutant Emission in a Megacity of China

Haze occurred in Zhengzhou, a megacity in the northern China, with the PM 2.5 as high as 254 μg m −3 on 25 December 2019, despite the emergency response measure of restriction on the emission of anthropogenic pollutants which was implemented on December 19 for suppressing local air pollution. Air pollutant concentrations, chemical compositions, and the origins of particulate matter with aerodynamic diameter smaller than 2.5 µm (PM 2.5 ) between 5–26 December were investigated to explore the reasons for the haze occurrence. Results show that the haze was caused by efficient SO 2 -to-suflate and NO x -to-nitrate conversions under high relative humidity (RH) condition. In comparison with the period before the restriction (5–18 December) when the PM 2.5 was low, the concentration of PM 2.5 during the haze (19–26 December) was 173 µg m −3 on average with 51% contributed by sulfate (31 µg m −3 ) and nitrate (57 µg m −3 ). The conversions of SO 2 -to-sulfate and NO x -to-nitrate efficiently produced sulfate and nitrate although the concentration of the two precursor gases SO 2 and NO x was low. The high RH, which was more than 70% and the consequence of artificial water-vapor spreading in the urban air for reducing air pollutants, was the key factor causing the conversion rates to be enlarged in the constriction period. In addition, the last 48 h movement of the air parcels on 19–26 December was stagnant, and the air mass was from surrounding areas within 200 km, indicating weather conditions favoring the accumulation of locally-originated pollutants. Although emergency response measures were implemented, high gas-to-particle conversions in stagnant and moisture circumstances can still cause severe haze in urban air. Since the artificial water-vapor spreading in the urban air was one of the reasons for the high RH, it is likely that the spreading had unexpected side effects in some certain circumstances and needs to be taken into consideration in future studies.