Double high pollution events in the Yangtze River Delta from 2015 to 2019: Characteristics, trends, and meteorological situations.

We investigated the spatial distribution and trend of double high pollution (DHP), in which the daily average concentration of fine particulate matter (PM2.5) was above 75 μg/m3 and the daily maximum 8-hour average ozone (MDA8 O3) concentration was above 160 μg/m3, in the Yangtze River Delta (YRD) region during 2015-2019, along with the meteorological and chemical characteristics during DHP and differences compared to high O3 pollution (HOP) and high PM2.5 pollution (HPP). In the YRD, Shanghai had the highest frequency of DHP at 7.6%, while Anhui had the least (2.1%). DHP mostly occurred in the northwest and along the Yangtze River in the east of the YRD, especially in spring (April) and autumn (October). MDA8 O3 level was relatively higher during DHP than HOP, while PM2.5 level was relatively higher during HPP than DHP. In 2015-2019, the total number of DHP events decreased in the YRD, but the changes in PM2.5 and O3 concentrations showed great spatial variations. DHP was often associated with a weak pressure field, under meteorological conditions with east winds, temperatures of 18.7-26.1 °C, relative humidity of 65.7-77.1%, sea level pressure of 1008.2-1019 hPa, wind speed of 1.4-2.4 m/s, and visibility of 3.1-7.5 km. Water-soluble ions (NO3-, NH4+, and SO42-) were the dominant components of PM2.5 during DHP at Nanjing and Changzhou City in 2019. Although the fraction of those ions during DHP and HPP were similar, the secondary conversion of NO2 and SO2 was stronger in HPP. The concentrations of those ions were lowest in HOP, with a higher fraction of sulfate than the other two types of pollution. The conversion of SO2 to sulfate was easier to occur than that of NO2 to nitrate under all the polluted conditions in the two cities.