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 (PM_2.5) 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 PM_2.5 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 PM_2.5 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 PM_2.5 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 SO₄^2-) were the dominant components of PM_2.5 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.