Contrasting Trends of Surface PM 2.5 , O 3 , and NO 2 and Their Relationships with Meteorological Parameters in Typical Coastal and Inland Cities in the Yangtze River Delta

The contrasting trends of surface particulate matter (PM 2.5 ), ozone (O 3 ), and nitrogen dioxide (NO 2 ) and their relationships with meteorological parameters from 2015 to 2019 were investigated in the coastal city of Shanghai (SH) and the inland city of Hefei (HF), located in the Yangtze River Delta (YRD). In both cities, PM 2.5 declined substantially, while O 3 and NO 2 showed peak values during 2017 when the most frequent extreme high-temperature events occurred. Wind speed was correlated most negatively with PM 2.5 and NO 2 concentrations, while surface temperature and relative humidity were most closely related to O 3 . All of the studied pollutants were reduced by rainfall scavenging, with the greatest reduction seen in PM 2.5 , followed by NO 2 and O 3 . By contrast, air pollutants in the two cities were moderately strongly correlated, although PM 2.5 concentrations were much lower and O x (O 3 + NO 2 ) concentrations were higher in SH. Additionally, complex air pollution hours occurred more frequently in SH. Air pollutant concentrations changed more with wind direction in SH. A more effective washout effect was observed in HF, likely due to the more frequent strong convection and thunderstorms in inland areas. This research suggests pertinent air quality control measures should be designed accordingly for specific geographical locations.