Informing City-Scale Air Pollution Episodes Using Hourly Time-Scale Measurements of PM_2.5 Molecular and Elemental Tracers at an Air Quality Research Supersite in Hong Kong

While general monitoring stations provide essential data on criteria air pollutants across a larger area, air quality research supersites offer comprehensive chemical composition measurements, albeit with limited spatial coverage. This study demonstrates the complementary roles of these two types of monitoring. Using data from the Hong Kong air quality monitoring network of 15 stations, we identified 37 city-scale haze episodes between November 2020 and May 2021, defined by PM2.5 concentrations exceeding 25 μg m–3. Hourly to bihourly measurements of PM2.5 chemical compositions─including molecular and elemental tracers─at a supersite enabled PM2.5 source identification and apportionment. Strong correlations in PM2.5 and trace element concentrations between the general stations and the supersite indicate spatial homogeneity of air pollution across Hong Kong during these episodes. Variations in the air mass origin and source intensity were found to significantly influence city-scale PM2.5 levels. Source apportionment based on the tracer measurements revealed dynamic changes in source contributions under different atmospheric conditions. Our findings demonstrate that detailed chemical characterization and source apportionment at a supersite can provide quantitative source insights into city-scale haze events, thus supporting the development of more targeted air pollution control strategies at a city level, such as Hong Kong and similar megacities.