Trends in secondary inorganic particles in Hong Kong, 1995–2020

Abstract

Hong Kong's secondary inorganic aerosol has changed over the last quarter century. It is important to assess changes in particle formation with a particular aim to assess its relationship with shifting emissions and regulatory policy over a period when air quality improved along with better visibility, likely the result of a decrease in ammonium containing aerosols. The concentrations of primary air pollutants, PM10, SO2 and NOx, declined over recent decades, while secondary pollutants SO42− and NH4+ paralleled these decreases, although NO3− shows an indistinct trend, yet the overall the sum of SO42−, NO3− and NH4+ one of decline. The importance of an automotive source led to increases in the NO3−:SO42− ratio. There is insufficient NH3 to neutralise the acid available from SO42− and NO3−, a deficit especially noticeable in June and July, when there is often not enough NH3 to form (NH4)2SO4, (NH4)HSO4 and NH4NO3 leaving free acid. Spatial variability between sites is less for aerosol SO42− and NO3− than for their precursors, and always more variable for nitrogen than sulfur. The extent of the transformation to the oxidized forms is increasing in Hong Kong, likely the result of less local SO2, and a greater proportion of SO42− transported long distances into the region. It is possible that a changing climate could lead to increased autumn ozone concentrations by 2030, so contributing to further oxidation of precursors to aerosol components. The increasing aerosol contribution via regional transport requires continued cooperation in regulatory policies to improve air quality across the Pearl River Delta.