Contribution of local emissions and transboundary air pollution to air quality in Hong Kong during El Niño-Southern Oscillation and heatwaves

Transboundary air pollution (TAP), which is strongly affected by weather and climate, is a primary contributor to air pollution. Both El Niño-Southern Oscillation (ENSO) events and heatwaves exert influences on TAP, thereby affecting air quality, which has yet to be well understood. This study applied an integrated statistical scheme to examine the contributions of local emission sources and TAP to particulate matter (PM), sulfur dioxide, and nitrogen dioxide during four ENSO events and 20 heatwaves from 2002 to 2016. Hong Kong was studied as an example because of its substantial TAP that can provide useful reference for cities with TAP. We found a remarkable and consistent reduction in the TAP contribution to pollutants during two El Niño events because of the associated rainfall increase, decreased frequency of northerly winds below 700 hPa, and overall stronger wind speed. Moreover, the TAP was the predominant contributor to the increase in the ambient PM concentration during the two La Niña events because of the associated decrease in rainfall, higher frequency of northerly winds below 900 hPa, and weaker wind speed, which were favorable to long-range transport and pollutant accumulation. The ten heatwaves associated with tropical cyclones within a distance of 1100 km from Hong Kong were accompanied by an increase in pollutant concentration, which was featured by the lower wind speed and less rainfall. Our findings highlight the long-term seasonal variability of TAP, and increases in TAP during the El Niño events and heatwaves.