Long-range Transport of Air Pollutants to Taiwan during the COVID-19 Lockdown in Hubei Province

ABSTRACT

Lockdown implemented to limit transmission of the COVID-19 virus in China reduced the emissions of air pollutants especially around Hubei Province, though restrictions were more widely applied across the country. Air over Taiwan may be affected by emissions from mainland China so the changes provided an opportunity to detect the effect of long-range pollutant transport to Taiwan, especially at remote sites. Small decreases were observed in NOx and PM_2.5 concentrations, when comparing 2020 with earlier years, as well as some hints of increases in O₃. Back trajectories from China were used to select days when this effect would be strongest and showed clearer evidence of reductions at remote sites in Taiwan. These were strongest for NOx and PM_2.5 in Kinmen and Matsu, close to the Fujian coast, but the effects were seen even on Magong in the Penghu archipelago and at Cape Fugui at the very north of Taiwan. Larger reductions on the coastal islands is not surprising given their proximity to major cities, but on Magong and at Cape Fugui, PM_2.5 concentrations are reduced by a few micrograms per cubic metre. Decreases in NOx are smaller and at Cape Fugui, barely amounting to a ppb. Changes in O₃ are distinctive, showing concentration increases of around 20% under trajectories deriving from Northern China, in keeping with observations that reductions in NOx lead to substantial increases in O₃. Overall, there was only a small reduction in NOx and PM in Taiwan. Although these changes in pollutant concentrations may be relatively large in remote areas, they are not likely to have a major effect on cumulative exposure in larger cities, where concentrations of these pollutants can be high. Nevertheless, the observations support calls for agreement over emission reductions to lessen pollutant transfer across the Taiwan Strait.