Air quality deterioration episode associated with typhoon over the complex topographic environment in central Taiwan

 Air pollution is typically at its lowest in Taiwan during summer. The mean concentrations of PM₁₀, PM_2.5, and daytime ozone (08:00–17:00 LST) during summer (June–August) over central Taiwan are 35–40 µg m^-3, 18–22 µg m^-3, and 30–42 ppb, respectively, between 2004 and 2019. Sampling analysis revealed that the contribution of organic carbon (OC) in PM_2.5 could exceed 30% in urban and inland mountain sites during July in 2017 and 2018. Frequent episodes of air quality deterioration occur over the western plains of Taiwan when an easterly typhoon circulation interacts with the complex topographic structure of the island. We explored an episode of air quality deterioration that was associated with a typhoon between 15 and 17 July 2018, using the Weather Research Forecasting with Chemistry (WRF-Chem) model. The results indicated that the continual formation of low-pressure systems or typhoons in the area between Taiwan and Luzon island in the Philippines provided a strong easterly ambient flow, which lasted for an extended period between 15 and 17 July. The interaction between the easterly flow and Taiwan’s Central Mountain Range (CMR) resulted in stable weather conditions and weak wind speed in western Taiwan during the study period. Numerical modeling also indicated that a lee side vortex easily formation and the wind direction could be changed from southwesterly to northwesterly over central Taiwan because of the interaction between the typhoon circulation and the CMR. The northwesterly wind coupled with a sea breeze was conducive to the transport of air pollutants, from the coastal upstream industrial and urban areas to the inland area. The dynamic process for the wind direction changed given a reasonable explanation why the observed SO₄^2- became the major contributor to PM_2.5 during the episode. SO₄^2- contribution proportions (%) to PM_2.5 at the coastal, urban, and mountain sites were 9.4 µg m^-3 (30.5%), 12.1 µg m^-3 (29.9%), and 11.6 µg m^-3 (29.7%), respectively. Moreover, the variation of the boundary layer height had a strong effect on the concentration level of both PM_2.5 and ozone. The combination of the lee vortex and land-sea breeze, as well as the boundary layer development, were the key mechanisms in air pollutants accumulation and transport. As typhoons frequently occur around Taiwan during summer and fall, and their effect on the island’s air quality merits further research attention.