Influences of sea breeze circulation and atmospheric stability on PM2.5 levels in an inland city of Taiwan

Air quality in central Taiwan is strongly influenced by meteorological conditions, particularly sea-land breeze circulation and atmospheric vertical stability, which govern the horizontal transport and vertical dispersion of pollutants. Douliu City, an inland urban area, experienced the highest PM 2.5 concentrations in the region. Despite nearly a 50% reduction in PM 2.5 levels from 2005 to 2020 due to emission control measures, stable atmospheric conditions accompanied by low average wind speeds (∼1.4 m s -1 ) continued to favor pollutant accumulation. This study combined long-term observations with an intensive autumn 2015 field campaign in Douliu, utilizing surface aerosol optical measurements and ground-based remote sensing instruments. Results indicated that daytime sea breezes transported pollutants from coastal to inland areas within 2–4 hours, increasing surface PM 2.5 concentrations from 39 to 54 μg m -3 and reducing AE 440–870 from 1.34 to 1.04 (i.e., coarser particle). At night, the planetary boundary layer descended from approximately 1 km to below 300 meters, resulting in elevated surface PM 2.5 concentrations and higher AE 450–700 values, which reflected the accumulation of fine-mode aerosols. Analyses of both surface and vertical measurements demonstrated that the effects of sea-land breeze circulation significantly influenced pollutant transport and accumulation, leading to higher PM 2.5 concentrations in the inland site compared to the coastal site. Moreover, atmospheric stability plays a critical role, as low wind speeds and temperature inversions favor pollutant accumulation and suppress the vertical dispersion of aerosols in inland regions. This study highlights the critical role of meteorology in shaping inland air quality and suggests that future research to focus on vertical profiling with lidar and unmanned aerial vehicles (UAVs) to better elucidate pollutant accumulation mechanisms. • Integrated surface and vertical observations clarify inland PM 2.5 transport, accumulation, and dispersion. • Sea–land breeze transport and temperature inversions enhanced PM 2.5 accumulation at a foothill city. • Long-term atmospheric stability was derived from station-based ΔT and explained the inland PM 2.5 accumulation features.