Observation-based investigation reveals major sources of heavy metals associated with fine particulate matter (PM2.5) in an East Asian urban area

Abstract

Exposure to elevated levels of fine particulate matter (PM_2.5) has been a major public health concern for decades. However, the specific sources of air toxics associated with PM_2.5 remain unclear. In this study, we investigate PM_2.5 pollution in Taichung, Taiwan—a representative East Asian metropolitan area— during March and November of 2021–2023 using an advanced two-stage positive matrix factorization (PMFxPMF) approach. This method enables detailed source apportionment of both bulk PM_2.5 and PM_2.5-bound heavy metals. Our analysis reveals that, during the East Asian winter monsoon seasons, regional transboundary pollution contributed 38 % to the PM_2.5 load, while local sources—such as carbonaceous aerosols, industrial processes, ammonium nitrate/chloride, and transformed sea spray—contributed between 9 % and 20 %. Furthermore, the formation of nitrate was the primary driver of air quality deterioration. Heavy metals constituted 1.2 %–1.5 % of PM_2.5 mass (0.24–0.32 μg m⁻³). By incorporating heavy metal fingerprints from two major local sources—coal-fired power plants and steel sintering facilities—as constraints in our PMF analysis, we reveal that ambient non-Fe heavy metals were mainly associated with suspended dust (34 %), implying significant health risk of dust exposure. Besides, vehicular pollution accounted for 14 % of non-Fe heavy metals, highlighting the need for a stronger control on non-exhaust vehicular emissions. Substantial contributions arose from coal combustion (9 %), steel sintering (5 %) and various industrial sources (22 %). Our results underscore the importance of accelerating the timeline for coal phaseout, and warrant a further investigation on the emissions of heavy metals from industrial activities.