Chemical Characteristics, Sources, and Transport Pathways of PM2.5 in a Mid-Altitude Industrial City of the Upper Yellow River Basin
摘要
In recent years, most studies on the chemical characteristics and sources of PM2.5 have focused on China's most densely populated regions, leaving mid-altitude cities in the Yellow River Basin relatively understudied. As a representative city with distinctive topography and industrial emissions, Wuhai City provides an important case for exploring air quality challenges faced by similar cities. This study investigated PM2.5 concentrations, chemical composition, potential sources, and transport pathways in Wuhai City based on sample collection and source analysis. PM2.5 concentrations were significantly higher in winter (45.3 µg/m3) and spring (43.0 µg/m3) than in autumn (23.3 µg/m3) and summer (20.7 µg/m3). The low annual PM2.5/PM10 ratio (0.29) indicates that coarse particles dominate. A distinct midday peak in PM2.5 was observed, a feature rarely reported elsewhere. Pollution episodes were classified as Type Ⅰ (PM2.5: 50–150 µg/m3, 91% of cases, year-round) and Type Ⅱ (> 150 µg/m3, 9% of cases, mainly spring and winter). Type Ⅰ was characterized by higher fractions of secondary inorganic ions (e.g., NO3⁻, SO42−, NH4+) and trace elements from anthropogenic sources, whereas Type Ⅱ was dominated by crustal elements (e.g., Si, Fe, Ca) and strongly influenced by long-range transported dust. Air mass trajectories showed that Type Ⅰ episodes were mainly linked to local emissions and regional transport, while Type Ⅱ originated from northwestern deserts and the Mongolian Plateau. Furthermore, mountain barrier and wind-topography interactions enhanced pollutant accumulation under stagnant conditions, exacerbating air pollution. These findings provide important insights for developing effective region-specific air quality management strategies.