| Summary: | Beijing–Tianjin–Hebei (BTH) region in China suffers frequent and heavy pollution from particulate matter with aerodynamic diameter ≤ 2.5 μm (PM2.5). Despite the extensive studies conducted in this region, there are insufficient data on the chemical composition, formation mechanism, potential sources, and regional transport contribution of PM2.5 in Zhangjiakou, Hebei province. In this study, the chemical and spatiotemporal variation in PM2.5 in Zhangjiakou were investigated at four urban sites in Zhangjiakou in 2019. The mean (± standard deviation, SD) observed PM2.5 concentration was 43.0 ± 34.2 μg/m3. The concentrations of PM2.5 and its major components were generally higher in the south than in the north, and in winter than in summer. The major chemical components were organic matter (OM, 31.8%), secondary inorganic ions (SIA, 25.7%), and mineral dust (14.7%). Higher NO3−/EC, SO42−/EC, sulphur oxidation ratios (SOR), and nitrogen oxidation ratios (NOR) were observed on pollution days, implying intense secondary production of nitrate during pollution periods. Heterogeneous reactions were important contributors to secondary sulphate formation. Secondary organic carbon (SOC) proportions (i.e., SOC/OC and SOC/EC ratios) decreased markedly from clean days to heavy pollution days in response to enhanced primary organic carbon (POC) emissions on pollution days. The source contributions varied among sites, seasons, and pollution levels according to positive matrix factorization (PMF) model; combustion and SIA sources were the key factors for air pollutant emission reduction. Small-scale and short-distance air mass transport from the northwestern and southern regions greatly impacted PM2.5 pollution according to potential source analysis. Weather Research and Forecast model (WRF) was coupled with Comprehensive Air Quality Model with Extensions (CAMx), and found that regional transport contributed significantly to PM2.5 pollution in Zhangjiakou (74.6%). The results of this study indicate that joint prevention and control of regional air pollution would meaningfully improve air quality in the BTH region.
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