Chemical composition, sources, and processes of winter haze in Chengdu, China: Insights from integrating the bulk chemical and single particle approaches

To understand the characteristics of haze in Chengdu—one of the cities in China typically suffering from this form of pollution—the chemical composition, sources, morphology, and evolution of fine particulate matter (PM_2.5) were analyzed in the most polluted winter by filter sampling and transmission electron microscopy with energy-dispersive X-ray spectroscopy. It was found that the average mass concentration of PM_2.5 during the whole observation period was 97.5 ± 40.4 μg m⁻³, and the contributions of major PM_2.5 components were: OM (31.7 %), NO₃⁻ (19.3 %), NH₄⁺ (11.1 %), and SO₄²⁻ (7.9 %). All single particles were divided into five types: OM, S-rich, mineral, soot, and fly ash/metal particles, most of which existed in the form of internally mixed. PM_2.5 sources included combustion (16.9 %), secondary aerosols (44.1 %), traffic emissions (33.2 %), and dust (5.8 %). Based on the PM_2.5 chemical composition, single particle analysis, and source apportionment results, we were able to conclude that traffic emissions and secondary aerosols jointly led to the evolution from non-polluted to moderately polluted days, while the evolution from moderately to heavily polluted days was mainly dominated by secondary aerosols. The internal mixing of particles increased significantly during the evolution from non-polluted to polluted days. The air masses originated from eastern Sichuan corresponded to the lowest pollution level and the highest contribution of carbonaceous components, while the air masses originated from the west corresponded to the highest pollution level and contribution of secondary inorganic species. Long-distance transmission was closely related to the evolution of pollution in Chengdu.