Sulfur enrichment in aerosol particles during haze and dust periods in urban Beijing.

Heterogeneous reactions on aerosol particles are important pathways for atmospheric sulfur transformation and secondary aerosol formation. Using high-resolution scanning electron microscopy coupled with energy-dispersive X-ray spectrometer, we investigated sulfur enrichment on individual particles during a persistent haze-to-dust transition in urban Beijing. Dust aerosols were dominated by mineral particles, whereas haze aerosols contained abundant sulfate and anthropogenic particles. During dust periods, all particle types exhibited weak sulfur enrichment, with sulfur contents generally below 5%. In contrast, carbonate/sulfate and clay-Na/Mg/Ca particles during haze periods showed intense sulfation, with average sulfur weight ratios reaching 56.4% and 23.2%, respectively. Nevertheless, equivalent pure sulfate particle concentrations remained comparable between haze and dust periods because of the much higher abundance of mineral dust particles during dust events. These findings reveal fundamentally different sulfur uptake pathways under haze and dust conditions and provide new insights into aerosol aging, dust-pollution interactions, and sulfur cycling under improving air quality in northern China.