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. Beijing has been suffering from frequent severe air pollution events, withconcentrations affected significantly by the mixed-layer height. Majorefforts have been made to study the physico-chemical properties,compositions, and sources of aerosol particles at ground level. However,little is known about the morphology, elemental composition, and mixingstate of aerosol particles above the mixed layer. In this work, we collectedindividual aerosol particles simultaneously at ground level (2 m aboveground) and above the mixed layer in urban Beijing (within the AtmosphericPollution and Human Health in a Chinese Megacity, APHH-Beijing, 2016 wintercampaign). The particles were analyzed offline by transmission electronmicroscopy coupled with energy dispersive X-ray spectroscopy. Our resultsshowed that the relative number contribution of mineral particles to allmeasured particles was much higher during non-haze periods (42.5 %) thanhaze periods (18.1 %); in contrast, internally mixed particlescontributed more during haze periods (21.9 %) than non-haze periods(7.2 %) at ground level. In addition, more mineral particles were found atground level than above the mixed-layer height. Around 20 % of individualparticles showed core–shell structures during haze periods, whereas only afew core–shell particles were observed during non-haze periods (2 %). Theresults showed that the particles above the mixed layer were more aged, witha larger proportion of organic particles originating from coal combustion.Our results indicate that a large fraction of the airborne particles abovethe mixed layer come from surrounding areas influenced by coal combustionactivities. This source contributes to the surface particle concentrationsin Beijing when polluted air is mixed down to the ground level.

Measurement report: Comparison of wintertime individual particles at ground level and above the mixed layer in urban Beijing