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

Abstract

Abstract. Beijing has been suffering from frequent severe air pollution events, with concentrations affected significantly by the mixed layer height. Major efforts have been made to study the physico-chemical properties, composition, and sources of aerosol particles at ground level. However, little is known on morphology, elemental composition, and mixing state of aerosol particles above the mixed layer. In this work, we collected individual aerosol particles simultaneously at ground level (2 m above ground) and above the mixed layer in urban Beijing (within the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-Beijing) 2016 winter campaign). The particles were analyzed off-line using transmission electron microscopy coupled with energy dispersive X-ray spectroscopy. Our results showed that the relative number contribution of mineral particles to all measured particles was much higher during non-haze periods (42.5 %) than haze periods (18.1 %); on the contrary, internally mixed particles contributed more during haze periods (21.9 %) than non-haze periods (7.2 %) at ground level. In addition, more mineral particles were found at ground level than above the mixed layer height. Around 20 % of individual particles showed core-shell structures during haze periods, whereas only a few core-shell particles were observed during non-haze periods (2 %). We found that the particle above the mixed layer tend to be more aged with a larger proportion of organic particles originated from coal combustion. Our results indicate that a significant fraction of the airborne particles above the mixed layer originated from surrounding areas influenced by coal combustion activities. This source contributes to the surface particle concentrations in Beijing when polluted air is mixed down to the ground level.

Surface chemistry of atmospheric nanoparticles during a haze episode in Beijing by TOF-SIMS

“…Previous studies of aerosol particles have mostly relied on bulk techniques such as gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), and inductively coupled plasma mass spectrometry (ICP-MS). Only the average concentration of chemical components (such as elements and water-soluble ions) could be got with the surface information destroyed during the extraction process (Li et al, 2020b;Wang et al, 2021). By a series of individual analysis techniques, the surface morphology, mixing state, chemical composition will be well investigated (Li et al, 2016;Kovochich et al, 2021).…”