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, compositions, and sources of aerosol particles at ground level. However, little is known about the 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 offline by 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 %); in contrast, 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 %). The results showed that the particles above the mixed layer were more aged, with a larger proportion of organic particles originating from coal combustion. Our results indicate that a large fraction of the airborne particles above the mixed layer come 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.
Highlights
Atmospheric aerosols emitted from anthropogenic or natural sources are composed of a variety of chemical components (Merikallio et al, 2011; Guo et al, 2014; Wang et al, 2016; Peng et al, 2016; Shao et al, 2017; Tao et al, 2017)
The hourly mean relative humidity (RH) ranged from 17 % to 97 %, with a 9 d mean of 50.3 %
The RH and PM2.5 were positively correlated according to the 216 groups of hourly data, suggesting that higher RH favors the formation of haze (Sun et al, 2014; Wang et al, 2016)
Summary
Atmospheric aerosols emitted from anthropogenic or natural sources are composed of a variety of chemical components (e.g., organic matter, black carbon, nitrate, sulfate, ammonium, metals, mineral dust) (Merikallio et al, 2011; Guo et al, 2014; Wang et al, 2016; Peng et al, 2016; Shao et al, 2017; Tao et al, 2017). Anthropogenic aerosols affect climate through cloud condensation nuclei activity (Kerminen et al, 2012), hygroscopic growth (Brock et al, 2016), and light scattering and absorption (Jacobson, 2001; Bond and Bergstrom, 2006; Merikallio et al, 2011; China et al, 2013; Peng et al, 2016; Bhandari et al, 2019b) They can have adverse effects on human health, for example, by carrying toxic and carcinogenic compounds (Chen et al, 2013; Shao et al, 2016, 2017). Atmospheric aerosol particles affect regional and global geochemical cycles as they are transported over long distances (Heald et al, 2006; Li et al, 2017c; Rodriguez-Navarro et al, 2018)
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