A conceptual framework for mixing structures in individual aerosol particles

Abstract

This study investigated the particle size‐ and age‐dependent mixing structures of individual particles in clean and polluted air. Aerosols were classified into eight components: sea salt, mineral dust, fly ash, metal, soot, sulfates, nitrates, and organic matter (OM). Based on our aerosol classification, a particle that consists of two or more aerosol components can be defined as an internally mixed particle. Otherwise, it is considered to be an externally mixed particle. Within the internally mixed particle class, we identified four heterogeneous mixing structures: core‐shell, dumbbell, OM coating, and dispersed OM, as well as one homogeneous‐like mixing structure. Homogeneous‐like mixing mainly occurred in fine particles (<1 µm), while the frequency of heterogeneously mixed particles increased with particle size. Our study demonstrated that particle mixing structures depend on particle size and location and evolve with time. OM‐coating and core‐shell structures are important indicators for particle aging in air as long as they are distant from specific emission sources. Long‐range transported particles tended to have core‐shell and OM‐coating structures. We found that secondary aerosol components (e.g., sulfates, nitrates, and organics) determined particle mixing structures, because their phases change following particle hydration and dehydration under different relative humidities. Once externally mixed particles are transformed into internally mixed particles, they cannot revert to their former state, except when semivolatile aerosol components are involved. Categorizing mixing structures of individual particles is essential for studying their optical and hygroscopic properties and for tracing the development of their physical or chemical properties over time.

Aerosol optical properties and direct radiative forcing based on measurements from the China Aerosol Remote Sensing Network (CARSNET) in eastern China

“…Long-term, ground-based observations have contributed greatly to our understanding of the spatial variations in aerosols and their effects on the Earth's climate (Holben et al, 2001;Kaufman et al, 2002;Sanap and Pandithurai, 2014;Z. Q. Li et al, 2016).…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…Core-shell morphology is prevalent configuration of tropospheric particles. They were also found in other sampling sites, but showed proportional difference; there are proportionally higher CS particles in mountain Tai, Greater Khingan and rural background sampling site, but less in urban and suburban-polluted air in Beijing and no CS particles in biomass and coal burning sourced particles [18].…”

“…These secondary particles are mainly composed of element C, O, S and minor N and are defined as S-rich particles [18,20]. We infer that they indeed are mixtures of sulfate, nitrate, and/or organic matter (OM) in the atmosphere [18].…”

“…The formation mechanism of the organic coated S-rich particles remains with some uncertainties. They may be formed by liquid-liquid phase separation under the Atmosphere 2018, 9, 40 7 of 10 various RH from high to low values [44] or by condensation of secondary organic compounds on the pre-existing inorganic aerosols [18,45].…”

“…A single-stage cascade impactor with a 0.5 mm diameter jet nozzle and an air flow of 1.0 L/min was applied. The collection efficiency of this kind of sampler is close to 100% on a 0.5 µm aerodynamic diameter if the particle density is 2 g/cm 3 [18]. To avoid the particle overlapping on subtracts, the sampling time ranged from 60 to 90 s. A kestral 4000 pocket weather meter (Nielsen-kellermann Inc., Boothwyn, PA, USA) was simultaneously applied to record the RH, temperature (T) and pressure (P).…”