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Remarkably, enhanced particulate nitrate (NO3−) concentrations occur in many environments during particulate matter (PM) pollution; however, information on the formation mechanism and alleviation strategies is still limited. Herein, to explore the NO3−formation mechanism and conditions, we measured the concentrations of water-soluble inorganic ions in PM1.0as well as the inorganic gas concentrations of HNO3, NO2, and NH3in Gimje, a highly dense livestock area, from June to July 2020 and January to February 2021. At the monitoring site, extremely high atmospheric NH3was measured with an hourly average of 96.9 ± 48.1 ppb, and the daily average of HNO3and PM1.0was 0.7 ± 0.7 ppb, and 20.1 ± 8.8 μg m−3, respectively. A clear increase in the NO3−concentration in PM1.0was observed on high pollution days (PM1.0≥ 20 μg m−3), suggesting that HNO3and NH3contributed to NO3−formation. Moreover, we applied the thermodynamic model ISORROPIA-II to predict the NO3−response to the reduction of total HNO3(TN), total NH3(TA), and SO42−. The results showed that controlling TN could be more effective in alleviating particulate NO3−than controlling SO42−and TA in the livestock area. We also compared this result to that of a nearby urban area, Jeonju. A similar result was observed, with efficient HNO3control, which reduced the NO3−concentration in Jeonju. These measurements and simulations indicated that NOxcontrol could be the most effective approach to reduce particulate NO3−concentrations in both livestock and urban areas. Our results provide a significant contribution to developing a strategy for alleviating particulate NO3−pollution.

Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM2.5) in two megacities, Seoul and Beijing, we collected PM2.5filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM2.5as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM2.5was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM2.5was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM2.5observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM2.5in polluted urban atmospheres.

Laboratory studies of liquid–liquid phase separation in particles containing organic species and inorganic salts of atmospheric relevance are reviewed. The oxygen-to-carbon elemental ratio (O:C) of the organic component appears to be the most useful parameter for estimating, to a first approximation, the occurrence of liquid–liquid phase separation and the separation relative humidity (SRH) in these particles. A trend of decreasing SRH for increasing O:C was found for simple organic–inorganic mixtures (<11 species). Phase separation in particles composed of laboratory-produced secondary organic material and sulphate species and in ambient particles is generally consistent with this trend. A further constraint is that liquid–liquid phase separation was always observed for O:C < 0.5 and was never observed for O:C ≥ 0.8. For organic materials of intermediate O:C ranging from 0.5 to 0.8, phase separation in simple organic–inorganic mixtures was influenced by the organic functional groups represented. The organic-to-inorganic mass ratio (OIR) affected the occurrence of liquid–liquid phase separation in a small number of cases. A dependence on salt type was observed with 87% of the studied organics exhibiting the following trend in SRH values: (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3, consistent with previous salting-out studies and the Hofmeister series. Liquid–liquid phase separation does not appear to be strongly influenced by the number of species making up the organic material. The morphology of phase separated particles appears to depend on composition, including O:C of the organic material, the inorganic salt and the OIR.