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Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health. Here a single-particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier was deployed during the Beijing 2022 Olympic Winter Games (OWG) to investigate the impacts of emission controls on particle mixing state and density. Our results show the dominance of carbonaceous particles comprising mainly total elemental carbon (Total-EC, 13.4 %), total organic carbon (Total-OC, 10.5 %) and Total-ECOC (47.1 %). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG, although those from primary emissions decreased. The composition of carbonaceous particles also changed significantly which was characterized by the decreases in EC mixed with nitrate and sulfate (EC-NS), EC mixed with potassium nitrate (KEC-N), and amine-containing particles and increase in ECOC mixed with nitrate and sulfate (ECOC-NS). This result indicates that emission controls during OWG reduced the mixing of EC with inorganic aerosol species and amines yet increased the mixing of EC with organic aerosol. The average effective density (ρeff) of aerosol particles (150–300 nm) was 1.15 g cm -3 during the non-Olympic Winter Games (nOWG), with higher values during OWG (1.26 g cm -3) due to the increase in secondary particle contribution. In addition, the ρeff of most particles increased with the increases in pollution levels and relative humidity, yet they varied differently for different types of particles, highlighting the impacts of aging and formation processes on the changes of particle density and mixing state. [ABSTRACT FROM AUTHOR]

Photochemical aging is a key atmospheric processing, yet the changes in physicochemical properties of organic aerosol (OA) during photochemical aging of primary emissions from cooking and burning are less understood. Here we conducted 12 burning (straw, wood, and coal) and cooking experiments to characterize the evolution of size distributions, volatility, and glass transition temperature (Tg) from fresh smoke to aged OA with an equivalent photochemical age of ∼1.5 days using an oxidation flow reactor‐thermodenuder‐aerosol mass spectrometer system. The mass spectra of OA showed significant changes during photochemical aging, for example, the rapid degradation of m/z 60 for straw and wood burning OA, and the large increase in f44 (fraction of m/z 44 in OA) for all OA. The contributions of non‐volatile compounds to the total OA in aged burning OA (1.6%–5.3%) decreased considerably compared with those in fresh burning smoke (2.3%–17.1%), suggesting that photochemical aging of primary emissions for ∼1.5 days produced more volatile secondary OA (SOA). Consistently, the pronounced formation of SOA below 150 nm was observed, and it showed more volatile properties than aged large particles. The Tg of OA under dry conditions (Tg,org) was estimated based on volatility distributions, and the results showed increased Tg,org during photochemical aging of biomass burning and coal combustion emissions, while decreased Tg,org for aged cooking OA. Overall, our results illustrate the different changes in size distributions, volatility, and Tg,org through photochemical aging of different primary emissions, which in turn affect their impacts on radiative forcing and human health. Plain Language Summary: The properties of organic aerosol (OA) from primary emissions can have rapid changes during photochemical aging, and hence affect the radiative forcing and health effects of OA. However, changes in physicochemical properties of OA during aging, for example, size distributions, volatility, and glass transition temperature (Tg), are less understood, particularly for the OA from different types of primary emissions. In this work, we characterized such changes in size distributions, volatility, and viscosity of OA from biomass burning, coal combustion, and cooking emissions during photochemical aging. We observed pronounced formation of secondary OA (SOA) below 150 nm after aging, which showed higher volatility than the aged larger OA particles. The aged biomass burning and coal combustion emissions showed lower volatilities and higher viscosities than fresh emissions. Our results demonstrate that the size distributions, volatility, and viscosity of OA in regions with high anthropogenic emissions can have significant changes in days and hence exert different climate and health effects. Key Points: The volatility and glass transition temperature of organic aerosol (OA) from primary emissions changed significantly during photochemical agingSecondary OA formed below 150 nm was more volatile than aged large particlesThe aged biomass burning and coal combustion emissions showed lower volatilities and higher viscosities than fresh emissions [ABSTRACT FROM AUTHOR]

Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health. Here a single particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier was deployed during the Beijing 2022 Olympic Winter Games (OWG) to investigate the impacts of emission controls on particle mixing state and density. Nearly 760,000 particles were detected, which were classified into seven major classes. Our results showed the dominance of carbonaceous particles comprising mainly Total-EC (13.4 %), Total-OC (10.5 %) and Total-ECOC (47.1 %). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG although those from primary emissions decreased. The composition of carbonaceous particles also changed significantly which was characterized by the decreases in EC-NS, KEC-N, and amine-containing particles, and increase in ECOC-NS. This result indicates that emission controls during OWG reduced the mixing of elemental carbon (EC) with inorganic aerosol species and amines, yet increased the mixing of EC with organic aerosol. The average effective density (ρeff) of aerosol particles (150–300 nm) was 1.20 g cm−3 with higher values during OWG (1.26 g cm−3) due to the increase in secondary particle contribution. The two types of fresh particles, i.e., Total-EC and high molecular weight organic matter presented the lowest ρeff (0.97 g cm−3 and 0.87 g cm−3, respectively). In addition, the ρeff of most particles increased as the increases in pollution levels and relative humidity, yet varied differently for different types of particles, highlighting the impacts of aging and formation processes on the changes of particle density and mixing state. [ABSTRACT FROM AUTHOR]