Previous single-particle studies in China were mainly conducted in polluted winters. The single-particle aerosol mass spectrometer (SPAMS) has been widely used to characterize mixing state, chemical compositions and formation mechanisms of aerosol particles (Bhave et al., 2002 Giorio et al., 2015 Li et al., 2014 Bi et al., 2011 Chen et al., 2017 Zhang et al., 2018). Although the effects of emission controls on chemical composition and formation mechanisms of fine particles have been extensively studied, their impacts on chemical and physical properties of single particles, e.g., mixing state and effective density ( ρ eff), are poorly understood. As a result, PM 2.5 decreased significantly during OWG (Liu et al., 2022). These measures included shutting down factories with high emissions, limiting the number of vehicles, stopping construction activities, and forbidding fireworks (The People's Government of Beijing Municipality, 2022). Similarly, the Chinese government imposed strict emission controls in Beijing and surrounding regions during the Olympic Winter Games (OWG) in Beijing in 2022. Short-term emission controls can improve air quality by reducing anthropogenic emissions temporarily, for example, the Beijing 2008 Summer Olympics (Wang et al., 2010 Okuda et al., 2011 Zhou et al., 2010), the 2014 Asia-Pacific Economic Cooperation (APEC) summit (Han et al., 2015 Gao et al., 2017 Ren et al., 2018), and the 2020 lockdown due to the corona virus disease (COVID-19) (Sun et al., 2020 Rajesh and Ramachandran, 2022 Zhang et al., 2022a). However, polluted events still occurred occasionally, particularly during wintertime with stagnant meteorological conditions and high anthropogenic emissions (Lei et al., 2021b Xu et al., 2022 Feng et al., 2022 Zhou et al., 2023). Beijing, experiencing severe pollution with high concentrations of fine particulate matter over the past decade (Huang et al., 2014 Guo et al., 2014 Sun et al., 2016), has had great success in air pollution control (Lei et al., 2021a Cheng et al., 2019), and the annual average concentration of PM 2.5 reached the Chinese National Ambient Air Quality Standard for the first time in 2021 (33 µg m −3). In addition, the ρ eff 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.Ītmospheric aerosols from both natural and anthropogenic sources exert strong influences on radiative forcing and human health (Buseck and Posfai, 1999 Anderson et al., 2003 Ramanathan et al., 2001 Prather, 2009), and the impacts depend strongly on their chemical and physical properties, e.g., composition, mixing state, and density. The average effective density ( ρ eff) 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. 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 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). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG, although those from primary emissions decreased. 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 %). 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. Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health.
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