Atmospheric particulate matter (PM) exerts significant impacts on air quality, climate change, and public health (IPCC, 2013; West et al., 2016). The megacities in developing countries, such as China and India, are facing severe air pollution, especially the fine particle (PM 2.5) problem because of their fast-growing economy and urbanization in past decades (Fu & Chen, 2017; Gurjar et al., 2016). To mitigate air pollution, the Chinese State Council implemented the "Air Pollution Prevention and Control Action Plan" in 2013 (Chinese State Council, 2013). As a consequence, anthropogenic emissions of major air pollutants decreased largely (e.g., 59% for SO 2 , 21% for NO x , and 33% for primary PM 2.5) in China during 2013-2017 (Q. Zhang & Geng, 2019). However, despite large reductions in primary emissions, heavy haze episodes still occur in

Black carbon aerosols absorb radiation and their absorptive strength is influenced by particle mixing structures and coating compositions. Liquid-liquid phase separation can move black carbon to organic particle coatings which affects absorptive capacity, but it is unclear which conditions favour this redistribution. Here we combine field observations, laboratory experiments, and transmission electron microscopy to demonstrate that liquid-liquid phase separation redistributes black carbon from inorganic particle cores to organic coatings under a wide range of relative humidity. We find that the ratio of organic coating thickness to black carbon size influences the redistribution. When the ratio is lower than 0.12, over 90% of black carbon is inside inorganic salt cores. However, when the ratio exceeds 0.24, most black carbon is redistributed to organic coatings, due to a change in its affinity for inorganic and organic phases. Using an optical calculation model, we estimate that black carbon redistribution reduces the absorption enhancement effect by 28–34%. We suggest that climate models assuming a core-shell particle structure probably overestimate radiative absorption of black carbon aerosols by approximately 18%.

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Databases of the morphological parameters of fresh size‐resolved soot particles and their systematic comparisons among various combustion sources are important to trace the soot aging process and evaluate their optical properties. Here, the mixing state, effective density (ρeff), monomer number and diameter (N and dp), and fractal dimension (Df) of size‐resolved soot particles from vehicle emissions (VE), biomass burning (BB), coal combustion (CC), tunnel air (TA) and urban air (UA) were characterized based on electron microscopy observations and analysis. We determined that freshly‐emitted soot particles from combustion sources contain not only bare‐like soot but also some coated‐soot particles. ρeff (157–689 kg/m3) decreased while N (46–1,500) and dp (24–42 nm) increased with the increased diameter of soot particles. The Df of BB and CC were independent of the diameter changes and fluctuated between 1.65 and 1.80, while the Df of VE and TA (1.62–1.71) increased and UA Df (1.87–1.80) decreased with increasing diameter. Based on Df obtained in this study, we found that the ensemble Df of VE, BB, CC, TA, and UA could not represent the finer (<150 nm) soot particles and coarser soot particles (>600 nm) in VE, TA, and UA. This study highlights that the morphological parameters of freshly‐emitted soot particles vary among different combustion sources and have a size effect, particularly for finer and coarser soot particles. The database of size‐resolved soot particles will be helpful to improve soot models based on numerical simulation and better evaluate atmospheric optical properties.

Coronavirus Disease 2019 (COVID-19) accidently appeared in Tibet on August 7, 2022, and broke the 920 consecutive epidemic-free days. The cities in Tibet completely kept lockdown to restrict the public...