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The long‐range transport of Asian dust and its complex interaction with anthropogenic pollutants have a significant yet poorly quantified effect on the global climate. In this study, dust events were observed based on an optical particle counter with a polarization detection module at an urban site in North China. The temporal variations in the size distribution and morphological changes of dust particles were analyzed. The results demonstrate that the dust events were induced by a deep Siberian low‐pressure anomaly and/or Mongolian cyclone in the low troposphere. The morphology of dust particles, as indicated by the depolarization ratio (δ = s/(s + p), the ratio of the parallel signal to the total backscattering signal), varied significantly at different transport heights. A decreasing tendency was found when dust was trapped within the boundary layer over polluted areas, particularly when the atmospheric loading of pollutants and the relative humidity were high. During a typical polluted dust case, the δ value of mineral dust (Dp = 5 μm) decreased by 39%, as the conditions were stagnant in East China, consistent with the Cloud‐Aerosol LIDAR with Orthogonal Polarization results. The morphological variation was evaluated according to the aspect ratio, presuming that the dust particles were ellipsoidal. The interaction between dust particles and water vapor could lead to an increase of at least 6% in volume when the relative humidity increased up to 90%. An aerosol classification based on δ values and size information could be performed to estimate the contributions of different aerosol types to pollution days. The results should be helpful for evaluating satellite‐borne and ground‐based light detection and ranging data analyses. Key Points: The occurrence of dust events in East Asia was related to a deep Siberian low‐pressure anomaly and/or a Mongolian cycloneThe single particle‐based depolarization ratio was observed to study the nonsphericity of dust particlesEnvironmental humidity and pollutant concentrations are the key factors that affect the production of polluted dust [ABSTRACT FROM AUTHOR]

Atmospheric loading of black carbon aerosols influences air quality and visibility, and their mixing state and aerosol chemistry were sensitive to both emission scenarios and regional transport. In this study, the aerodynamic size-resolved mixing state of refractory black carbon aerosols (rBC) was investigated using a novel tandem system consisting of an aerodynamic aerosol classifier (AAC), a single particle soot photometer (SP2), and single-particle-aerosol mass spectrometry (SPA-MS) during the XXIV Olympic Winter Games (OWG) at an urban site in Beijing. Particles with aerodynamic diameter (D ae) of 200 nm and 300 nm were selected by AAC to focus on the effect of relative differences in morphology on the coating thickness, coating compositions, and optical properties of rBC particles. We found that rBC-containing particles with D ae of 300 nm were characterized by a rBC core with a count median diameter (CMD) of 108 ± 4 nm that typically have regular morphology (χ : m e a n ∼ 1) , higher relative coating thickness (RCT: mean ∼1.9), and strong light absorption enhancement (E abs : mean ∼1.5). The rBC-containing particles with D ae = 200 nm normally had irregular shapes (χ : m e a n ∼ 1.4) and weak E abs (E abs : mean ∼1.37). Classification based on air mass clustering and air pollution level revealed that the coating thickness of rBC particles only exhibited unimodal patterns under clean air conditions and distinct bimodal distributions under heavy pollution conditions, implying multiple origins of rBC particles, even during OWG periods. The coating thickness and E abs of rBC particles were greater when influenced by the higher RH air from east. Furthermore, the chemical compositions of the rBC coating quantitatively determined by SPA-MS were used to estimate the possible origins of the D ae = 300 nm, featured by a thicker coating thickness and stronger light absorption enhancement. Our results showed that rBC particles were primarily mixed with organics, nitrate, and sulfate during the XXIV OWG period. The organic components have a limited role in increasing rBC coatings under polluted conditions, while the alkali metal ions (K and Na) associated with traffic emissions and the secondary inorganic species favor the formation of thick coatings. Higher RH makes limited contributions to rBC mixed with sulfate and organics only (BCOC*_S) and pure BC. The relatively fresh BC could directly mix with organics and sulfate at low RH levels while evolving to mix with nitrate at high RH conditions. In addition to high concentrations of locally emitted precursors and high RH that tend to form thick coatings of nitrate-related rBC aerosols, secondary transport from the west and southwest also contributes to rBC aerosols mixed with nitrate with relatively thick coatings. • rBC with an aerodynamic diameter (D ae) of 300 nm typically have regular morphology. • rBC of regular shape(D ae = 300 nm)has thicker coatings and stronger absorption enhancement. • During the OWG, rBC particles were primarily mixed with organics, nitrate and sulfate. • rBC particles had multiple sources, even in the OWG periods. • The organic components have a limited role in increasing rBC coatings under polluted conditions. [ABSTRACT FROM AUTHOR]

The mixing state of black carbon aerosols (BC) has the largest uncertainty for climate forcing evaluation. In this study, a tandem observation, combining a centrifugal particle mass analyzer (CPMA) and a single particle soot photometer (SP2), was conducted to investigate the mixing state of BC in a coastal city in eastern China. First, particles with specific masses (1.0 fg, 2.1 fg, 4.5 fg, 9.7 fg, and 20.1 fg) were selected using CPMA and consecutively measured by SP2 to determine the corresponding BC mass (M BC) of the particle. The mass ratio (M R) of the coating matter to the BC core was used to indicate the coating thickness and aging degree of BC. The results showed that the M R value obtained from the CPMA-SP2 system was well correlated with the mixing state condition derived from the traditional lag-time method. However, the lag-time method tends to misclassify thickly to thinly coated BC as particles with larger masses. The M R generally increased as the mass of BC-containing particles increased. M R showed a clear diurnal pattern with a minimum at 8:00 and a maximum at 13:00 due to photochemical process during the daytime. The larger BC-containing particles had a much weaker diurnal pattern because other secondary processes may contribute more to the coatings for large BC-containing particles. The M R maintained a high value when the air mass was stagnant in the local area with high pollutant concentrations and high relative humidity (RH), while the M R was at its lowest value when the air mass was derived from the marine area with high RH and low pollutant concentrations. These observations suggest that the increase in M R is the result of the combined effect of high RH and high pollutant concentrations. [Display omitted] • Traditional lag-time method tends to misclassify thickly to thinly coated BC as particles with larger masses. • The increase of BC's coating needs the joint action of pollutants precursors, aging time and humidity. • The coating thickness of BC with larger masses had weaker diurnal pattern. [ABSTRACT FROM AUTHOR]