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Black carbon (BC) aerosols in the atmosphere play a significant role in climate systems due to their strong ability to absorb solar radiation. The lifetime of BC depends on atmospheric transport, aging and consequently on wet scavenging processes (in-cloud and below-cloud scavenging). In this study, sequential rainwater samples in eight rainfall events collected in 2 mm interval were measured by a tandem system including a single particle soot photometer (SP2) and a nebulizer. The results showed that the volume-weighted average (VWA) mass concentrations of refractory black carbon (rBC) in each rainfall event varied, ranging from 10.8 to 78.9 µg/L. The highest rBC concentrations in the rainwater samples typically occurred in the first fraction from individual rainfall events. The geometric mean median mass-equivalent diameter (MMD) decreased under precipitation, indicating that r BC with larger sizes was relatively aged and preferentially removed by wet scavenging. A positive correlation (R 2 = 0.73) between the VWA mass concentrations of r BC in rainwater and that in ambient air suggested the important contribution of scavenging process. Additionally, the contributions of in-cloud and below-cloud scavenging were distinguished and accounted for 74% and 26% to wet scavenging, respectively. The scavenging ratio of rBC particles was estimated to be 0.06 on average. This study provides helpful information for better understanding the mechanism of rBC wet scavenging and reducing the uncertainty of numerical simulations of the climate effects of rBC. [Display omitted] [ABSTRACT FROM AUTHOR]

The mixing state of refractory black carbon (rBC) aerosols is regarded as a major uncertainty in climate-forcing assessments. This study measured the size distribution and mixing state of rBC using a single-particle soot photometer in the winter of 2022 at an urban site in Beijing. During the campaign, the mean mass concentration of rBC was 0.79 ± 0.63 μg m−3, much lower than that observed in Beijing in the winter of 2013. The number and mass median diameter of the rBC cores were 97 and 200 nm, respectively. The average relative coating thicknesses (RCT) of rBC-containing particles was 1.45 ± 0.27 and average light absorption enhancement (Eabs) was 1.43 ± 0.22. Similar to Eabs, RCT and secondary inorganic aerosols (SIA), especially nitrate, increased with increasing relative humidity. This suggests that higher SIA mass fraction in aerosol with a higher relative humidity in the environment results in a thicker coating and black carbon ageing, thus enhancing light absorption. RCT and Eabs increased rapidly with the increase of PM 1 loading but increased slowly when the PM 1 mass concentration exceeded 60 μg m−3. This indicates that a further increase in the NR-PM 1 species at high PM loading does not increase the thickness of the coatings; therefore, there is no further increase in Eabs. The effects of the chemical composition of the Eabs obtained using the Lindeman, Merenda, and Gold method showed that nitrate plays a prominent role in the enhancement of light absorption, followed by SOA. The rBC mass concentration exhibited clear day-low and night-high patterns which were influenced by the synergetic effects of the boundary layer and emissions. RCT and Eabs presented similar diurnal variations, peaking in the afternoon, which was consistent with the pattern of SIA. This suggests that the SIA produced via photochemical reactions supports the ageing of black carbon-containing particles, leading to light-absorption enhancement. The back-trajectory analysis showed the highest values of rBC mass concentration, RCT, and Eabs appeared in the S cluster suggested that the rBC-containing particles transported from south Beijing were more aged with a thicker coating. While the rBC-containing particles from northern and northwestern Beijing had lower RCT and Eabs values. • Nitrate is the dominant contributor to the coatings on BC and prominent role in light-absorption enhancement. • RCT and Eabs increased rapidly as PM 1 loading increased, but slowly when the PM 1 mass concentration exceeded 60 μg m−3. • The rBC- containing particles transported from south Beijing are more aged with a thicker coating. • ∼15% of the NR-PM 1 mass was bound to BC particles. [ABSTRACT FROM AUTHOR]

The light absorption enhancement (E abs) of black carbon (BC) caused by non-BC materials is an important source of uncertainty in radiative forcing estimate, yet remains poorly understood in relatively polluted environment such as the megacity Beijing. Here BC absorption enhancement at 630 nm was in-situ measured using a thermodenuder coupled with a soot particle aerosol mass spectrometer and a single scattering albedo monitor in Beijing in summer. The project average (±1σ) E abs was 1.59 (±0.26), suggesting a significant amplification of BC absorption due to coating materials. E abs presented a clear daytime increase due to enhanced photochemical processing, and a strong dependence on the mass ratios of non-BC coatings to BC (R BC). Our results showed that the increase in E abs as a function of R BC was mainly caused by the increased contributions of secondary aerosol. Further analysis showed that the BC absorption enhancement in summer in Beijing was mainly associated with secondary formation of nitrate, sulfate and highly oxidized secondary organic aerosol (SOA), while the formation of freshly and less oxidized SOA appeared not to play an important role. Image 1 • A significant BC absorption enhancement (∼1.6) in summer in Beijing. • Light absorption enhancement depends strongly on the amount of BC coatings. • SIA and highly oxidized SOA play the most impacts on absorption enhancement. [ABSTRACT FROM AUTHOR]

The physical properties of black carbon (BC) including the mass loading, size distribution and mixing state were in-situ characterized by aircraft measurements using a single particle soot photometer (SP2) in the lower troposphere over Beijing area. The flights were conducted in both late spring and winter during the daytime with well-developed planetary boundary layer (PBL). The BC mass in the PBL (BC PBL) in late spring showed no apparent vertical gradient nor correlation with the PBL height (PBLH) due to strong convective mixing; in winter the BC PBL was more concentrated near ground and anti-correlated with the PBLH due to dilution effect of the dominant cleaner northerly air masses at higher altitude. The BC mass loading at height h , C(h) within the PBL can be extended from the surface level (C0) in late spring; for levels above the PBL, C(h) can be parameterized by applying an exponential decline function C(h) = C0*exp(h/hs) , with the scale height (hs) of 0.31 ± 0.16 km and 0.66 ± 0.24 km for late spring and winter respectively. This parameterization excluded the profiles for: turbulent conditions when the BC mass was efficiently vented upwards and diluted, expressed as C(h) = C0 up to the top of the PBL; or in periods of strong southerly advection, when the entire column was significantly influenced by regional transport from the polluted south regions. The BC core mass median diameters (MMD) were commonly populated at 205–220 nm in both seasons, with additional mode of MMD ∼195 nm also frequently observed in late spring. The bulk relative coating thickness of BC (coated diameter divided by uncoated core diameter D p /D c) in the PBL mostly populated at 2.0–2.2 but at ∼1.2–1.6 in the lower free troposphere (FT). The mass absorption cross section of BC at 550 nm (MAC550), constrained by the SP2 measurements, was largely influenced by the coating thickness, was relatively consistent in the PBL at ∼8.6 m2 g−1, but reduced to 7–7.5 m2 g−1 in the FT or turbulent condition due to decreased coatings. The BC was found to exhibit smaller particle size in the FT but larger in the PBL, which may imply larger BC have been scavenged by low-level clouds. The BC particles trapped in the PBL or regionally transported from polluted region represent the most absorbing element in the particulate matter population and should be particularly considered in evaluating the radiative forcing impact of aerosols over this region. • Vertical profiles of black carbon physical properties in the lower troposphere were in-situ characterized over Beijing. • Black carbon at different layers was influenced by seasonal emission and meteorological conditions. • These results will improve modelling on black carbon radiative forcing and constrain remote sensing absorption measurement. [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]

Black carbon (BC), which is a by-product with incomplete combustion of carbonaceous materials, can be used as an indicator of combustion emissions and is an important climate forcer. In this study, a spatial–temporal synthesis of BC aerosols and the affecting factors was conducted in urban Beijing. As observed, BC showed a spatial pattern with high concentration in south and low in north. BC concentration evidently decreased by approximately 61% between 2005 and 2017. From 2015 to 2017, the mass ratio of BC/PM 2.5 dropped by 28%, which suggested a more efficient effect of control measures to BC than PM 2.5. The BC/CO ratio dropped by 22%, which indicated the decreasing emission from fossil fuel sources. With regard to BC loading, the spectral dependence of absorption aerosol exhibited significant seasonal variations. High absorption Ångström exponent (α) was observed during heating season, which reflected the increasing contribution of brown carbon (BrC) to light absorption. Backward trajectory analysis showed that the levels of BC and PM 2.5 were high in Cluster-South and Cluster-West. BrC absorption was high in Cluster-West, Cluster-Northwest and Cluster-Northeast, due to the biomass and coal burning for domestic heating and aging processes on a regional scale. The effects of emission control and transport variability on pollutant variation were estimated on the basis of the cluster analysis. Results indicated that the effect of emission reduction was the major reason for the decrease of BC from 2015 to 2017, which resulted in a 34% reduction of BC concentration. Meanwhile, transport variability caused a 15% reduction. • A decrease by 61% in BC concentration was observed in urban Beijing during 2005–2017. • Approximately 34% of the decrease in BC concentration was attributed to emission control. • BrC contributed a considerable part of total aerosol light absorption, particularly during heating season. [ABSTRACT FROM AUTHOR]