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The addition of coating to the black carbon (BC) enhances its absorption as more light is focused by the coating "lens." The absorption enhancement factor (Eabs) of BC is difficult to quantify due to an inadequate representation of its mixing structure and the interaction with radiation. Here, by tracking the evolution of the fresh BC particles in the ambient, we found a transitional stage of the particle Eabs with the non‐BC‐to‐BC mass ratio (RBC) at ∼2, below which there were insufficient coating materials to encapsulate the BC core and the absorption enhancement was not significant (∼14%). When the RBC >∼ 2, obvious absorption enhancement occurred as the BC cores were fully covered. Secondary inorganic species played the most critical role in the coating materials to enhance the "lensing effect." We suppose the particle‐resolved core‐shell Mie model can be applied in the Eabs prediction for most cases. Plain Language Summary: Black carbon is the dominant light‐absorbing component of atmospheric aerosols, significantly affecting the global climate. In the atmospheric aging process, such as condensation and oxidation, fresh BC is gradually coated. The light absorption of BC is significantly associated with the amount of its coating material since the coating could act as "lens" to refract more light toward the BC core. However, this "lensing effect" for BC particles is poorly quantified, bringing uncertainties when evaluating the aerosol radiative forcing. In this study, we found there was a transitional behavior for the light absorption by BC particles which was related to the particle mixing states. When the non‐BC‐to‐BC mass ratio for particles is larger than 2, the coating materials are insufficient to encapsulate the BC core and cannot act as the coating "lens." Our study provides a step forward in understanding the global radiation effect of BC. Key Points: A transitional stage of the absorption enhancement (Eabs) during the evolution of black carbon (BC) was directly observed in the ambientBy tracking the evolution of fresh BC, the predicted Eabs based on the particle‐resolved model showed consistent transitional stageThe secondary inorganic species acted the most critical role in the coating materials to enhance the absorption of BC‐containing particles [ABSTRACT FROM AUTHOR]

• The BC-containing particles were dominated by BC-nitrate particles (BC N, 52.3%) followed by BC-nitrate-sulfate particles (BC NS, 22.4%). • The enhanced mixing state of BC with nitrate was mainly due to the increase of ambient nitrate mass concentration. • The enhanced mixing state of BC with nitrate reduced the light absorption ability of aged BC particles. Black carbon (BC) plays an important role in air quality and climate change, which is closely associated with its mixing state and chemical compositions. In this work the mixing state of BC-containing single particles was investigated to explore the evolution process of ambient BC particles using a single particle aerosol mass spectrometer (SPAMS) in March 2018 in Zhengzhou, China. The BC-containing particles accounted for 61.4% of total detected ambient single particles and were classified into five types including BC-nitrate (BC-N, 52.3%) as the most abundant species, followed by BC-nitrate-sulfate (BC-NS, 22.4%), BCOC (16.8%), BC-fresh (BC-F, 4.5%) and BC-sulfate particles (BC-S, 4.0%). With enhancement of the ambient nitrate concentration, the relative peak area (RPA) of nitrate in BC-N and BC NS particles both increased, yet only the number fraction (N f) of BC N particles increased while the N f of BC-NS particles decreased, suggesting that the enhanced mixing state of BC with nitrate was mainly due to the increase in the ambient nitrate mass concentration. In addition, the N f of BC-N decreased from 65.3% to 28.4% as the absorbing Ångström exponents (AAE) of eBC increased from 0.75 to 1.45, which indicated the reduction of light absorption ability of aged BC particles with the enhanced formation of BC-N particles. The results of this work indicated a change in the mixing state of BC particles due to the dominance of nitrate in PM 2.5 , which also influenced the optical properties of aged BC particles. [Display omitted] [ABSTRACT FROM AUTHOR]

Black carbon (BC) strongly absorbs solar radiation, contributing to global warming. Absorption enhancement of BC particles is difficult to quantify due to an inadequate representation of their complex morphology and mixing structures, as well as interaction with radiation. Here, we apply a 3D method accounting for detailed BC mixing structures to predict the absorption enhancement of individual BC particles (Eabs) and the total BC particle population (Eabs, bulk). The diverse range of mixing structures in individual BC particles leads to variable Eabs that could hardly be predicted by empirical approximations. We find that the volume proportion of the BC embedded in coating (F) determines Eabs when the particle to BC core diameter ratio (Dp/Dc) is larger than 2.0. Our findings reveal the potential mechanism behind the differences in observed and modeled Eabs, bulk. The framework builds a bridge connecting the microscopic mixing structure of individual BC particle with Eabs, bulk. Plain Language Summary: Absorption by black carbon (BC) in the atmosphere strongly affects radiative balance and global climate. The large discrepancies in observed and modeled BC absorption enhancements raise a hot debate. Through applying a new 3D shape model based on electron microscope observations, we propose a new framework that estimates BC absorption enhancement through accounting for mixing structure diversity in individual particles. Our results reveal that the diverse range of mixing structures in individual particles in ambient air leads to complex absorption enhancement that could hardly be predicted by the empirical approximation. The bulk absorption enhancements based on diverse mixing structures provide an explanation for the globally disparate results from laboratory and field observations. The new framework linking microphysical structures to bulk BC optical properties can be used to improve assessment of climate impact. Key Points: The impact on absorption enhancement by embedded fraction is significantly enhanced when BC particle to core diameter ratio >2.0The embedded fraction and coating thickness in aged BC particles can lead to significant differences in bulk absorption enhancementA new bridge is built to connect the microscopic mixing structure of individual aged BC particles with bulk absorption enhancement of BC [ABSTRACT FROM AUTHOR]

The complex particle morphology of black carbon (BC) induces considerable uncertainties in constraining its optical properties. In particular, the diversity of how the BC core and coating are combined is yet to be resolved due to the lack of direct ambient quantification. In this study, by concurrent measurements of the electrical mobility diameter (Dm) and optical volume‐equivalent diameter (Dve), the dynamic shape factor (χ = Dm/Dve) of ambient BC‐containing particles at different mixing states was quantified for the first time. BC in urban Beijing showed a widely distributed χ of 1.4 ± 0.43. Particles with larger refractory BC (rBC) cores showed higher non‐sphericity and higher χ. A ten‐fold increase in the volume ratio of the coating over rBC (VR) could decrease the fraction of fractal BC by 36%. This study proposed a χ‐VR space to describe the particle shape, which is able to include all ambient quantified BC morphologies in optical model. Plain Language Summary: Black carbon (BC) is an important short‐lived light‐absorbing aerosol component in the atmosphere. Its optical properties are complicated by the complex particle morphology, such as the non‐BC substances associated with it and the combination of BC and non‐BC substances within particle. The previous quantification of particle morphology could not distinguish the information specifically for ambient BC, nor the single BC particle characterization can represent full diversities for all particles. This study used concurrent measurements of volume‐equivalent and mobility diameter of BC‐containing particles (BCc) to characterize the coating content and particle shape online for all BC particles in the ambient environment. As a result, we can include all the measured particle morphology information in optical models. Particles with larger BC cores showed higher non‐sphericity, with a greater discrepancy between the measured absorbing properties and that calculated by the Mie theory. Key Points: The morphology of ambient black carbon‐containing particles was characterized by volume‐equivalent and mobility diameter measurementsBlack carbon with higher coatings showed more spherical shapesA space using coatings and particle shape was developed to incorporate morphology information for all black carbon‐containing particles [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]

Reducing the uncertainty in aerosol radiative forcing requires a comprehensive understanding of the factors affecting black carbon (BC) light absorption. In this study, the characteristics and influencing factors of light absorption enhancement (E abs) of refractory BC (rBC) were investigated by conducting intensive measurements at an urban site in northwest China during the early summer of 2018. On average, the absorption of rBC was enhanced by 34% as a result of the internal mixing of rBC with other aerosol components. Secondary inorganic aerosols (SIAs) were found to have considerable effects on the E abs of rBC. The E abs showed a robust linear relationship with the bulk nitrate/rBC mass ratio in fine particles, with an increase of 3% per nitrate/rBC ratio unit. A notable increase in E abs from dusk to the next morning was observed, in accordance with the diurnal variations in nitrate and sulfate, indicating the excellent contribution of non-photochemical formation of SIAs to E abs. This fact was further supported by the positive correlation of the nitrate/rBC and sulfate/rBC ratios with relative humidity (RH) rather than photochemical indicators. This study indicates that the aqueous and/or heterogeneous formation of SIAs is likely the dominant aging pathway leading to the high E abs of rBC. • Light absorption enhancement (E abs) of BC by coating was observed in northwest China. • Nitrates and sulfates highly affected the E abs , especially nitrates. • The aqueous and/or heterogeneous formation of nitrates and sulfates played a dominant role in E abs. [ABSTRACT FROM AUTHOR]

Black carbon (BC) particles are responsible for substantial radiative heating of the atmosphere. However, the climate‐relevant properties of BC particles in the free troposphere (FT) are poorly constrained over longer time scales. Here, we report on in situ, intensive field campaigns deploying a single‐particle soot photometer during summer and winter seasons at the Jungfraujoch (3,580 m a.s.l., Switzerland), a site often located in the lower FT. The refractory BC (rBC) mass size distribution was very stable across the two seasons. BC was generally internally mixed (i.e., heavily coated with other aerosol material) in summer. However, against general expectations, BC was predominantly externally mixed in winter, with a high occurrence of negligibly‐to‐thinly coated BC. This strong seasonality in lower‐FT BC mixing state has not been previously observed and would substantially influence the lifetime, loading, and climate effects of free‐tropospheric BC. Future measurements should investigate the generality of these results. Plain Language Summary: Black carbon (BC) aerosols (i.e., soot particles) play an important role in global climate change by causing atmospheric warming. To understand this role, we need to know what types of BC particles are found in the atmosphere; how big are they and how are they mixed with other aerosols? This type of information is lacking for BC particles found in the free troposphere (the atmospheric layer immediately above the mixed boundary layer of air at the Earth's surface). We performed measurements during winter and summer seasons at Jungfraujoch in the Swiss Alps (3,580 m a.s.l.) in order to fill this gap in knowledge. We discovered a large and unexpected seasonal contrast; in summer at this site, BC aerosols are heavily coated with other aerosol material, whereas in winter the BC is largely uncoated. This seasonal contrast has important implications for our understanding of how long BC particles stay in the atmosphere (coated particles are removed faster), and our estimates of BC climate impacts (coated particles absorb more sunlight). Key Points: Laser‐induced‐incandescence measurements revealed no seasonal variability in black carbon (BC) mass size distributions at JungfraujochBC was heavily coated in summer but largely uncoated in winter, a seasonality that has not been reported beforeThese measurements have important implications for the aging timescales and lifetime of BC in the free troposphere [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]

In-situ measurements of aerosol optical properties were conducted at Mt. Huang from September 23 to October 28, 2012. Low averages of 82.2, 10.9, and 14.1 Mm−1 for scattering coefficient (σ sp, neph, 550), hemispheric backscattering coefficient (σ hbsp, neph, 550), and absorption coefficient (σ ap, 550), respectively, were obtained. Atmospheric aging process resulted in the increase of σ ap, 550 but the decrease of the single scattering albedo (ω 550) at constant aerosol concentration. However, the proportion of non-light-absorbing components (non-BCs) was getting higher during the aging process, resulting in the increase of aerosol diameter, which also contributed to relatively higher σ sp, neph, 550 and ω 550. Diurnal cycles of σ sp, neph, 550 and σ ap, 550 with high values in the morning and low values in the afternoon were observed closely related to the development of the planetary boundary layer and the mountain-valley breeze. BC mixing state, represented by the volume fraction of externally mixed BC to total BC (r), was retrieved by using the modified Mie model. The results showed r reduced from about 70% to 50% when the externally mixed non-BCs were considered. The periodical change and different diurnal patterns of r were due to the atmospheric aging and different air sources under different synoptic systems. Local biomass burning emissions were also one of the influencing factors on r. Aerosol radiative forcing for different mixing state were evaluated by a "two-layer-single-wavelength" model, showing the cooling effect of aerosols weakened with BC mixing state changing from external to core-shell mixture. Unlabelled Image [ABSTRACT FROM AUTHOR]

Recent studies worldwide have demonstrated that coating on atmospheric black carbon (BC) aerosol enhances its absorption potential. However, studies quantifying the light-absorption enhancement (E abs) of BC are extremely limited over the Indian region. The northwestern Indo-Gangetic Plain (IGP) region witnesses annual episodes of substantial crop-residue burning during the post-monsoon season (PoMS), which has been attracting wide attention due to its impact on regional air quality and aerosol characteristics. Chemical aging of these biomass burning (BB) emissions occurs during its long-range transport across the IGP by the weak westerlies, and such atmospheric processing affects their chemical, optical, and radiative properties over downwind locations. This study examines the impact of atmospheric aging of biomass burning emissions on BC light-absorption enhancement (E abs) using extensive, simultaneous measurements of single particle refractory BC and near-real-time mass spectrometry measurements of non-refractory submicron aerosols from an IGP outflow site. It was observed that organics were the dominant submicron aerosol species, and positive matrix factorization analysis of the organics mass spectra suggested that a large fraction of organics (64%) was secondary in nature. The BC particles in the IGP outflow (mass median diameter ∼ 0.18 ± 0.01 μm) were found to be thickly coated with volume-weighted bulk relative coating thickness in the range of 1.3–1.8. The evolution of biomass-burning organic aerosols (BBOA) indicated the presence of significant amounts of aged BBOA during the PoMS. Further, the oxidation products from the evolution of BB emissions contributed to enhanced coatings (∼ 30–70%) on BC. Mie core-shell theory calculations were then used to assess the changes in BC light absorption characteristics due to the coating of non-refractory mass on its cores. It was found that thick coatings on BC led to significant light-absorption enhancement (E abs) (bulk E abs in the range of 1.14–1.94 at 550 nm, depending on the extent of coating). Increased E abs values of ≥1.8 were seen when the coating on BC exceeded 60% of its core size. The modeled bulk E abs at 550 nm depicted a better association with highly oxidized secondary organic aerosols (SOA) than sulfate among the secondary aerosol species, suggesting an SOA-dominant regime. The present findings suggest that the critical impact of the aging of BC and co-emitted species from widespread BB emissions in the IGP outflow should be considered when evaluating the regional radiative implications. • Evidence for the presence of aged biomass-burning organic aerosols in the IGP outflow. • Aged biomass-burning organic aerosols contributed to enhanced coatings on BC cores. • BC absorption potential is enhanced by a factor > 1.5 due to non-refractory coatings. • A higher fraction of secondary organic aerosols resulted in enhanced absorption. [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]

Recent reports indicate that the absorption enhancement of black carbon (BC) and brown carbon (BrC) particles is determined by the evolution of morphology and mixing state during the atmospheric processing. In this study, laboratory-generated BC-BrC mixture particles (BC-BrC) were exposed to sulfuric acid, ammonia/triethylamine, and water vapor sequentially to investigate the alteration in light absorption, morphology and mixing state during simulated atmospheric processing. Condensation of sulfuric acid and exposure to ammonia or triethylamine at 5% relative humidity decreased light absorption cross section of BC-BrC by 13%–26%, but subsequent hydration at 85% relative humidity led to an increase by 5%–20%. Our results show that surface reactions and hydration can significantly alter the light absorption cross section of BC-BrC. Our observation suggests that the restructuring of BC-BrC induced by neutralization reactions on the BC-BrC surface was responsible for the reduction in light absorption cross section at low relative humidity. On the other hand, the formation of an aqueous coating through water uptake by the hydrophilic inorganic coating at high relative humidity caused an absorption enhancement. [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]

Abstract: Intensive firecracker/firework displays during Chinese New Year (CNY) release fine particles and gaseous pollutants into the atmosphere, which may lead to serious air pollution. We monitored ambient PM2.5 and black carbon (BC) concentrations at a regional background site in the Yellow River Delta region during the CNY in 2011. Our monitoring data and MOUDI images showed that there was a haze event during the CNY. Daily average PM2.5 concentration reached 183μgm−3 during the CNY, which was six times higher than that before and after the CNY. Similarly, the black carbon (BC) concentrations were elevated during the CNY. In order to confirm whether the firecracker/firework related emission is the main source of the haze particles, we further analyzed the morphology and chemical composition of individual airborne particles collected before, during and after the CNY by using transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM/EDS). We found that sulfate and organic-rich particles were dominant in the atmosphere before and after the CNY. In contrast, K-rich sulfates and other metal (e.g., Ba-rich, Al-rich, Mg-rich, and Fe-rich) particles were much more abundant than ammoniated sulfate particles during the CNY. These data suggest that it was the aerosol particles from the firecracker/firework emissions that induced the regional haze episode during the CNY. In individual organic and K-rich particles, we often found more than two types of nano-metal particles. These metal-bearing particles also contained abundant S but not Cl. In contrast, fresh metal-bearing particles from firecrackers generated in the laboratory contained abundant Cl with minor amounts of S. This indicates that the firecracker/firework emissions during the CNY significantly changed the atmospheric transformation pathway of SO2 to sulfate. [Copyright &y& Elsevier]

Abstract: Black carbon (BC) is the dominant light-absorbing aerosol component in the atmosphere and plays an important role in atmospheric pollution and climate change. The light-absorbing properties of BC rely on particle size, shape, composition, as well as the BC mixing state with other aerosol components, thus more thorough exploration of BC aerosol characteristics is critical in understanding its atmospheric sources and effects. In this study, a newly-developed Single Particle Soot Photometer (SP2) was deployed in Shenzhen, China, for continuous BC measurements to obtain the important information about size distribution and mixing state of BC under severe air pollution conditions of China. The mean BC mass concentrations were found to be 6.0 and 4.1 μg m−3 at an urban site (UT) in the fall and winter, respectively, while it is much lower (2.6 μg m−3) at a rural site (BG) in the fall. The mass size distributions of BC in volume equivalent diameter (VED) at the three sites showed a similar lognormal pattern, with the peak diameter at BG (222 nm) slightly larger than at the UT (210 nm) site. As to mixing state, the average percentage of internally mixed BC at the UT site was detected to be 40% and 46% in the fall and winter, respectively, while that at the BG site in the fall was only a slightly higher (47%), which implies that fresh local fossil fuel combustions were still significant at this rural site. The analysis of extremely high BC concentrations (>20 μg m−3) at UT indicates that they were a complex of comparable contributions from both local fresh emissions and regional transport under unfavorable meteorology. Other characteristics of BC aerosol and their influencing factors in Shenzhen were also discussed. [Copyright &y& Elsevier]

To investigate the effects of biomass burning and photochemical oxidation on the black carbon (BC) mixing state and light absorption, a tandem centrifugal particle mass analyzer (CPMA) and single-particle soot photometer (SP2) system was used to measure ambient fine particulate BC during EXPLORE-YRD 2018 campaign (EXP eriment on the e L ucidation of the atmospheric O xidation capacity and aerosol fo R mation, and their E ffects in Yangtze River Delta). The mass ratio (M R) of the non-BC coating to BC core, a morphology-independent parameter to quantify the mixing state of BC-containing aerosols, is derived. Two types of periods were identified, namely non-significant biomass-burning period (NB), and enhanced biomass burning period (EB). The M R was higher during EB (2.3 ± 0.5), compared with during the NB (2.0 ± 0.5). M R showed bimodal diurnal variation with peaks in the early morning and early afternoon, mainly due to the biomass burning and photochemical processes, respectively. The light-absorbing enhancement ranged from 1.0 to 1.19 when M R was from 1.4 to 3.4. The relationship of rBC mixing state and its optical properties was parameterized, which can further be used in the regional model. • BC coating-to-core Mass ratio was quantified using a tandem CPMA - SP2 system, with an average of 2.2 ± 0.5 during the campaign. • Photochemical processing played a dominant role in changing coating amount and light absorption of BC in the summer daytime. • The relationship of rBC mixing state and its optical properties (E abs) was parameterized. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the most important aerosol light-absorbing component, and its effect on radiation forcing is determined by its microphysical properties. In this study, two microphysical parameters of refractory BC (rBC), namely, size distribution and mixing state, in urban Beijing from 2013 to 2019 were investigated to understand the effects of source changes over the past years. The mass equivalent diameter of rBC (D c) exhibited bimodal lognormal distributions in all seasons, with the major modes accounting for most (>85%) of the rBC masses. The mass median diameter (MMD) was obviously larger in winter (209 nm) than in summer (167 nm) likely due to the contribution of more rBC with larger D c from solid fuel combustion and enhanced coagulation of rBC in polluted winter. More rBC particles were thickly coated in winter, with the number fraction of thickly coated rBC (f coatBC) ranging within 29%–48% compared with that of 12%–14% in summer. However, no evidential increase in BC light-absorption capability was observed in winter. This finding was likely related to the lower absorption efficiency of larger rBC in winter, which partly offset the coating-induced light enhancement. Two stage of decreases in MMD and f coatBC were observed, accompanied with a persistent decrease in rBC loading, thereby reflecting the discrepant effects of source control measures on rBC loading and physical properties. The control measures in the earlier stage before 2016 was more efficient to reduce the rBC loading but slightly influenced the microphysical properties of rBC. As of 2016, the reduction in rBC concentration slowed down because of its low atmospheric loading. However, rBC showed a more obvious decrease in its core size and became less coated. The decrease in f coatBC may have weakened the BC absorption and accelerated the decrease in light absorption resulting from the reduction in rBC loading. Image 1 Decreases in BC core size and coating fraction were observed in Beijing from 2013 to 2019 in response to clean air actions. [ABSTRACT FROM AUTHOR]

Interactions between clouds and black carbon (BC) represent a significant uncertainty in aerosol radiative forcing. To investigate the influence of cloud processing on the scavenging of BC, concurrent measurement of individual cloud droplet residue particles (cloud RES) and interstitial particles (cloud INT) throughout a cloud event was deployed at Mt. Tianjing (1690 m a.s.l.) in southern China. An aethalometer (AE-33), a single particle aerosol mass spectrometer (SPAMS) and a scanning mobility particle sizer (SMPS) were used to investigate the mass concentration of equivalent BC (EBC), size-resolved number of BC-containing particles, and size-resolved number concentration of submicron particles in real-time, respectively. The number-based SEs of the submicron particles varied between 2.7 and 31.1%. Mass scavenging efficiency (MSE) ranged from 4.7% to 52.6% for EBC, consistent with the number-based SE (from 11.3% to 59.6%) of the BC-containing particles throughout the cloud event. Several factors that may influence the SEs of the BC-containing particles are considered and examined. SEs are most likely determined by a single factor, i.e., liquid water content (LWC), with R2 > 0.8 in a power function throughout the cloud event. Stage-resolved investigation of SEs further reveals that particle size matters more than other factors in the cloud formation stage, whereas there is an increasing role of the mixing state in the development and stability stage. We also observed lower SEs for the BC-containing particles internally mixed organics, consistent with previous literature. • The stage-resolved scavenging efficiency of BC particles is firstly reported. • Scavenging efficiency (SE) is dominantly controlled by liquid water content (LWC). • Key factors influencing the in-cloud SE of BC-containing particles are assessed. [ABSTRACT FROM AUTHOR]

Black carbon (BC) exerts important radiative effects over regions with intensive emissions. This study presents in-situ aircraft measurements of BC vertical profiles including mass loading, size distribution and mixing state, spanning a range of pollution levels in both warm and cold seasons over Beijing. The development of planetary boundary layer (PBL) influenced the properties of pollutants at low levels, and regional transport from the southwest elevated the pollution at higher altitudes. Thicker coatings of BC were associated with higher pollution in the PBL, where interactions between BC and other substances intensively took place. Considering the mixing state of BC, the absorption efficiency could be potentially increased by up to 86% and 60% in the PBL and lower free troposphere, respectively. Including a column-integrated absorption enhancement, the in-situ constrained absorption aerosol optical depth at wavelength 870 nm (AAOD 870) improved the agreement with AERONET by 28%, but the in-situ measurement remained 19% lower. A radiative transfer model finds a BC heating rate of 0.1–0.3 K/d and 0.5–3.1 K/d for less and more polluted environments respectively, and the BC coating effect could positively introduce a +0.1–4.2 Wm-2 radiative forcing. The presence of aerosol layer enhanced the positive vertical gradient of heating rate by redistributing the actinic flux. In particular, this gradient was further enhanced by introducing thickly-coated BC at higher level during the regional transport events, which may promote the temperature inversion and further depress the PBL development on polluted days. • BC mass loading for less polluted Beijing was consistent with urban EU/NA. • BC heating rate in the PBL increased to 0.5-3.1K/d on polluted days and had positive vertical gradient. • BC coatings enhanced the columnar absorption up to 80% with positive radiative forcing +0.1–4.2 Wm-2. [ABSTRACT FROM AUTHOR]

The accurate derivation of the proportion and absorption enhancement of black carbon (BC)-containing aerosols in the atmosphere is critical to assess their effect on air quality and climate. Here, using the field measured size-resolved volatility shrink factor, BC bulk mass concentration and the BC mass fraction in BC-containing particles in winter Beijing, we retrieved and quantified both the number and mass concentration of (1) non-BC, (2) internally mixed BC and (3) externally mixed BC of ambient fine aerosol particles. The reliability of the retrieval method has been evaluated by comparing with the simultaneously measured data. The number fraction of BC-containing particles accounts for 60–78% of ambient fine particles, with internally (both BC core and coating materials) and externally mixed BC of 51–64% and 9–23%, respectively. Only for nucleated particles on clean days, when nucleation is a major source of aerosol particles, did the non-BC component dominate (54%). A large amount of aerosols are BC-containing particles, with mass fraction of 32–52%, suggesting the dominant role of BC in elevating mass concentration of particulate matter (PM) in a polluted urban area. We also show that the BC particles are thickly coated with coating thickness (characterized by D p /D c , ratio of the BC diameter before and after heating at 300 °C) of 1.6–2.2, implying efficient aging of BC particles in polluted urban area. Our results imply a large proportion of BC-containing particles in the atmosphere, which could help towards understanding the role of BC on regional haze formation and climate forcing. Image 1 • Mixing state of BC was retrieved using a new method. • BC-containing particles account for large proportions of ambient fine aerosols. • The BC-containing particles are thickly coated. Main findings of this work: 1. BC-containing particles account for large proportion. 2. The BC-containing particles are thickly coated. [ABSTRACT FROM AUTHOR]

A single particle soot photometer (SP2) was deployed in urban Nanjing, located in the Yangtze River Delta (China), to investigate the mixing state and sources of ambient refractory black carbon (rBC) from 26 January to 25 February 2014, along with an in-situ measurement of submicron aerosol chemical species by an aerodyne aerosol chemical speciation monitor (ACSM). The results showed that anthropogenic activities associated with firework emissions can be a significant source for rBC-containing particles during the period of the Chinese New Year, resulting from the evident peaks of rBC at midnight. During the residual periods, namely regular day (RD), the diurnal cycles of rBC presented two typical peaks that can be attributed to a synergistic influence of local traffic emissions and boundary layer changes throughout a day. Three coating factors, including organics, sulfate, and nitrate (-rich), were resolved using a positive matrix factorization (PMF) approach to explain the potential contribution of non-rBC coatings (i.e., organics, sulfate, and nitrate) to the coating thickness of rBC-containing particles. As the results show, organic aerosols (OAs) might be a major contributor to the coating thickness of rBC-coating particles during the whole period. The relative coating thickness (a ratio between coated particle size to BC core) exhibited a positive relationship with sulfate, indicative of the favorable coating factor during the episode caused by firework emissions. Source apportionment of rBC was performed via a multiple linear regression between the total rBC mass and each ACSM-PMF factor (rBC-ACSM-PMF). On average, biomass burning emissions accounted for 43%, being the largest contributor during the RD period, whereas local traffic emissions played a major role during the new year time. [ABSTRACT FROM AUTHOR]

Direct in-situ measurements of aerosol mixing state, optical properties, and chemical composition were performed in summertime of 2014 at Nanjing, China. Aerosols were predominantly internally mixed, with an average effective density of 1.30–1.63 g cm−3 for 50–230 nm particles, increasing with size. Externally mixed, relatively fresh black carbon (BC) was only episodically observed, with a second mode peaking at 0.51–0.91 g cm−3. For particles of 110, 140, 185 and 230 nm, BC accounted for 1.7 ± 1.2%, 4.8 ± 3.0%, 5.3 ± 3.3%, and 5.1 ± 3.3% of the particle mass, while being present in 26.4 ± 5.3%, 58.1 ± 27.7%, 59.8 ± 25.4%, and 62.4 ± 27.9% of the particle number concentration, indicating that BC was heavily coated and may contribute significantly to atmospheric aerosol population. Substantial BC absorption enhancement was observed with E abs of 1.41 ± 0.39, 1.42 ± 0.40 and 1.35 ± 0.38 at 405, 532 and 781 nm, respectively. High volatile aerosol components, in particular nitrate, were found to play vital roles in BC's absorption enhancement. High E abs values were associated with elevated NO x and RH. A clear diurnal pattern was observed for E abs supporting a significant impact from traffic emissions, which stood in contrast with previous studies reporting very thin coating and negligible absorption enhancement for traffic emitted BC. High concentrations of NO x co-emitted with BC from traffic sources and its conversion to particulate nitrate likely contributed to the aging and increased absorption of BC particles, which was even enhanced under high RH above the deliquescence point of ammonium nitrate. Therefore, our results indicated that the mitigation of NO x emissions from traffic was critical for reducing the positive radiative forcing induced by BC, especially under high RH conditions. Image 1 • Mixing state, optical properties, and composition of BC were measured in Nanjing. • E abs (BC) of 1.41 ± 0.39, 1.42 ± 0.40 and 1.35 ± 0.38 at 405, 532 and 781 nm were observed. • NO x co-emitted with BC from traffics may lead to the BC aging and observed E abs (BC). [ABSTRACT FROM AUTHOR]

The impact of mixing state of particles from biomass burning on aerosol optical properties (aerosol optical depth (AOD) and single-scattering albedo (SSA)) is studied over the Euro- Mediterranean region during the severe fire event in the Balkans between 20 and 31 July 2007, also characterized by high dust concentrations. When the mixing state is resolved in chemistry-transport models, chemical compounds are grouped for computational reasons, and internal mixing is assumed within each group. Up to six groups are defined here (dust, black carbon, two inorganic groups and two organic groups). The influence of different grouping assumptions is studied here and compared to the influence of the distribution of black carbon (BC) in particles ("pure homogeneous" representation and "core-shell" one), and the influence of modeling the water absorbed by both inorganic and organic compounds. The comparisons of simulated AODs to observations from the surface network AERONET show that AOD is slightly underestimated when aerosol compounds are assumed to be externally mixed and slightly overestimated when they are assumed to be internally mixed. The mixing state of dust with other compounds, as well as the distribution of BC in particles, strongly influence the optical properties. The impact of the mixing state on AOD is higher than the impact of the distribution of BC in particles, reaching 8–12% on average over the fire regions and 16% in the fire plume. Analysis related to the impact of particle mixing state and BC distribution on SSA shows results similar to AOD. The impact of the mixing state on SSA can reach −8.5% over the fire regions and it is higher than the impact of the BC representation, which is lower than 2%. At the location of fires, water absorbed by inorganics and organics is shown to influence the AOD by about 2%, which is in the lower range of the influence of water on AOD over the region (between 0 and 40%). This low influence of water on AOD during fire is due to assumptions made in the modelling, where most of secondary organic aerosols formed during fires are assumed to be hydrophobic. • Water absorbed by inorganics and organics enhances optical properties. • Mixing-state representation matters more than black-carbon distribution in particles. • The mixing state of dust needs to be considered separately from other compounds. [ABSTRACT FROM AUTHOR]