Black Carbon Aging Research Articles

Impacts of elevated anthropogenic emissions on physicochemical characteristics of black-carbon-containing particles over the Tibetan Plateau

Abstract. Black carbon (BC) in the Tibetan Plateau (TP) region has distinct climate effects that strongly depend on its mixing state. The aging processes of BC in the TP are subject to emissions from various regions, resulting in considerable variability of its mixing state and physicochemical properties. However, the mechanism and magnitude of this effect are not yet clear. In this study, field observations on physicochemical properties of BC-containing particles (PMBC) were conducted in the northeast (Xihai) and southeast (Lulang) regions of the TP to investigate the impacts of transported emissions from lower-altitude areas on BC characteristics in the TP. Large spatial discrepancies were found in the chemical composition of PMBC. Both sites showed higher concentrations of PMBC when they were affected by transported air masses outside the TP but with diverse chemical composition. Source apportionment for organic aerosol (OA) suggested that primary OA in the northeastern TP was attributed to hydrocarbon OA (HOA) from anthropogenic emissions, while it was dominated by biomass burning OA (BBOA) in the southeastern TP. Regarding secondary aerosol, a marked enhancement in nitrate fraction was observed on aged BC coating in Xihai when the air masses were brought by updrafts and easterly winds from lower-altitude areas. With the development of boundary layer, the enhanced turbulent mixing promoted the elevation of anthropogenic pollutants. In contrast to Xihai, the thickly coated BC in Lulang was mainly caused by elevation and transportation of biomass burning plumes from south Asia, showing a large contribution of secondary organic aerosol (SOA). The distinct transported emissions lead to substantial variations of both chemical composition and light absorption ability of BC across the TP. The thicker coating and higher mass absorption cross-section (MAC) of PMBC in air masses elevated from lower-altitude regions reveal the promoted BC aging processes and their impacts on the mixing state and light absorption of BC in the TP. These findings emphasize the vulnerability of plateau regions to influences of elevated emissions, leading to significant changes in BC concentration, mixing states and light absorption across the TP, all of which need to be considered in the evaluation of BC radiative effects for the TP region.

Enhanced Aging of Black Carbon under Recent Clean Air Actions and Future Carbon Neutrality Scenario in China.

China has implemented strict emission control measures, but it is unclear how they affect black carbon (BC) aging and light absorption. Here, we use the Community Atmosphere Model version 6 (CAM6) with the four-mode version of the Modal Aerosol Module coupled with machine learning (MAM4-ML) to simulate BC aging during 2011-2018 and 2050/2100 following a carbon neutrality scenario (SSP1-2.6), respectively. During 2011-2018, the mass ratio of coatings to BC (RBC) widely increased (5.4% yr-1) over the east of China. The increased secondary organic aerosol (SOA) coatings dominate (88%) the increased RBC, while the sulfate coatings decrease. The drivers of BC coating changes come from the different magnitudes of emission reductions in secondary aerosol precursors (i.e., volatile organic compounds (VOCs) and SO2) and BC. During 2011-2018, the increased RBC enhances the BC mass absorption cross section (MAC, 0.7% yr-1). In 2050/2100 for SSP1-2.6, emission control leads to further increased RBC (95/145%) and BC MAC (12/17%). For both 2011-2018 and 2050/2100, the enhanced BC MAC partly offsets the declining direct radiative effect (DRE) of BC due to direct emission reduction. As a result, the full impact of direct emission reductions of BC on BC DRE is only 75% for 2011-2018 and 90/94% for 2050/2100.

Optical properties and radiative forcing of carbonaceous aerosols in a valley city under persistent high temperature

Carbonaceous aerosols were collected in the valley city of Baoji city in Northern China in August 2022. The light absorption characteristics and influencing factors of black carbon (BC) and brown carbon (BrC) were analyzed, and their radiative forcing was estimated. The results showed that the light absorption of secondary brown carbon [AbsBrC,sec (370)] was 7.5 ± 2.4 Mm−1, which was 2.5 times that of primary brown carbon [AbsBrC,pri (370), 3.0 ± 1.2 Mm−1]. During the study period, the absorption Ångström exponent (AAE) of aerosol was 1.6, indicating that there was obvious secondary aerosol formation or carbonaceous aerosol aging in the valley city of Baoji. Except for secondary BrC (BrCsec), the light absorption coefficient (Abs) and mass absorption efficiency (MAE) of BC and primary BrC (BrCpri) during the persistent high temperature period (PHT) were higher than those during the normal temperature period (NT) and the precipitation period (PP), which indicated that the light absorption capacity of black carbon and primary brown carbon increased, while the light absorption capacity of secondary brown carbon decreased under persistent high temperature period. Secondary aerosols sulfide (SO42−), nitrate (NO3−) and secondary organic carbon (SOC) are important factors for promoting the light absorption enhancemen of BC and BrCpri and photobleaching of BrCsec during persistent high temperature period. The Principal Component Analysis-Multiple Linear Regression (PCA-MLR) model showed that traffic emissions was the most important source of pollution in Baoji City. Based on this, the secondary source accelerates the aging of BC and BrC, causing changes in light absorption. During PHT, the radiative forcing of BC and BrCpri were enhanced, while the radiative forcing of BrCsec was weakened, but the positive radiative forcing generated by them may aggravate the high-temperature disaster.

Impacts of α-pinene ozonolysis products on the morphology and optical properties of black carbon

Black carbon (BC) is known to absorb solar radiation and thus have a warming impact on climate. The atmospheric aging of fresh BC has been suggested to enhance BC's light absorption, but the extent of variation observed varies largely in field and laboratory measurements. Biogenic VOCs with much higher abundance than anthropogenic emissions are reactive towards atmospheric oxidants, and act as major sources of secondary organic aerosol (SOA) in the atmosphere. In this study, we performed experiments of BC aging in the presence of condensable organic vapors generated from the ozonolysis of α-pinene. Significant changes in the particle size, mass, effective density, morphology as well as optical properties are observed upon coating of BC by α-pinene ozonolysis products. While the BC aging rate depends on the initial concentrations of precursors forming condensable materials and the particle size, the extent of transformation of BC at a given size is mainly controlled by the volume equivalent coating thickness. The highly fractal BC core is converted to a more compact morphology for a coating thickness of ∼25–30 nm depending on the initial particle size. The absorption and scattering of light by BC also increase by more than 1.29- and 5.30- fold during the coating process. The enhancement of optical properties can be attributed to both formation of coatings and restructuring of the BC core, with the latter accounting for a large fraction of optical enhancement. Our results indicate that the ozonolysis of α-pinene has profound effects on the morphology and optical properties of BC, with a magnitude comparable to sulfuric acid and ammonium nitrate, but much higher than other reported anthropogenic hydrocarbons. Although present in relatively low concentrations, α-pinene oxidation reactions can considerably enhance the impacts of BC on air quality and climate forcing.

Aging of biomass burning emissions in the Indo-Gangetic Plain outflow: Implications for black carbon light-absorption enhancement

Recent studies worldwide have demonstrated that coating on atmospheric black carbon (BC) aerosol enhances its absorption potential. However, studies quantifying the light-absorption enhancement (Eabs) 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 (Eabs) 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 (Eabs) (bulk Eabs in the range of 1.14–1.94 at 550 nm, depending on the extent of coating). Increased Eabs values of ≥1.8 were seen when the coating on BC exceeded 60% of its core size. The modeled bulk Eabs 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.

Evaluating BC Aging Processes in the Community Atmosphere Model Version 6 (CAM6)

AbstractThe uncertainty of the climatic effect of Black carbon (BC) remains large. One critical uncertainty source that needs to be captured is BC aging. Here we use the Community Atmosphere Model version 6 (CAM6) configured with the four‐mode version of the Modal Aerosol Module (MAM4) to evaluate the modeled BC aging process with recent laboratory and in‐situ measurements over China. As revealed by the comparison of BC aging timescale and number fraction of aged BC against recent measurements, the modeled condensation aging timescale is estimated to be about 0.8 hr (17%) faster than the chamber measurement, and the diurnal variations of modeled BC aging degree are typically higher than observations mainly due to the fast increase in modeled BC aging degree during daytime. Further analysis shows that the condensation aging dominates (>70%) BC aging across China. More specifically, the condensation of secondary organic aerosol (SOA) vapor contributes most to BC aging over China. Slowing down BC aging increases the modeled surface BC concentration over remote Western China and BC burden, but hardly changes surface BC concentration over Eastern China. Our results suggest that BC aging representation in the MAM4 needs to be further improved toward slowing down the BC aging rate, especially the condensation aging by SOA, to improve the BC simulation over remote areas and its impact on BC transport in MAM4.

Mixing state of refractory black carbon in the residual layer over megacity

Black carbon (BC) is one important component contributing to global warming and its climate-related impacts strongly depend on mixing state. Previous observations at ground level indicated BC aging was at a fast rate in daytime with efficient photochemical reactions, while BC aging significantly weakened at night. Here we present evidences that BC aging still occurs efficiently at night in the residual layer (RL). The ratio of “thickly” coated refractory BC (rBC) in total rBC (fBC) increased from 51.3% at 00:00 LST to 61.5% at 07:00 LST at the CITIC station, which located in the RL at night, with an increasing rate of 1.4% per hour. Such an increasing rate was even higher than that during noontime (11:00 to 15:00 LST, 0.7% per hour). Similar trend also reflected in the coating thickness (Dp/Dc) of rBC particles, which increased from 1.52 at 00:00 LST to 1.63 at 07:00 LST. The aging of rBC in the RL at night enhances light absorption of rBC particles correspondingly; calculated absorption enhancement (Eabs) increased from 1.64 at 00:00 LST to 1.79 in at 07:00 LST. Further analysis indicated that the Eabs depends not only on the Dp/Dc of rBC particles, but also on its size. An increase in the size of rBC particles in polluted episode can also enhance the Eabs. Combined observations of development of boundary layer and pollutants at the CITIC station suggested that rBC particles were upwards transported in daytime and trapped in the RL at night, where they were aged efficiently. These results will improve our understanding on rBC aging in the atmosphere, and hence help to evaluate its radiative forcing.

Differential mobilization and sequestration of sedimentary black carbon in the East China Sea

Black carbon (BC) derived from incomplete combustion of biomass and fossil fuels on land can be mobilized and transported to the ocean. Burial of BC in the ocean sequesters atmospheric CO2 into a long-term carbon sink, likely exerting a positive influence on mitigating global warming. However, the abundances, sources, and burial of sedimentary BC in marine sediments remain poorly constrained, hindering us from accurately understanding the mobilization and sequestration of BC and its roles in the ocean carbon cycle. Here, we investigate concentrations and isotopes (13C and 14C) of BC among grain size-fractionated surface sediments along a across-shelf transect from the Yangtze River prodelta to the Okinawa Trough to decipher the fate of BC in the East China Sea (ECS). Our results show that the bulk BC concentrations decrease firstly from the Yangtze River prodelta to the outer shelf and then increase to the Okinawa Trough. Grain size-fractionated BC concentrations vary along the transect, which we mainly attribute to the differential mobilization of BC driven by hydrodynamic processes. We argue that BC is aged during the mobilization, which results in an older 14C ages of BC found seaward. After considering biomass- and fossil-derived BC apportionments based on 14C balance calculation, we think that BC aging may be verified by more fossil-derived BC burial in the Okinawa Trough. We estimate that BC may account for ∼15% of sedimentary organic carbon (SOC), and up to ∼30% of terrestrial SOC buried in the ECS. BC burial fluxes decrease along the transect, and are heterogeneous in different size fractions, indicating differential sequestration of BC in the shelf and trough. We further estimate that 685 Gg/yr of BC is sequestered in the ECS, and 491 Gg/yr in the prodelta area, with ∼30% being continental biomass-derived BC. We suggest that increasing biomass-derived BC production on land and burying it in the ocean may serve as a powerful means for sequestrating atmospheric CO2, potentially contributing to carbon neutrality.

Weakened Haze Mitigation Induced by Enhanced Aging of Black Carbon in China

A great challenge for haze pollution mitigation with the existing emission control measures in China is ozone (O3) increase. The chemical processes leading to weakened haze mitigation are still poorly understood. Our work identifies the enhanced aging chemistries of black carbon (BC) with increasing O3 as an essential driver to weaken haze mitigation based on field observations during autumn/winter haze periods in 2014 and 2018 in North China Plain. The enhanced atmospheric oxidation capacity induced by increasing O3 promotes the initial aging of accumulated fresh BC from continuous emission under haze pollution conditions and consequently improves the hygroscopicity of BC-containing particles to provide more particulate surfaces and volumes for aqueous and heterogeneous chemistries. The enhanced BC aging amplifies PM2.5 concentrations by ∼20%, which can be broken by concurrent reductions in multipollutant emissions (i.e., BC, nitrogen oxides, and volatile organic compounds), especially from residential and industrial sources. Moreover, enhanced BC aging implies an adverse effect of O3 increase on climate change. Observationally enhanced BC aging will help to constrain estimations of the interactions among O3 increase, haze pollution, and climate warming in recent years in China.

The effect of black carbon aging from NO2 oxidation of SO2 on its morphology, optical and hygroscopic properties

Atmospheric aging of black carbon (BC) leads to changes in its physiochemical properties, exerting complex effects on environment and climate. In this study, we have conducted laboratory chamber experiments to investigate the effects of BC aging on its morphology, hygroscopicity and optical properties by exposing monodisperse fresh BC particles to ambient ubiquitous species of nitrogen dioxide (NO2), sulfur dioxide (SO2) and ammonia (NH3) in absence of UV light. We show a rapid aging from highly fractal to compacted aggregates for the monodisperse BC particles with an initial diameter of 150 nm, with decline in the dynamic shape factor (χ) from about 1.8 to nearly 1. The effective density of the monodisperse BC particles increases from ∼0.54 to 1.50 g cm−3 accordingly. The aging process leads to that the light scattering, absorption, and single scattering albedo of the monodisperse BC particles are strongly enhanced by factors of 7.0, 1.8 and 3.0 respectively. By comparing with the BC aging from other mechanisms, we reveal a critical role of the composition of the coating materials on BC in determining its light absorption enhancement. Moreover, due to strong water uptake capacity of the aged BC particles, the light absorption enhancement (Eabs) could be 40–60% higher at humid atmosphere compared with dry conditions. This BC aging process from NO2 oxidation of SO2 may occur commonly in polluted regions and thus considerably alter its effects on regional air quality and climate.

Intercomparison of equivalent black carbon (eBC) and elemental carbon (EC) concentrations with three-year continuous measurement in Beijing, China

Due to the lack of black carbon (BC) measurement data in some cases, elemental carbon (EC) is often used as a surrogate of BC, with a simple assumption that they are interchangeable. Such assumption will inevitably lead to uncertainties in radiative forcing estimation and health impact assessment. In order to quantitatively and systematically evaluate the relationship between BC and EC as well as factors responsible for their difference, 3-year collocated equivalent BC (eBC) and EC measurements with 1-h resolution were performed in Beijing, China continuously from 2016 to 2019. EBC concentration was measured by the multi-wavelength aethalometer (AE-33) based on optical analysis, while EC concentration was determined by semi-continuous OC/EC analyzer with thermal-optical method. The results showed that around 90% of eBC concentration was higher than that of EC, with average difference between eBC and EC as 1.21 μg m−3 (accounting for 33% of average eBC in Beijing). EBC and EC concentrations exhibited strong correlation (r = 0.90) during the whole study period, but the slopes (or eBC/EC ratio) and correlation coefficients varied across seasons (spring: 1.67 and 0.94; summer: 0.91 and 0.65; fall: 1.15 and 0.88; winter: 1.09 and 0.91, respectively). Based on the information from shell/core ratios by Single Particle Soot Photometer (SP2), source apportionment results by positive matrix factorization model, and chemical composition of PM2.5, the differences between eBC and EC concentrations were found to be primarily related to BC aging process and secondary components as evidenced by strong positive correlation with secondary species (e.g., secondary organic carbon and nitrate). This study provided seasonal specific conversion factors of eBC and EC in Beijing and helpful reference for other areas, which will contribute new knowledge of carbonaceous aerosol and reduce uncertainty in assessing future climate change and health studies of BC.

Measurement report: Strong light absorption induced by aged biomass burning black carbon over the southeastern Tibetan Plateau in pre-monsoon season

Abstract. During the pre-monsoon season, biomass burning (BB) activities are intensive in southern Asia. Facilitated by westerly circulation, those BB plumes can be transported to the Tibetan Plateau (TP). Black carbon (BC), the main aerosol species in BB emissions, is an important climate warming agent, and its absorbing property strongly depends on its size distribution and mixing state. To elucidate the influence of those transported BB plumes on the TP, a field campaign was conducted on the southeast edge of the TP during the pre-monsoon season. It was found that the transported BB plumes substantially increased the number concentration of the atmospheric BC particles by a factor of 13 and greatly elevated the number fraction of thickly coated BC from 52 % up to 91 %. Those transported BC particles had slightly larger core size and much thicker coatings than the background BC particles. However, the coating mass was not evenly distributed on BC particles with different sizes. The smaller BC cores were found to have larger shell / core ratios than the larger cores. Besides, the transported BB plumes strongly affected the vertical variation in the BC's abundance and mixing state, resulting in a higher concentration, larger number fraction, and higher aging degree of BC particles in the upper atmosphere. Resulting from both increase in BC loading and aging degree, the transported BB plumes eventually enhanced the total light absorption by a factor of 15, of which 21 % was contributed by the BC aging, and 79 % was contributed from the increase in BC mass. Particularly, the light absorption enhancement induced by the aging process during long-range transport has far exceeded the background aerosol light absorption, which implicates a significant influence of BC aging on climate warming over the TP region.

A comprehensive evaluation of aerosol extinction apportionment in Beijing using a high-resolution time-of-flight aerosol mass spectrometer

An aerosol mass spectrometer (AMS) was used to measure the chemical composition of non-refractory submicron particles (NR-PM1) in Beijing from 2012 to 2013. The average concentration of NR-PM1 was 56 μg·m−3, with higher value of 106 μg·m−3 when Beijing was influenced by air masses from south in winter. Organics was the primary chemical component with a concentration of 26 μg·m−3, accounting for 46% of the total NR-PM1. The ratio of NO3−/SO42− was utilized to identify the relative contribution of stationary and traffic related resource to PM pollution. When NR-PM1 concentration was between 50 and 200 μg·m−3, NO3−/SO42−was larger than 1, indicating traffic resource contributed more than stationary resource during the aerosol growth. A new method was developed to calculate aerosol extinction coefficient (σ) as a function of aerosol optical depth (AOD) and the mixing layer height (MLH). σ derived from the new method showed a statistically significant correlation with that obtained from traditional method, which was calculated using visibility (y = 0.99x + 85 R2 = 0.69). Multiple linear regressions in dependence of chemical component were performed to evaluate light extinction apportionment. Under the overall condition, NR-PM1 contributed about 88% to the whole aerosol light extinction; organics, ammonium chloride, ammonium nitrate, ammonium sulfate, black carbon contributed 30%, 6%, 24%, 26% and 6% of the NR-PM1 light extinction, respectively. By further comparing the light extinction apportionment under the different dominated air masses, we concluded that the organics and ammonium sulfate contributed more in polluted days (36% and 23%) than that in clean days (21% and 21%). Mass ratio (MR) between NR-PM1 and black carbon (MR = massNR-PM1/massBC) was used to identify black carbon aging degree, and the result showed that aerosol mass extinction efficiency increased rapidly after MR reached about 7 in the process of black carbon aging.

Investigation of the wet removal rate of black carbon in East Asia: validation of a below- and in-cloud wet removal scheme in FLEXible PARTicle (FLEXPART) model v10.4

Abstract. Understanding the global distribution of atmospheric black carbon (BC) is essential for unveiling its climatic effect. However, there are still large uncertainties regarding the simulation of BC transport due to inadequate information about the removal process. We accessed the wet removal rate of BC in East Asia based on long-term measurements over the 2010–2016 period at three representative background sites (Baengnyeong and Gosan in South Korea and Noto in Japan). The average wet removal rate, represented by transport efficiency (TE), i.e., the fraction of undeposited BC particles during transport, was estimated to be 0.73 in East Asia from 2010 to 2016. According to the relationship between accumulated precipitation along trajectory and TE, the wet removal efficiency was lower in East and North China but higher in South Korea and Japan, implying the importance of the aging process and frequency of exposure to below- and in-cloud scavenging conditions during air mass transport. Moreover, the wet scavenging in winter and summer showed the highest and lowest efficiency, respectively, although the lowest removal efficiency in summer was primarily associated with a reduced BC aging process because the in-cloud scavenging condition was dominant. The average half-life and e-folding lifetime of BC were 2.8 and 7.1 d, respectively, which is similar to previous studies, but those values differed according to the geographical location and meteorological conditions of each site. Next, by comparing TE from the FLEXible PARTicle (FLEXPART) Lagrangian transport model (version 10.4), we diagnosed the scavenging coefficients (s−1) of the below- and in-cloud scavenging scheme implemented in FLEXPART. The overall median TE from FLEXPART (0.91) was overestimated compared to the measured value, implying the underestimation of wet scavenging coefficients in the model simulation. The median of the measured below-cloud scavenging coefficient showed a lower value than that calculated according to FLEXPART scheme by a factor of 1.7. On the other hand, the overall median of the calculated in-cloud scavenging coefficients from the FLEXPART scheme was highly underestimated by 1 order of magnitude, compared to the measured value. From an analysis of artificial neural networks, the convective available potential energy, which is well known as an indicator of vertical instability, should be considered in the in-cloud scavenging process to improve the representative regional difference in BC wet scavenging over East Asia. For the first time, this study suggests an effective and straightforward evaluation method for wet scavenging schemes (both below and in cloud), by introducing TE along with excluding effects from the inaccurate emission inventories.

Effects of primary particle size on light absorption enhancement of black carbon aerosols using the superposition T-matrix method

Light absorption enhancement (Eabs) of black carbon (BC) aerosol following atmospheric aging is one of the most challenging issues in the assessment of aerosol radiative forcing. BC Eabs is constrained by complex particle morphologies; however, large uncertainties continue to occur due to certain morphological parameters, including primary particle size. The values of Eabs during BC aging is quantified with diverse primary particle sizes using the superposition T-matrix method (STM). The results show that the uncertainty of absorption enhancement due to the primary particle size of fully aged BC particles ranges from ~10% to 20%, while the uncertainties arising from varied BC volume-equivalent size and fractal dimension are ~20–30% and ~8–12%, respectively. The optical properties of BC particles with volume-equivalent radii ranging from 50 to 70 nm were largely influenced (up to ~50%) by inappropriate assumptions regarding primary particle size. The specific assumptions of primary particle size in optical modeling plays an important role in constraining BC Eabs.

Simulated aging processes of black carbon and its impact during a severe winter haze event in the Beijing-Tianjin-Hebei region

Black carbon (BC) can mitigate or worsen air pollution by perturbing meteorological conditions. BC aging processes strongly influence the evolution of the particle size, concentration, and optical properties of BC, which determine its influence on meteorology. Here, we use the online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model to quantify the role of BC aging processes, including physical processes (PP) and absorption enhancement (AE), in causing BC-induced meteorological changes and their associated feedbacks to PM2.5 (particulate matter less than 2.5 μm in diameter) and O3 concentrations during a severe haze event in the Beijing-Tianjin-Hebei (BTH) region during 21–27 February 2014. Our results show that, compared to those from the simulation without PP, the simulated near-surface BC concentration and BC mass loading in the BTH region decreased by 6.6% and 12.1%, respectively, when PP were included. PP increased the proportion of large BC (particle diameter greater than 0.312 μm) below 1000 m from 28 to 33% to 59–64% in the BTH region. When both PP and AE were included in the simulation, the reduction in PBL height due to the BC-PBL interaction was 116.3 m (20.7%), compared to reductions of 75.7 m (13.5%) without AE and 66.6 m (11.9%) without PP and AE. However, during this haze event, anomalous northeasterly winds were produced by the direct radiative effect of BC, which further affected aerosol mixing and transport. Due to their combined impacts on multiple meteorological factors, the direct radiative effects of BC without PP and AE, without AE, and with PP and AE increased the surface concentrations of PM2.5 by 8.3 μg m−3 (by 6.1% relative to the mean value), 6.1 μg m−3 (4.5%) and 9.6 μg m−3 (7.0%), respectively, but decreased the surface O3 concentrations by 2.8 ppbv (7.4%), 4.0 ppbv (9.0%) and 5.0 ppbv (10.8%) on average in the BTH region during 21–27 February 2014.

Impact of aging process on atmospheric black carbon aerosol properties and climate effects

Black carbon (BC) is a key driver of global warming, exerting a strong but highly uncertain effect on the global climate. A key challenge of quantifying BC’s radiative effect is predicting the impact of aging process on BC’s properties. BC is mainly emitted from the incomplete combustion of fossil fuel and biomass burning. After being emitted, it undergoes complex chemical and physical transformations in the atmosphere, through condensation, coagulation and heterogeneous oxidation, which is referred to as BC aging. During aging, BC is gradually coated by other chemical species, which dramatically affects BC properties and alters the corresponding climate effects. The objective of this review is to discuss the current knowledge of BC aging from both laboratory and field measurements. This review first summarizes the methods used in the laboratory experiments and field observations. Then it reviews the effects of aging process on BC mixing state, which comprises both morphology and chemical composition. On this basis, changes to BC light absorption, hygroscopicity, cloud condensation nuclei (CCN) and ice nucleation (IN) activity during the aging process are discussed; these properties are closely related to BC climate effects. Laboratory studies on BC aging are always conducted using single-precursor systems, which do not capture the complex composition of the real atmosphere. In field studies, some state-of-the-art online instruments have been deployed to measure BC properties. However, since the field studies are usually conducted at fixed stations, it is difficult to dynamically track the entire BC aging process. Atmospheric aging process converts BC particles from externally mixed to internally mixed, with other chemical species coated on them. The morphology evolution in the aging process contains three stages: void filling, collapse and subsequent growth, which results in BC morphology changing from the initially chain-like shapes to spherical core-shell structures. The chemical composition of coatings is affected by both emission source and extent of aging, showing an increasing proportion of secondary species during the aging process. BC light absorption is enhanced because of coatings that accumulate during aging. However, the absorption enhancement factors reported in previous studies show large discrepancy because of differences in aging degree, morphology and chemical composition. The particle-to-particle heterogeneity in mixing state can also lead to different enhancement factors for particle populations. Along with the enhancement of light absorption, BC particles hygroscopicity and CCN activity are also enhanced during aging process. However, observational evidence is still lacking. The IN activity of BC is still an open question, and whether aging process enhances or suppresses the IN activity of BC is still debatable. Based on this review of previous studies, we recommend further research on the following topics: (1) Characterize the aging process for BC from different emission sources under realistic combustion conditions. (2) Identify the key factors affecting BC aging in various atmospheric environments. (3) Explore the role of BC particles in regional air pollution. (4) Conduct further studies on the interaction between BC and cloud formation.

Significant restructuring and light absorption enhancement of black carbon particles by ammonium nitrate coating

Field observations have suggested that particulate nitrate can promote the aging of black carbon (BC), yet the mechanisms of the aging process and its impacts on BC’s light absorption are undetermined. Here we performed laboratory simulation of internal mixing of flame-generated BC aggregates with ammonium nitrate. Variations in particle size, mass, coating thickness, effective density, dynamic shape factor, and optical properties were determined online by a suite of instruments. With the development of coatings, the particle size initially decreased until reaching a coating thickness of ∼10 nm and then started increasing, accompanied by an increase in effective density and a decrease in dynamic shape factor, reflecting the transformation of BC particles from highly fractal to near-spherical morphology. This is partially attributable to the restructuring of BC cores to more compact forms. Exposing coated particles to elevated relative humidity (RH) led to additional BC morphology changes, even after drying. Particle light absorption and scattering were also amplified with ammonium nitrate coating, increasing with coating thickness and RH. For BC particles with a 17.8 nm coating, absorption and scattering were increased by 1.5- and 7.9-fold when cycled through 70% RH (5-70-5% RH), respectively. The irreversible restructuring of the BC core caused by condensation of ammonium nitrate and water altered both absorption and scattering, with a magnitude comparable to or even exceeding the effects of increased coating. Results show that ammonium nitrate is among the most efficient coating materials with respect to modifying BC morphology and optical properties compared with other inorganic and organic species investigated previously. Accordingly, mitigation of nitrate aerosols is necessary for the benefits of both air pollution control and reducing the impacts of BC on visibility impairment and radiative forcing on climate change. Our results also pointed out that the effect of BC core restructuring needs to be considered when evaluating BC’s light absorption enhancement.

Contrasting size-resolved hygroscopicity of fine particles derived by HTDMA and HR-ToF-AMS measurements between summer and winter in Beijing: the impacts of aerosol aging and local emissions

Abstract. The effects of aerosols on visibility through scattering and absorption of light and on climate through altering cloud droplet concentration are closely associated with their hygroscopic properties. Here, based on field campaigns in winter and summer in Beijing, we compare the size-resolved hygroscopic parameter (κgf) of ambient fine particles derived by an HTDMA (hygroscopic tandem differential mobility analyzer) to that (denoted as κchem) calculated by an HR-ToF-AMS (high-resolution time-of-flight aerosol mass spectrometer) measurements using a simple rule with the hypothesis of uniform internal mixing of aerosol particles. We mainly focus on contrasting the disparity of κgf and κchem between summer and winter to reveal the impact of atmospheric processes/emission sources on aerosol hygroscopicity and to evaluate the uncertainty in estimating particle hygroscopicity with the hypothesis. We show that, in summer, the κchem for 110, 150, and 200 nm particles was on average ∼10 %–12 % lower than κgf, with the greatest difference between the values observed around noontime when aerosols experience rapid photochemical aging. In winter, no apparent disparity between κchem and κgf is observed for those >100 nm particles around noontime, but the κchem is much higher than κgf in the late afternoon when ambient aerosols are greatly influenced by local traffic and cooking sources. By comparing with the observation from the other two sites (Xingtai, Hebei and Xinzhou, Shanxi) of north China, we verify that atmospheric photochemical aging of aerosols enhances their hygroscopicity and leads to 10 %–20 % underestimation in κchem if using the uniform internal mixing assumption. The effect is found more significant for these >100 nm particles observed in remote or clean regions. The lower κchem likely resulted from multiple impacts of inappropriate application of the density and hygroscopic parameter of organic aerosols in the calculation, as well as influences from chemical interaction between organic and inorganic compounds on the overall hygroscopicity of mixed particles. We also find that local/regional primary emissions, which result in a large number of externally mixed BC (black carbon) and POA (primary organic aerosol) in urban Beijing during traffic rush hour time, cause a 20 %–40 % overestimation of the hygroscopic parameter. This is largely due to an inappropriate use of density of the BC particles that is closely associated with its morphology or the degree of its aging. The results show that the calculation can be improved by applying an effective density of fresh BC (0.25–0.45 g cm−3) in the mixing rule assumption. Our study suggests that it is critical to measure the effective density and morphology of ambient BC, in particular in those regions with influences of rapid secondary conversion/aging processes and local sources, so as to accurately parameterize the effect of BC aging on particle hygroscopicity.

Fossil fuel combustion and biomass burning sources of global black carbon from GEOS-Chem simulation and carbon isotope measurements

Abstract. We identify sources (fossil fuel combustion versus biomass burning) of black carbon (BC) in the atmosphere and in deposition using a global 3-D chemical transport model GEOS-Chem. We validate the simulated sources against carbon isotope measurements of BC around the globe and find that the model reproduces mean biomass burning contribution (fbb; %) in various regions within a factor of 2 (except in Europe, where fbb is underestimated by 63 %). GEOS-Chem shows that contribution from biomass burning in the Northern Hemisphere (fbb: 35±14 %) is much less than that in the Southern Hemisphere (50±11 %). The largest atmospheric fbb is in Africa (64±20 %). Comparable contributions from biomass burning and fossil fuel combustion are found in southern (S) Asia (53±10 %), southeastern (SE) Asia (53±11 %), S America (47±14 %), the S Pacific (47±7 %), Australia (53±14 %) and the Antarctic (51±2 %). fbb is relatively small in eastern Asia (40±13 %), Siberia (35±8 %), the Arctic (33±6 %), Canada (31±7 %), the US (25±4 %) and Europe (19±7 %). Both observations and model results suggest that atmospheric fbb is higher in summer (59 %–78 %, varying with sub-regions) than in winter (28 %–32 %) in the Arctic, while it is higher in winter (42 %–58 %) and lower in summer (16 %–42 %) over the Himalayan–Tibetan Plateau. The seasonal variations of Atmosphericfbb are relatively flat in North America, Europe and Asia. We conducted four experiments to investigate the uncertainties associated with biofuel emissions, hygroscopicity of BC in fresh emissions, the aging rate and size-resolved wet scavenging. We find that doubling biofuel emissions for domestic heating north of 45∘ N increases fbb values in Europe in winter by ∼30 %, reducing the discrepancy between observed and modeled atmospheric fbb from −63 % to −54 %. The remaining large negative discrepancy between model and observations suggests that the biofuel emissions are probably still underestimated at high latitudes. Increasing the fraction of thickly coated hydrophilic BC from 20 % to 70 % in fresh biomass burning plumes increases the fraction of hydrophilic BC in biomass burning plumes by 0 %–20 % (varying with seasons and regions) and thereby reduces atmospheric fbb by up to 11 %. Faster aging (4 h e-folding time versus 1.15 d e-folding time) of BC in biomass burning plumes reduces atmospheric fbb by 7 % (1 %–14 %, varying with seasons and regions), with the largest reduction in remote regions, such as the Arctic, the Antarctic and the S Pacific. Using size-resolved scavenging accelerates scavenging of BC particles in both fossil fuel and biomass burning plumes, with a faster scavenging of BC in fossil fuel plumes. Thus, atmospheric fbb increases in most regions by 1 %–14 %. Overall, atmospheric fbb is determined mainly by fbb in emissions and, to a lesser extent, by atmospheric processes, such as aging and scavenging. This confirms the assumption that fbb in local emissions determines atmospheric fbb in previous studies, which compared measured atmospheric fbb directly with local fbb in bottom-up emission inventories.