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The light-absorbing organic aerosol (OA), known as brown carbon (BrC), has important radiative impacts; however, its sources and evolution after emission remain to be elucidated. In this study, the light absorption at multiple wavelengths, mass spectra of OA and microphysical properties of black carbon (BC) were characterized at a typical suburban environment in Beijing. The absorption of BC is constrained by its size distribution and mixing state, and the BrC absorption is obtained by subtracting the BC absorption from the total aerosol absorption. Aerosol absorption was further apportioned to BC, primary BrC and secondary BrC by applying the least correlation between secondary BrC and BC. The multilinear regression analysis on the factorized OA mass spectra indicated that the OA from traffic and biomass burning emission contributed to primary BrC. Importantly, the moderately oxygenated OA (O / C = 0.62) was revealed to highly correlate with secondary BrC. These OA had higher nitrogen content, in line with the nitrogen-containing functional groups detected by the Fourier transform infrared spectrometer. The photochemical processes were found to reduce the mass absorption cross section (MAC) of primary OA, reducing its contribution to total absorption by 20 %, at the same time increasing MAC for secondary OA, which showed a 30 % enhancement in contribution to total absorbance, implying the concurrent whitening and darkening of BrC. This provides field evidence that the photochemically produced secondary nitrogen-containing OA can considerably compensate for some bleaching effect on the primary BrC, hereby causing radiative impacts. [ABSTRACT FROM AUTHOR]

The mass concentration of atmospheric particulate matter (PM) has been continuously decreasing in the Beijing-Tianjin-Hebei region. However, health endpoints do not exhibit a linear correlation with PM mass concentrations. Thus, it is urgent to clarify the prior toxicological components of PM to further improve air quality. In this study, we analyzed the long-term oxidative potential (OP) of water-soluble PM 2.5 , which is generally considered more effective in assessing hazardous exposure to PM in Beijing from 2018 to 2022 based on the dithiothreitol assay and identified the crucial drivers of the OP of PM 2.5 based on online monitoring of air pollutants, receptor model, and random forest (RF) model. Our results indicate that dust, traffic, and biomass combustion are the main sources of the OP of PM 2.5 in Beijing. The complex interactions of dust particles, black carbon, and gaseous pollutants (nitrogen dioxide and sulfur dioxide) are the main factors driving the OP evolution, in particular, leading to the abnormal rise of OP in Beijing in 2022. Our data shows that a higher OP is observed in winter and spring compared to summer and autumn. The diurnal variation of the OP is characterized by a declining trend from 0:00 to 14:00 and an increasing trend from 14:00 to 23:00. The spatial variation in OP of PM 2.5 was observed as the OP in Beijing is lower than that in Shijiazhuang, while it is higher than that in Zhenjiang and Haikou, which is primarily influenced by the distribution of black carbon. Our results are of significance in identifying the key drivers influencing the OP of PM 2.5 and provide new insights for advancing air quality improvement efforts with a focus on safeguarding human health in Beijing. [ABSTRACT FROM AUTHOR]

Considering human activities are the most direct and important factors leading to the accumulation and loss of soil black carbon (BC), a detailed understanding of soil BC in different functional zones and reasonable soil management are the keys to determine whether soil is the carbon source or carbon sink. In this study, carbon concentrations and isotope compositions of black carbon (BC%, δ13CBC) and organic carbon (OC, δ13COC) were determined in topsoil from different functional zones (roadside grass verges, parks, residential areas, mines, arable lands, woodlands and wastelands) in urban and suburban of Beijing, China. The organic carbon (OC) content in soils of the urban area (averaging 1.67%) is higher than in suburban areas (averaging 1.05%). Black carbon is not evenly distributed across different functional zones, but is more concentrated and variable in urban areas (0.11–2.43%, averaging 0.73%) than in suburban areas (0.03–0.70%, averaging 0.18%). The ratios of BC% to OC% in suburban areas (0.04–0.40) are smaller than in urban areas (0.15–0.98). The δ13CBC of topsoil (–26.35...–20.85‰) reflects that coal combustion has a strong impact on soil BC accumulation in Beijing. The carbon isotope differences between OC and BC of the suburban and urban topsoils are positive (averaging +0.56‰) and negative (averaging –0.41‰), respectively. This suggests that urban areas are seriously influenced by human activities which lead to a large amount of fossil fuel combustion, but suburban areas have received a uniform deposition of atmospherically transported finer BC aerosols. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is known to have major impacts on both human health and climate. The populated megacity represents the most complex anthropogenic BC emissions where the sources and related impacts are very uncertain. This study provides source attribution and characterization of BC in the Beijing urban environment during the joint UK–China APHH (Air Pollution and Human Health) project, in both winter (November–December 2016) and summer (May–June 2017). The size-resolved mixing state of BC-containing particles was characterized by a single-particle soot photometer (SP2) and their mass spectra was measured by a soot particle aerosol mass spectrometer (SP-AMS). The refractory BC (rBC) mass loading was around a factor of 2 higher in winter relative to summer, and more variable coatings were present, likely as a result of additional surface emissions from the residential sector and favourable condensation in the cold season. The characteristics of the BC were relatively independent of air mass direction in summer, whereas in winter air masses from the Northern Plateau were considerably cleaner and contained less-coated and smaller BC, but the BC from the Southern Plateau had the largest core size and coatings. We compare two online source apportionment methods using simultaneous measurements made by the SP2, which measures physical properties of BC, and the chemical approach using the positive matrix factorization (PMF) of mass spectra from the SP-AMS for the first time. A method is proposed to isolate the BC from the transportation sector using a mode of small BC particles (core diameter Dc<0.18 µ m and coating thickness ct < 50 nm). This mode of BC highly correlated with NOx concentration in both seasons (∼14 ng m -3 BC ppb -1 NOx) and corresponded with the morning traffic rush hour, contributing about 30 % and 40 % of the total rBC mass (35 % and 55 % in number) in winter and summer respectively. The BC from coal burning or biomass burning was characterized by moderate coatings (ct = 50–200 nm) contributing ∼20 %–25 % of rBC mass. Large uncoated BC particles (Dc>0.18 µ m and ct < 50 nm) were more likely to be contributed by coal combustion, as these particles were not present in urban London. This mode was present in Beijing in both winter (∼30 %–40 % rBC mass) and summer (∼40 % rBC mass) but may be dominated by the residential and industrial sector respectively. The contribution of BC thickly coated with secondary species (ct > 200 nm) to the total rBC mass increased with pollution level in winter but was minor in summer. These large BC particles importantly enhanced the absorption efficiency at high pollution levels – in winter when PM 1 > 100 µ g m -3 or BC > 2 µ g m -3 , the absorption efficiency of BC increased by 25 %–70 %. The reduction of emissions of these large BC particles and the precursors of the associated secondary coating will be an effective way of mitigating the heating effect of BC in urban environments. [ABSTRACT FROM AUTHOR]

Biomass burning (BB) is an important source of brown carbon (BrC) and black carbon (BC), which are two key highly absorbent substances in atmospheric particles and can have a substantial positive impact on the climate radiative forcing. This study presents the light absorption properties of BC and BrC in PM2.5 during the winter in Beijing, with a discussion on the regional transportation of the light absorption of BC and BrC. Relatively high levels of the light absorption coefficient (Abs λ) of BC, BrC, and the chemical compounds were found during haze episodes. The average Abs BC at λ = 880 and Abs BrC at λ = 370 during the haze period were as high as 4.4 and 2.9 times higher than those during the clean periods. The biomass burning tracer levoglucosan was significantly correlated with Abs BC880 (R 2 = 0.53, P < 0.001), Abs BrC370 (R 2 = 0.47, P < 0.001) and Abs BC(BB) (R 2 = 0.69, P < 0.001). The average contributions of biomass burning to organic carbon (OC) and Abs BC were 33% and 48%, respectively, indicating that biomass burning was an important source of light-absorbing substances in the atmosphere. Concentration-weighted trajectory (CWT) analyses using TrajStat software also demonstrated that regional transport of biomass burning emissions from the northwestern and southwestern areas, which cover the intense fire spots from VIIRS, had a considerable influence on the light absorption properties of PM 2.5 and even haze formation in Beijing during the winter. • Fairly high levels of the Abs BC and Abs BrC found during haze episodes. • Light absorption of BrC gave equal roles to that of BC near the UV–Vis band but was dominant at visible wavelengths. • Regional biomass burning was an important source for light absorbing substance in PM 2.5 during winter in the North China Plain region. [ABSTRACT FROM AUTHOR]

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

Black carbon (BC) forms an important component of particulate matter globally, due to its impact on the climate, environment and human health. Megacities such as Beijing are hotspots of BC emissions and due to rapid urbanisation and development their sources are uncertain and rapidly changing. Therefore, identifying and quantifying these emission sources is critical for effective policy making and achieving the desired reduction in air pollution.In this study, at an urban location in Beijing, we present the first ever direct measurements of BC fluxes using eddy correlation. The measurements were made as part of a large joint UK-China field experiment, during winter 2016 (November-December) and summer 2017 (May-June). We focus on characterising BC properties, particularly its mixing state, through analysis of the size and coating content of individual particles. This allows us to attribute individual particles to potential sources. We then use eddy correlation to separately calculate BC fluxes for each of the different sources. The Single Particle Soot Photometer (SP2) was used to uniquely quantify the morphology independent mass of single refractory BC particles and their coating content. Black carbon was measured at 5 Hz from an inlet placed at 102 m height, adjacent to a 3D sonic anemometer. From the covariance between concentrations and vertical wind speed emission fluxes can be calculated over flux footprints of several square kilometres, using the eddy covariance method.In this paper, BC characterisation and flux analysis will be presented for both winter and summer seasons using four weeks of continuous data, which includes several severe haze pollution events. The analysis identified different habits of BC containing particles in summer and winter, which indicates a variation in their source origin (traffic, coal and biomass burning). The fluxes will provide further insight into complex environment of Beijing, allowing us to identify and quantify contribution of local emission sources. Furthermore, we have recently carried out similar flux emission studies in Delhi during Nov 2018. Initial data from this experiment will be presented, along with a preliminary comparison analysis of the emission trends in both megacities. [ABSTRACT FROM AUTHOR]