Wintertime Optical Properties of Primary and Secondary Brown Carbon at a Regional Site in the North China Plain

Light absorption of black and brown carbon in eastern China

Black carbon (BC) and brown carbon (BrC) are important light-absorbing components of carbonaceous aerosols in the atmosphere. While BC concentrations and properties have been widely studied, BrC concentrations and its radiative balance have been rarely studied due to the lack of information on the light-absorbing properties. In this study, a novel Bayesian inference model (BI) and conventional BC and BrC models were both applied to systematically study the BC and BrC properties at a roadside site in Yantai based on the data from a 7-wavelength aethalometer and total carbon (TC) analyzer. TC was resolved into BC, BrC and white carbon (WtC) by the BI model, in which BC dominates the TC (43.4 %), followed by WtC (29.5 %) and BrC (27.1 %) during autumn and winter. Significant agreements in BC and babs, BC (880) (R > 0.95) were achieved by BI and traditional BC model. BC source apportionment shows a clear contribution of biomass burning in cold seasons, especially on cold winter nights. Further separations of babs, BrC (370) shows distinct seasonal variations of babs, BrC, pri (370) and babs, BrC, sec (370). While babs, BrC, pri (370) diel distribution was similar across different seasons, babs,BrC,sec(370) shows a significant contribution in winter, especially during nighttime, possibly due to the enhanced nighttime NO3 chemistry via aqueous reactions. Hourly resolved mass absorption efficiency (MAE) shows a clear BrC photobleaching process during noontime. Unlike HKUST where clear darkening-photobleaching BrC process was observed, the morning darkening-photobleaching processes were interfered by traffic emissions, suggesting that the formation/emission process of BrC surpassed the photobleaching process in the morning.

Impact of mixed benzene site exposure on bioaccessibility in simulated lung fluids and health risk assessment

Benzene series (BTEX) are predominant volatile organic compounds (VOCs) in petroleum production and downstream industrial sites, primarily enter human lungs via inhalation, posing significant health risks. To address the critical limitation of existing risk assessments that focus solely on individual components, this study investigated benzene and ethylbenzene based on contamination characteristics of petroleum refineries in Northwest China. An innovative in vitro membrane oxygenator system was developed to simulate single and mixed exposure scenarios at three soil concentrations: low (5 mg/kg), medium (10 mg/kg), and high (20 mg/kg), respectively. Health risk indices including inhalation risk (Inh), hazard quotient (HQ), and lifetime cancer risk (LCR) were calculated using bioaccessibility-adjusted parameters to precisely compare risk variations across exposure modes. Results demonstrated significant synergistic effects in gas-liquid mass transfer kinetics under mixed exposure (P < 0.05), the simulated lung fluid bioaccessibility of benzene and ethylbenzene was also significantly higher (P < 0.05), likely due to their intermolecular cosolvency. Risk assessment results indicated that Inh, HQ, and LCR indices in mixed exposure were significantly higher (P < 0.05) than in single exposure, with a concentration-dependent contribution to health risks. At low concentrations, benzene's health risk indices increased by 185 %-284 %, while ethylbenzene's increased by approximately 63 %-68 %, indicating a synergistic effect in mixed exposure scenarios. This study offers a new methodological basis for health risk assessment of BTEX mixed exposure at petrochemical sites.

Insights into the formation of secondary organic aerosols from agricultural residue burning emissions: A review of chamber-based studies

Organic aerosols (OA) are a significant component of fine particulate matter in the ambient air and are formed through primary and secondary processes. Primary organic aerosols (POA) are directly released from sources, while secondary organic aerosols (SOA) are formed through the oligomerization and/or oxidation of volatile organic compounds (VOCs) in the atmosphere. Recently, there has an increasing attention on the SOA budgets, their formation pathways, and photochemical evolution due to their impacts on climate and human health. Biomass burning (BB) is a significant source of OA, contributing around 5-30 % to the SOA burden globally. Agricultural residue burning (ARB) is a type of BB that contributes ∼10 % of total atmospheric OA mass worldwide, whereas it contributes higher in Asian regions like China and India. ARB emits a significant amount of air pollutants, including VOCs, into the atmosphere. However, there is inadequate information on the transformation of ARB emissions to SOA due to limited laboratory studies. The present review focuses on the formation mechanism of SOA from ARB emissions, summarizing the current state of the art about ARB precursors and their oxidation products from chamber-based studies, including measurement methods and analytical instrumentation. The review also discusses the role of different types of oxidants in OA mass enhancement, factors affecting the overall SOA yield, and the uncertainties involved in the process.

Aromatic Nitration Enhances Absorption of Biomass Burning Brown Carbon in an Oxidizing Urban Environment

Brown carbon (BrC) from biomass burning constitutes a significant portion of light-absorbing components in the atmosphere. Although the aging of BrC surrogates from biomass burning has been studied in many laboratory settings, BrC aging behavior in real-world urban environments is not well understood. In this study, through a combination of online dynamic monitoring and offline molecular characterization, the ambient optical aging of BrC was linked to its dynamic changes in molecular composition. Enhanced light absorption by BrC was consistently observed during the periods dominated by oxygenated biomass burning organic aerosol (BBOA), in contrast to periods dominated by primary emissions or secondary formation in aqueous-phase. This enhancement was linked to the formation of nitrogen-containing compounds during the ambient aging of BBOA. Detailed molecular characterization, alongside analysis of environmental parameters, revealed that an increased atmospheric oxidizing capacity, marked by elevated levels of ozone and nighttime NO3 radicals, facilitated the formation of nitrated aromatic BrC chromophores. These chromophores were primarily responsible for the enhanced light absorption during the ambient aging of BBOA. This study elucidates the nitration processes that enhance BrC light absorption for ambient BBOA, and highlights the crucial role of meteorological conditions. Furthermore, our findings shed light on the chemical and optical aging processes of biomass burning BrC in ambient air, offering insights into its environmental behavior and effects.

Evolution of black carbon and brown carbon during summertime in Southwestern China: An assessment of control measures during the 2023 Chengdu Summer World University Games

The 31st FISU Summer World University Games (SWUG) was held in Chengdu, southwestern China, from July 22 to August 8, 2023. A series of control measures were carried out to ensure good air quality during the SWUG, providing an opportunity to investigate the atmospheric behaviors of light-absorbing aerosols under such a substantial disturbance caused by the control measures. To assess the impacts of emission controls on primary pollutants, a field campaign was conducted at a rural site in Chengdu to investigate the characterization of equivalent black carbon (eBC). The changes of eBC concentrations before, during, and after the SWUG were characterized. The sources of eBC were resolved, and the impacts of atmospheric processes on the absorption capacity were also investigated. During the SWUG, the eBC concentration decreased by 12.1 % and 25.3 % compared with those before and after the SWUG. A fossil fuel combustion (eBCff) and a biomass burning (eBCbb) originated eBC were resolved using the aethalometer model. Both eBCff and eBCbb decreased during the SWUG, indicating the effectiveness of control measures. After the SWUG, the influence of biomass burning emissions became more and more significant, and the contribution of brown carbon (BrC) to light absorption at 370-660 nm increased by 52, 19, 7, 6, and 17 % compared to those during the SWUG. As the biomass burning emitted aerosols aged, the absorption Ångström exponent and babs(BrC370nm) decreased gradually, which was mainly due to the photobleaching of the chromophores during the daytime. eBCff was mainly affected by strong wind, while high eBCbb concentration was mainly attributed to the gradual accumulation of biomass-burning emissions near the observation site. The results show the significant reduction of eBC with the implementation of the air pollution mitigation campaign, and provide insights on the impacts of atmospheric processes on BC optical properties during summertime.

Applying the total carbon-black carbon approach method to investigate the characteristics of primary and secondary carbonaceous aerosols in ambient PM2.5 in northern Taiwan

Ambient fine particulate matter (PM2.5) comprises a diverse array of carbonaceous species, and the impact of carbonaceous aerosols (CA) extends to both long-term and short-term effects on human health and the environment. Understanding the distinctive composition of CA is crucial for gaining insights into the origins of airborne particulate matter. Due to their diverse physicochemical properties and intricate heterogeneous reactions, CA often exhibits temporal and spatial variations. Ground-based and highly time-resolved apportionment methods play a vital role in discerning CA emissions. This study utilized high-time resolution data of total carbon (TC) and black carbon (BC) for CA apportionment in northern Taiwan. The advanced numerical model (TC-BC(λ)), coupled with continuous measurement data, facilitated CA allocation based on optical absorption characteristics, organic or elemental carbon composition, and the distinction between primary and secondary origins. Primary carbonaceous aerosols dominated the monitoring site, accounting for 67.5 % compared to the 32.5 % contribution from secondary forms of CA. The summer season exhibited a maximum increase in secondary organic aerosols (SOA) at 41.5 %. Diurnal variations for primary emissions, such as BCc and primary organic aerosols (POA), showed marked peaks for BCff and POAnon-abs during morning rush hours. In contrast, BCbb and POABrC displayed bimodal peaks with increased concentrations during evening hours. Conversely, SOA exhibited significantly different diurnal trends, with SOABrC peaking late at night due to aqueous phased reactions and a noontime peak of SOAnon-abs observed due to photo-oxidation processes. Furthermore, the study employed backward trajectory analysis and concentration-weighted trajectories (CWTs) to examine the long-range transport of CA, identifying potential sources, origins, and transport patterns of CA components to the receptor site in Taiwan during different seasons.

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.

Spatiotemporal distribution, light absorption characteristics, and source apportionments of black and brown carbon in China

Black carbon (BC) and brown carbon (BrC) are aerosols that absorb light and thereby contribute to climate change. In this study, the light absorption properties and spatiotemporal distributions of equivalent BC (eBC) and BrC aerosols were determined based on continuous measurements of aerosol light absorption from January to August 2017, using a seven-channel aethalometer at 49 sampling sites in China. The source apportionments of BC and BrC were identified using the BC/PM2.5, absorption Ångström exponent, the concentration-weighted trajectory method, and the random forest model. Based on the results, BC was the dominant light absorber, whereas BrC was responsible for a higher proportion of the light absorption in northern compared to southern China. The light absorption of BrC was highest in winter (34.3 Mm-1), followed by spring (19.0 Mm-1) and summer (3.6 Mm-1). The combustion of liquid fuels accounted for over 50 % of the light absorption coefficient of BC in most cities and the importance of carbon monoxide (CO) and nitrogen dioxide (NO2) was over 10 % for BC emitted by liquid fuel combustion, based on the random forest model. The contribution of solid fuel combustion to BC in the north was larger than that in the southern regions as coal combustion and crop residue burning are important emission sources of BC in most northern cities. The contribution of primary BrC to light absorption was high in some northern cities, whereas that of secondary BrC was prevalent in some southern cities. The diurnal variations in secondary BrC were affected by changes in odd oxygen and relative humidity, which promoted the photobleaching of the chromophores and aqueous-phase reactions of secondary BrC.

Source apportionment, health risk assessment, and trajectory analysis of black carbon and light absorption properties of black and brown carbon in Delhi, India

Black Carbon (BC) is an important atmospheric pollutant, well recognized for adverse health and climatic effects. The present work discusses the monthly and seasonal variations of BC sources, health risks, and light absorption properties. The measurement was done from January to December 2021 using a seven wavelength aethalometer. Annual average BC concentration during the study period was 12.2 ± 8.8 μg/m3 (ranged from 1.9 - 52.2 μg/m3). Results represent highest BC concentration during winter (W), followed by post-monsoon (P-M), summer (S), and monsoon (M) seasons where the fossil fuel (FF) combustion is the major source during W, S, and M seasons and biomass burning (BB) during the P-M season. The health risk assessment revealed that individuals in Delhi are exposed to BC levels equivalent to inhaling the smoke from 36 passively smoked cigarettes (PSC) everyday. The risk is highest during W reaching upto 71 PSC and minimum during M i.e., 9 PSC. The light absorption properties were calculated for BC (AbsBC) and Brown carbon (AbsBrC). AbsBC and varied from 229-89 Mm-1 between 370-950 nm and AbsBrC varied from 87-12 Mm-1 between 370-660 nm. AbsBC contributed substantially to total absorption at all wavelengths, while AbsBrC contribution is quite significant in the UV region only. Trajectory analysis confirmed significant influence of regional sources (e.g., biomass-burning aerosols from northwest and east direction) on air quality, health risks, and light absorption properties of BC over Delhi especially during the P-M season. The BB events of Punjab, Haryana, Uttar Pradesh, and eastern Pakistan seems to have significant influence on Delhi's air quality predominantly during P-M season.

Impact of peri-urban forest fires on air quality and aerosol optical and chemical properties: The case of the August 2021 wildfires in Athens, Greece

Wildfires occurring near urban areas are known to have exceedingly detrimental impacts on the environment, air quality, economy and human health. In this framework, this study examines the effects of peri-urban forest fires on atmospheric chemical composition, and aerosol physical-optical properties in Athens, Greece, during August 2021. Satellite imagery and air mass trajectories showed advection of intense smoke plumes over Athens from three forest fires persisting for 10 days in the greater Athens area and in Central Greece (Euboea). During August 1-20, 2021, daily PM2.5 concentrations ranged from 8.9 to 78.7 μg m-3, and were associated with high OC levels (2.3-27.8 μg m-3), while BC and BCbb concentrations on smoke-impacted days were 2.6 μg m-3 and 1.0 μg m-3, respectively (2-3 times higher than August mean levels). During the peak of biomass burning (BB) smoke transport over Athens, daily-average scattering and absorption coefficients at short wavelengths maximized at 313 Mm-1 and 171 Mm-1, respectively. There was also a large impact of ambient BrC (brown carbon) absorption (60 Mm-1), while the OC/EC ratio exhibited characteristically low values (3-4), linked to flaming combustion (modified combustion efficiency of 0.97-0.99). The absorption Ångström exponent (1.38) and single scattering albedo (0.74) indicated highly absorbing BB aerosol, deviating from the normal summer patterns. BB-tracers like nssK+ displayed strong correlations with OC, EC and BC concentrations, as well as with scattering and absorption coefficients. However, forest fires drastically modified the levels of additional chemical species, with enhancements observed for Ca2+, NO3-, Cl-, and for organic aerosol (OA) components such as BBOA and less-oxidized oxygenated OA (LO-OOA). Since under climate change conditions, the Mediterranean is anticipated to experience a dramatic rise in the frequency and intensity of wildfires, the results highlight the necessity for prevention and mitigation policies to safeguard urban air quality.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.

Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the "Air Pollution Complex"

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the "air pollution complex" was first proposed by Professor Xiaoyan TANG in 1997. For papers published in 2021 on air pollution (only papers included in the Web of Science Core Collection database were considered), more than 24 000 papers were authored or co-authored by scientists working in China. In this paper, we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years, including studies on (1) sources and emission inventories, (2) atmospheric chemical processes, (3) interactions of air pollution with meteorology, weather and climate, (4) interactions between the biosphere and atmosphere, and (5) data assimilation. The intention was not to provide a complete review of all progress made in the last few years, but rather to serve as a starting point for learning more about atmospheric chemistry research in China. The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established, provided robust scientific support to highly successful air pollution control policies in China, and created great opportunities in education, training, and career development for many graduate students and young scientists. This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances, whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China, to hopefully be addressed over the next few decades.

Broadband spectrum characteristics and radiative effects of primary brown carbon from wood pyrolysis

Brown carbon (BrC), known as light-absorbing organic aerosol in the near-ultraviolet (UV) and short visible region, plays a significant role in the global and regional climate change. A detailed understanding of the spectral optical properties of BrC is beneficial for reducing the uncertainty in radiative forcing calculation. In this work, the spectral properties of primary BrC were investigated by using a four-wavelength broadband cavity-enhanced albedometer with central wavelengths at 365, 405, 532 and 660 nm. The BrC samples were generated by the pyrolysis of three types of wood. During the pyrolysis process, the measured average single scattering albedo (SSA) at 365 nm was about 0.66 to 0.86, where the average absorption Ångström exponent (AAE) was between 5.8 and 7.8, and the average extinction Ångström exponent (EAE) was within 2.1 to 3.5. The full spectral measurement of SSA (300-700 nm) was realized by an optical retrieval method and the retrieved SSA spectrum was directly applied to evaluate aerosol direct radiative forcing (DRF) efficiency. The DRF efficiency over ground of various primary BrC emissions increased from 5.3 % to 68 % as compared to the non-absorbing organic aerosol assumption. A decrease of about 35 % in SSA would cause the DRF efficiency over ground to change from cooling effect to warming effect (from -0.33 W/m² to +0.15 W/m²) in the near-UV band (365-405 nm). The DRF efficiency over ground of strongly absorptive primary BrC (lower SSA) contributed 66 % more than weakly absorptive primary BrC (higher SSA). These findings proved the importance of broadband spectral properties of BrC, which are substantial for radiative forcing evaluation of BrC and should be considered in global climate models.

Significant Promotion of Light Absorption Ability and Formation of Triplet Organics and Reactive Oxygen Species in Atmospheric HULIS by Fe(III) Ions

Metal ions are key components in atmosphere that potentially affect the optical properties and photochemical reactivity of atmospheric humic-like substances (HULIS), while this mechanism is still unclear. In this study, we demonstrated that atmospheric HULIS coupled with Fe³⁺, Cu²⁺, Zn²⁺, and Al³⁺ exhibited distinct optical properties and reactive intermediates from that of HULIS utilizing three-dimensional fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. The HULIS components showed light absorption that increased by 56% for the HULIS-Fe³⁺ system, fluorescence blue shift, and fluorescence quenching, showing a certain dose-effect relationship. These are mainly attributed to the fact that the highly oxidative HULIS chromophores have a stronger complexing ability with Fe³⁺ ions than the other metal ions. In addition, triplet organics (promoting ratio: 53%) and reactive oxygen species (promoting ratio: 82.6%) in the HULIS-Fe³⁺ system showed obvious generation promotion. Therefore, the main assumption of the photochemical mechanisms of atmospheric HULIS in the HULIS-Fe³⁺ system is that Fe³⁺ ions can form ³HULIS*-Fe³⁺ complexation with photoexcited ³HULIS* and then transition to the ground state through energy transfer, electron transfer, or nonradiative transition, accompanied by the formation of singlet oxygen and hydroxyl radicals. Our results provide references for evaluating the radiative forcing and aging effect of metal ions on atmospheric aerosols.

Two-year-long high-time-resolution apportionment of primary and secondary carbonaceous aerosols in the Los Angeles Basin using an advanced total carbon-black carbon (TC-BC(λ)) method

In recent years, carbonaceous aerosols (CA) have been recognized as a significant contributor to the concentration of particles smaller than 2.5 μm (i.e., PM_2.5), with a negative impact on public health and Earth's radiative balance. In this study, we present a method for CA apportionment based on high-time-resolution measurements of total carbon (TC), black carbon (BC), and spectral dependence of absorption coefficient using a recently developed Carbonaceous Aerosol Speciation System (CASS). Two-year-long CA measurements at two different locations within California's Los Angeles Basin are presented. CA was apportioned based on its optical absorption properties, organic or elemental carbon composition, and primary or secondary origin. We found that the secondary organic aerosols (SOA), on average, represent >50 % of CA in the study area, presumably resulting from the oxidation of anthropogenic and biogenic volatile organic components. Remarkable peaks of SOA in summer afternoons were observed, with a fractional contribution of up to 90 %. On the other hand, the peak of primary emitted CA, consisting of BC and primary organic aerosol (POA), contributed >80 % to the CA during morning rush hours on winter working days. The light absorption of BC dominated over the brown carbon (BrC), which contributed to 20 % and 10 % of optical absorption at the lower wavelength of 370 nm during winter nights and summer afternoons, respectively. The highest contribution of BrC, up to 50 %, was observed during the wildfire periods. Although the uncertainty levels can be high for some CA components (such as split between primary emitted and secondary formed BrC during winter nights), further research focused on the optical properties of CA at different locations may help to better constrain the parameters used in CA apportionment studies. We believe that the CASS system combined with the apportionment method presented in this study can offer simplified and cost-effective insights into the composition of carbonaceous aerosols.

Vehicle exhausts contribute high near-UV absorption through carbonaceous aerosol during winter in a fast-growing city of Sichuan Basin, China

Carbonaceous aerosols pose significant climatic impact, however, their sources and respective contribution to light absorption vary and remain poorly understood. In this work, filter-based PM_2.5 samples were collected in winter of 2021 at three urban sites in Yibin, a fast-growing city in the south of Sichuan Basin, China. The composition characteristics of PM_2.5, light absorption and source of carbonaceous aerosol were analyzed. The city-wide average concentration of PM_2.5 was 87.4 ± 31.0 μg/m³ in winter. Carbonaceous aerosol was the most abundant species, accounting for 42.5% of the total PM_2.5. Source apportionment results showed that vehicular emission was the main source of PM_2.5 during winter, contributing 34.6% to PM_2.5. The light absorption of black carbon (BC) and brown carbon (BrC) were derived from a simplified two-component model. We apportioned the light absorption of carbonaceous aerosols to BC and BrC using the Least Squares Linear Regression with optimal angstrom absorption exponent of BC (AAE_BC). The average absorption of BC and BrC at 405 nm were 51.6 ± 21.5 Mm^-1 and 17.7 ± 8.0 Mm^-1, respectively, with mean AAE_BC = 0.82 ± 0.02. The contribution of BrC to the absorption of carbonaceous reached 26.1% at 405 nm. Based on the PM_2.5 source apportionment and the mass absorption cross-section (MAC) value of BrC at 405 nm, vehicle emission was found to be the dominant source of BrC in winter, contributing up to 56.4%. Therefore, vehicle emissions mitigation should be the primary and an effective way to improve atmospheric visibility in this fast-developing city.

Decrease of atmospheric black carbon and CO2 concentrations due to COVID-19 lockdown at the Mt. Waliguan WMO/GAW baseline station in China

The Coronavirus Disease 2019 (COVID-19) lockdown policy reduced anthropogenic emissions and impacted the atmospheric chemical characteristics in Chinese urban cities. However, rare studies were conducted at the high mountain site. In this work, in-situ measurements of light absorption by carbonaceous aerosols and carbon dioxide (CO₂) concentrations were conducted at Waliguan (WLG) over the northeastern Tibetan Plateau of China from January 3 to March 30, 2020. The data was employed to explore the influence of the COVID-19 lockdown on atmospheric chemistry in the background-free troposphere. During the sampling period, the light absorption near-infrared (>470 nm) was mainly contributed by BC (>72%), however, BC and brown carbon (BrC) contributed equally to light absorption in the short wavelength (∼350 nm). The average BC concentrations in the pre-, during and post-lockdown were 0.28 ± 0.25, 0.18 ± 0.16, and 0.28 ± 0.20 μg m^-3, respectively, which decreased by approximately 35% during the lockdown period. Meanwhile, CO₂ also showed slight decreases during the lockdown period. The declined BC was profoundly attributed to the reduced emissions (∼86%), especially for the combustion of fossil fuels. Moreover, the declined light absorption of BC, primary and secondary BrC decreased the solar energy absorbance by 35, 15, and 14%, respectively. The concentration weighted trajectories (CWT) analysis suggested that the decreased BC and CO₂ at WLG were exclusively associated with the emission reduction in the eastern region of WLG. Our results highlighted that the reduced anthropogenic emissions attributed to the lockdown in the urban cities did impact the atmospheric chemistry in the free troposphere of the Tibetan Plateau.

Light absorption and source apportionment of water soluble humic-like substances (HULIS) in PM2.5 at Nanjing, China

Humic-like substances (HULIS), as important components of brown carbon (BrC), play an important role in climate change. In this study, one-year PM_2.5 samples from 2017 to 2018 were collected at Nanjing, China and the water soluble HULIS and other chemical species were analyzed to investigate the seasonal variations, optical properties and possible sources. The HULIS concentrations exhibited highest in winter and lowest in summer. The annual averaged HULIS concentration was 2.61 ± 1.79 μg m^-3, accounting for 45 ± 13% of water-soluble organic carbon (WSOC). The HULIS light absorption coefficient at 365 nm (Abs_{365, HULIS}) averagely accounted for 71 ± 19% of that of WSOC, suggesting that HULIS are the main light-absorbing components in WSOC. The annual averaged Ångström absorption exponent and mass absorption efficiency of HULIS at 365 nm were 5.22 ± 0.77 and 1.71 ± 0.70 m² g^-1. Good correlations between HULIS with levoglucosan and K⁺ suggested biomass burning (BB) influence on HULIS. High concentrations of HULIS and secondary species (e.g., NO₃^-, SO₄^2-, NH₄⁺, C₂O₄^2-) were found in present of high relative humidity, indicating strong aqueous phase secondary HULIS formation. Secondary HULIS produced from anthropogenic and biogenic precursors were quantified based on the positive matrix factorization (PMF) model and the results showed that both fossil (55%) and biogenic (45%) emission sources made great contributions to HULIS. Fossil fuel combustion significantly contributed to HULIS formation throughout the whole year, which were enriched with more secondary HULIS (30%) than primary HULIS (25%). Strongest BB contribution (39%) was found in winter and biogenic SOA contribution (32%) was found in summer. A multiple linear regression (MLR) method was further applied to obtain specific source contributions to Abs_{365, HULIS} and the results showed that strong light-absorbing chromophores were produced from anthropogenic precursors. Our results highlight the anthropogenic SOA and fossil fuels combustion contributions to HULIS in addition to the biggest contributor, BB, in urban area in China.

Hourly emission estimation of black carbon and brown carbon absorption from domestic coal burning in China

Residential coal combustion (RCC) emission demonstrates obvious daily variation, while no real-time estimation of air pollutants from RCC has been reported, as the limitation of hourly activity data and emission factors. With a dilution sampling system, a high-precision electronic balance, and an Aethalometer Model AE33, a real-time monitoring platform for RCC emission was established. Hourly emission factors (EFs) of BC and absorption emission factors (AEFs) of BrC from eleven kinds of chunk coals and nine kinds of honeycomb coals burning in China were obtained. The monthly and hourly coal consumption amounts were calculated with the activity data from literature reviews and a field survey. The first hourly BC and absorption cross section of BrC emission inventories from RCC were established in China. The historical emission trends (2003-2017) indicated that the policy has rapidly controlled the emission of BC and ACS_BrC from RCC in urban area (26.7% and 31.8% decreased, respectively in 2013). While in rural areas, their emission continually increased by 1.2% ~ 5.3% until more strict law enacted in 2017. Emissions of BC and ACS_BrC in winter seasons were 60.1 Gg and 1064.1 Gm², which accounted for 54.3% and 55.1% of the total BC and ACS_BrC emissions correspondingly. The peak values of hourly emission of BC and ACS_BrC (in 370 nm) normally appeared at 19:00-23:00, accounting for 43.0% and 41.5% of their total daily emission. The low emission periods were at cooking times including 7:00, 12:00, and 17:00 of a day and the whole emission of BC and ACS_BrC for the three periods accounted for 1.8% and 2.3% of their daily emission. This high-resolution BC and ACS_BrC emission inventories can be useful for future modeling works on the formation and evolution of a haze event, the smoke aging and transportation, as well as corresponding climate and human health effects.

Primary nature of brown carbon absorption in a frigid atmosphere with strong haze chemistry

Severe haze hovered over Harbin during the heating season of 2019-2020, making it one of the ten most polluted Chinese cities in January of 2020. Here we focused on the optical properties and sources of brown carbon (BrC) during the extreme atmospheric pollution periods. Enhanced formation of secondary BrC (BrC_sec) was evident as relative humidity (RH) became higher, accompanied with a decrease of ozone but concurrent increases of aerosol water content and secondary inorganic aerosols. These features were generally similar to the characteristics of haze chemistry observed during winter haze events in the North China Plain, and indicated that heterogeneous reactions involving aerosol water might be at play in the formation of BrC_sec, despite the low temperatures in Harbin. Although BrC_sec accounted for a substantial fraction of brown carbon mass, its contribution to BrC absorption was much smaller (6 vs. 28%), pointing to a lower mass absorption efficiency (MAE) of BrC_sec compared to primary BrC. In addition, emissions of biomass burning BrC (BrC_BB) were inferred to increase with increasing RH, coinciding with a large drop of temperature. Since both the less absorbing BrC_sec and the more absorbing BrC_BB increased as RH became higher, the MAE of total BrC were largely unchanged throughout the measurement period. This study unfolded the contrast in the source apportionment results of BrC mass and absorption, and could have implications for the simulation of radiative forcing by brown carbon.

Evolution of organic carbon during COVID-19 lockdown period: Possible contribution of nocturnal chemistry

Carbonaceous aerosol is one of the main components of atmospheric particulate matter, which is of great significance due to its role in climate change, earth's radiation balance, visibility, and human health. In this work, carbonaceous aerosols were measured in Shijiazhuang and Beijing using the OC/EC analyzer from December 1, 2019 to March 15, 2020, which covered the Coronavirus Disease 2019 (COVID-19) pandemic. The observed results show that the gas-phase pollutants, such as NO, NO2, and aerosol-phase pollutants (Primary Organic Compounds, POC) from anthropogenic emissions, were significantly reduced during the lockdown period due to limited human activities in North China Plain (NCP). However, the atmospheric oxidation capacity (Ox/CO) shows a significantly increase during the lockdown period. Meanwhile, additional sources of nighttime Secondary Organic Carbon (SOC), Secondary Organic Aerosol (SOA), and babs, BrC(370 nm) are observed and ascribed to the nocturnal chemistry related to NO3 radical. The Potential Source Contribution Function (PSCF) analysis indicates that the southeast areas of the NCP region contributed more to the SOC during the lockdown period than the normal period. Our results highlight the importance of regional nocturnal chemistry in SOA formation.

Optical properties of mountain primary and secondary brown carbon aerosols in summertime

Brown carbon (BrC) can affect atmospheric radiation due to its strong absorption ability from the near ultraviolet to the visible range, thereby influencing global climate. However, given the complexity of BrC's chemical composition, its optical properties are still poorly understood, especially in mountainous areas. In this study, the black carbon (BC) tracer method is used to explore the light-absorbing properties of primary and secondary BrC at Mount Hua, China during the 2018 summer period. The primary BrC absorption contributes to 10-15% of the total BrC absorption at a wavelength of 370 nm. From the positive matrix factorization analysis, traffic emissions are found to be a major source of primary BrC absorption (44%), followed by industry and biomass-burning emissions (29%). The secondary BrC accounts for 87% of the total BrC absorption at a wavelength of 370 nm, indicating that BrC is dominated by secondary formation. The observation of a higher secondary BrC absorption diurnal pattern at Mount Hua can be affected by secondary BrC in the residual layer after sunrise and the formation of light-absorbing chromophores by photochemical oxidation in the afternoon. The estimated average mass absorption efficiencies of primary and secondary BrC (MAE_{_pri} and MAE_{_sec}, respectively) are 0.4 m²/g and 2.1 m²/g at wavelengths of 370 nm, respectively, indicating a stronger light-absorbing ability for secondary BrC than for primary BrC. There is no significant difference in MAE_{_pri} within a daily variation, but the daytime MAE_{_sec} value is higher than that during the night. Our study shows that secondary BrC is important to light absorption in mountainous areas.

Apportionment of black and brown carbon spectral absorption sources in the urban environment of Athens, Greece, during winter

This study examines the spectral properties and source characteristics of absorbing aerosols (BC: Black Carbon; BrC: Brown Carbon, based on aethalometer measurements) in the urban background of Athens during December 2016-February 2017. Using common assumptions regarding the spectral dependence of absorption due to BC (AAE_BC = 1) and biomass burning (AAE_bb = 2), and calculating an optimal AAE_ff value for the dataset (1.18), the total spectral absorption was decomposed into five components, corresponding to absorption of BC and BrC from fossil-fuel (ff) combustion and biomass burning (bb), and to secondary BrC estimated using the BC-tracer minimum R-squared (MRS) method. Substantial differences in the contribution of various components to the total absorption were found between day and night, due to differences in emissions and meteorological dynamics, while BrC and biomass burning aerosols presented higher contributions at shorter wavelengths. At 370 nm, the absorption due to BC_ff contributed 36.3% on average, exhibiting a higher fraction (58.1%) during daytime, while the mean BC_bb absorption was estimated at 18.4%. The mean absorption contributions due to BrC_ff, BrC_bb and BrC_sec were 6.7%, 32.3% and 4.9%, respectively. The AbsBC_ff,370 component maximized during the morning traffic hours and was strongly correlated with NO_x (R² = 0.76) and CO (R² = 0.77), while a similar behavior was seen for the AbsBrC_ff,370 component. AbsBC_bb and AbsBrC_bb levels escalated during nighttime and were highly associated with nss-K⁺ and with the organic aerosol (OA) components related to fresh and fast-oxidized biomass burning (BBOA and SV-OOA) as obtained from ACSM measurements. Multiple linear regression was used to attribute BrC absorption to five OA components and to determine their absorption contributions and efficiencies, revealing maximum contributions of BBOA (33%) and SV-OOA (21%). Sensitivity analysis was performed in view of the methodological uncertainties and supported the reliability of the results, which can have important implications for radiative transfer models.

Evolution of the chromophore aerosols and its driving factors in summertime Xi'an, Northwest China

Atmospheric chromophores have photo-sensitiveness that can participate in photochemical reactions, so they may have the potential to make an important contribution in organic aerosols aging. This study attempts to explain the effects of oxidation reaction and photochemical reaction on atmospheric chromophores. For this study, the summer period (higher sunshine intensity) was selected to observe the mechanisms by the online excitation emission matrix (EEM) fluorescence. The results showed that a lot of secondary organic aerosols were produced in the afternoon, but a large portion of them is non-chromophore. We observed that the secondary chromophores of highly-oxygenated humic-like substances (HULIS) were produced, which suggests a degradation product of less-oxygenated HULIS. The photochemical reaction and oxidation reaction were the important reactions that occur in the afternoon, which drives the oxidation state evolution of the atmospheric chromophores. Atmospheric oxidation processes are the mainly driving reaction for the transformation of atmospheric chromophore. The aged aerosol has a lower fluorescence index and a high degree of humification. It is speculated that the aerosol from night to morning is in the accumulation process dominated by local sources, and then it is mainly in the process of being gradually aged at noon and afternoon. This study will guide to better understand the atmospheric chemical processes of chromophore aerosols and provide guidance for the EEM approach to trace the aerosol aging in the atmosphere.

Diurnal evolutions and sources of water-soluble chromophoric aerosols over Xi'an during haze event, in Northwest China

Atmospheric brown carbon and their chemical behavior potentially impacts the climate and air quality. Due to lack of researches on the atmospheric chromophores by using online experimental instrument, so using the offline EEM approaches to study their types, sources and chemical processes. In this study, PILS-EEM-TOC system (Particle into liquid sampler coupled with excitation-emission matrix and total organic carbon) was developed in order to distinguish the hourly evolutions and sources of water-soluble chromophoric organic matters in atmospheric fine particles. The results suggested that the sources of atmospheric chromophores in winter were primary combustion (~90%) and coal burning, followed by biomass burning and cooking emissions in Xi'an (Northwest China). These atmospheric chromophores decay under the combined action of solar radiation and atmospheric oxidants. Meanwhile, the secondary chromophores were mainly highly-oxygenated humic-like substance (HULIS), produced by atmospheric oxidation reactions with the highest peak in the afternoon. The partly secondary chromophores can also be generated through the Maillard-like reaction in the morning, which depends on the relative humidity of the atmosphere. These findings made a deeper understanding of the sources and transformation of atmospheric brown carbon aerosols.

Light absorption of black carbon and brown carbon in winter in North China Plain: comparisons between urban and rural sites

The light absorption black carbon (BC) and brown carbon (BrC) are two important sources of uncertainties in radiative forcing estimate. Here we investigated the light absorption enhancement (E_abs) of BC due to coated materials at an urban (Beijing) and a rural site (Gucheng) in North China Plain (NCP) in winter 2019 by using a photoacoustic extinctiometer coupled with a thermodenuder. Our results showed that the average (±1σ) E_abs was 1.32 (±0.15) at the rural site, which was slightly higher than that at the urban site (1.24 ± 0.15). The dependence of E_abs on coating materials was found to be relatively limited at both sites. However, E_abs presented considerable increases as a function of relative humidity below 70%. Further analysis showed that E_abs during non-heating period in Beijing was mainly caused by secondary components, while it was dominantly contributed by enhanced primary emissions in heating season at both sites. In particular, aerosol particles mixed with coal combustion emissions had a large impact on E_abs (>1.40), while the fresh traffic emissions and freshly oxidized secondary OA (SOA) had limited E_abs (1.00-1.23). Although highly aged or aqueous-phase processed SOA coated on BC showed the largest E_abs, their contributions to the bulk absorption enhancement were generally small. We also quantified the absorption of BrC and source contributions. The results showed the BrC absorption at the rural site was nearly twice that of urban site, yet absorption Ångström exponents were similar. Multiple linear regression analysis highlighted the major sources of BrC being coal combustion emissions and photochemical SOA at both sites with additional biomass burning at the rural site. Overall, our results demonstrated the relatively limited winter light absorption enhancement of BC in different chemical environments in NCP, which needs be considered in regional climate models to improve BC radiative forcing estimates.

Substantial decreases of light absorption, concentrations and relative contributions of fossil fuel to light-absorbing carbonaceous aerosols attributed to the COVID-19 lockdown in east China

To prevent spreads of Coronavirus disease-2019 (COVID-19), China adopted the lockdown measures in late January 2020, providing a platform to study the response of air quality and atmospheric chemical and physical properties to strict reduced emissions. In this study, the continuous measurements of aerosol light absorption were conducted in Nanjing, east China, from January 3 to March 31, 2020. Our results showed that the contribution of black carbon (BC) to light absorption at the different wavelengths was more than 75% and the rest light absorption was contributed by brown carbon (BrC), which was mainly originated from primary emissions. Secondary BrC absorption, which was mainly produced by photochemical oxidation, constituted a minor fraction (2-7%) of the total absorption. Compared with the sampling in the pre-lockdown, the significant decreases of BC (43%) and secondary BrC absorption (31%) were found during the lockdown period, resulting in a substantial decrease of solar energy absorbance by 36% on a local scale. The control measures also changed the diurnal variations of light absorption. Due to the reduced emissions, the relative fraction of fossil fuel to BC also dropped from 78% in the pre-lockdown to 71% in the lockdown. The concentrations of BC, PM_2.5 and NO₂ decreased 1.1 μg m^-3, 33 μg m^-3 and 9.1 ppb whereas O₃ concentration increased 9.0 ppb during the COVID-19 lockdown period. The decreased concentrations of BC, PM_2.5 and NO₂ were mainly contributed by both emission reduction (51-64%) and meteorological conditions (36-49%). Our results highlighted that the balance of control measures in alleviation of particulate matter (PM) and O₃ pollution, and meteorology should be seriously considered for improvement of air quality in this urban city of China.

Light Absorption Properties of Organic Aerosol from Wood Pyrolysis: Measurement Method Comparison and Radiative Implications

Growing evidence indicates that organic aerosol (OA) is a significant absorber of solar radiation. Such absorptive OA is known as "brown carbon" (BrC). However, a formal analytical method for BrC is currently lacking although several methods have been applied to determine its absorption properties. Reported imaginary refractive index (k_OA) values from various combustion sources span 2 orders of magnitude. Measurement methods are an important factor affecting this k_OA variation. In this work, isolated OA from wood pyrolysis was used to compare four methods to determine absorbing properties of OA. The generated aerosol was lognormally distributed, spherical, and nearly pure organic matter. Optical closure was considered as the reference method. k_OA calculated from the extract bulk light absorbance measurement was comparable to that determined by optical closure. k_OA and mass absorption cross section obtained by online and offline filter-based transmission measurements were similar, but 3.5 to 5.0 times greater than those determined by optical closure. Absorption Ångström Exponents determined by the four methods were comparable and ranged from 6.1 to 6.8. A clear-sky radiative transfer model implied that using the optical parameters derived from different methods in the full climate model could produce different radiative impacts of primary OA emissions.