Brown Carbon Light Absorption Research Articles

An Overestimation of Light Absorption of Brown Carbon in Ambient Particles Caused by Using Filters with Large Pore Size

An Overestimation of Light Absorption of Brown Carbon in Ambient Particles Caused by Using Filters with Large Pore Size

Chromophoric Fingerprinting of Brown Carbon from Residential Biomass Burning

Emissions from residential biomass burning are a significant source of atmospheric brown carbon (BrC). In this study, we used liquid chromatography-photodiode array-high resolution tandem mass spectrometry to investigate the chromophoric fingerprinting of BrC emitted from residential biomass burning. In total, 59 major chromophores were identified, which accounted for 49–85% of the total light absorption of BrC (averaged between 300 and 500 nm) for different samples. These chromophores include furans, lignin pyrolysis products, coumarins, flavonoids, stilbenes, N-containing aromatic compounds, and unsubstituted or oxygenated polycyclic aromatic hydrocarbons, of which some are newly reported as BrC species (e.g., stilbenes and substituted phenyl cyanates). Among the chromophores identified, seven are common to all samples while some are specific to certain biofuel or burning conditions. For instance, 3,3′-dimethoxyquercetin from wheat burning, p-hydroxybenzaldehyde and apigenin from maize smoldering, 4-nitro-2-vinylphenol from maize flaming, and nodakenetin and anthanthrene from wood flaming are specific to the fuel type or burning condition. The identified optical markers will be essential for understanding atmospheric chemical and optical processes of biomass burning BrC in future studies, while the source-specific chromophore profiles developed in this study are a prerequisite for apportioning the biofuel types and burning conditions, which is a key for estimating source-specific radiative forcing.

Significant light absorption of brown carbon during the 2020 California wildfires

In this study, the contribution of brown carbon (BrC) to the absorption aerosol optical depth (AAOD) during the August to October 2020 California wildfires in Fresno, Monterey, and the University of California Santa Barbara (UCSB) was investigated using Aerosol Robotic Network (AERONET) column measurements with Moderate Resolution Imaging Spectroradiometer (MODIS) fire pixel counts. There was an approximate three to five times increase in AAOD and fine-mode aerosols during intensive wildfires in August–October 2020 compared to the wildfires in the previous 18 years (2002–2019). Substantial daily variation in the contribution of BrC to AAOD was correlated with the fire pixel counts (correlation coefficients of 0.63, 0.40, and 0.57 at Fresno, Monterey, and UCSB, respectively). This variation was influenced by regional topography, atmospheric conditions, and distance from the fire. Between August and October 2020, the average contribution of BrC to AAOD at 440 nm due to wildfires was 35.3 ± 5.6, 35.1 ± 6.8, and 40.6 ± 9.5% at Fresno, Monterey, and UCSB, respectively. This was approximately twice as high as for those sites without a direct wildfire influence. The BrC contribution with direct wildfire influence over the period of January–December 2020 at Fresno, Monterey, and UCSB (32.8 ± 7.5, 31.6 ± 7.9, and 40.0 ± 3.5%, respectively) and from 2002 to 2019 (30.7 ± 8.3, 28.5 ± 4.8, and 35.7 ± 14.6%, respectively) was approximately 20% greater than other BrC sources including vehicles, fossil fuel combustion, and residential heating.

Regional Differences in the Light Absorption Properties of Fine Particulate Matter Over the Tibetan Plateau: Insights From HR‐ToF‐AMS and Aethalometer Measurements

AbstractThe Tibetan Plateau (TP) has received wide scientific concern in recent decades owing to its important impacts on regional climatic and cryospheric changes, hydrological cycles, and environments. However, our understanding of the spatial distribution of the chemical and optical properties of aerosols is still limited based on prior single‐site observations. In this study, aerosol light absorptions from black carbon (BC) and brown carbon (BrC) were explored and compared among three remote TP sites, including Qomolangma Station (QOMS) on the southern TP, Nam Co Station (NamCo) on the central TP, and Waliguan Observatory on the northeastern TP. Although the aerosol mass concentration at QOMS was less than half of that at Waliguan, the light absorption coefficient of aerosols at QOMS was nearly 5 times higher than that at Waliguan, mainly as a result of more light‐absorbing carbonaceous aerosols on the southern TP due to long‐range transported biomass burning emissions. Specifically, BC dominated aerosol light absorption at all wavelengths, whereas BrC contributed ∼30% of the light absorption at 370 nm at QOMS and ∼20% at Waliguan and NamCo. A major contributor to BrC light absorption at QOMS was biomass burning‐related organic aerosols apportioned from aerosol mass spectrometry measurements. Understanding the significant regional differences in aerosol light absorption properties on the TP is useful for evaluating the regional climatic and environmental effects and validating the model results on aerosol radiative forcing.

Polycyclic Aromatic Carbon: A Key Fraction Determining the Light Absorption Properties of Methanol-Soluble Brown Carbon of Open Biomass Burning Aerosols.

The composition and radiative forcing of light-absorbing brown carbon (BrC) aerosol remain poorly understood. Polycyclic aromatics (PAs) are BrC chromophores with fused benzene rings. Understanding the occurrence and significance of PAs in BrC is challenging due to a lack of standards for many PAs. In this study, we quantified polycyclic aromatic carbon (PAC), defined as the carbon of fused benzene rings, based on molecular markers (benzene polycarboxylic acids, BPCAs). Open biomass burning aerosols (OBBAs) of 22 rainforest plants were successively extracted with water and methanol for the analysis of water- and methanol-soluble PAC (WPAC and MPAC, respectively). PAC is an important fraction of water- and methanol-soluble organic carbon (WSOC and MSOC, respectively). WPAC/WSOC ranged from 0.03 to 0.18, and MPAC/MSOC was even higher (range: 0.16-0.80). The priority polycyclic aromatic hydrocarbons contributed less than 1% of MPAC. The mass absorption efficiency (MAE) of MSOC showed a strong linear correlation with MPAC/MSOC (r = 0.60-0.95, p < 0.01). The absorption Ångström exponent (AAE) of methanol-soluble BrC showed a strong linear correlation with the degree of aromatic condensation of MPAC, which was described by the average number of carboxylic groups of BPCA (r = -0.79, p < 0.01). This result suggested that PAC was a key fraction determining the light absorption properties (i.e., light absorptivity and wavelength dependence) of methanol-soluble BrC in OBBAs.

Vertical evolution of black and brown carbon during pollution events over North China Plain

The vertical distribution of carbonaceous aerosol impacts climate change, air quality and human health, but there is a lack of in-situ vertical observations of black (BC) and brown carbon (BrC). Thus, the characteristic of vertical profiles of BC concentration, particle number concentration (PNC), O3 concentration and optical absorption of BC and BrC were observed in a suburban site over North China Plain, where heavy pollution of PM2.5 and O3 always occurred in winter and summer, respectively. In winter, during a heavy pollution episode, the BC and PNC was near uniformly distributed within mixing layer (ML) (15.2 ± 6.7 μg m−3 and 678 ± 227 p cm−3, respectively) and decreased with altitude above the ML. The BC heating rate reached about 0.13 K h−1 during the heaviest pollution day. In summer, the BC concentration (2.9 ± 1.3 μg m−3) in ML during the middle O3 pollution events was higher than that (1.7 ± 0.6 μg m−3) during the light O3 pollution. The light absorption coefficients of BC at 880 nm and BrC at 375 nm measured in the early morning were lower than that in the daytime, and the contribution of BrC to total light absorption of carbonaceous aerosols was in the range of 27–47%. In addition, BC was effectively transported to high altitude than BrC in the daytime. The light absorption of secondary BrC in the daytime was higher 10–20% than that in the early morning. Simultaneously, the contribution of secondary BrC to the total BrC light absorption at 375 nm was range from 32% to 68% within 1000 m.

Multigeneration Production of Secondary Organic Aerosol from Toluene Photooxidation.

Photooxidation of volatile organic compounds (VOCs) produces secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC) via multiple reaction steps/pathways, reflecting significant chemical complexity relevant to gaseous oxidation and subsequent gas-to-particle conversion. Toluene is an important VOC under urban conditions, but the fundamental chemical mechanism leading to SOA formation remains uncertain. Here, we elucidate multigeneration SOA production from toluene by simultaneously tracking the evolutions of gas-phase oxidation and aerosol formation in a reaction chamber. Large size increase and browning of monodisperse sub-micrometer seed particles occur shortly after initiating oxidation by hydroxyl radical (OH) at 10-90% relative humidity (RH). The evolution in gaseous products and aerosol properties (size/density/optical properties) and chemical speciation of aerosol-phase products indicate that the aerosol growth and browning result from earlier generation products consisting dominantly of dicarbonyl and carboxylic functional groups. While volatile dicarbonyls engage in aqueous reactions to yield nonvolatile oligomers and light-absorbing nitrogen heterocycles/heterochains (in the presence of NH3) at high RH, organic acids contribute to aerosol carboxylates via ionic dissociation or acid-base reaction in a wide RH range. We conclude that toluene contributes importantly to SOA/BrC formation from dicarbonyls and organic acids because of their prompt and high yields from photooxidation and unique functionalities for participation in aerosol-phase reactions.

Brownness of Organic Aerosol over the United States: Evidence for Seasonal Biomass Burning and Photobleaching Effects.

Light-absorptivity of organic aerosol may play an important role in visibility and climate forcing, but it has not been assessed as extensively as black carbon (BC) aerosol. Based on multiwavelength thermal/optical analysis and spectral mass balance, this study quantifies BC for the U.S. Interagency Monitoring of Protected Visual Environments (IMPROVE) network while developing a brownness index (γBr) for non-BC organic carbon (OC*) to illustrate the spatiotemporal trends of light-absorbing brown carbon (BrC) content. OC* light absorption efficiencies range from 0 to 3.1 m2 gC-1 at 405 nm, corresponding to the lowest and highest BrC content of 0 and 100%, respectively. BC, OC*, and γBr explain >97% of the variability of measured spectral light absorption (405-980 nm) across 158 IMPROVE sites. Network-average OC* light absorptions at 405 nm are 50 and 28% those for BC over rural and urban areas, respectively. Larger organic fractions of light absorption occur in winter, partially due to higher organic brownness. Winter γBr exhibits a dramatic regional/urban-rural contrast consistent with anthropogenic BrC emissions from residential wood combustion. The spatial differences diminish to uniformly low γBr in summer, suggesting effective BrC photobleaching over the midlatitudes. An empirical relationship between BC, ambient temperature, and γBr is established, which can facilitate the incorporation of organic aerosol absorptivity into climate and visibility models that currently assume either zero or static organic light absorption efficiencies.

Brown Carbon in Primary and Aged Coal Combustion Emission.

Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by the UV-visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE365) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m2 g-1, which was significantly higher than that for aged OA (0.24 ± 0.18 m2 g-1). Secondary OA (SOA) dominated aged OA and the decreased MAE365 after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE365 of SOA (0.14 ± 0.08 m2 g-1) was much lower than that of POA. A comparison of MAE365 of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.

PM1 chemical composition and light absorption properties in urban and rural areas within Sichuan Basin, southwest China.

Sichuan Basin is encircled by high mountains and plateaus with the heights ranging from 1km to 3km, and is one of the most polluted regions in China. However, the dominant chemical species and light absorption properties of aerosol particles is still not clear in rural areas. Chemical composition in PM1 (airborne particulate matter with an aerodynamic diameter less than 1μm) and light-absorbing properties were determined in Chengdu (urban) and Sanbacun (rural) in western Sichuan Basin (WSB), Southwest China. Carbonaceous aerosols and secondary inorganic ions (NH4+, NO3- and SO42-) dominate PM1 pollution, contributing more than 85% to PM1 mass at WSB. The mean concentrations of organic and elemental carbon (OC, EC), K+ and Cl- are 19.69μgm-3, 8.00μgm-3, 1.32μgm-3, 1.16μgm-3 at the rural site, which are 26.2%, 65.3%, 34.7% and 48.7% higher than those at the urban site, respectively. BrC (brown carbon) light absorption coefficient at 405nm is 63.90±27.81Mm-1 at the rural site, contributing more than half of total absorption, which is about five times higher than that at urban site (10.43±4.74Mm-1). Compared with secondary OC, rural BrC light absorption more depends on primary OC from biomass and coal burning. The rural MAEBrC (BrC mass absorption efficiency) at 405nm ranges from 0.6 to 5.1m2g-1 with mean value of 3.5±0.8m2g-1, which is about three times higher than the urban site.

Brown carbon light absorption over an urban environment in northern peninsular Southeast Asia.

Light-absorbing organic carbon (or brown carbon, BrC) has been recognized as a critical driver in regional-to-global climate change on account of its significant contribution to light absorption. BrC sources vary from primary combustion processes (burning of biomass, biofuel, and fossil fuel) to secondary formation in the atmosphere. This paper investigated the light-absorbing properties of BrC such as site-specific mass absorption cross-section (MACBrC), absorption Ångström exponent (AAEBrC), and the absorbing component of the refractive index (kBrC) by using light absorption measurements from a 7-wavelength aethalometer over an urban environment of Chiang Mai, Thailand in northern peninsular Southeast Asia (PSEA), from March to April 2016. The contribution of BrC to total aerosol absorption (mean ± SD) was 46±9%, 29±7%, 24±6%, 20±4%, and 15±3% at 370, 470, 520, 590, and 660nm, respectively, highlighting the significant influence of BrC absorption on the radiative imbalance over northern PSEA. Strong and significant associations between BrC light absorption and biomass-burning (BB) organic tracers highlighted the influence of primary BB emissions. The median MACBrC and kBrC values at 370nm were 2.4m2g-1 and 0.12, respectively. The fractional contribution of solar radiation absorbed by BrC relative to BC (mean ± SD) in the 370-950nm range was estimated to be 34±7%, which can significantly influence the regional radiation budget and consequently atmospheric photochemistry. This study provides valuable information to understand BrC absorption over northern PSEA and can be used in model simulations to reassess the regional climatic impact with greater accuracy.

High light absorption and radiative forcing contributions of primary brown carbon and black carbon to urban aerosol

To investigate the seasonal variation of black carbon (BC) and brown carbon (BrC) light absorption at an urban site in Xi'an, China, an annual measurement was conducted by using a 7-wavelengths Aethalometer (model AE33). The results showed the aerosol absorptions were observed approximately 5–8 times greater in winter (395 Mm−1 at 370 nm and 99 Mm−1 at 880 nm, respectively) than those in autumn (49 Mm−1 at 370 nm and 18 Mm−1 at 880 nm, respectively). BC was the dominant light absorbing carbonaceous component at all wavelengths (from 370 nm to 950 nm). BrC absorption (370 nm) maximized during winter with 41% of the total aerosol absorption, and was lower in autumn (16%). The light absorption contributions of primary BrC (BrCpri) were higher than that of secondary BrC (BrCsec) in four seasons, with more than 67% of total BrC light absorption. The seasonal absorption Ångström exponents (AAEs) of BrC, BrCpri, and BrCsec were directly derived with the highest values (3.91, 3.69, and 4.94, respectively) in wintertime. The BrCpri contributed much higher solar absorptions percentage to BC than BrCsec. The results underlined the primary emissions of urban BrCpri and BC, implying that the strategies of energy efficiency enhancement and energy structure reformation will be very important in reducing primary emissions in China. Such investigations should be conducted in other developing countries with severe air pollution.

Identification of PM2.5 sources contributing to both Brown carbon and reactive oxygen species generation in winter in Beijing, China

Better understanding of fine particulate matter (PM2.5) composition and sources associated with brown carbon (BrC) and reactive oxygen species (ROS) are essential for developing effective air quality control strategies to mitigate climate warming and protect human health. To identify the common chemical composition and sources associated with BrC and ROS, BrC light absorption and hydroxyl radical (·OH), one of the most reactive ROS, were measured for the same batch of filter samples collected in Beijing, China during a winter sampling period covering days with different pollution levels, together with detailed chemical composition analysis, including major components and individual organic and inorganic source-specific tracers. The results showed that both light absorption of BrC and ·OH generation by per mass PM2.5 decreased with increase of PM2.5 mass concentrations, which could be primarily attributed to the predominant roles played by secondary inorganic aerosols in the increased PM2.5 mass, while both of them increased with the fractions of water-soluble organic carbon (WSOC) in PM2.5. WSOC and combustion-related sources were found to be the common and major contributors to both BrC and ·OH generation in Beijing winter. However, BrC exhibited better correlations with fossil source WSOC, while ·OH better correlated with biomass burning source related WSOC. During the study period, fossil sources especially coal combustion were more important sources for BrC, while biomass burning played a relatively stronger role in ·OH generation.

Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes.

The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.

Discrepancies between brown carbon light-absorption properties retrieved from online and offline measurements

We compared the imaginary part of the refractive indices (k) of brown carbon (BrC) retrieved from online and solvent-extraction offline light-absorption measurements. BrC with variable light-absorp...

N‐Alkanes and PAHs in the Southeastern Tibetan Plateau: Characteristics and Correlations With Brown Carbon Light Absorption

AbstractThe samples of atmospheric total suspended particle (TSP) were obtained at the remote Tibetan site (named Lulang) from November 2015 to November 2016. The seasonal characteristics of n‐alkanes, polycyclic aromatic hydrocarbons (PAHs) variations, and their correlations to brown carbon (BrC) light absorption in TSP were investigated. Elevated TSP n‐alkanes and PAHs levels were observed in winter (2.8 and 10 times higher than in monsoon, respectively), largely due to the seasonal contributions of wintertime home heating and long‐range transport. The total‐PAHs was found to correlate strongly with the methanol‐soluble BrC (MeS‐BrC) light absorption coefficient (babs) (R = 0.80) compared to water‐soluble BrC (WS‐BrC) (R = 0.70) at 365 nm. The BrC solar absorptions percentages to those of element carbon (EC) in the range of ultraviolet (300–400 nm) were high in both winter and premonsoon seasons, which can affect the vulnerable environmental radiative balance and atmospheric photochemistry.

Water/Methanol-Insoluble Brown Carbon Can Dominate Aerosol-Enhanced Light Absorption in Port Cities.

Light absorption enhancement (Eabs) of black carbon (BC) is a key factor in global climate models and is impacted by brown carbon (BrC) and the lensing effect of coatings. We conducted an in-depth field study on Eabs for ambient aerosols at a monitoring point in Shanghai, China, by real-time aerosol optical property monitoring and high-performance liquid chromatography/diode array detector/quadrupole-time-of-flight mass spectrometry (HPLC/DAD/Q-ToF-MS) analysis. The results showed Eabs at λ = 530 nm caused by the lensing effect was about 1.39 ± 027, accounting for 18.84% of the total light absorption. In this study, BrC is classified as soluble BrC (soluble in both water and methanol) or insoluble BrC (insoluble in both water and methanol). Soluble BrC accounted for 13.68 ± 11.15% of the total aerosol light absorption. For the first time, we concluded that insoluble BrC can contribute more than 60 and 97% of total aerosol and BrC light absorption in port cities, respectively. The molecular analysis of soluble BrC identified N-containing aromatic compounds (4-nitrophenol, 4-nitrocatechol, methyl nitrophenol, methyl nitrocatechols, and nitro-1-naphthol) commonly observed in biomass burning emissions or biomass burning-impacted atmospheres. A series of components (C16H26O3S, C17H28O3S, C18H30O3S, and C19H32O3S) were determined to be emissions from nearby cargo ships filled with heavy fuel oil (HFO), which further confirmed that insoluble BrC emitted from cargo ships could be the largest contributor to Eabs. This study confirms the global significance of evaluating HFO used in port cities in climate models. The control measures of cargo ship emission should be considered for the related environmental and health issues in port cities.

Changes in light absorption by brown carbon in soot particles due to heterogeneous ozone aging in a smog chamber

Light absorption by brown carbon (BrC) is dynamic due to atmospheric aging processes, leading to complex and poorly constrained effects on photochemistry and climate. In this study, a smog chamber was used to simulate the heterogeneous ozone (O3) aging of soot particles. Twelve aging times and seven O3 concentrations were set to investigate the effects of aging degree on BrC light absorption. The results showed that light absorption by BrC was enhanced after O3 aging, but followed a non-monotonic trend with an initial increase and subsequent decrease. An aging time of 60 min and O3 concentration of 1.2 ppm were optimal for enhancing BrC absorption, where the contribution of BrC to total absorption and the contribution of BrC relative to black carbon absorption at 370 nm of ozonized soot were 23.0 ± 1.8% and 30.0 ± 3.0%, respectively, much greater than those of fresh soot (8.1 ± 1.1% and 8.8 ± 1.3%, respectively). The absorption Ångström exponent (AAE) and delta C (ΔC) of ozonized soot at 60 min ranged from 1.18 ± 0.01 to 1.31 ± 0.03 and from 13.5 ± 7.0 to 24.3 ± 13.5 μg m−3, respectively, and were greater than those of fresh soot (1.12 ± 0.02 and 8.0 ± 0.8 μg m−3), but also showed non-monotonic trends, suggesting the formation of BrC during O3 aging. Comparative results indicated that AAE might be a better BrC indicator for soot than ΔC. The non-monotonic trend was tentatively explained by changes in organic carbon, oxygenated functional groups and conjugated structures, as well as polycyclic aromatic hydrocarbon (PAH) degradation and oxygenated PAH formation. The relative intensities of oxidative formation and degradation of chromophores may determine BrC evolution during O3 aging. This study will be useful for clarifying BrC evolution in the atmosphere and estimating its radiative forcing.

Light Absorbing Properties of Primary and Secondary Brown Carbon in a Tropical Urban Environment.

Brown carbon (BrC) has significant climatic impact, but its emission sources and formation processes remain under-represented in climate models. However, there are only limited field studies to quantify the light absorption properties of specific types of primary and secondary organic aerosols (POAs and SOAs) in different environments. This work investigates the light absorption properties of the major OA components in Singapore, a well-developed city in the tropical region, where air quality can be influenced by multiple local urban sources and regional biomass burning events. The source-specific mass absorption cross-section (MAC) and wavelength dependence of different BrC components were quantified based on highly time-resolved aerosol chemical composition and absorption measurements. In particular, the combustion-related emission sources were the primary contributors to BrC light absorption and they were moderately absorbing. The SOA materials, which were freshly formed under atmospheric conditions with industrial influences, were also moderately light absorptive. The aged SOA components that were composed of aged regional emissions, including biomass burning and coal combustion emissions from nearby regions, were weakly light absorbing, highlighting the possibility of photobleaching of BrC during their atmospheric aging and dispersion. Lastly, our estimations illustrate that typical urban POAs and SOAs can contribute up to approximately 36-58% of the BrC absorption, even in some urban locations that are influenced by biomass burning emissions.

Characterizing chemical composition and light absorption of nitroaromatic compounds in the winter of Beijing

Abstract Nitroaromatic compounds (NACs) were believed as one of the major light absorption components in brown carbon (BrC) and important fractions of organic aerosols. To understand the chemical compositions and light absorption of NACs in the highly polluted atmospheres, a field campaign was conducted in the winter of urban Beijing. Ambient PM2.5 samples were collected and a total of 12 NACs were identified and quantified using ultrahigh performance liquid chromatography coupled to Orbitrap mass spectrometer. Light absorption of methanol-extracted BrC and individual NAC in PM2.5 were also measured using UV–Vis spectrophotometer. The average concentration of quantified NACs was 173 ± 137 ng/m3, ranging from 19 to 585 ng/m3, which was much higher than those in summer in Beijing or in winter in other areas. Nitrophenols and nitrocatechols are the dominant species among the identified NACs in the winter of Beijing. Four pollution episodes were observed during the observation. The NAC concentrations increased from 62 ng/m3 during clean days to 224 ng/m3 during pollution episodes, with both secondary formation and biomass burning emissions as important sources. Particulate BrC was estimated to contribute as high as 45% of the total light absorption of carbonaceous aerosols at 370 nm based on the online aethalometer measurement. The identified NACs contributed about 17% of the total absorption by methanol-extracted BrC at 370 nm, though the mass concentrations of quantified NACs only accounted for 0.6% of the organic matter. This indicates the high absorption capability of NACs, which needs to be taken into consideration when studying the aerosol light absorption in urban environments. Moreover, the maximum absorption values of NACs among the range of 300–400 nm made them important to atmospheric photochemistry reactions. The linkage between chemical compositions and light absorption of NACs deserves to be further investigated to understand the light absorption mechanisms and environmental impacts of organic aerosols.