The molecular composition, optical properties, and oxidative potential of methanol-soluble organic carbon (MSOC, i.e., brown carbon) generated during different stages (initial vs stable) of residential coal combustion were systematically investigated. The findings demonstrated notable disparities in MSOC, characterized by relatively higher abundance of unsaturated aromatic substances produced in the initial combustion stage, whereas more highly oxygenated polar and S-containing compounds were produced in the stable combustion stage for bituminous coals. Additionally, results indicated that MSOC generated in the initial combustion stage exhibited stronger light-absorbing capacity than that generated in the stable combustion stage, which was positively correlated with aromaticity. Spearman's rank correlations were utilized to screen 507 potential light-absorbing molecules, predominantly comprising CHO and CHON compounds with aromatic and condensed aromatic structures. The oxidative potential of MSOC generated in the stable combustion stage was higher than that generated in the initial combustion stage, which may be attributed to its abundance of S-containing compounds. In summary, the initial stage of residential coal combustion releases a substantial quantity of light-absorbing pollutants, contributing to light absorption by atmospheric aerosols, while the high-temperature stable combustion stage releases large quantities of S-containing compounds, posing health risks.

Strong influence of NO-NO2-Ox on brown carbon (BrC) absorption characteristics: Seasonal and diurnal variations.

pH-dependent reaction kinetics between glyoxal and ammonium sulfate in simulated cloud droplets.

Interactions between black, brown, blue and green carbon from terrestrial to marine ecosystems: A critical review

Seasonal variation and optical properties of brown carbon and nitroaromatic compounds in PM2.5 in the urban area of Beijing

Abstract. Warming climate is predicted to increase forest fires which can be a major source of black and brown carbon (BC and BrC) into the atmosphere. Unlike North American forest fires, very limited studies have characterized North Eurasian biomass burning (BB) emissions. In this work, we determined the emission factors (EF) of carbonaceous aerosols and characterized light absorption of BrC emitted from boreal and peat burning through offline filter extraction method. The results were compared to African savanna emissions. Effects of atmospheric dilution and oxidative aging on BrC absorptivity were investigated for selected BB emissions sampled into an environmental chamber. Organic carbon (OC) and elemental carbon (EC) EFs of fresh BB emissions ranged between 1.30–89.9 and 0.01–4.80 g kg−1 respectively. Methanol soluble OC (MSOC) represented more than 92 % of fresh BB emissions, irrespective of fuel type, and consisted of weakly absorbing BrC with imaginary refractive index at 550 nm (kMSOC_550) ranging from 0.002 to 0.011. Water soluble OC (WSOC) fractions varied among fresh BB emissions but overall exhibited higher mass absorption efficiencies at 365 nm (MAE365) than MSOC. Dilution-related evaporative loss in environmental chamber resulted in less volatile OC, making them less soluble in methanol. Photochemical and dark oxidative aging further increased the low volatility OC fractions of the organics along with its oxidation state. Our estimated OC-EC emission factors and kMSOC for fresh BB emissions can be used for future modelling purposes. Further online measurements are needed to account for non-soluble strong BrC in aged BB emissions.

Abstract Brown carbon (BrC) is a type of light‐absorbing organic carbon and its structural characteristics have significant effects on atmospheric radiative forcing and global climate change. In this work, the main absorbing components of BrC were separated into three fractions and each fraction was analyzed individually and systematically. The compositional and structural characteristics of BrC across different wavelengths were studied, and the potential BrC compounds were revealed. Overall, nearly 100 light‐absorbing compounds were identified. The result showed that aromatic and heterocyclic compounds were main contributors to BrC absorption in the wavelength from 220 to 450 nm, which play an important role in radiative forcing. Interestingly, carbonyl groups dominated the BrC absorption in the wavelength from 200 to 220 nm. Although BrC compounds have minimal direct radiative impact due to limited solar radiation at such a wavelength, it can still play important roles in atmospheric photochemistry by participating in light‐induced oxidation process and lead to severe photochemical pollution. Additionally, carboxyl‐rich alicyclic molecules and lignin‐like substances all promoted the absorptivity of BrC, and nitrogenous substituents further enhanced such a process. These results were further extrapolated to various environmental conditions, which indicated that urban BrC probably contained more carbonyl groups. Conversely, more heterocyclic substances were observed in the forest and ocean areas, which illustrated high burden of radiative forcing in these areas. Overall, our work reveals that the BrC absorption is largely dependent on their molecule properties in different wavelengths, and such a correlation can guide and facilitate the BrC absorption analysis.

Abstract Residential activities are major contributors to global carbonaceous aerosol emissions, particularly in densely populated and low‐middle‐income countries such as India. Assessing the climatic impacts of these aerosols requires a detailed understanding of their optical properties and atmospheric abundance. Despite a strong understanding of cooking activities, residential water and space heating emissions in India remain highly uncertain. Using the field measurements, this study quantifies emission factors (EFs) and optical properties of emissions from water and space heating in India. The measured PM 2.5 , elemental carbon (EC), and organic carbon (OC) EFs for heating range between 2.2–20.7, 0.4–2.2, and 0.8–5.5 g kg −1 , respectively, are ∼2 fold higher than those for cooking. PM 2.5 EFs from firewood (20.7 ± 8.5 g kg −1 ) are larger than those during the crop residue (7.1 ± 5.7 g kg −1 ) and dung cake (17.4 ± 9.8 g kg −1 ) burning for water heating. These emissions exhibit significant absorption, corroborated by low values of single‐scattering albedo at 532 nm ranging from 0.17 to 0.96. A large absorption Angström exponent of 1.34–2.57 suggests the presence of brown carbon. The study estimates emissions of 1,239 (±264), 309 (±88), and 88 (±30) Gg yr −1 for PM 2.5 , OC, and EC, respectively, from residential heating activities in India. Spatially, emission patterns from heating differ from those for cooking, with high emissions in northern hilly regions, the Indo‐Gangetic plains, and western and southern India. The derived EFs, optical properties, and high‐resolution emissions enhance the understanding of aerosol climate impacts, offering insights for regional mitigation strategies for air pollution.

Phenolic compoundsare some of the most abundant emissions of biomassburning during wildfires. Catechol, the most abundant isomer of benzenediolin biomass burning emissions, undergoes oxidation in the aqueous phaseof cloud droplets to form secondary organic aerosol (SOA), includingproducts that absorb light at visible wavelengths, called brown carbon(BrC) chromophores. After cloud evaporation, the remaining submicronSOA particles are susceptible to further oxidant- and light-drivenprocessing. Here, we investigate the multiphase ozone oxidation ofthe reaction mixture from the aqueous OH-initiated oxidation of catechol,i.e., simulated OH-driven processing in clouds, using a coated-wallflow-tube apparatus. Reactive uptake of ozone was determined for thinfilms from the OH-driven processing of catechol with and without furtherirradiation of the thin films, i.e., simulated light-driven processingafter cloud evaporation, at low and moderate relative humidity (RH).The experimental time series were reproduced using kinetic multilayermodeling, which, along with qualitative microscopy experiments, providedinsights into the diffusivity of these materials. After OH-drivenprocessing, the thin films exhibited uptake coefficients of 2 ×10–6 and 9 × 10–6 at 0 and50% RH, respectively, and 4 h of exposure to 130 ppb of ozone. AfterOH- and light-driven processing, the uptake coefficients were lower,2 × 10–7 at 0% RH and 4 × 10–6 at 50% RH, for the same ozone exposure. Consequently, the reactionmixture of catechol was plasticized upon uptake of water vapor butvitrified under UV irradiation. Kinetic multilayer modeling showsthat slower ozone diffusion at low RH and after light-driven processingcan lead to an increase in the atmospheric lifetime of reactive speciesfrom less than 1 h to more than a day.

Light-absorbing carbonaceous aerosols (LACs), including black carbon (BC) and brown carbon (BrC), significantly influence Earth's radiative balance and global climate. However, their atmospheric aging processes and associated optical evolution remain insufficiently understood. In this study, in situ photochemical aging of ambient LACs under varying relative humidity (RH) conditions was simulated using an oxidation flow reactor (OFR). The distinct absorption evolution of BC and BrC was investigated, and the underlying mechanisms were explored. BC absorption primarily decreased under low-RH aging but significantly increased under high-RH aging. This contrasting behavior can be attributed to RH-dependent changes in BC coating processes: the dominant loss of preexisting coatings at low RH versus enhanced formation of secondary species that preferentially coat BC under high RH. Notably, BC absorption enhancement is more sensitive to nitrate, ammonium, and secondary organic aerosol (SOA) formation than to sulfate. BrC exhibited optical bleaching under both RH conditions; however, the bleaching rate was substantially accelerated under high RH at comparable photochemical ages within the range of below 5 equiv atmospheric aging days. This is primarily due to a 2-fold increase in the aqueous-phase photo-oxidative degradation of BrC chromophores derived from biomass-burning sources, whereas nonbiomass BrC showed RH-independent bleaching. These findings show that RH strongly modulates the chemical and optical aging of LACs, with important implications for their direct radiative forcing and better representation in climate models.

Abstract. Light-absorbing organic carbon, collectively known as brown carbon (BrC), significantly influences climate and air quality, particularly in urban environments like Dhaka, Bangladesh. Despite their significance, the contributions and transformation pathways of phenolic compounds – major precursors of brown carbon (BrC) – are still insufficiently understood in the South Asian megacities. This study addresses this gap by investigating the surface morphology of PM2.5, quantifying seven phenolic BrC precursors, and exploring the aqueous-phase formation pathway of nitrophenols at two urban sites (Dhaka South and Dhaka North) from July 2023 to January 2024. Phenolic compounds, including phenol, methylphenols, methoxyphenol, hydroxyphenol, and nitrophenol were identified and quantified using gas chromatography–flame ionization detector (GC-FID). PM2.5 surface morphology and elemental composition were analyzed via Field Emission Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (FESEM-EDX), and functional groups were characterized using Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR-FTIR). Results revealed that PM2.5 particles were predominantly spherical or chain-like with carbonaceous elements (C, O, N, S), mineral dust, and trace metals. The dominant functional groups included aromatic conjugate double bond, carbonyl, and nitro group. Aqueous-phase nitration of 2-hydroxyphenol under acidic conditions, analyzed via UV-Vis spectroscopy, demonstrated an alternative nitrophenol formation pathway. Among the detected compounds, 2-hydroxyphenol and 4-nitrophenol showed the highest average concentrations (2.31 ± 1.39 and 2.20 ± 1.21 µg m−3, respectively). Seasonal variations showed elevated nitrophenol levels during winter, especially in Dhaka South (4.54 ± 2.94 µg m−3). These findings highlight the quantification of phenolic precursors and the role of aqueous-phase reactions in BrC formation, providing valuable insights for future atmospheric modeling and air quality management strategies in South Asia.

Light absorption and molecular composition of brown carbon in Nanjing, China: Large contribution of biomass burning and secondary formation.

Observation of Brown carbon in PM2.5 revealing effects of source-dependence and aging on light absorption.

This study, based on global AERONET observation data from 2023, employs a synergistic inversion algorithm that integrates aerosol optical, microphysical, and chemical properties to retrieve the global distribution of aerosol parameters. We find that the global annual mean aerosol optical depth (AOD), fine-mode AOD (AODf), coarse-mode AOD (AODc), absorbing aerosol optical depth (AAOD), single scattering albedo (SSA) are 0.20, 0.15, 0.04, 0.024, and 0.87, respectively. From the perspective of spatial distribution, in densely populated urban areas, AOD is mainly determined by AODf, while in the areas dominated by natural sources, AODc contributes more. Combined with the optical and microphysical properties, fine-mode aerosols dominate optical contributions, whereas coarse-mode aerosols dominate volume contributions. In terms of chemical components, fine-mode aerosols at most global sites are primarily carbonaceous. The mass concentrations of black carbon (BC) exceed 10 mg m−2 in parts of South Asia, Southeast Asia, and the Arabian Peninsula, while the mass fraction of brown carbon (BrC) accounts for more than 16% in regions such as the Sahara, Western Africa, and the North Atlantic Ocean reference areas. The dust (DU) dominates in coarse mode, with the annual mean DU fraction reaching 86.07% in the Sahara. In coastal and humid regions, the sea salt (SS) and water content (AWc) contribute significantly to the aerosol mass, with fractions reaching 13.13% and 34.39%. The comparison of aerosol properties in the hemispheres reveals that the aerosol loading in the Northern Hemisphere caused by human activities is higher than in the Southern Hemisphere, and the absorption properties are also stronger. We also find that the uneven distribution of global observation sites leads to a significant underestimation of aerosol absorption and coarse-mode features in global mean values, highlighting the adverse impact of observational imbalance on the assessment of global aerosol properties. By combining analyses of aerosol optical, microphysical, and chemical properties, our study offers a quantitative foundation for understanding the spatiotemporal distribution of global aerosols and their emission contributions, providing valuable insights for climate change assessment and air quality research.

Secondary production of brown carbon particles with an agglomerate structure

Spatiotemporal Variability and Radiative Forcing of Secondary Brown Carbon over India

Abstract. Benzothiazoles (BTs), widely used as vulcanization accelerators in the rubber industry, have frequently been identified in the atmosphere, especially in areas with heavy traffic. BTs can undergo gas-phase oxidation in the atmosphere, which contributes to secondary aerosol mass. However, given their certain water solubility, the atmospheric fate of BTs associated with aqueous-phase transformations is unclear. In this study, the reactions of benzothiazole (BT), 2-methylbenzothiazole (MBT), and 2-chlorobenzothiazole (CBT) with hydroxyl radicals (OH) were investigated. The rate constants of BT, MBT, and CBT reacted with OH radicals were determined to be (8.0 ± 1.8), (7.6 ± 1.7), and (7.6 ± 1.9) × 109 M-1s-1 at initial pH 2 and (9.7 ± 2.7), (9.8 ± 2.7), and (9.4 ± 2.7) × 109 M-1s-1 at initial pH 10, respectively. Lifetimes ranging from several minutes to several hours were estimated under mean OH concentrations in various atmospheric aqueous phases, which are significantly shorter than those estimated under mean OH concentrations in the gas phase. The nanoparticle tracing analysis (NTA) directly shows the formation of nanoparticles from the aqueous-phase photooxidation of the selected BTs. Data analysis of liquid chromatography Orbitrap mass spectrometry (LC-Orbitrap MS) identifies many multifunctional oligomers. Changes in optical property support the formation of oligomers and suggest that the products have the potential to contribute to the atmospheric brown carbon. In addition, higher yields of sulfate are formed after the reactions. It is highlighted that the aqueous-phase oxidation of BTs can contribute to the secondary aerosol mass in the ambient atmosphere, particularly in polluted regions where concentrations of BTs are comparable to those of benzenes, potentially altering the chemical composition and optical properties of atmospheric particles.

The formation of brown carbon (BrC) in atmospheric aerosols significantly influences air quality and climate, yet the underlying chemistry governing its interfacial genesis remains poorly understood. Here, we reveal that the intense electric field at the air-water interface of deliquescent nitrite aerosols promotes the oxidation of methoxyphenol, a key biomass burning constituent. The formation of conjugated oligomers, such as C16H14O6 and C15H13NO7, exhibits substantial light absorption (mass absorption coefficients ∼4 m2 g-1)─a level that surpasses many previously reported pathways. The aging rate can be increased by up to 2 orders of magnitude compared to bulk solutions due to selective radical-radical coupling polymerization. This is driven by promoted intermolecular energy transfer between direction-oriented excited methoxyphenol molecules and reduced energy barriers via stabilized dimer intermediates in the presence of a strong interfacial electric field, as evidenced by high-resolution spectroscopy and quantum calculations. Our study underscores the catalytic role of electric fields in the browning process, reshaping our understanding of atmospheric photochemical aging and highlighting an overlooked driver of BrC formation with far-reaching implications for aerosol reactivity and atmospheric processes.

Abstract. Brown carbon (BrC) aerosols have attracted considerable attention due to their significant climatic effects, yet their sources, optical properties, and seasonal behavior remain poorly understood in remote high-altitude regions. In this study, year-long fine particular-matter (PM2.5) samples were collected at a receptor site in the northeastern Tibetan Plateau (TP) to investigate the optical and chemical properties and sources of water-soluble BrC (WS-BrC). The annual average PM2.5 concentration was 10.3 ± 7.4 µg m−3 with clear seasonal variation (spring > winter > fall > summer). Organic aerosol (OA) was the major component across all seasons with an annual contribution of 37.7 % to the total PM2.5 mass, followed by sulfate (21.3 %), nitrate (12.1 %), and other species. Backward trajectory analysis indicated that aerosols were mainly transported from the northeast and east of the sampling site. The seasonal mass absorption efficiency of WS-BrC at the wavelength of 365nm (MAE365) were 0.92 ± 0.54 m2g−1 in spring, 0.40 ± 0.24 m2 g−1 in summer, 0.81 ± 0.46 m2 g−1 in fall, and 0.97 ± 0.49 m2 g−1 in winter, exhibiting a relatively weak light absorption throughout the year with the strongest photobleaching in summer. Notably, WS-BrC light absorption was positively correlated with the oxidation degree of OA during spring and winter, but negatively correlated in summer and fall, suggesting different chemical aging processes and sources of BrC. These findings enhance our understanding of BrC behavior on the TP and contribute to assessments of its climatic impacts in this high-altitude region.

Unraveling the absorption properties and driving mechanisms of atmospheric brown carbon in coastal air masses.