Single-particle volatility and implications for brown carbon absorption in Beijing, China

Brown carbon from biomass burning imposes strong circum-Arctic warming

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

Brown carbon (BrC) consists of those organic compounds in atmospheric aerosols that absorb solar radiation and may play an important role in planetary radiative forcing and climate. However, little is known about the production and loss mechanisms of BrC in the atmosphere. Here, we study how the light absorptivity of BrC from wood smoke and secondary BrC generated from the reaction of ammonium sulfate with methylglyoxal changes under photolytic aging by UVA radiation in the aqueous phase. Owing to its chemical complexity, BrC is separated by molecular weight using size exclusion chromatography, and the response of each molecular weight fraction to aging is studied. Photolytic aging induced significant changes in the light absorptivity of BrC for all molecular weight fractions; secondary BrC was rapidly photoblenched, whereas for wood smoke BrC, both photoenhancement and photobleaching were observed. Initially, large biomass burning BrC molecules were rapidly photoenhanced, followed by slow photolysis. As a result, large BrC molecules dominated the total light absorption of aged biomass burning BrC. These experimental results further support earlier observations that large molecular weight BrC compounds from biomass burning can be relatively long-lived components in atmospheric aerosols, thus more likely to have larger impacts on aerosol radiative forcing and could serve as biomass burning tracers.

Abstract. Carbonaceous aerosols significantly affect global radiative forcing and climate through absorption and the scattering of sunlight. Black carbon (BC) and brown carbon (BrC) are light-absorbing carbonaceous aerosols. The direct radiative effect (DRE) of BrC is uncertain. A recent study suggests that BrC absorption is comparable to BC in the upper troposphere over biomass burning regions and that the resulting radiative heating tends to stabilize the atmosphere. Yet current climate models do not include proper physical and chemical treatments of BrC. In this study, we derived a BrC global biomass burning emission inventory on the basis of the Global Fire Emissions Database version 4 (GFED4), developed a module to simulate the light absorption of BrC in the Community Atmosphere Model version 5 (CAM5) of the Community Earth System Model (CESM), and investigated the photobleaching effect and convective transport of BrC on the basis of Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry Project (DC3) measurements. The model simulations of BC were also evaluated using HIAPER (High-Performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) measurements. We found that globally BrC is a significant absorber, the DRE of which is 0.10 W m−2, more than 25 % of BC DRE (+0.39 W m−2). Most significantly, model results indicated that BrC atmospheric heating in the tropical mid and upper troposphere is larger than that of BC. The source of tropical BrC is mainly from wildfires, which are more prevalent in the tropical regions than higher latitudes and release much more BrC relative to BC than industrial sources. While BC atmospheric heating is skewed towards the northern mid-latitude lower atmosphere, BrC heating is more centered in the tropical free troposphere. A possible mechanism for the enhanced convective transport of BrC is that hydrophobic high molecular weight BrC becomes a larger fraction of the BrC and less easily activated in a cloud as the aerosol ages. The contribution of BrC heating to the Hadley circulation and latitudinal expansion of the tropics is likely comparable to BC heating.

The light-absorbing properties of atmospheric brown carbon (BrC) are poorly understood due to its complex chemical composition. Here, a black-carbon-tracer method was coupled with source apportionments of organic aerosol (OA) to explore the light-absorbing properties of primary and secondary BrC from the North China Plain (NCP). Primary emissions of BrC contributed more to OA light absorption than secondary processes, and biomass burning OA accounted for 60% of primary BrC absorption at λ = 370 nm, followed by coal combustion OA (35%) and hydrocarbon-like OA (5%). Secondary BrC absorption was high in the early morning and later decreased due to the bleaching of chromophores. Nighttime aqueous-phase chemistry promoted the formation of secondary light-absorbing compounds and the production of strongly absorbing particles. Source analysis showed that the NCP region was the most important source for primary and secondary BrC subtypes at the study site. The mean direct radiative forcing for BrC was 0.15 W m-2 (0.11 W m-2 and 0.04 W m-2 for the primary and secondary fractions, respectively). This study provides new information on the optical properties of primary and secondary BrC and highlights the importance of atmospheric oxidation on BrC absorption.

Abstract. Traditional gas and particle phase chemical markers used to identify the presence of biomass burning (BB) emissions were measured for a large forest fire near Boulder, Colorado. Correlation of the organic matter mass spectroscopic m/z 60 with measured particle light absorption properties found no link at 532 nm, and a strong correlation at 404 nm. Non-black carbon absorption at 404 nm was well correlated to the ratio of the mass fractions of particulate organic matter (POM) that was m/z 60 (f60) to m/z 44 (f44). The f60 to f44 ratio did not fully explain the variability in non-BC absorption, due to contributions of brown carbon (BrC) absorption and absorption due to internal mixing of POM with black carbon (BC). The absorption Ångstrom exponent (ÅAbs) showed a good correlation to f60/f44; however the best correlation resulted from the mass absorption efficiency (MAE) of BrC at 404 nm (MAEPOM-404 nm) and f60/f44. This result indicates that the absorption of POM at low visible and UV wavelengths is linked to emissions of organic matter that contribute to the m/z 60 mass fragment, although they do not contribute to 532 nm absorption. m/z 60 is often attributed to levoglucosan and related compounds. The linear relationship between MAEPOM-404 nm and f60/f44 suggests that the strength of BrC absorption for this fire can be predicted by emissions of f60-related organic matter.

<strong class="journal-contentHeaderColor">Abstract.</strong> The optical properties, chemical composition, and potential chromophores of brown carbon (BrC) aerosol particles were studied during typical summertime and wintertime at a kerbside in downtown Karlsruhe, a city in central Europe. The average absorption coefficient and mass absorption efficiency at 365 nm (Abs<span class="inline-formula">₃₆₅</span> and MAE<span class="inline-formula">₃₆₅</span>) of methanol-soluble BrC (MS-BrC) were lower in the summer period (1.6 <span class="inline-formula">±</span> 0.5 Mm<span class="inline-formula">⁻¹</span>, 0.5 <span class="inline-formula">±</span> 0.2 m<span class="inline-formula">²</span> g<span class="inline-formula">⁻¹</span>) than in the winter period (2.8 <span class="inline-formula">±</span> 1.9 Mm<span class="inline-formula">⁻¹</span>, 1.1 <span class="inline-formula">±</span> 0.3 m<span class="inline-formula">²</span> g<span class="inline-formula">⁻¹</span>). Using a parallel factor (PARAFAC) analysis to identify chromophores, two different groups of highly oxygenated humic-like substances (HO-HULIS) dominated in summer and contributed 96 <span class="inline-formula">±</span> 6 % of the total fluorescence intensity. In contrast, less-oxygenated HULIS (LO-HULIS) dominated the total fluorescence intensity in winter with 57 <span class="inline-formula">±</span> 12 %, followed by HO-HULIS with 31 <span class="inline-formula">±</span> 18 %. Positive matrix factorization (PMF) analysis of organic compounds detected in real time by an online aerosol mass spectrometer (AMS) led to five characteristic organic compound classes. The statistical analysis of PARAFAC components and PMF factors showed that LO-HULIS chromophores were most likely emitted from biomass burning in winter. HO-HULIS chromophores could be low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOCs) in summer. Five nitro-aromatic compounds (NACs) were identified by a chemical ionization mass spectrometer (C<span class="inline-formula">₇</span>H<span class="inline-formula">₇</span>O<span class="inline-formula">₃</span>N, C<span class="inline-formula">₇</span>H<span class="inline-formula">₇</span>O<span class="inline-formula">₄</span>N, C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₅</span>N, C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₄</span>N, and C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₃</span>N), which contributed 0.03 <span class="inline-formula">±</span> 0.01 % to the total organic mass but can explain 0.3 <span class="inline-formula">±</span> 0.1 % of the total absorption of MS-BrC at 365 nm in winter. Furthermore, we identified 316 potential brown carbon molecules which accounted for 2.5 <span class="inline-formula">±</span> 0.6 % of the organic aerosol mass. Using an average mass absorption efficiency (MAE<span class="inline-formula">₃₆₅</span>) of 9.5 m<span class="inline-formula">²</span>g<span class="inline-formula">⁻¹</span> for these compounds, we can estimate their mean light absorption to be 1.2 <span class="inline-formula">±</span> 0.2 Mm<span class="inline-formula">⁻¹</span>, accounting for 32 <span class="inline-formula">±</span> 15 % of the total absorption of MS-BrC at 365 nm. This indicates that a small fraction of brown carbon molecules dominates the overall absorption. The potential BrC molecules assigned to the LO-HULIS component had a higher average molecular weight (265 <span class="inline-formula">±</span> 2 Da) and more nitrogen-containing molecules (62 <span class="inline-formula">±</span> 1 %) than the molecules assigned to the HO-HULIS components. Our analysis shows that the LO-HULIS, with a high contribution of nitrogen-containing molecules originating from biomass burning, dominates aerosol fluorescence in winter, and HO-HULIS, with fewer nitrogen-containing molecules as low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOC), dominates in summer.

Abstract. The Amazon rainforest is a sensitive ecosystem experiencing the combined pressures of progressing deforestation and climate change. Its atmospheric conditions oscillate between biogenic and biomass burning (BB) dominated states. The Amazon further represents one of the few remaining continental places where the atmosphere approaches pristine conditions during occasional wet season episodes. The Amazon Tall Tower Observatory (ATTO) has been established in central Amazonia to investigate the complex interactions between the rainforest ecosystem and the atmosphere. Physical and chemical aerosol properties have been analyzed continuously since 2012. This paper provides an in-depth analysis of the aerosol's optical properties at ATTO based on data from 2012 to 2017. The following key results have been obtained. The aerosol scattering and absorption coefficients at 637 nm, σsp,637 and σap,637, show a pronounced seasonality with lowest values in the clean wet season (mean ± SD: σsp,637=7.5±9.3 M m−1; σap,637=0.68±0.91 M m−1) and highest values in the BB-polluted dry season (σsp,637=33±25 M m−1; σap,637=4.0±2.2 M m−1). The single scattering albedo at 637 nm, ω0, is lowest during the dry season (ω0=0.87±0.03) and highest during the wet season (ω0=0.93±0.04). The retrieved BC mass absorption cross sections, αabs, are substantially higher than values widely used in the literature (i.e., 6.6 m2 g−1 at 637 nm wavelength), likely related to thick organic or inorganic coatings on the BC cores. Wet season values of αabs=11.4±1.2 m2 g−1 (637 nm) and dry season values of αabs=12.3±1.3 m2 g−1 (637 nm) were obtained. The BB aerosol during the dry season is a mixture of rather fresh smoke from local fires, somewhat aged smoke from regional fires, and strongly aged smoke from African fires. The African influence appears to be substantial, with its maximum from August to October. The interplay of African vs. South American BB emissions determines the aerosol optical properties (e.g., the fractions of black vs. brown carbon, BC vs. BrC). By analyzing the diel cycles, it was found that particles from elevated aerosol-rich layers are mixed down to the canopy level in the early morning and particle number concentrations decrease towards the end of the day. Brown carbon absorption at 370 nm, σap,BrC,370, was found to decrease earlier in the day, likely due to photo-oxidative processes. BC-to-CO enhancement ratios, ERBC, reflect the variability of burnt fuels, combustion phases, and atmospheric removal processes. A wide range of ERBC between 4 and 15 ng m−3 ppb−1 was observed with higher values during the dry season, corresponding to the lowest ω0 levels (0.86–0.93). The influence of the 2009/2010 and 2015/2016 El Niño periods and the associated increased fire activity on aerosol optical properties was analyzed by means of 9-year σsp and σap time series (combination of ATTO and ZF2 data). Significant El Niño-related enhancements were observed: in the dry season, σsp,637 increased from 24±18 to 48±33 M m−1 and σap, 637 from 3.8±2.8 to 5.3±2.5 M m−1. The absorption Ångström exponent, åabs, representing the aerosol absorption wavelength dependence, was mostly &lt;1.0 with episodic increases upon smoke advection. A parameterization of åabs as a function of the BC-to-OA mass ratio for Amazonian aerosol ambient measurements is presented. The brown carbon (BrC) contribution to σap at 370 nm was obtained by calculating the theoretical BC åabs, resulting in BrC contributions of 17 %–29 % (25th and 75th percentiles) to σap 370 for the entire measurement period. The BrC contribution increased to 27 %–47 % during fire events under El Niño-related drought conditions from September to November 2015. The results presented here may serve as a basis to understand Amazonian atmospheric aerosols in terms of their interactions with solar radiation and the physical and chemical-aging processes that they undergo during transport. Additionally, the analyzed aerosol properties during the last two El Niño periods in 2009/2010 and 2015/2016 offer insights that could help to assess the climate change-related potential for forest-dieback feedbacks under warmer and drier conditions.

Organic carbon is a significant constituent of PM2.5. The organic carbon percentage in indoor microenvironments is higher than in the ambient environment. Water soluble organic carbon (WSOC) is responsible for altering hygroscopicity, hence determining the ability of particles to act as cloud condensation nuclei (CCN). In this study, quartz fiber filters were collected from two restaurants (R1 and R2 ) using an Airmetrix low-volume sampler at a 5 lpm flow rate. R1 was a closed restaurant where dining tables and the cooking locations were adjacent to each other. R2 was a semi-indoor environment; two sides of the dining place were open with adjoining dining tables, and cooking locations. Organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were estimated using DRI thermal optical analyzer and total organic carbon analyzer instruments. All these constituents can exfiltrate outdoors and alter the climate. Absorption coefficient (babs)&amp;#160; and Mass Absorption Efficiency (MAE) were calculated for Water-soluble carbon at 365 nm. Average OC and EC concentrations were very high in R1, i.e. 160.31 &amp;#181;g/m3 and 14.99 &amp;#181;g/m3,respectively. It was lower in R2, i.e., 29.61 &amp;#181;g/m3 and 4.88 &amp;#181;g/m3,respectively. As there was a direct biomass burning source, i.e., cooking, the major portion of the OC was POC. In R1, the average POC was ~67%, and in R2, ~60% of the total OC. The average WSOC percentages in the R1 and R2 were 8.01% and 25.21%, respectively. MAE values were comparable, i.e., 0.04 and 0.03 in R1 and R2, as the nature of the source was similar. Brown Carbon (BrC) absorption peaks at 365 nm were observed in both locations, confirming its presence. A negative correlation was observed between babs and WSOC, indicating BrC as the main absorption component of the WSOC. The effective carbon ratio (ECR= SOC/[POC+EC]) was calculated to estimate the impact of the particles on the local radiative energy balance. The values were 0.42 in R1 and 0.51 in R2. Aerosol generated from both locations was more absorbing in nature than scattering. In the R1, the aerosol had more absorption capability than in R2 . This study did not quantify the amount of&amp;#160; WSOC and BrC from different food and cooking fuels. But, from this study, it was noticed that cooking-related aerosols are absorbing in nature and can exfiltrate outdoors and alter the local climate.Keywords: Water Soluble Organic Carbon, Mass Absorption Effiency , Effective Carbon Ratio, Brown Carbon

Abstract. A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 °C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC), and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an Ångström exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorption attributable to BrC estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC, with maximum contributions of up to 92 % of total aerosol absorption at 405 nm and up to 58 % of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30 % of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (EC ∕ OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that BrC grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC ∕ OC ratio decreases.

Light absorption of organic carbon emitted from burning wood, charcoal, and kerosene in household cookstoves

Abstract. Western wildfires have a major impact on air quality in the US. In the fall of 2016, 107 test fires were burned in the large-scale combustion facility at the US Forest Service Missoula Fire Sciences Laboratory as part of the Fire Influence on Regional and Global Environments Experiment (FIREX). Canopy, litter, duff, dead wood, and other fuel components were burned in combinations that represented realistic fuel complexes for several important western US coniferous and chaparral ecosystems including ponderosa pine, Douglas fir, Engelmann spruce, lodgepole pine, subalpine fir, chamise, and manzanita. In addition, dung, Indonesian peat, and individual coniferous ecosystem fuel components were burned alone to investigate the effects of individual components (e.g., “duff”) and fuel chemistry on emissions. The smoke emissions were characterized by a large suite of state-of-the-art instruments. In this study we report emission factor (EF, grams of compound emitted per kilogram of fuel burned) measurements in fresh smoke of a diverse suite of critically important trace gases measured using open-path Fourier transform infrared spectroscopy (OP-FTIR). We also report aerosol optical properties (absorption EF; single-scattering albedo, SSA; and Ångström absorption exponent, AAE) as well as black carbon (BC) EF measured by photoacoustic extinctiometers (PAXs) at 870 and 401 nm. The average trace gas emissions were similar across the coniferous ecosystems tested and most of the variability observed in emissions could be attributed to differences in the consumption of components such as duff and litter, rather than the dominant tree species. Chaparral fuels produced lower EFs than mixed coniferous fuels for most trace gases except for NOx and acetylene. A careful comparison with available field measurements of wildfires confirms that several methods can be used to extract data representative of real wildfires from the FIREX laboratory fire data. This is especially valuable for species rarely or not yet measured in the field. For instance, the OP-FTIR data alone show that ammonia (1.62 g kg−1), acetic acid (2.41 g kg−1), nitrous acid (HONO, 0.61 g kg−1), and other trace gases such as glycolaldehyde (0.90 g kg−1) and formic acid (0.36 g kg−1) are significant emissions that were poorly characterized or not characterized for US wildfires in previous work. The PAX measurements show that the ratio of brown carbon (BrC) absorption to BC absorption is strongly dependent on modified combustion efficiency (MCE) and that BrC absorption is most dominant for combustion of duff (AAE 7.13) and rotten wood (AAE 4.60): fuels that are consumed in greater amounts during wildfires than prescribed fires. Coupling our laboratory data with field data suggests that fresh wildfire smoke typically has an EF for BC near 0.2 g kg−1, an SSA of ∼ 0.91, and an AAE of ∼ 3.50, with the latter implying that about 86 % of the aerosol absorption at 401 nm is due to BrC.

The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH/dichloromethane (MeOH/DCM, 1:1, v/v), MeOH/DCM (1:2, v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ~95 %), followed by MeOH and MeOH/DCM mixtures (< 90 %), and the DMF extracts had significant (p < 0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (> 280 °C) are less soluble in MeOH (~80 %) than in DMF (~90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Source apportionment results indicated that the MeOH insoluble OC mainly came from unburned fossil fuels and polymerization processes of aerosol organics. These results highlight the necessity of replacing MeOH with DMF for further investigations on structures and light absorption of low-volatile BrC.

The atmospheric abundance of black carbon (BC) and brown carbon (BrC) aerosol particles has been measured using aethalometry at four locations (Kamloops, BC; Prince George, BC; Joussard, AB; East Trout Lake, SK) in Western Canada through the 2023 and 2024 wildfire seasons (June to November). Intense wildfires led to higher median (Interquartile = Q 3‐ Q1) BC mass concentrations in 2023 (0.45 (0.80–0.19), 0.55 (1.22–0.20), 0.28 (0.69–0.00), 0.16 (0.37–0.00) μg m −3 ) than in 2024 (0.31 (0.60–0.00), 0.25 (0.51–0.00), 0.18 (0.53–0.00) μg m −3 ) for Kamloops, Prince George, Joussard, and East Trout Lake, respectively (with no data for East Trout Lake in 2024). The season‐ and site‐wide means for the aerosol Absorption Angstrom Exponent for 2023 and 2024 are 1.7 and 1.5, respectively, indicating the presence of BrC in both years. The Trajectory Fire Interception Method (TFIM), which determines the spatial and temporal overlap between atmospheric back trajectories and satellite fire counts was used to calculate the average atmospheric transport time of fire plumes for the heavily wildfire‐affected sites in 2023 (Prince George, Kamloops, Joussard). Consistent with laboratory studies of multiphase oxidative aging of smoke particles, the TFIM approach demonstrates that the BrC absorption (at 375 nm, normalized to BC) decreases by about a factor of two as the average atmospheric transport time increases from 15 hr to greater than 46 hr. This corresponds to a BrC whitening e ‐folding timescale of 27 hr averaged across the three sites. Contrary to past studies that have analyzed individual fire plumes, an innovative aspect of this work is that the BrC aging timescale is derived from data accumulated continuously during the entire 2023 wildfire season at multiple locations. In addition, the aging timescale includes smoke with origins in the boreal forest, whereas most prior studies have been conducted at lower latitudes.

To investigate the pollution characteristics and sources of atmospheric brown carbon (BrC) in Chongming Island, a background site of the Yangtze River Delta (YRD) region in China, PM2.5 samples collected from December 2018 to January 2019 were analyzed to determine their chemical compositions and optical properties. The results showed that the light absorption coefficient (Abs365,M) of BrC extracted by methanol at 365 nm was (5.39±3.33) M-1·m-1, which was 1.3 times of the water extracted BrC. Both increased significantly with the increase of pH values, suggesting that less acidic conditions can enhance the light absorption ability of BrC. In winter, both Abs365 and MAE365 (mass absorption efficiency) were higher in the nighttime than in the daytime. A strong linear correlation observed between Abs365 and levoglucosan (R2=0.72) indicated that many light absorbing substances in Chongming Island were derived from biomass burning emissions. During the campaign, nitro-aromatic compounds (NACs) and PAHs accounted for (1.5±1.1) ng·m-3 and (8.3±4.7) ng·m-3, respectively, contributing to 0.1% and 0.067% of the absorption of the total BrC at 365 nm, respectively. Positive matrix factorization (PMF) analysis further showed that biomass and fossil fuel combustions were the main sources of BrC in Chongming Island in winter, accounting for 56% of the total BrC, followed by secondary formation, accounting for 24% of the total BrC, with road dust contributing only 6%.