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Excitation-emission matrix (EEM) fluorescence spectroscopy has been widely used to characterize chemical components of brown carbon (BrC), yet the molecular basics and formation mechanisms of chromophores decomposed by parallel factor (PARAFAC) analysis are not fully understood. Here, water-soluble organic carbon (WSOC) in aerosols from Karachi, Pakistan, were characterized with EEM spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Three PARAFAC components were identified, including two humic-like (C1 and C2), and one protein-like (C3) species. Among them, the C2 shows the longest emission maxima (~494 nm), and correlates tightly with the mass absorption efficiency at 365 nm (MAE365), the character of BrC. Molecular families associated with each of the three components were determined by Spearman correlation analysis between FT-ICR MS peaks and PARAFAC component intensities. The C1 and C2 components are associated with nitrogen-enriched compounds, despite that C2 more with higher aromaticity, higher N content, and highly oxygenated compounds. The formulas associated with C3 include fewer nitrogen-containing species, with a lower unsaturated degree and oxidation state. A dominant oxidation pathway for the formation of C1 and C2 components was suggested, notwithstanding their different precursor types. A large number of formulas associated with C2 were found to be located in the “potential BrC” region, overlapped with BrC-associated formulas, and readily correlated tightly with MAE365. This suggests that the compounds illuminating C2 may have also contributed substantially to the BrC light absorption. These findings were important for future studies using the EEM-PARAFAC method to explore the compositions, processes, and sources of atmospheric BrC.

Organosulfur compounds (OrgSs), especially organosulfates, have been widely reported to be present in large quantities in particulate organic matter found in various atmospheric environments. Despite hundreds of organosulfates and their formation mechanisms being previously identified, a large fraction of OrgSs remain unexplained at the molecular level, and a better understanding of their formation pathways and critical environmental parameters is required to explain the variations in their concentrations. In this study, the abundance and molecular composition of OrgSs in fine particulate samples collected in Guangzhou were reported. The results revealed that the ratio of the annual average mass of organic sulfur to total particulate sulfur was 33 ± 12 %, and organic sulfur had positive correlations with SO2 (r=0.37, p) and oxidant (NOx + O3, r=0.40, p). A Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed that more than 80 % of the sulfur-containing formulas detected in the samples had the elemental composition of o/(4s+3n)≥1, indicating that they were largely in the form of oxidized organosulfates or nitrooxy organosulfates. Many OrgSs that were previously tentatively identified as having biogenic or anthropogenic origins were also present in freshly emitted aerosols derived from combustion sources. The results indicated that the formation of OrgSs through an epoxide intermediate pathway could account for up to 46 % of OrgSs from an upper bound estimation, and the oxidant levels could explain 20 % of the variation in the mass of organic sulfur. The analysis of our large dataset of FT-ICR MS results suggested that relative humidity, oxidation of biogenic volatile organic compounds via ozonolysis, and NOx-related nitrooxy organosulfate formation were the major reasons for the molecular variation of OrgSs, possibly highlighting the importance of the acid-catalyzed ring-opening of epoxides, oxidation processes, and heterogeneous reactions involving either the uptake of SO2 or the heterogeneous oxidation of particulate organosulfates into additional unrecognized OrgSs.

Organosulfur compounds (OrgSs), especially organosulfates, have been widely reported at large quantities in particulate organic matter found in various atmospheric environments. Despite various kinds of organosulfates and their formation mechanisms being previously identified, a large fraction of OrgSs remain unexplained at the molecular level, impeding further knowledge on additional formation pathways and critical environmental parameters that help to explain their concentrations. In this work, the abundance and molecular composition of OrgSs in fine particulate samples collected in Guangzhou was reported. Our results revealed that organic sulfur can averagely contribute to 30 % of total particulate sulfur, and showed positively correlations with the SO2 (r = 0.37, p < 0.05) and oxidants (NOx+O3, r = 0.40, p < 0.01). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) results presented that more than 80 % by number of the detected OrgSs in our samples have the elemental composition of O/(4S+3N) ≥ 1, indicating that they were largely in the form of oxidized organosulfates and/or nitrooxy organosulfates. Many OrgSs, which are tentatively attributed to previously identified biogenic and anthropogenic origins, were also present in aerosols derived from freshly-emitted combustion sources. Results show that the formation of OrgSs through an epoxide intermediate pathway could be as much as 46 %, and the oxidants levels could explain 20 % variation of organic sulfur mass. The analysis from our large FT-ICR MS dataset suggests that relative humidity, oxidation of biogenic volatile organic compounds via ozonolysis, and NOx-related nitrooxy organosulfate formations were the major reasons for the molecular variation of OrgSs, possibly highlighting the importance of heterogeneous reactions involving either the uptake of SO2 or the heterogeneous oxidations of particulate organosulfates into additional unrecognized OrgSs.

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 Angstrom 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.

Understanding the intra-city variation of PM2.5 is important for air quality management and exposure assessment. In this study, to investigate the spatiotemporal variation of PM2.5 in Guangzhou, we developed land use regression (LUR) models using data from 49 routine air quality monitoring stations. The R2, adjust R2 and 10-fold cross validation R2 for the annual PM2.5 LUR model were 0.78, 0.72 and 0.66, respectively, indicating the robustness of the model. In all the LUR models, traffic variables (e.g., length of main road and the distance to nearest ancillary) were the most common variables in the LUR models, suggesting vehicle emission was the most important contributor to PM2.5 and controlling vehicle emissions would be an effective way to reduce PM2.5. The predicted PM2.5 exhibited significant variations with different land uses, with the highest value for impervious surfaces, followed by green land, cropland, forest and water areas. Guangzhou as the third largest city that PM2.5 concentration has achieved CAAQS Grade II guideline in China, it represents a useful case study city to examine the health and economic benefits of further reduction of PM2.5 to the lower concentration ranges. So, the health and economic benefits of reducing PM2.5 in Guangzhou was further estimated using the BenMAP model, based on the annual PM2.5 concentration predicted by the LUR model. The results showed that the avoided all cause mortalities were 992 cases (95% CI: 221–2140) and the corresponding economic benefits were 1478 million CNY (95% CI: 257–2524) (willingness to pay approach) if the annual PM2.5 concentration can be reduced to the annual CAAQS Grade I guideline value of 15 μg/m3. Our results are expected to provide valuable information for further air pollution control strategies in China.

Continental outflow influences concentration and chemical composition of marine aerosols, which has an important impact on regional biogeochemistry and climate. Aerosols sampling for molecular marker study was conducted from June 2015 to May 2016 at Xieyang Island in the northern South China Sea (SCS), to learn the impact of outflow from Indo-China Peninsular (ICP) and South China (SC). Xieyang Island was under the influence of air masses from ICP and SC nearly 70% of the year. Levels of anhydrosugars were higher during October to early March. They were lower in April and May, although biomass burning events in ICP were most intensive and half of air masses back trajectories passed through ICP in this period. Significant correlations between levels of anhydrosugars, polycyclic aromatic hydrocarbons, n-alkanes, high molecular weight n-fatty acids and terephthalic acid were observed, suggesting co-emissions of these compounds. Source types of aerosol primary organic matter (POM) tracked by these markers, namely open burning of municipal wastes, fossil fuel combustion, higher plant emissions and biomass burning, largely contributed in fall and winter with SC to be an important source region. However, levels of sugar alcohols (a group of biogenic aerosol tracers) were higher in warm seasons with ICP and SCS to be the main source regions, and were poorly correlated with levels of other molecular markers. Carbon preference index of n-alkanes also increased in warm seasons, indicating enhanced higher plant wax emissions. It suggested that there was a close link between biogenic emissions of sugar alcohols (or n-alkanes) and the growing activities of related organisms. Besides continental outflow, there were marine sources of steranes, hopanes (fossil sources tracers) and mannitol (a sugar alcohol), since their levels were higher for aerosol samples basically influenced by air masses originated from SCS.

Radiocarbon (14C) analysis is a powerful tool that can unambiguously distinguish fossil and non-fossil sources of carbonaceous particles. However, one of the big challenges of this method is to isolate elemental carbon (EC) or black carbon (BC) for 14C analysis. Hydropyrolysis (hypy) has proven to be an effective method for separating BC in environmental matrices. The potential of hypy for isolation of EC from atmospheric aerosols is evaluated using typical combustion products from non-fossil (biomass), fossil fuel (coal and petroleum), and ambient aerosol samples collected in Beijing and Guangzhou. Using solid state nuclear magnetic resonance (NMR) along with measurement of carbon content and 14C, hypy conditions of 15 MPa hydrogen pressure and 550 °C temperature was confirmed to effectively separate EC from aerosol samples. Consequently, a comparison study of EC 14C in aerosol samples separated using the two-step heating method (CTO-375), thermal-optical method and hypy was conducted. The results show that hypy is an effective and stable approach for matrix-independent 14C quantification of EC in aerosols.

Chromophores represent an important portion of light-absorbing species, i.e. brown carbon. Yet knowledge on what and how chromophores contribute to aerosol light absorption is still sparse. To address this problem, we examined soluble independent chromophores in a set of year-round aerosol samples from Bangkok. The water-soluble chromophores identified via excitation-emission matrix (EEM) spectroscopy and follow-up parallel factor analysis could be mainly assigned as humic-like substances and protein-like substances, which differed in their EEM pattern from that of the methanol-soluble fraction. The emission wavelength of chromophores in environmental samples tended to increase compared with that of the primary combustion emission, which could be attributed to secondary formation or the aging process. Fluorescent indices inferred that these light-absorbing chromophores were not significantly humified and comprised a mixture of organic matter of terrestrial and microbial origin, while these inferences exhibited a refutation with primary biomass burning and coal combustion results. A multiple linear regression analysis revealed that larger chromophores that were oxygen-rich and highly aromatic with high molecular weights, were the key contributors of light absorption, preferably at longer emission wavelength (λmax > 500 nm). Positive matrix factorization analysis further suggested that up to 60 % of these responsible chromophores originated from biomass burning emissions.

An increasing number of alternative flame retardants (FRs) are being introduced, following the international bans on the use of polybrominated diphenyl ether (PBDE) commercial mixtures. FRs' production capacity has shifted from developed countries to developing countries, with China being the world's largest producer and consumer of FRs. These chemicals are also imported with e-waste to China. Therefore, it is important to understand the current status of regulated brominated FRs, their phase-out in China, and their replacement by alternatives. In this study, a broad suite of legacy and alternative FRs, including eight PBDEs, six novel brominated FRs (NBFRs), two dechlorane plus variants (DPS), and 12 organophosphate FRs (OPFRs) were evaluated in the air of 10 large Chinese cities in 2018. OPFRs are the most prevalent FRs in China, exhibiting a wide range of 1-612 ng/m3, which is several orders of magnitude higher than PBDEs (1-1827 pg/m3) and NBFRs (1-1428 pg/m3). BDE 209 and DBDPE are the most abundant compounds in brominated FRs (>80%). The North China Plain (NCP, excluding Beijing), Guangzhou, and Lanzhou appear to be three hotspots, although with different FR patterns. From 2013/2014 to 2018, levels of PBDEs, NBFRs, and DPs have significantly decreased, while that of OPFRs has increased by 1 order of magnitude. Gas-particle partitioning analysis showed that FRs could have not reached equilibrium, and the steady-state model is better suited for FRs with a higher log KOA (>13). To facilitate a more accurate FR assessment in fine particles, we suggest that, in addition to the conventional volumetric concentration (pg/m3), the mass-normalized concentration (pg/g PM2.5) could also be used.

Understanding the intra-city variation of PM

Water-soluble brown carbon (ws-BrC) exerts an important influence on climate change, but its emission sources and optical properties remain poorly understood. In this study, we isolated two ws-BrC proxies, water-soluble organic carbon (WSOC) and humic-like substance carbon (HULIS-C), from particulate matter collected in Guangzhou, China, during December 2012 for the measurement of dual carbon isotopes (14C and 13C) and light absorption. The mass absorption efficiencies of WSOC and HULIS-C at 365nm were 0.81±0.16 and 1.33±0.21m2g-1C, respectively. The 14C results showed that two-thirds of WSOC and HULIS-C were derived from non-fossil sources (e.g., biomass burning and biogenic emission), and the remaining third was derived from fossil sources. The δ13C values of WSOC and HULIS-C were -23.7±1.2‰ and -24.2±0.9‰, respectively, underlining the limited influences of C4 plants and natural gas on ws-BrC. Fitting the data to a multiple linear regression, we further concluded that approximately 80% and 10% of the light absorption at 365nm was due to non-fossil and fossil carbon, respectively. Non-fossil sources of ws-BrC, such as the burning of agricultural residue, were responsible for the light absorption recorded in Guangzhou.

Few efforts have been made to elucidate the influence of weather conditions on the fate of semivolatile organic compounds (SOCs). Here, daily fine particulate matter (PM2.5) during clear, haze, and fog episodes collected in the winter in Beijing, China was analyzed for polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants (BFRs), and organophosphate flame retardants (OPFRs). The total concentrations of PAHs, OPFRs, and BFRs had medians of 45.1 ng/m3 and 1347 and 46.7 pg/m3, respectively. The temporal pattern for PAH concentrations was largely dependent on coal combustion for residential heating. OPFR compositions that change during colder period were related to enhanced indoor emissions due to heating. The mean concentrations of SOCs during haze and fog days were 2–10 times higher than those during clear days. We found that BFRs with lower octanol and air partition coefficients tended to increase during haze and fog episodes, be removed from PM2.5 during clear episodes, or both. For PAHs an...

Brown carbon (BrC) is a class of unidentified organic compounds that efficiently absorb solar radiation in the ultraviolet (UV) wavelengths, and its effects on climate are poorly understood. Measurement of the light absorption properties of BrC in liquid extracts is a commonly used BrC analytical method, but the optical characteristics of water-soluble BrC may be affected by pH and solvent. In this work, we investigated the effects of concentration, pH, and solvent on water-soluble BrC from ambient aerosols, biomass burning, diesel exhaust, and a humic substance standard. The results showed that pH can affect the light absorption properties of water-soluble BrC, whereas concentration had little effect, except low concentrations dissolved in methanol. Therefore, the pH of humic-like substances (HULIS) should be adjusted to the same value as water-soluble carbon (WSOC) for calculating the light-absorption contribution of HULIS to WSOC. The light absorptivity of water-soluble BrC dissolved in methanol was higher than that in water. Considering the pH and concentration effects, extraction of WSOC with a particle: water ratio of 0.25 mg/mL is proposed as well as to get a reference pH for light absorption analysis.

Brown carbon (BrC) draws increasing attention due to its effects on climate and other environmental factors. In China, household coal burned for heating and cooking purposes releases huge amounts of carbonaceous particles every year; however, BrC emissions have rarely been estimated in a persuasive manner due to the unavailable emission characteristics. Here, seven coals jointly covering geological maturity from low to high were burned in four typical stoves as both chunk and briquette styles. The optical integrating sphere (IS) method was applied to measure the emission factors (EFs) of BrC and black carbon (BC) via an iterative process using the different spectral dependence of light absorption for BrC and BC and using humic acid sodium salt (HASS) and carbon black (CarB) as reference materials. The following results have been found: (i) the average EFs of BrC for anthracite coal chunks and briquettes are 1.08 ± 0.80 and 1.52 ± 0.16 g kg−1, respectively, and those for bituminous coal chunks and briquettes are 8.59 ± 2.70 and 4.01 ± 2.19 g kg−1, respectively, reflecting a more significant decline in BrC EFs for bituminous coals than for anthracites due to briquetting. (ii) The BrC EF peaks at the middle of coal's geological maturity, displaying a bell-shaped curve between EF and volatile matter (Vdaf). (iii) The calculated BrC emissions from China's residential coal burning amounted to 592 Gg (1 Gg = 109 g) in 2013, which is nearly half of China's total BC emissions. (iv) The absorption Ångström exponents (AAEs) of all coal briquettes are higher than those of coal chunks, indicating that the measure of coal briquetting increases the BrC ∕ BC emission ratio and thus offsets some of the climate cooling effect of briquetting. (v) In the scenario of current household coal burning in China, solar light absorption by BrC (350–850 nm in this study) accounts for more than a quarter (0.265) of the total absorption. This implies the significance of BrC to climate modeling.

Humic-like substances (HULIS) are complex mixtures that are highly associated with brown carbon (BrC) and are important components of biomass burning (BB) emissions. In this study, we investigated the light absorption, emission factors (EFs), and amounts of HULIS emitted from the simulated burning of 27 types of regionally important rainforest biomass in Southeast Asia. We observed that HULIS had a high mass absorption efficiency at 365 nm (MAE365), with an average value of 2.6 ± 0.83 m2 g−1 C. HULIS emitted from BB accounted for 65% ± 13% of the amount of water-soluble organic carbon (WSOC) and 85% ± 10% of the light absorption of WSOC at 365 nm. The EFs of HULIS from BB averaged 2.3 ± 2.1 g kg−1 fuel, and the burning of the four vegetation subtypes (herbaceous plants, shrubs, evergreen trees, and deciduous trees) exhibited different characteristics. The differences in EFs among the subtypes were likely due to differences in lignin content in the vegetation, the burning conditions, or other factors. The light absorption characteristics of HULIS were strongly associated with the EFs. The annual emissions (minimum–maximum) of HULIS from BB in this region in 2016 were 200–371 Gg. Furthermore, the emissions from January to April accounted for 99% of the total annual emissions of HULIS, which is likely the result of the burning activities during this season. The most significant emission regions were Cambodia, Burma, Thailand, and Laos. This study, which evaluated emissions of HULIS by simulating open BB, contributes to a better understanding of the light-absorbing properties and regional budgets of BrC in this region.

We investigated the fluorescence and chemical-structural characteristics of dissolved brown carbon (BrC) in smoke particulates emitted from the combustion of biomass and fossil fuels (coal and vehicle exhaust) by excitation-emission matrix (EEM) spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). Six components were resolved by parallel factor analysis (PARAFAC) of the water-soluble and methanol-soluble organic carbon (MSOC) fractions, respectively. These fluorescent components varied among sources. Combined with FT-ICR MS ion groups, we found that the fluorescent components agreed well with the functional groups, particularly with nitrogen (N)- and sulfur (S)-containing groups. Among the six PARAFAC components (P1–6) retrieved from the water-soluble organic carbon (WSOC) fraction, except for the P3 component, the other components exhibited different values among the three types of emission sources tested. Vehicle exhaust was characterized by high P1 and P6 components, which are mainly associated with aromatic organosulfate compounds, and a high P5 component, mainly associated with sulfonates; coal combustion was characterized by a high P4 component, which is associated with nitrooxy-organosulfate (nitrooxy-OS) compounds; and biomass burning was characterized by the P2 component. Similar results were observed in the case of the MSOC fraction. This study reveals the source contribution and possible structures of previously unclear excitation-emission matrix (EEM) fluorescent components in combustion-derived aerosols. These are the first findings of this type and are potentially applicable to further studies on EEM-based source apportionment of dissolved BrC in aerosols.

The molecular composition and structure of polar organic matters (POM) in particles emitted from various vessels and excavators were characterized using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). POM was extracted by purified water and was discussed by elemental composition which was divided into three groups, namely CHO, CHON and S-containing compounds (CHONS and CHOS). The results showed that: (i) CHO (accounting for 49 % of total POM relative peak response) was the most abundant group for all tested off-road engines, followed by CHON (33 %) and CHOS (35 %) for diesel- and HFO (heavy fuel oil)-fueled off-road equipment, respectively. (ii) The abundance and structure of the CHON group in water extracts were different in terms of engine type and load. The fraction of relative peak response of CHON was highest for excavator emissions under the working mode compared to other modes (idling and moving). Furthermore, dinitrophenol and methyl dinitrophenol were the most abundant emission species for excavators with high rated speed, while nitronaphthol and methyl nitronaphthol were more important for low rated speed vessels. (iii) The composition and structure of S-containing compounds was directly influenced by fuel oil characteristics (sulfur content and aromatic ring), with much more condensed aromatic rings in S-containing compounds observed for HFO-fueled vessels, while more abundant aliphatic chains were observed in emissions from diesel equipment. Overall, higher fractions of condensed hydrocarbons and aromatic rings in POM emitted from vessels using HFO caused strong optical absorption capacity. And different structures existing in POM could provide a direction to qualitative and quantities the exact organic compounds as tracers to distinguish the emission from diesel or HFO- fueled off-road engines.

The Asia-Pacific Economic Cooperation (APEC) summit took place in Beijing, China, 5-11 November 2014, during which numerous measures were performed to control the air pollution, and consequently, the sky of Beijing was so clean that the public called it "APEC blue." The concentrations before, during, and after the APEC summit are 14.4+/-6.81 mu g C/m(3), 6.66+/-2.99 mu g C/m3, and 32.3+/-10.6 mu g C/m(3), respectively, for organic carbon (OC), and 2.27+/-1.17 mu g C/m(3), 0.76+/-0.52 mu g C/m(3), and 4.99+/-1.74 mu g C/m(3), respectively, for elemental carbon (EC). We quantify the contributions of fossil and nonfossil sources to the OC and EC using radiocarbon. Results show that the contribution of nonfossil sources is 56+/-1% (before APEC), 61+/-1% (during APEC), and 48+/-1% (after APEC), respectively, for OC, and 36+/-4% (before APEC), 46+/-1% (during APEC), and 33+/-4% (after APEC), respectively, for EC. Comparing to the period before APEC, 70% and 60% of fossil EC and OC and 60% and 50% of nonfossil EC and OC are reduced, respectively, implying that the control on the nonfossil sources has considerable contribution to the good air quality in Beijing. Both EC and OC mass loadings during the APEC summit would have increased by 60% if the biomass-burning activities were not taken into account for control. In such a case, the atmospheric visibility would decrease 20% at least and the blue sky thereby would likely not have been visible during the summit.

Fine carbonaceous aerosols (CAs) is the key factor influencing the currently filthy air in megacities of China, yet seldom study simultaneously focuses on the origins of different CAs species using specific and powerful source tracers. Here, we present a detailed source apportionment for various CAs fractions, including organic carbon (OC), water-soluble OC (WSOC), water-insoluble OC (WIOC), elemental carbon (EC) and secondary OC (SOC) in the largest cities of North (Beijing, BJ) and South China (Guangzhou, GZ), respectively, using the measurements of radiocarbon and anhydrosugars. Results show that non-fossil fuel sources such as biomass burning and biogenic emission make a significant contribution to the total CAs in Chinese megacities: 56 ± 4 % in BJ and 46 ± 5 % in GZ, respectively. The relative contributions of primary fossil carbon from coal and liquid petroleum combustions, primary non-fossil carbon and secondary organic carbon (SOC) to total carbon are 19, 28 and 54 % in BJ, and 40, 15 and 46 % in GZ, respectively. Non-fossil fuel sources account for 52 % in BJ and 71 % in GZ of SOC, respectively. These results suggest that biomass burning has a greater influence on regional particulate air pollution in North China than in South China. We observed an unabridged haze bloom–decay process in South China, which illustrates that both primary and secondary matter from fossil sources played a key role in the blooming phase of the pollution episode, while haze phase is predominantly driven by fossil-derived secondary organic matter and nitrate.

Atmospheric brown carbon (BrC) is an important constituent of light-absorbing organic aerosols with many unclear issues. Here, the light-absorption properties of BrC with different polarity characteristics at a regional site of Pearl River Delta Region during 2016–2017, influenced by sources and molecular compositions, were revealed using radiocarbon analysis and Fourier transform ion cyclotron resonance mass spectrometry. Humic-like substance (HULIS), middle polar (MP), and low polar (LP) carbon fractions constitute 46 ± 17%, 30 ± 7%, and 7 ± 3% of total absorption coefficient from bulk extracts, respectively. Our results show that the absorption proportions of HULIS and MP to the total BrC absorption are higher than their mass proportions to organic carbon mass, indicating that HULIS and MP are the main light-absorbing components in water-soluble and water-insoluble organic carbon fractions, respectively. With decreases in non-fossil HULIS, MP, and LP carbon fractions (66 ± 2%, 52 ± 2%, and 36 ± 3%, respectively), the abundances of unsaturated compounds and mass absorption efficiency at 365 nm of three fractions decreased synchronously. Increases in both non-fossil carbon and levoglucosan in winter imply that the enhanced light-absorption could be attributed to elevated levels of biomass burning organic aerosols (BBOA), which increases the number of light-absorbing nitrogen-containing compounds. Moreover, the major type of potential BrC in HULIS and MP carbon fractions are oxidized BBOA, but the potential BrC chromophores in LP are mainly associated with primary BBOA. This study reveals that biomass burning has adverse effects on radiative forcing and air quality, and probably indicates the significant influences of atmospheric oxidation reactions on the forms of chromophores.

Oxidative potential (OP) is a promising and integrative metric for assessing health effects associated with exposure to ambient particulate matter. Source apportionment of water-soluble OP in aerosols is the basis for formulating pollution control policies. In this study, we collected total suspended particulate (TSP) samples in Bangkok, on the Indochinese Peninsula, where pollution levels are high due to annual biomass burning. We analyzed the chemical composition of the samples and performed dithiothreitol (DTT) assays to measure their OP. We observed higher TSP mass and chemical species concentrations among samples collected in the dry seasons due to biomass burning and fossil fuel combustion. The seasonally averaged volume-normalized OP (OPv) was highest in the dry I season (January to March), which was consistent with higher TSP concentrations during this period. The mass-normalized OP (OPm) decreased as TSP concentration increased due to the unaccounted fractions dominating the increase in TSP and/or the inhibiting effects of redox-active metals. Transition metals (e.g., Cu, V, and Ni), hopanes/steranes, and carbonaceous species (organic carbon and elemental carbon) correlated well with OPm. Source apportionment using the Positive Matrix Factorization model (PMF) and multiple linear regression analysis revealed that, although biomass burning accounted for the highest contribution (mean: 26%) to TSP concentration, the dominant source of water-soluble OPv of TSP was fossil fuel combustion, which included land fossil fuel combustion (63%) and ship emissions (16%). This significant contribution of fossil fuel combustion was determined by the transition metals Cu and V which were the main drivers of DTT activity in TSP.

Water-soluble brown carbon (BrC) plays an important role in climate change by influencing aerosol radiative forcing. There is little information on aerosol BrC over the South China Sea (SCS). In this study, water-soluble organic carbon (WSOC) in a round-year set of aerosol samples from a remote island in the northern SCS were characterized for optical properties. In-depth information about the sources and input pathways of water-soluble BrC was obtained using molecular markers and statistic tools. The highest WSOC concentrations, light absorption coefficients at 365 nm (Abs365) and mass absorption efficiencies at 365 nm (MAE365) were observed in winter when atmospheric outflow from mainland China and the northern Indo-China Peninsula prevailed. Through the year, primary emissions from biomass burning and urban secondary organic aerosols (SOA) & waste combustion, respectively, were observed to be associated with higher MAE365 (2.47 ± 0.40 m2 g−1 and 1.97 ± 0.22 m2 g−1) and to be the main contributors to Abs365 (22.0 ± 3.6% and 31.6 ± 3.6%), while biogenic SOA showed little contribution. For the first time, microorganism/plankton primary emissions, mainly from the sea, was identified to be an important contributor to water-soluble BrC (13.6 ± 4.2% of Abs365, MAE365: 0.98 ± 0.30 m2 g−1), especially in spring (31% of Abs365). This implies that emissions from microorganism/plankton warrants careful consideration in the assessment of global aerosol light absorbance.

Water-soluble brown carbon (ws-BrC) exerts an important influence on climate change, but its emission sources and optical properties remain poorly understood. In this study, we isolated two ws-BrC proxies, water-soluble organic carbon (WSOC) and humic-like substance carbon (HULIS-C), from particulate matter collected in Guangzhou, China, during December 2012 for the measurement of dual carbon isotopes (

Humic-like substances (HULIS) are a class of high molecular weight, light-absorbing compounds that are highly related to brown carbon (BrC). In this study, the sources and compositions of HULIS isolated from fine particles collected in Beijing, China during the 2014 Asia-Pacific Economic Cooperation (APEC) summit were characterized based on carbon isotope (13C and 14C) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, respectively. HULIS were the main light-absorbing components of water-soluble organic carbon (WSOC), accounting for 80.2 ± 6.1% of the WSOC absorption capacity at 365 nm. The carbon isotope data showed that HULIS had a lower non-fossil contribution (53 ± 4%) and were less enriched with 13C (−24.2 ± 0.6‰) relative to non-HULIS (62 ± 8% and −20.8 ± 0.3‰, respectively). The higher relative intensity fraction of sulfur-containing compounds in HULIS before and after APEC was attributed to higher sulfur dioxide levels emitted from fossil fuel combustion, whereas the higher fraction of nitrogen-containing compounds during APEC may have been due to the relatively greater contribution of non-fossil compounds or the influence of nitrate radical chemistry. The results of investigating the relationships among the sources, elemental compositions, and optical properties of HULIS demonstrated that the light absorption of HULIS appeared to increase with increasing unsaturation degree, but decrease with increasing oxidation level. The unsaturation of HULIS was affected by both sources and aging level.

Brown carbon (BrC) draws increasing attention due to its effects on climate and other fields. In China, household coal burned for heating/cooking purposes releases huge amounts of carbonaceous particles every year; however, BrC emissions have rarely been estimated in a persuasive manner due to the unavailable emission characteristics. Here 7 coals jointly covering geological maturity from low to high were burned in 4 typical stoves at both chunk and briquette styles. The optical integrating sphere (IS) method was applied to measure the emission factors (EFs) of BrC and BC via an iterative process using the different spectral dependence of light absorption for BrC and BC. It is found that (i) the average EFs of BrC for anthracite coal chunks and briquettes are (1.08 ± 0.80) g kg−1 and (1.52 ± 0.16) g kg−1, respectively, and those for bituminous coal chunks and briquettes are (8.59 ± 2.70) g kg−1 and (4.01 ± 2.19) g kg−1, respectively, reflecting a more significant decline of BrC EFs for bituminous coals than for anthracites due to briquetting, (ii) the BrC EF peaks at the middle of coal's geological maturity, displaying a bell shaped curve between EF and volatile matter (Vdaf), (iii) the calculated BrC emissions from China's residential coal burning amounted to 592 Gg (1 Gg = 109 g) in 2013, which is nearly half of China's total BC emissions, (iv) absorption Ångström exponent (AAEs) of all coal briquettes are higher than those of coal chunks, indicating that the measure of coal briquetting increases the BrC / BC emission ratio and thus offsets some of the climate cooling effect of briquetting, and (v) in the scenario of current household coal burning in China, solar light absorption by BrC (350–850 nm in this study) accounts for more than a quarter (0.265) of the total absorption. This implies the significance of BrC to climate modeling.