Signatures of Biomass Burning Aerosols in the Plume of a Saltmarsh Wildfire in South Texas

Characterization of biomass burning tracers in particulate matter at 12 sites in China: Significant increase of coal combustion emitted levoglucosan in northern China during winter

Biomass burning (BB) is the largest contributor to carbonaceous aerosols globally. Specific organic tracers can track BB particles and identify BB types. At present, there is limited information on the composition of BB tracers on a continental scale. In this study, we conducted year-round sampling of particulate matter (PM) at 12 sites in China. Nine BB tracers were measured in PM with aerodynamic diameters <1.1 μm (PM<1.1), in the range of 1.1-3.3 μm (PM1.1-3.3), and > 3.3 μm (PM>3.3). The annual average concentration of these nine BB tracers (∑9 BB tracers) in the total PM was 366 ng m-3 with the majority of levoglucosan (66 %). The concentration of ∑9 BB tracers was higher in northern China than in southern China, especially in winter. ∑9 BB tracers were most enriched in PM<1.1 (50-61 % in mass), followed by PM1.1-3.3 and PM>3.3. The highest concentrations of ∑9 BB tracers were observed in winter, while satellite-recorded fire spots were intensive in autumn and spring. The mismatch of seasonal trends between them indicated that the high levels of BB tracers in winter was not due to open BB. The composition of 4-hydroxybenzoic acid, syringic acid and vanillic acid suggested that the burning of crop residues and softwoods were the major BB types in China. The ratio of levoglucosan to mannosan could neither identify the major BB types in China nor distinguish between BB and coal combustion. Correlation analysis and the PMF model demonstrated that non-BB sources contributed 7 %-58 % to levoglucosan at the 12 sites, with coal combustion being the predominant non-BB source in China, especially in northern urban sites during winter. Our findings suggest that caution should be taken in application of these organic tracers to identify BB types and estimate BB aerosols.

Chemical Fingerprinting of Biomass Burning Organic Aerosols from Sugar Cane Combustion: Complementary Findings from Field and Laboratory Studies

Agricultural fires are a major source of biomass-burning organic aerosols (BBOAs) with impacts on health, the environment, and climate. In this study, globally relevant BBOA emissions from the combustion of sugar cane in both field and laboratory experiments were analyzed using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry. The derived chemical fingerprints of fresh emissions were evaluated using targeted and nontargeted evaluation approaches. The open-field sugar cane burning experiments revealed the high chemical complexity of combustion emissions, including compounds derived from the pyrolysis of (hemi)cellulose, lignin, and further biomass, such as pyridine and oxime derivatives, methoxyphenols, and methoxybenzenes, as well as triterpenoids. In comparison, laboratory experiments could only partially model the complexity of real combustion events. Our results showed high variability between the conducted field and laboratory experiments, which we, among others, discuss in terms of differences in combustion conditions, fuel composition, and atmospheric processing. We conclude that both field and laboratory studies have their merits and should be applied complementarily. While field studies under real-world conditions are essential to assess the general impact on air quality, climate, and environment, laboratory studies are better suited to investigate specific emissions of different biomass types under controlled conditions.

Effects of Air Pollutants from Wildfires on Downwind Ecosystems: Observations, Knowledge Gaps, and Questions for Assessing Risk

Wildfires have increased in frequency and area burned, trends expected to continue with climate change. Among other effects, fires release pollutants into the atmosphere, representing a risk to human health and downwind terrestrial and aquatic ecosystems. While human health risks are well studied, the ecological impacts to downwind ecosystems are not, and this gap may present a constraint on developing an adequate assessment of the ecological risks associated with downwind wildfire exposure. Here, we first screened the scientific literature to assess general knowledge about pathways and end points of a conceptual model linking wildfire generated pollutants and other materials to downwind ecosystems. We found a substantial body of literature on the composition of wildfire derived pollution and materials in the atmosphere and subsequent transport, yet little observational or experimental work on their effects on downwind ecological end points. This dearth of information raises many questions related to adequately assessing the ecological risk of downwind exposure, especially given increasing wildfire trends. To guide future research, we pose eight questions within the well-established US EPA ecological risk assessment paradigm that if answered would greatly improve ecological risk assessment and, ultimately, management strategies needed to reduce potential wildfire impacts.

Quantifying international and interstate contributions to primary ambient PM2.5 and PM10 in a complex metropolitan atmosphere

We quantify the contributions of long-range and regionally transported aerosols to ambient primary PM_2.5 and PM₁₀ in a representative United States industrialized/urban atmosphere via detailed elemental analysis and chemical mass balance (CMB) modeling after identifying their presence using a variety of publicly available satellite data/information, software products, and synoptic-scale aerosol models. A year-long study in Houston, Texas identified North African dust as the principal long-range global source of primary particulate matter (PM). CMB estimated transatlantic dust from the Sahara-Sahel region to be dominant in the summer months contributing an average of 3.5 μg m^-3 to PM_2.5 and 7.9 μg m^-3 to PM₁₀ during May-August, i.e., the active Saharan dust season. Biomass burning was the chief source of regionally transported PM impacting air quality on different occasions throughout the year depending on the fire location. Four major biomass combustion events affecting air quality in Texas were calculated to contribute an average of 1.3 μg m^-3 to PM_2.5 and 1.4 μg m^-3 to PM₁₀ in corresponding samples whose origins were tracked to Canada, southeastern states of USA, and Central America using fire maps, HYSPLIT back trajectories, and the Navy Aerosol Analysis and Prediction System global aerosol model. Elemental concentrations and signature ratios revealed significant mixing of potassium, rare earth metals, and vanadium from proximal and distal crustal (natural) sources with anthropogenically emitted PM. This demonstrates the need to isolate the non-mineral components of these metals to employ them as tracers for primary PM emitted by biomass burning, petroleum refineries, and oil combustion. Transboundary contributions to primary PM_2.5 were 1.5 μg m^-3 and 3.1 μg m^-3 to PM₁₀ adding 16% to annual average mass concentration of both size fractions demonstrating that local sources were primarily responsible for ambient air quality with non-trivial contributions from international and interstate sources. Rigorously identifying and quantifying aerosol sources assists in improving air quality management policies designed to protect public health and comply with ever-decreasing federal PM standards that allow state agencies to exclude contributions that are not reasonably controllable or preventable from regulatory decisions and actions.

Seasonal variations and potential sources of biomass burning tracers in particulate matter in Nanjing aerosols during 2017-2018

Biomass burning (BB) is an important source of atmospheric particulate matter and can adversely affect air quality, visibility, human health, and climate change. To study the characteristics and potential source regions of BB tracers in PM_2.5, a liquid chromatography-mass spectrometry instrument (HPLC-MS/MS) is applied in this study to develop and validate a method to determine organic tracers of BB in 397 aerosol samples. The total mean concentrations of 17 tracers measured in 2017 and 2018 were 333.32 ng m^-3 and 243.45 ng m^-3, respectively. Among them, the concentration of levoglucosan was the highest among all the tracers, with 325.63 ng m^-3 in 2017 and 237.47 ng m^-3 in 2018. The BB tracers showed obvious seasonal variations characteristics, most of which were abundant in winter. However, the concentrations of 3,4-dimethoxyacetic acid and sinapinic acid were higher in summer and spring than that in the other seasons. There were obvious differences in the Potential Source Contribution Factor (PSCF) model results of the BB's potential source area annually and in different seasons. The results of the potential source analysis showed that Beijing-Tianjin-Hebei had a great impact on the Nanjing air quality in 2017. Finally, five source factors for BB were identified based on the Positive Matrix Factor (PMF) model, and these were cellulose, hardwood, softwood, grass, and secondary formed. During the 2 years, cellulose was the largest contributor to biomass burning. Owing to the different fire conditions each year, the contribution of the five factors to the BB tracers was also different. For example, the contribution of softwood to the BB tracers was greater in 2018 (8.4%) than in 2017 (5.2%), while the contributions of hardwood and cellulose did not change significantly.

Organic aerosol compositions and source estimation by molecular tracers in Dushanbe, Tajikistan

To elucidate the molecular composition and sources of organic aerosols in Central Asia, carbonaceous compounds, major ions, and 15 organic molecular tracers of total suspended particulates (TSP) were analyzed from September 2018 to August 2019 in Dushanbe, Tajikistan. Extremely high TSP concentrations (annual mean ± std: 211 ± 131 μg m^-3) were observed, particularly during summer (seasonal mean ± std: 333 ± 183 μg m^-3). Organic carbon (OC: 11.9 ± 7.0 μg m^-3) and elemental carbon (EC: 5.1 ± 2.2 μg m^-3) exhibited distinct seasonal variations from TSP, with the highest values occurring in winter. A high concentration of Ca²⁺ was observed (11.9 ± 9.2 μg m^-3), accounting for 50.8% of the total ions and reflecting the considerable influence of dust on aerosols. Among the measured organic molecular tracers, levoglucosan was the predominant compound (632 ± 770 ng m^-3), and its concentration correlated significantly with OC and EC during the study period. These findings highlight biomass burning (BB) as an important contributor to the particulate air pollution in Dushanbe. High ratios of levoglucosan to mannosan, and syringic acid to vanillic acid suggest that mixed hardwood and herbaceous plants were the main burning materials in the area, with softwood being a minor one. According to the diagnostic tracer ratio, OC derived from BB constituted a large fraction of the primary OC (POC) in ambient aerosols, accounting for an annual mean of nearly 30% and reaching 63% in winter. The annual contribution of fungal spores to POC was 10%, with a maximum of 16% in spring. Measurements of plant debris, accounting for 3% of POC, divulged that these have the same variation as fungal spores.

Discrepant oxidation behavior of ferric ion and hydroxyl radical on syringic acid and vanillic acid in atmospheric Fenton-like system

Syringic acid (SA) and vanillic acid (VA) from biomass combustion are not only the potential sources of atmospheric brown carbon (BrC) but also the traceable markers of biomass burning in smoke particles. In this work, the Fenton-like oxidation in a mixed system containing SA and VA was studied under some typical conditions in atmospheric aqueous. The influence of scavenger, Fe³⁺ concentration, H₂O₂ concentration, SA concentration, pH and oxygen was discussed. Our results revealed that despite SA and VA have similar structures, Fe³⁺ and HO sever as their main oxidation sources, respectively. The addition of SA could heighten the HO yield obviously compared with conventional Fenton-like oxidation in atmospheric water, and this performance was attributed to the strong reducibility to Fe³⁺. In addition, SA accelerated the oxidation of VA and caused a 4.7-fold elevation in the initial rate. These results demonstrate that the process may change the amount of SA and VA and then disturb their mass ratio, which is important for aerosol source characterization work.

Tracers from Biomass Burning Emissions and Identification of Biomass Burning

The major organic compositions from biomass burning emissions are monosaccharide derivatives from the breakdown of cellulose, generally accompanied by small amounts of straight-chain, aliphatic, oxygenated compounds, and terpenoids from vegetation waxes, resins/gums, and other biopolymers. Levoglucosan from cellulose can be utilized as a specific or general indicator for biomass combustion emissions in aerosol samples. There are other important compounds, such as dehydroabietic acid, syringaldehyde, syringic acid, vanillic acid, vanillin, homovanillic acid, 4-hydroxybenzoic acid, and p-coumaric acid, which are additional key indicators of biomass burning. In this review, we will address these tracers from different types of biomass burning and the methods used to identify the sources in ambient aerosols. First, the methods of inferring biomass burning types by the ratio method are summarized, including levoglucosan/mannose, syringic acid/vanillic acid, levolgucosan/K+, vanillic acid/4-hydroxybenzoic acid, levoglucosan/OC, and levoglucosan/EC to infer the sources of biomass burning, such as crop residual burning, wheat burning, leaf burning, peatland fire, and forest fire in Asia. Second, we present the source tracer ratio methods that determine the biomass combustion types and their contributions. Finally, we introduce the PCA (Principal component analysis) and PMF (Positive matrix factor) methods to identify the type of biomass burning and its contributions according to emission factors of different species in various plants such as softwood, hardwood, and grass.

Kinetics and mechanism of syringic acid degradation initiated by hydroxyl radical and sulphate radical in the aqueous phase

Syringic acid (Syr) is an abundant component in aerosol particles. Multiphase photo-oxidation in aerosol phase provides an important oxidation pathway for Syr in the atmosphere. In this work, we studied the multiphase degradation of Syr by measuring rate coefficients of its reactions with potential radical oxidants (OH and SO₄^-) in aqueous solutions and by theoretical calculations, and degradation mechanisms by identifying the (intermediate) products. Rate coefficients, in 10⁹ M^-1 s^-1, were obtained as 32 ± 2 (pH 3) and 25 ± 2 (pH 6) for reactions with OH radical, and 1.7 ± 0.1 (pH 3) and 0.9 ± 0.02 (pH 6) for reactions with SO₄^-. Reactions of Syr with OH and SO₄^- were all in diffusion-control limit. Rate coefficients' difference under pH 6 and pH 3 in SO₄^- reaction was caused by Coulomb's force between negatively charged species. Theoretical calculations showed that the reaction of Syr with OH starts mainly by hydrogen atom transfer (HAT) from phenolic groups and secondly by OH addition to the aromatic ring. No product was identified in the reaction of Syr and OH radical at pH 3 due probably to the rapid mineralization of phenoxy radical formed from HAT, while products after OH additions were identified for a reaction at pH 6. On the other hand, reaction of Syr with SO₄^- starts by single-electron transfer (SET), forming Syr⁺, which can undergo hydrolysis, sulfation, and dimerization with Syr and other aromatic intermediates, etc. Dimerization products from the phenoxy-type radical were not found here.

Aromatic acids as biomass-burning tracers in atmospheric aerosols and ice cores: A review

Biomass burning (BB) is one of the largest sources of carbonaceous aerosols with adverse impacts on air quality, visibility, health and climate. BB emits a few specific aromatic acids (p-hydroxybenzoic, vanillic, syringic and dehydroabietic acids) which have been widely used as key indicators for source identification of BB-derived carbonaceous aerosols in various environmental matrices. In addition, measurement of p-hydroxybenzoic and vanillic acids in snow and ice cores have revealed the historical records of the fire emissions. Despite their uniqueness and importance as tracers, our current understanding of analytical methods, concentrations, diagnostic ratios and degradation processes are rather limited and scattered in literature. In this review paper, firstly we have summarized the most established methods and protocols for the measurement of these aromatic acids in aerosols and ice cores. Secondly, we have highlighted the geographical variability in the abundances of these acids, their diagnostic ratios and degradation processes in the environments. The review of the existing data indicates that the concentrations of aromatic acids in aerosols vary greatly with locations worldwide, typically more abundant in urban atmosphere where biomass fuels are commonly used for residential heating and/or cooking purposes. In contrast, their concentrations are lowest in the polar regions which are avoid of localized emissions and largely influenced by long-range transport. The diagnostic ratios among aromatic acids can be used as good indicators for the relative amounts and types of biomass (e.g. hardwood, softwood and herbaceous plants) as well as photochemical oxidation processes. Although studies suggest that the degradation processes of the aromatic acids may be controlled by light, pH and hygroscopicity, a more careful investigation, including closed chamber studies, is highly appreciated.

Aerosols as a source of dissolved black carbon to the ocean

Dissolved black carbon (DBC) is the largest known slow-cycling organic carbon pool in the world's oceans. Atmospheric deposition could significantly contribute to the oceanic DBC pool, but respective information is lacking. Here we estimate that, during the dust outbreak season, the atmospheric dry deposition of water-soluble black carbon (WSBC) is ~ 40% of the riverine input to the China coastal seas. The molecular composition of atmospheric WSBC determined by ultrahigh-resolution mass spectrometry, reveals similar soil-derived sources as for riverine discharge. WSBC is significantly positively correlated with water-soluble organic carbon (WSOC) in marine aerosols, and water-soluble black carbon contributes on average 2.8 ± 0.65% to the total WSOC. Based on this relationship, the global atmospheric deposition of DBC to the ocean is estimated to be 1.8 ± 0.83 Tg yr^-1. Anticipated future changes in biomass burning and dust mobilization might increase these numbers, with consequences for regional ecosystems and global carbon reservoirs.The contribution of atmospheric deposition to the oceanic dissolved black carbon pool (DBC) is unclear. Here, the authors show that water-soluble black carbon is positively correlated with water-soluble organic carbon in marine aerosols, and that atmospheric deposition is a significant source of oceanic DBC.