Development of a method for fast analysis of phenolic molecular markers in biomass burning particles using high performance liquid chromatography/atmospheric pressure chemical ionisation mass spectrometry

Comprehensive understanding on sources of high levels of fine particulate nitro-aromatic compounds at a coastal rural area in northern China

Nitro-aromatic compounds (NACs) are among the major components of brown carbon (BrC) in the atmosphere, causing negative impacts on regional climate, air quality, and ecological health. Due to the extensive origins, it is still a challenge to figure out the contributions and originating regions for different sources of atmospheric NACs. Here, field observations on fine particulate NACs were conducted at a coastal rural area in Qingdao, China in the winter of 2018 and 2019. The mean total concentrations of fine particulate nitro-aromatic compounds were 125.0 ± 89.5 and 27.7 ± 21.1 ng/m3 in the winter of 2018 and 2019, respectively. Among the measured eleven NACs, nitrophenols and nitrocatechols were the most abundant species. Variation characteristics and correlation analysis showed that humidity and anthropogenic primary emissions had significant influences on the NAC abundances. In this study, two tracing methods of the improved spatial concentration weighted trajectory (SCWT) model and the receptor model of positive matrix factorization (PMF) were combined to comprehensively understand the origins of NACs in fine particles at coastal Qingdao. Four major sources were identified, including coal combustion, biomass burning, vehicle exhaust, and secondary formation. Surprisingly, coal combustion was responsible for about half of the observed nitro-aromatic compounds, followed by biomass burning (∼30%). The results by SCWT demonstrated that the coal combustion dominated NACs mainly originated from the Shandong peninsula and the areas to the north and southwest, while those dominated by biomass burning primarily came from local Qingdao and the areas to the west.

Source apportionment of gaseous Nitrophenols and their contribution to HONO formation in an urban area

Nitrophenols (NPs) have significant impacts on human health, climate, and atmospheric chemistry. Despite numerous measurements of particulate NPs, still little is known about their gaseous atmospheric abundances, sources, and fate. Here, four gaseous NPs [2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), and 2-Methyl-4-nitrophenol (2-Me-4-NP)] were continuously monitored during late Spring at an urban site in Houston, Texas. Among the four NPs, 4-NP showed the highest abundance, followed by 2-Me-4-NP, 2-NP, and 2,4-DNP with average concentrations of 1.07 ± 0.19 ppt, 0.47 ± 0.12 ppt, 0.41 ± 0.16 ppt, and 0.27 ± 0.09 ppt, respectively. The positive matrix factorization (PMF) model identified seven sources: industrial NPs, secondary formation, phenol sources, acetonitrile source, natural gas/crude oil, traffic, and petrochemical industries/oil refineries. A zero-dimensional photochemical box model was used to simulate the observed 2-NP and 2,4-DNP. A 50.0% and 70.0% jNO2 was found to be consistent with the measured 2-NP and 2,4-DNP. This yields a nitrous acid (HONO) production of 7.5 ± 2.5 ppt/h from 06:00 to 18:00 Central Standard Time (CST) from both NPs. An extrapolation including other known NPs suggests a maximum HONO formation of 13.8 ppt/h. The results of this study suggest that using PMF analysis supplemented by photochemical box model provides identification of the NPs sources and their atmospheric implication to HONO formation.

Offline analysis of secondary formation markers in ambient organic aerosols by liquid chromatography coupled with time-of-flight mass spectrometry

The present study provides a comprehensive assessment of the quantitative analysis by high-performance liquid chromatography coupled with dual orthogonal electrospray ionization time-of-flight mass spectrometry (HPLC-ESI-TOF-MS) of pinene markers, biomass-burning related phenols, and other relevant carboxylic acids in atmospheric aerosol samples. Significant insights into the quantitative determination are offered on the basis of systematic experiments targeting the optimization of chromatographic separation, ionization source, and mass spectrometer performance. After testing three analytical columns, the best separation of the compounds of interest was achieved on a Poroshell 120 ECC18 column (4.6 × 50 mm, 2.7 µm) thermostated at 35 °C, operating in gradient elution mode with 0.1% acetic acid in water and acetonitrile at a 0.8 mL min^-1 flow rate. Optimal operational conditions for the ESI-TOF-MS instrument were identified as a 350 °C drying gas temperature, 13 L min^-1 drying gas flow rate, 60 psig nebulizer pressure, 3000 V for the ion transfer capillary, 60 V for the skimmer, and 150 V for the fragmentor. Additionally, the matrix effect on the ESI efficiency and the spike recovery factors of the compounds were tested. Method quantification limits can go as low as in the 0.88-48.0 μg L ^- 1 (3.67-200 pg m ^- 3_, at 120 m³ of sampled air) range. The developed method was shown to be reliable for the quantification of the targeted compounds in real atmospheric aerosol samples. The accuracy in the molecular mass determination of less than 5 ppm and the acquisition in the full scan mode were shown to bring additional insights into the organic constituents in atmospheric aerosols.

Particulate nitrated aromatic compounds from corn straw burning: Compositions, optical properties and potential health risks

Nitrated aromatic compounds (NACs) are important components of brown carbon (BrC), and their health and climate effects are of wide concern. Biomass burning is a major contributor to NACs in the atmosphere, yet NACs emitted from biomass burning are poorly constrained. In this study particulate NACs from open burning of corn straws were characterized in terms of their compositions, light absorption and toxic equivalents. 1, 6-dinitropyrene was the most abundant species among the measured nitropolycyclic aromatic hydrocarbons (NPAHs) with a share of 13.4% in total NPAHs, while 4-nitrocatechol was the most abundant nitrophenol (NP) species and accounted for 25.4% of measured NPs. 2-nitropyrene, widely used as a marker of secondary formation of NPAHs, was found to be the second most abundant NPAHs (13.3% of the total NPAHs) in the particulate matter (PM) primarily emitted from corn straw burning, and thus is inappropriate to be an indicator of the secondary formation. The measured primary NACs could only explain a negligible part (0.2%) of the light absorption by BrC. Although the concentrations of 9 toxic NACs were less than one-third of the 16 USEPA priority PAHs, their benzo(a)pyrene toxic equivalency quotients however were approximately 10 times that of the 16 PAHs. This study suggests that in comparison of PAHs from straw burning, NACs should be given greater attention due to their potentially higher toxic effects.

Polar Nitrated Aromatic Compounds in Urban Fine Particulate Matter: A Focus on Formation via an Aqueous-Phase Radical Mechanism

Polar nitrated aromatic compounds (pNACs) are key ambient brown carbon chromophores; however, their formation mechanisms, especially in the aqueous phase, remain unclear. We developed an advanced technique for pNACs and measured 1764 compounds in atmospheric fine particulate matter sampled in urban Beijing, China. Molecular formulas were derived for 433 compounds, of which 17 were confirmed using reference standards. Potential novel species with up to four aromatic rings and a maximum of five functional groups were found. Higher concentrations were detected in the heating season, with a median of 82.6 ng m^-3 for Σ₁₇pNACs. Non-negative matrix factorization analysis indicated that primary emissions particularly coal combustion were dominant in the heating season. While in the non-heating season, aqueous-phase nitration could generate abundant pNACs with the carboxyl group, which was confirmed by their significant association with the aerosol liquid water content. Aqueous-phase formation of 3- and 5-nitrosalicylic acids instead of their isomer of 4-hydroxy-3-nitrobenzoic acid suggests the existence of an intermediate where the intramolecular hydrogen bond favors kinetics-controlled NO₂^• nitration. This study provides not only a promising technique for the pNAC measurement but also evidence for their atmospheric aqueous-phase formation, facilitating further evaluation of pNACs' climatic effects.

Emissions and light absorption of PM2.5-bound nitrated aromatic compounds from on-road vehicle fleets

Vehicle emissions are an important source of nitrated aromatic compounds (NACs) in particulate size smaller 2.5 μm (PM_2.5), which adversely affect human health and biodiversity, especially in urban areas. In this study, filter-based PM_2.5 samples were collected during October 14-19, 2019, in a busy urban tunnel (approximately 35,000 vehicles per day) in south China to identify PM_2.5-bound NACs. Among them, 2,8-dinitrodibenzothiophene, 3-nitrodibenzofuran and 2-nitrodibenzothiophene were the most abundant nitrated polycyclic aromatic hydrocarbons (NPAHs), while 2-methyl-4-nitrophenol, 2,4-dinitrophenol, 3-methyl-4-nitrophenol and 4-nitrophenol were the most abundant nitrophenols (NPs). The observed mean fleet emission factors (EFs) of NPAHs and NPs were 2.2 ± 2.1 and 7.7 ± 4.1 μg km^-1, and were 2.9 ± 2.7 and 10.2 ± 5.4 μg km^-1 if excluding electric and liquefied petroleum gas vehicles, respectively. Regression analysis revealed that diesel vehicles (DVs) had NPAH-EFs (55.3 ± 5.3 μg km^-1) approximately 180 times higher than gasoline vehicles (GVs) (0.3 ± 0.2 μg km^-1), and NP-EFs (120.6 ± 25.8 μg km^-1) approximately 30 times higher than GVs (4.1 ± 0.2 μg km^-1), and thus 89% NPAH emissions and 56% NP emissions from the onroad fleets were contributed by DVs although DVs only accounted for 3.3% in the fleets. Methanol solution-based light absorption measurements demonstrated that the mean incremental light absorption for methanol-soluble brown carbon at 365 nm was 6.8 ± 2.2 Mm^-1, of which the 44 detected NACs only contributed about 1%. The mean EF of the 7 toxic NACs was approximately 3% that of the 16 priority PAHs; However, their benzo(a)pyrene toxic equivalence quotients (TEQ_BaP) could reach over 25% that of the PAHs. Moreover, 6-nitrochrysene mainly from DVs contributed 93% of the total TEQ_BaP of the NACs. This study demonstrated that enhancing DV emission control in urban areas could benefit the reduction of exposure to air toxins such as 6-nitrochrysene.

Variabilities in Primary N-Containing Aromatic Compound Emissions from Residential Solid Fuel Combustion and Implications for Source Tracers

Nitroaromatic compounds (NACs) not only are strongly absorbing chromophores but also adversely affect human health. NACs can be emitted from incomplete combustions and can derive secondarily through photochemical reactions. Here, emission experiments were conducted for 31 fuel-stove combinations to elucidate variations in, and influencing factors of, NAC emission factors (EF_∑NACs) and to explore potential tracers for different combustion sources. EF_∑NACs varied by 2 orders of magnitude among different combinations. Differences in fuel type contributed more than the stove difference to the observed variation. EF_∑NACs for biomass pellets was approximately 66% lower than that for raw biomass, although the bulk organic and brown carbon EFs were 95% lower. 2-Nitro-1-naphthol was the most abundant individual compound, followed by 4-nitrocatechol, while acid compounds (salicylic acid and benzoic acid) were low in abundance (<1%). Substantially different profiles were observed between coal and biomass burning emissions. Biomass burning had more single-ring-based phenolic compounds with more 4-nitrocatechol, while in coal combustion, more two-ring products were produced. This study demonstrated much lower ratios of 2-nitro-1-naphthol/4-nitrocatechol for biomass in both traditional (2.0 ± 3.5) and improved stoves (3.0 ± 2.1) than for coals (15 ± 6). Coal and biomass burning differed in not only EF_∑NACs but also compound profile, consequently leading to distinct health and climate impacts; moreover, the ratio of 2-nitro-1-naphthol/4-nitrocatechol may be used in source apportionment of NACs.

Determination of trace concentrations of simple phenols in ambient PM samples

Phenols are hazardous, but yet ubiquitous in the environment, including in atmospheric aerosols due to combustion emissions. There, phenols are subjected to secondary transformations, producing even more toxic nitrophenolic air pollutants. However, primary simple phenols, i.e. those containing only hydroxyl, methyl and methoxy substituents are not easy to detect. Trace concentrations, semi-volatile character and poorly ionizable functional groups prevent us from their determination by the most common analytical techniques, such as gas and liquid chromatography with mass spectrometric detection (GC/LC-MS). Here, we present a new derivatization method for MS/MS detection with positive ion electrospray ionization (+ESI-MS/MS) of simple phenols in atmospheric particulate matter (PM) extracts. The method is sensitive, selective, and robust, and requires no sample concentration step, which is critical due to the volatile character of the target analytes. After derivatization with dansyl chloride, phenol, catechol, cresols and guaiacol were detected in urban PM samples from Ljubljana, Slovenia. This method finally enables to study the abundance of primary phenols in atmospheric PM from different sources, which will improve understanding of secondary aerosol (trans)formation pathways and allow for more targeted mitigation strategies in respect to airborne phenolic pollutants.

Nitrogen Enrichment during Soil Organic Matter Burning and Molecular Evidence of Maillard Reactions

Wildfires in forested watersheds dramatically alter stored and labile soil organic matter (SOM) pools and the export of dissolved organic matter (DOM). Ecosystem recovery after wildfires depends on soil microbial communities and revegetation and therefore is limited by the availability of nutrients, such as nitrogen-containing and labile, water-soluble compounds. However, SOM byproducts produced at different wildfire intensities are poorly understood, leading to difficulties in assessing wildfire severity and predicting ecosystem recovery. In this work, water-extractable organic matter (WEOM) from laboratory microcosms of soil burned at discrete temperatures was characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to study the impacts of fire temperature on SOM and DOM composition. The molecular composition derived from different burn temperatures indicated that nitrogen-containing byproducts were enriched with heating and composed of a wide range of aromatic features and oxidation states. Mass difference-based analysis also suggested that products formed during heating could be modeled using transformations along the Maillard reaction pathway. The enrichment of N-containing SOM and DOM at different soil burning intensities has important implications for ecosystem recovery and downstream water quality.

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.

Substantial emissions of nitrated aromatic compounds in the particle and gas phases in the waste gases from eight industries

Nitrated aromatic compounds, the ubiquitous nitrogen-containing organic pollutants, impact the environment and organisms adversely. As industrial raw materials and intermediates, nitrated aromatic compounds and their aromatic precursors are widely employed in the industrial production activities. Nevertheless, their emission from industrial waste gases has so far not been studied extensively. In this study, the concentrations of 12 nitrated aromatic compounds in the particle and gas phases downwind of 16 factories encompassing eight industries (i.e., pharmaceutical, weaving and dyeing, herbicide, explosive, painting, phenolic resin, paper pulp and polystyrene foam industries), were determined by ultra-high-performance liquid chromatography-mass spectrometry. Their concentrations in the particle and gas phases from different factories ranged from 114.7 ± 63.5 to 296.6 ± 62.5 ng m^-3 and 148.7 ± 7.4 to 309.8 ± 26.2 ng m^-3, respectively, thus, exhibiting significantly high concentrations as compared to the background sites. Among the 12 detected species, 4-nitrophenol, 5-nitrosalicylic acid, 3-nitrosalicylic acid and 4-methyl-2,6-dinitrophenol were observed to be the predominant species, with total fractions up to 47.9-72.3% and 63.1-70.3% in the particle and gas phases, respectively. Their emission profiles with respect to the industrial activities exhibited large discrepancies as compared to the combustion sources, thus, indicating different formation mechanisms. The emission ratios of particulate nitrated aromatic compounds owing to the industrial activities were estimated between 0.5 ± 0.2 and 4.3 ± 1.5 ng μg^-1, which were higher than or comparable to those from various combustion sources. The findings from this study confirm the industrial emission to be an important source of nitrated aromatic compounds in the atmosphere. The substantial emissions of nitrated aromatic compounds from various industries reported in this study provide the fundamental basis for further emission estimation and pollution control.

Physiological Role of the Previously Unexplained Benzenetriol Dioxygenase Homolog in the Burkholderia sp. Strain SJ98 4-Nitrophenol Catabolism Pathway

4-Nitrophenol, a priority pollutant, is degraded by Gram-positive and Gram-negative bacteria via 1,2,4-benzenetriol (BT) and hydroquinone (HQ), respectively. All enzymes involved in the two pathways have been functionally identified. So far, all Gram-negative 4-nitrophenol utilizers are from the genera Pseudomonas and Burkholderia. But it remains a mystery why pnpG, an apparently superfluous BT 1,2-dioxygenase-encoding gene, always coexists in the catabolic cluster (pnpABCDEF) encoding 4-nitrophenol degradation via HQ. Here, the physiological role of pnpG in Burkholderia sp. strain SJ98 was investigated. Deletion and complementation experiments established that pnpG is essential for strain SJ98 growing on 4-nitrocatechol rather than 4-nitrophenol. During 4-nitrophenol degradation by strain SJ98 and its two variants (pnpG deletion and complementation strains), 1,4-benzoquinone and HQ were detected, but neither 4-nitrocatechol nor BT was observed. When the above-mentioned three strains (the wild type and complementation strains with 2,2'-dipyridyl) were incubated with 4-nitrocatechol, BT was the only intermediate detected. The results established the physiological role of pnpG that encodes BT degradation in vivo. Biotransformation analyses showed that the pnpA-deleted strain was unable to degrade both 4-nitrophenol and 4-nitrocatechol. Thus, the previously characterized 4-nitrophenol monooxygenase PnpA_SJ98 is also essential for the conversion of 4-nitrocatechol to BT. Among 775 available complete genomes for Pseudomonas and Burkholderia, as many as 89 genomes were found to contain the putative pnpBCDEFG genes. The paucity of pnpA (3 in 775 genomes) implies that the extension of BT and HQ pathways enabling the degradation of 4-nitrophenol and 4-nitrocatechol is rarer, more recent, and likely due to the release of xenobiotic nitroaromatic compounds. IMPORTANCE An apparently superfluous gene (pnpG) encoding BT 1,2-dioxygenase is always found in the catabolic clusters involved in 4-nitrophenol degradation via HQ by Gram-negative bacteria. Our experiments reveal that pnpG is not essential for 4-nitrophenol degradation in Burkholderia sp. strain SJ98 but instead enables its degradation of 4-nitrocatechol via BT. The presence of pnpG genes broadens the range of growth substrates to include 4-nitrocatechol or BT, intermediates from the microbial degradation of many aromatic compounds in natural ecosystems. In addition, the existence of pnpCDEFG in 11.6% of the above-mentioned two genera suggests that the ability to degrade BT and HQ simultaneously is ancient. The extension of BT and HQ pathways including 4-nitrophenol degradation seems to be an adaptive evolution for responding to synthetic nitroaromatic compounds entering the environment since the industrial revolution.

Kinetics and Mass Yields of Aqueous Secondary Organic Aerosol from Highly Substituted Phenols Reacting with a Triplet Excited State

Biomass burning emits large amounts of phenols, which can partition into cloud/fog drops and aerosol liquid water (ALW) and react to form aqueous secondary organic aerosol (aqSOA). Triplet excited states of organic compounds (³C*) are likely oxidants, but there are no rate constants with highly substituted phenols that have high Henry's law constants (K_H) and are likely important in ALW. To address this gap, we investigated the kinetics of six highly substituted phenols with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6-4.6) × 10⁹ M^-1 s^-1, while values at pH 5 are 2-5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials, allowing rate constants of other phenols to be predicted. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making hydroxyl radicals, while Cu(II) inhibits phenol decay. Mass yields of aqueous SOA from triplet reactions are large and range from 59 to 99%. Calculations using our data along with previous oxidant measurements indicate that phenols with high K_H can be an important source of aqSOA in ALW, with ³C* typically the dominant oxidant.

Characterization of benzene polycarboxylic acids and polar nitroaromatic compounds in atmospheric aerosols using UPLC-MS/MS

The molecular characterization of water-soluble organic compounds (WSOC), a large fraction of the organic mass found in the atmospheric aerosols, is important to better understand emissions and atmospheric processes influencing the particulate pollution in most urban areas. This study deals with the development of a routine method using ultrahigh pressure liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (UPLC-ESI-MS/MS) for rapid analysis of primary and secondary organic compounds distributed among three classes: (i) benzene polycarboxylic acids; (ii) nitroaromatic acids and (iii) nitrophenols in ambient particles. Using an UPLC HSS T3 column with a mobile phase consisting of formic acid/acetonitrile under gradient elution, all target analytes were eluted within a total time of 12 min. Although some targeted analytes with different m/z were not resolved, a quantitation of these compounds was carried by distinct multiple reaction monitoring (MRM) transitions. Quality parameters of the method were established. The method was proven to be sensitive with limits of detection ranged from 0.02 to 0.89 ng/mL. Together with a simple sample preparation and the use of labeled internal standards, the method was confirmed to be robust and reliable to determine a large number of organic tracers in atmospheric particulate matter samples. The analytical procedure was also applied to assess the abundance and characteristics of target analytes in PM2.5 emitted from diesel and gasoline-powered engines, and Urban Dust and Diesel Particulate Matter Standard Reference Materials (SRM 1649b and SRM 1650b, respectively). The obtained results suggest that trimellitic, 4-hydroxyphthalic and 4-nitrophthalic acids may be used as potential tracers for diesel engine emissions. Clear differences in distribution of target species were observed between urban PM2.5 affected by the traffic and biomass burning emissions.

Emissions of fine particulate nitrated phenols from various on-road vehicles in China

Nitrated phenols are receiving increasing attention due to their adverse impacts on the environment and human health. Previous measurements have revealed the non-ignorable contribution of vehicle exhaust to atmospheric nitrated phenols in urban areas. However, there is a lack of comprehensive understanding of the emission characteristics and the total emission of nitrated phenols from current on-road traffic. This study investigated the emissions from eight passenger vehicles, eight trucks, and two taxis, with fuel types including diesel, gasoline, and compressed natural gas. Exhaust emissions were collected and measured using a mobile measurement system on two testing routes. Twelve nitrated phenols in the collected fine particulate matter were detected using ultrahigh performance liquid chromatography-mass spectrometry. Overall, the emission profiles of fine particulate nitrated phenols varied with vehicle load and fuel type. The 4-nitrophenol and its methyl derivatives were dominant nitrated phenol species emitted by the vehicles with proportions of 38.4%-68.0%, which is significantly different from the proportions of nitrated phenols emitted from biomass burning and coal combustion. The emission factors also exhibited large variations across vehicle type, fuel type, and emission standards, with relatively low values for gasoline vehicles and taxis fueled by compressed natural gas and high values for diesel vehicles. Based on the emission factors of nitrated phenols from different vehicles, the estimated total emission of nitrated phenols from on-road vehicles in China was 58.9 Mg (-86%-85% within 95% confidence interval), with diesel trucks contributing the most substantial fractions. This work highlights the very high level of emissions of nitrated phenols from diesel vehicles and provides an essential basis for atmospheric modeling and effective pollution control.

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.

Emissions of fine particulate nitrated phenols from the burning of five common types of biomass

Nitrated phenols are among the major constituents of brown carbon and affect both climates and ecosystems. However, emissions from biomass burning, which comprise one of the most important primary sources of atmospheric nitrated phenols, are not well understood. In this study, the concentrations and proportions of 10 nitrated phenols, including nitrophenols, nitrocatechols, nitrosalicylic acids, and dinitrophenol, in fine particles from biomass smoke were determined under three different burning conditions (flaming, weakly flaming, and smoldering) with five common types of biomass (leaves, branches, corncob, corn stalk, and wheat straw). The total abundances of fine nitrated phenols produced by biomass burning ranged from 2.0 to 99.5 μg m^-3. The compositions of nitrated phenols varied with biomass types and burning conditions. 4-nitrocatechol and methyl nitrocatechols were generally most abundant, accounting for up to 88-95% of total nitrated phenols in flaming burning condition. The emission ratios of nitrated phenols to PM_2.5 increased with the completeness of combustion and ranged from 7 to 45 ppmm and from 239 to 1081 ppmm for smoldering and flaming burning, respectively. The ratios of fine nitrated phenols to organic matter in biomass burning aerosols were comparable to or lower than those in ambient aerosols affected by biomass burning, indicating that secondary formation contributed to ambient levels of fine nitrated phenols. The emission factors of fine nitrated phenols from flaming biomass burning were estimated based on the measured mass fractions and the PM_2.5 emission factors from literature and were approximately 0.75-11.1 mg kg^-1. According to calculations based on corn and wheat production in 31 Chinese provinces in 2013, the total estimated emission of fine nitrated phenols from the burning of corncobs, corn stalks, and wheat straw was 670 t. This work highlights the apparent emission of methyl nitrocatechols from biomass burning and provides basic data for modeling studies.

Molecular markers of biomass burning in arctic aerosols

Biomass burning is one of the most important sources of organic matter in the atmosphere as it affects the absorption and scattering of solar radiation, creates cloud condensation nuclei and possibly influences ice and snow albedo. Here we created and validated an analytical method using HPLC/(-)-ESI-MS/MS to determine phenolic compounds (PCLCs): vanillic acid, isovanillic acid, homovanillic acid, syringic acid, syringaldehyde, ferulic acid, p-coumaric acid, and coniferyl aldehyde at trace levels in particulate matter. We analyzed eighteen high-volume air samples from Ny Ålesund (Svalbard) collected during the boreal spring and summer of 2010. Biomass burning molecules including PCLCs (<0.49 μm, mean atmospheric concentration 6 pg m(-3)), levoglucosan (0.004 to 0.682 ng m(-3)) and acrylamide (32 fg m(-3) to 166 fg m(-3)) were present in the sampled aerosols. Levoglucosan concentrations, an unambiguous cellulose combustion tracer, derived from 2010 Russian fires. PCLCs levels in the Ny Alesund atmosphere in different size fractions reflected both long-range transport linked to biomass burning and a terrigenous local source.

Contribution of nitrated phenols to wood burning brown carbon light absorption in Detling, United Kingdom during winter time

We show for the first time quantitative online measurements of five nitrated phenol (NP) compounds in ambient air (nitrophenol C6H5NO3, methylnitrophenol C7H7NO3, nitrocatechol C6H5NO4, methylnitrocatechol C7H7NO4, and dinitrophenol C6H4N2O5) measured with a micro-orifice volatilization impactor (MOVI) high-resolution chemical ionization mass spectrometer in Detling, United Kingdom during January-February, 2012. NPs absorb radiation in the near-ultraviolet (UV) range of the electromagnetic spectrum and thus are potential components of poorly characterized light-absorbing organic matter ("brown carbon") which can affect the climate and air quality. Total NP concentrations varied between less than 1 and 98 ng m(-3), with a mean value of 20 ng m(-3). We conclude that NPs measured in Detling have a significant contribution from biomass burning with an estimated emission factor of 0.2 ng (ppb CO)(-1). Particle light absorption measurements by a seven-wavelength aethalometer in the near-UV (370 nm) and literature values of molecular absorption cross sections are used to estimate the contribution of NP to wood burning brown carbon UV light absorption. We show that these five NPs are potentially important contributors to absorption at 370 nm measured by an aethalometer and account for 4 ± 2% of UV light absorption by brown carbon. They can thus affect atmospheric radiative transfer and photochemistry and with that climate and air quality.

Development of a liquid chromatographic method based on ultraviolet-visible and electrospray ionization mass spectrometric detection for the identification of nitrocatechols and related tracers in biomass burning atmospheric organic aerosol

RATIONALE

Studying the chemical composition of biomass burning aerosol (BBA) is very important in order to assess their impact on the climate and the biosphere. In the present study, we focus on the characterization of some newly recognized biomass burning aerosol tracers including methyl nitrocatechols, nitroguaiacols and 4-nitrocatechol, but also on nitrophenols, methyl nitrophenols and nitrosalicylic acids, using liquid chromatography tandem mass spectrometry.

METHODS

For the purpose of their separation and detection in atmospheric aerosol, a new chromatographic method was initially developed based on reversed-phase chromatography coupled with ultraviolet/visible (UV/Vis) detection. The method was afterwards transferred to a liquid chromatography/electrospray ionization linear ion trap mass spectrometry (LC/ESI-LITMS) system in order to identify the targeted analytes in winter aerosol from the city of Maribor, Slovenia, using their chromatographic retention times and characteristic (-)ESI product ion (MS(2) ) spectra.

RESULTS

The fragmentation patterns of analytes obtained with LITMS are presented. Additional nitro-aromatic compounds (m/z 168 and 182) closely related to the targeted nitrocatechols and nitroguaiacols were detected in the aerosol. According to their MS(2) spectra these compounds could be attributed to methyl homologues of methyl nitrocatechols and nitroguaiacols.

CONCLUSIONS

The proposed LC/MS method results in a better separation and specificity for the targeted analytes. Several nitro-aromatic compounds were detected in urban BBA. The LC/MS peak intensity of the newly detected methyl nitrocatechols and nitroguaiacols is comparable to that of the methyl nitrocatechols, which also qualifies them as suitable molecular tracers for secondary biomass burning aerosol.