Nitrosalicylic Acids Research Articles

Characterization of atmospheric nitroaromatic compounds in Southwest China by direct injection liquid chromatography-tandem mass spectrometry analysis of aerosol extracts

Nitroaromatic compounds (NACs) in ambient particles are of great concern due to their adverse effects on human health and climate. However, investigations on the characteristics and potential sources of NACs in Southwest China are still scarce. In this study, a field sampling campaign was carried out in the winter of 2022 at a suburban site in Mianyang, Southwest China. A direct injection liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to rapidly determine 10 NACs in fine particulate matter (PM2.5) extracts. The method was sensitive for the quantification of the NACs, with a limit of quantification (LOQ) in the range of 0.092–0.52 ng mL−1. Then, the developed method was applied to determine the concentrations of nitrophenols (NPs), nitrocatechols (NCs), nitrosalicylic acids (NSAs), and nitronaphthol in PM2.5 in Mianyang. The average concentration of total NACs was 78.2 ± 31.2 ng m−3, with daily concentrations ranging from 20.7 to 127.9 ng m−3. Among the measured NACs, 4-nitrocatechol was the most abundant, accounting for 57.8% of the NACs in winter. The five NPs compounds together contributed to 14% of the NACs, which was lower than in other Chinese cities due to the warm climate in winter in Southwest China. NSAs and nitronaphthol each accounted for less than 5% of the NACs. Three major sources of NACs were identified based on the principal component analysis, including vehicle emissions, biomass burning, and secondary formation. The significant correlation between individual NACs and NO2 supported their secondary formation sources. The good correlation between NPs and cloud amount further suggested that gas-phase oxidation was the possible NPs formation mechanism. Our findings revealed the important role of nitrocatechols in NACs in Southwest China, implying that more measures should be taken to control biomass burning and aromatic volatile organic compounds emissions to reduce the level of NACs.

Nitroaromatic compounds in six major Chinese cities: Influence of different formation mechanisms on light absorption properties

Nitroaromatic compounds (NACs) are important nitrogen organics in aerosol with strong light-absorbing and chemically reactive properties. In this study, NACs in six Chinese megacities, including Harbin (HB), Beijing (BJ), Xi'an (XA), Wuhan (WH), Chengdu (CD), and Guangzhou (GZ), were investigated for understanding their sources, gas-particle partitioning, and impact on BrC absorption properties. The concentrations of ΣNACs in PM2.5 in the six cities ranged from 9.15 to 158.8 ng/m3 in winter and from 2.02 to 9.39 ng/m3 in summer. Nitro catechols (NCs), nitro phenols (NPs), and nitro salicylic acids (NSAs) are the main components in ΣNACs, with NCs being dominant in particulate phase and NPs being dominant in the gas phase. Correlation analysis between different pollutant species revealed that coal and biomass combustions were the major sources of NACs in the northern cities during wintertime, while secondary formation dominated NACs in the southern cities during summertime. The contribution of ΣNACs to brown carbon (BrC) light absorption ranged from 0.85 to 7.98 % during the wintertime and 2.07–6.44 % during the summertime. The mass absorption efficiency at 365 nm (MAE365) were highest for 4-nitrocatechol (4NC, 17.4–89.0 m2/g), 4-methyl-5-nitrocatechol (4M5NC, 15.0–76.9 m2/g), and 4-nitroguaiacol (4NG, 11.7–59.8 m2/g). The formation of NCs and NG through oxidation and nitration of catechol and guaiacol led to a significant increase in aerosol light absorption. In contrast, NPs and NSAs formed by the photonitration and photooxidation in liquid phase showed high polarity but low light absorption ability, and the proportions of (NPs + NSAs) in the light absorption of ΣNACs were lower than 15.3 % in the six megacities.

Characterization of nitrated aromatic compounds in fine particles from Nanjing, China: Optical properties, source allocation, and secondary processes

Recently, nitrated aromatic compounds (NACs) have received much attention due to their role as key chromophores of brown carbon (BrC) and their impact on human health and the climate. In this study, a method for detection of 12 NACs in the atmosphere was developed and applied to the detection of 191 atmospheric samples in the northern suburbs of Nanjing in 2017. The average concentration of total NACs in Nanjing was 26.48 ng m−3, which was lower than that in North China. The total NACs also showed obvious seasonal variation, with the highest concentration in winter (51.99 ng m−3) and the lowest concentration in summer (11.26 ng m−3). Moreover, the contribution of subcomponents of NACs also changed with the seasons. Nitrophenols (NPs) and nitrocatechols (NCs) were most abundant in winter, while nitrosalicylic acids (NSAs) were more abundant in summer, accounting for 30%, 27%, and 85%, respectively. The reason for this result may be due to the different sources of dominance of NACs in different seasons. The light absorption of NACs to water-soluble BrC was mainly concentrated in the 300–400 nm range, and its contribution reached the maximum at 310 nm. NPs and NCs had the highest contribution to BrC among all NACs in winter, with a range of 25–54% and 3–59%, respectively. The Positive Matrix Factorization (PMF) was used to analyze the main sources of NACs in different seasons. Secondary generation was the largest source in summer, accounting for 43.5%, and biomass combustion contributed the most in autumn, accounting for 36.7%. NACs are affected by temperature, especially in summer, and the subcomponents vary in temperature dependence. The secondary generation process of NACs is affected by NO2 and O3, especially when NO2 is greater than 40 μg m−3 and O3 is less than 220 μg m−3.

Atmospheric Nitrated Phenolic Compounds in Particle, Gaseous, and Aqueous Phases During Cloud Events at a Mountain Site in North China: Distribution Characteristics and Aqueous‐Phase Formation

AbstractNitrated phenolic compounds in the atmosphere are receiving increasing attention due to their light absorption and biological toxicity. In this study, particulate, gaseous, and cloud water samples were simultaneously collected during cloud events at the summit of Mount Tai in northern China in spring, summer, and winter and the contents of 11 nitrated phenolic compounds were determined. The seasonal average concentrations of the total nitrated phenolic compounds in particles were in the range of 7.3–27.1 ng m−3, a little lower than those measured in the gas‐phase (18.3–70.6 ng m−3). Their concentrations in cloud water were at the levels of 168.4–438.5 μg L−1. 4‐Nitrophenol and nitrosalicylic acids were the dominant compounds in particles, while 4‐nitrophenol and 2,4‐dinitrophenol were the most abundant in the gas phase and cloud water samples. During cloud events, most nitrated phenolic compounds were mainly distributed in the particle phase, except dinitrophenols which were mainly distributed in the gas phase in winter. The field‐derived effective Henry's law coefficients were several orders of magnitude higher than their theoretical values in pure water. Moreover, the measured concentrations of particulate nitrated phenolic compounds were substantially greater than the theoretical predictions, especially in spring. The above results indicate that nitrated phenolic compounds were partly formed via aqueous‐phase reactions inside the cloud droplets or on the wet particle surfaces, which changed their distribution patterns. The much higher ratios of 2,4‐dinitrophenol to the sum of 4‐nitrophenol and 5‐nitrosalicylic acid in cloud water than those in particles further confirm the enhanced formation via aqueous processes.

Nitrated monoaromatic hydrocarbons (nitrophenols, nitrocatechols, nitrosalicylic acids) in ambient air: levels, mass size distributions and inhalation bioaccessibility

Nitrated monoaromatic hydrocarbons (NMAHs) are ubiquitous in the environment and an important part of atmospheric humic-like substances (HULIS) and brown carbon. They are ecotoxic and with underresearched toxic potential for humans. NMAHs were determined in size-segregated ambient particulate matter collected at two urban sites in central Europe, Ostrava and Kladno, Czech Republic. The average sums of 12 NMAHs (Σ12NMAH) measured in winter PM10 samples from Ostrava and Kladno were 102 and 93 ng m−3, respectively, and 8.8 ng m−3 in summer PM10 samples from Ostrava. The concentrations in winter corresponded to 6.3–7.3% and 2.6–3.1% of HULIS-C and water-soluble organic carbon (WSOC), respectively. Nitrocatechols represented 67–93%, 61–73% and 28–96% of NMAHs in PM10 samples collected in winter and summer at Ostrava and in winter at Kladno, respectively. The mass size distribution of the targeted substance classes peaked in the submicrometre size fractions (PM1), often in the PM0.5 size fraction especially in summer. The bioaccessible fraction of NMAHs was determined by leaching PM3 samples in two simulated lung fluids, Gamble’s solution and artificial lysosomal fluid (ALF). More than half of NMAH mass is found bioaccessible, almost complete for nitrosalicylic acids. The bioaccessible fraction was generally higher when using ALF (mimics the chemical environment created by macrophage activity, pH 4.5) than Gamble’s solution (pH 7.4). Bioaccessibility may be negligible for lipophilic substances (i.e. log KOW > 4.5).

Characterization of the light-absorbing properties, chromophore composition and sources of brown carbon aerosol in Xi'an, northwestern China

Abstract. The impact of brown carbon aerosol (BrC) on the Earth's radiative forcing balance has been widely recognized but remains uncertain, mainly because the relationships among BrC sources, chromophores and optical properties of aerosol are poorly understood. In this work, the light absorption properties and chromophore composition of BrC were investigated for samples collected in Xi'an, northwestern China, from 2015 to 2016. Both absorption Ångström exponent (AAE) and mass absorption efficiency (MAE) show distinct seasonal differences, which could be attributed to the differences in sources and chromophore composition of BrC. Three groups of light-absorbing organics were found to be important BrC chromophores, including compounds that have multiple absorption peaks at wavelengths > 350 nm (12 polycyclic aromatic hydrocarbons and their derivatives) and compounds that have a single absorption peak at wavelengths < 350 nm (10 nitrophenols and nitrosalicylic acids and 3 methoxyphenols). These measured BrC chromophores show distinct seasonal differences and contribute on average about 1.1 % and 3.3 % of light absorption of methanol-soluble BrC at 365 nm in summer and winter, respectively, about 7 and 5 times higher than the corresponding carbon mass fractions in total organic carbon. The sources of BrC were resolved by positive matrix factorization (PMF) using these chromophores instead of commonly used non-light-absorbing organic markers as model inputs. Our results show that vehicular emissions and secondary formation are major sources of BrC (∼ 70 %) in spring, coal combustion and vehicular emissions are major sources (∼ 70 %) in fall, biomass burning and coal combustion become major sources (∼ 80 %) in winter, and secondary BrC dominates (∼ 60 %) in summer.

Composition and mass size distribution of nitrated and oxygenated aromatic compounds in ambient particulate matter from southern and central Europe – implications for the origin

Abstract. Nitro-monoaromatic hydrocarbons (NMAHs), such as nitrocatechols, nitrophenols and nitrosalicylic acids, are important constituents of atmospheric particulate matter (PM) water-soluble organic carbon (WSOC) and humic-like substances (HULIS). Nitrated and oxygenated derivatives of polycyclic aromatic hydrocarbons (NPAHs and OPAHs) are toxic and ubiquitous in the ambient air; due to their light absorption properties, together with NMAHs, they are part of aerosol brown carbon (BrC). We investigated the winter concentrations of these substance classes in size-resolved PM from two urban sites in central and southern Europe, i.e. Mainz (MZ), Germany, and Thessaloniki (TK), Greece. The total concentration of 11 NMAHs (∑11NMAH concentrations) measured in PM10 and total PM were 0.51–8.38 and 12.1–72.1 ng m−3 at the MZ and TK sites, respectively, whereas ∑7OPAHs were 47–1636 and 858–4306 pg m−3, and ∑8NPAHs were ≤90 and 76–578 pg m−3, respectively. NMAHs contributed 0.4 % and 1.8 % to the HULIS mass at MZ and TK, respectively. The mass size distributions of the individual substances generally peaked in the smallest or second smallest size fraction i.e. <0.49 or 0.49–0.95 µm. The mass median diameter (MMD) of NMAHs was 0.10 and 0.27 µm at MZ and TK, respectively, while the MMDs of NPAHs and OPAHs were both 0.06 µm at MZ and 0.12 and 0.10 µm at TK. Correlation analysis between NMAHs, NPAHs, and OPAHs from one side and WSOC, HULIS, sulfate, and potassium from the other suggested that fresh biomass burning (BB) and fossil fuel combustion emissions dominated at the TK site, while aged air masses were predominant at the MZ site.

Size distributions of nitrated phenols in winter at a coastal site in north China and the impacts from primary sources and secondary formation

Nitrated phenols in particulate matters are among the major components of brown carbon, harm plant growth and human health. To understand the size distributions of nitrated phenols in the polluted coastal region and the factors influencing these distributions, size-resolved particulate matters were collected from a rural site in the coastal city of Qingdao, China, in January 2019, and analyzed for the presence of 11 nitrated phenols. The average concentrations of total nitrated phenols in fine- and coarse-mode particles were 123.6 and 37.2 ng m−3, respectively. 4-Nitrophenol was found to be the dominant nitrated phenol, followed by 3-methyl-6-nitrocatechol, 3-methyl-4-nitrophenol, and 4-nitrocatechol. On average, maximum concentrations of nitrated phenols were in condensation-mode particles, whereas a minor concentration peak of nitro-salicylic acids was present in droplet-mode particles. In addition, a minor concentration peak of 4-methyl-2,6-dinitrophenol was noticed in coarse-mode particles. Comparisons of the size distributions under different situations confirmed that both primary emissions and secondary formation had significant effects on the abundances and particle-sizes of nitrated phenols. Coal combustion in residential villages and firework burning during a festival led to a sharp increase of nitrated phenols in condensation-mode particles, whereas dust promoted their heterogeneous formation in coarse-mode particles, and high humidity in the coastal area facilitated their aqueous formation in droplet-mode particles.

Nitrated phenols and the phenolic precursors in the atmosphere in urban Jinan, China

Nitrated phenols are a major class of brown carbon in the atmosphere and have adverse effects on human and plants health. They are emitted from combustion sources or produced by oxidation of phenolic precursors. In this study, fine particulates, total suspended particulates, and gas-phase samples were collected in urban Jinan in winter, spring, and summer, and UHPLC-MS analysis was used to determine 8 phenolic compounds and 12 nitrated phenols in these samples. The seasonal average concentrations of total phenolic compounds and total nitrated phenols were in the ranges of 2.6–18.7 ng m−3 and 13.5–105.4 ng m−3, respectively. The concentrations of phenolic compounds and nitrated phenols were highest in winter, followed (in decreasing order) by spring, and summer. Phenol and salicylic acid were the most abundant phenolic species in both gaseous and particulate samples. 4-Nitrophenol was the most abundant nitrated phenols in particulate matters, followed by 4-nitrocatechol and 5-nitrosalicylic acid, while 4-nitrophenol and 2,4-dinitrophenol were the dominant species in the gas phase. The distributions of phenolic compounds and nitrated phenols in fine and coarse particles and in gas and particle phases were largely dependent on the aerosol size distribution, the ambient temperature, and the compound volatility. More of them were distributed in fine particles and gas-phase in summer than in spring. It was found that phenol, catechol, methyl-catechols, 4-nitrophenol, and methyl-nitrophenols mainly derived from coal combustion, while biomass burning was the main source of cresols, 2,6-dimethyl-4-nitrophenol, 4-nitrocatechol, and methyl-nitrocatechols. In addition, secondary formation contributed the largest fraction of nitrosalicylic acids and vehicle exhaust was the major source of cresols, 2,6-dimethyl-4-nitrophenol, and 4-methyl-2,6-dinitrophenol. Further correlation analysis revealed positive correlations between nitrated phenols and corresponding phenolic precursors, indicating the important roles that phenolic precursors played in the secondary formation and abundance of nitrated phenols in the atmosphere.

Light absorption properties of brown carbon (BrC) in autumn and winter in Beijing: Composition, formation and contribution of nitrated aromatic compounds

Nitro-aromatic compounds (NACs) are an important part of brown carbon because of their strong light absorbing properties in the visible and near ultraviolet regions. In this study, the concentration and formation mechanism of eight NACs, as well as their relative contribution to the light absorption of methanol-extracted BrC, were determined from fine particle samples collected in Beijing using ultra-high performance liquid chromatography coupled with high resolution mass spectrometry and ultraviolet–visible light spectrophotometry. The average concentrations of the eight NACs were 20.27 ± 12.40 ng m−3 and 74.17 ± 50.66 ng m−3 in autumn and winter, respectively. Among the detected NACs, 4-nitrophenol (4NP) and 4-nitrocatechol (4NC) were the most abundant compounds. The correlation analysis showed that the NACs other than 2,4-dinitrophenol (2,4-DNP) are strongly intercorrelated. Significant correlations among nitro catechols (NCs), nitro salicylic acids (NSAs) and NO2 support their proposed origin from secondary formation in the presence of •OH + NO2 (daytime process) or •NO3+NO2 (nighttime process). Significant correlations among nitrate, sulfate and NSAs were also found, indicating that the nitration of salicylic acid by nitric acid and sulfuric acid was another possible mechanism formation of NSAs. The methanol-extracts in winter had much higher light absorption than those in autumn, which is in agreement with the trend of NACs but contrary to the trends of PM2.5, water soluble ions, organic carbon and elemental carbon. The average absorption Ångström exponents (AAEs) during autumn and winter were 4.62 and 5.55, respectively, suggesting that secondary transformation were the main sources of BrC. The light absorption of NACs accounted for 1.17% and 3.18% of the total methanol-soluble BrC light absorption at λ = 365 nm in autumn and winter, respectively, while their masses only accounted for 0.16% and 0.44% of the OC, respectively, demonstrating that although NACs are not the major contributor to BrC, but they are strong BrC chromophores, and their influence on solar radiation should be included in future modeling work.

Seasonal variation and sources of derivatized phenols in atmospheric fine particulate matter in North China Plain

Qualitative and quantitative analyses of derivatized phenols in Beijing and in Xinglong were performed from 2016 to 2017 using gas chromatography-mass spectrometry. The results showed substantially more severe pollution in Beijing. Of the 14 compounds detected, the total average concentration was 100 ng/m3 in Beijing, compared with 11.6 ng/m3 in Xinglong. More specifically, concentration of nitro-aromatic compounds (NACs) (81.9 ng/m3 in Beijing and 8.49 ng/m3 in Xinglong) was the highest, followed by aromatic acids (14.6 ng/m3 in Beijing and 2.42 ng/m3 in Xinglong) and aromatic aldehydes (3.62 ng/m3 in Beijing and 0.681 ng/m3 in Xinglong). In terms of seasonal variation, the highest concentrations were found for 4-nitrocatechol in winter in Beijing (79.1 ± 63.9 ng/m3) and 4-nitrophenol in winter in Xinglong (9.72 ± 8.94 ng/m3). The analysis also revealed diurnal variations across different seasons. Most compounds presented higher concentrations at night in winter because of the decreased boundary layer height and increased heating intensity. While some presented higher levels during the day, which attributed to the photo-oxidation process for summer and more biomass burning activities for autumn. Higher concentrations appeared in winter and autumn than in spring and summer, which resulted from more coal combustions and adverse meteorological conditions. The significant correlations among NACs indicated similar sources of pollution. Higher correlations presented within each subgroup than those between the subgroups. Good correlations between levoglucosan and nitrophenols, nitrocatechols, nitrosalicylic acids, with correlation coefficients (r) of 0.66, 0.69 and 0.69, respectively, indicating an important role of biomass burning among primary sources.

Characterization and possible sources of nitrated mono- and di-aromatic hydrocarbons containing hydroxyl and/or carboxyl functional groups in ambient particles in Nagoya, Japan

By understanding the sources and temporal variations of nitrated mono- and di-aromatic hydrocarbons (nitroaromatic hydrocarbons, NAHCs) in ambient particles, we can evaluate their impact on the climate, environment, and human health. This work quantifies (for the first time) a series of NAHCs (3,5-dinitrosalicylic acid, 2,4-dinitro-1-naphthol, 4-nitrophthalic acid, and 2-hydroxy-5-nitrobenzyl alcohol) in ambient particles collected from an urban site in Nagoya, Japan. The data were collected throughout the day/night cycles during the summer and fall of 2013. To evaluate the possible sources of the NAHCs, the observed NAHC concentrations were compared with organic tracers of anthropogenic secondary organic aerosols (ASOAs) and biomass burning. Nitrocatechols were mainly influenced by emissions from biomass burning during the fall, and by secondary formation from anthropogenic volatile organic compounds (AVOCs) during the summer. In both seasons, AVOC oxidation mainly produced nitrosalicylic acids, 4-nitro-1-naphthol, and 4-nitrophthalic acid, whereas biomass burning in the fall produced a significant amount of 4-nitro-1-naphthol. The concentrations of 3,5-dinitrosalicylic acid and 4-nitrophthalic acid were highly correlated with those of other ASOA tracers, and were strongly related to wind direction, oxidant concentration, and solar radiation. Therefore, these NAHCs are suggested as new tracers of ASOA. Meanwhile, although 2-hydroxy-5-nitrobenzyl alcohol was generated by AVOCs photo-oxidation during the day, its concentration remained high at night, indicating additional nighttime sources such as nitrate radical reactions. Our results suggest that different NAHCs could have different sources, large seasonal and day-to-night variations in their major ones.

Snow scavenging and phase partitioning of nitrated and oxygenated aromatic hydrocarbons in polluted and remote environments in central Europe and the European Arctic

Abstract. Nitrated and oxygenated polycyclic aromatic hydrocarbons (N/OPAHs) are emitted in combustion processes and formed in polluted air. Their environmental cycling through wet deposition has hardly been studied. Fresh snow samples at urban and rural sites in central Europe, as well as surface snow from a remote site in Svalbard, were analysed for 17 NPAHs, 8 OPAHs, and 11 nitrated mono-aromatic hydrocarbons (NMAHs), of which most N/OPAHs as well as nitrocatechols, nitrosalicylic acids, and 4-nitroguaiacol are studied for the first time in precipitation. In order to better understand the scavenging mechanisms, the particulate mass fractions (θ) at 273 K were predicted using a multi-phase gas-particle partitioning model based on polyparameter linear free energy relationships. ∑NPAH concentrations were 1.2–17.6 and 8.8–19.1 ng L−1 at urban and rural sites, whereas ∑OPAHs were 79.8–955.2 and 343.3–1757.4 ng L−1 at these sites, respectively. 9,10-anthraquinone was predominant in snow aqueous and particulate phases. NPAHs were only found in the particulate phase with 9-nitroanthracene being predominant followed by 2-nitrofluoranthene. Among NMAHs, 4-nitrophenol showed the highest abundance in both phases. The levels found for nitrophenols were in the same range or lower than those reported in the 1980s and 1990s. The lowest levels of ∑N/OPAHs and ∑NMAHs were found at the remote site (3.5 and 390.5 ng L−1, respectively). N/OPAHs preferentially partitioned in snow particulate phase in accordance with predicted θ, whereas NMAHs were predominant in the aqueous phase, regardless of θ. It is concluded that the phase distribution of non-polar N/OPAHs in snow is determined by their gas-particle partitioning prior to snow scavenging, whereas that for polar particulate phase substances, i.e. NMAHs, is determined by an interplay between gas-particle partitioning in the aerosol and dissolution during in- or below-cloud scavenging.

Observations of fine particulate nitrated phenols in four sites in northern China: concentrations, source apportionment, and secondary formation

Abstract. Filter samples of fine particulate matters were collected at four sites in northern China (urban, rural, and mountain) in summer and winter, and the contents of nine nitrated phenols were quantified in the laboratory with the use of ultra-high-performance liquid chromatography coupled with mass spectrometry. During the sampling periods, the concentrations of particulate nitrated phenols exhibited distinct temporal and spatial variation. On average, the total concentration of particulate nitrated phenols in urban Jinan in the wintertime reached 48.4 ng m−3, and those in the summertime were 9.8, 5.7, 5.9, and 2.5 ng m−3 in urban Jinan, rural Yucheng and Wangdu, and Mt. Tai, respectively. The elevated concentrations of nitrated phenols in wintertime and in urban areas demonstrate the apparent influences of anthropogenic sources. The positive matrix factorization receptor model was then applied to determine the origins of particulate nitrated phenols in northern China. The five major source factors were traffic, coal combustion, biomass burning, secondary formation, and aged coal combustion plume. Among them, coal combustion played a vital role, especially at the urban site in the wintertime, with a contribution of around 55 %. In the summertime, the observed nitrated phenols were highly influenced by aged coal combustion plumes at all of the sampling sites. Meanwhile, in remote areas, contributions from secondary formation were significant. Further correlation analysis indicates that nitrosalicylic acids were produced mostly from secondary formation that was dominated by NO2 nitration.

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 PM2.5 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 PM2.5 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.

Liquid chromatography tandem mass spectrometry method for characterization of monoaromatic nitro-compounds in atmospheric particulate matter

Nitrogen-containing organic compounds in the atmosphere have drawn attention owing to their impact on aerosol chemistry and physics and their potential adverse effects on the biosphere. Among them, nitrocatechols and their homologs have recently been associated with biomass burning. In the present study, nitrocatechols, nitrophenols, nitroguaiacols and nitrosalicylic acids (NSAs) were simultaneously quantified for the first time by using a new analytical method based on liquid chromatography/tandem mass spectrometry, which was systematically optimized and validated. Several analyte specific issues regarding the sample preparation and chromatographic analysis were addressed in order to ensure method sensitivity, precision, and accuracy. Sample matrix effects were thoroughly investigated in order to ensure method specificity. The method was found to be sensitive with limits of detection ranging from 0.1 to 1.0μgL−1, and with accuracy generally between 90 and 104%. The relative standard deviations for repeatability and intermediate precision were better than 4% and 9%, respectively. The method was applied to the analysis of winter and summer PM10 samples from the city of Ljubljana, Slovenia. Aerosol concentrations as high as 152 and 134ngm−3 were obtained for the major aerosol nitro-aromatics: 4-nitrocatechol (4NC) and methyl-nitrocatechols (MNCs), respectively. Up to 500-times higher concentrations of 4NC and MNCs were found in winter compared to summer aerosols. The correlation analysis for winter samples showed that 4NC, MNCs, and NSAs are strongly inter-correlated (R2=0.84–0.96). Significant correlations between these analytes and anhydrosugars support their proposed origin from biomass burning. The studied nitro-aromatics were found to constitute a non-negligible fraction (around 1%) of the organic carbon.

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

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. 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. 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. 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.

Molybdenum (VI) binded to humic and nitrohumic acid models in aqueous solutions. Salicylic, 3-nitrosalicylic, 5-nitrosalicylic and 3,5 dinitrosalicylic acids: part 2

In this work electrochemical and Ultraviolet-Visible studies were performed in solutions of salicylic acid models of humic and nitrohumic acids, a laboratory artifact, and molybdenum in order to determine the affinity of these models towards the metal ion. Molybdenum, which plays a very important role in the soil chemistry, and together with humic substances, impart fertility to soil and water and is a key element in the activity of nitrogenase. The obtained results showed that at least one complexed species is present at the pH range of 6.3 to 8.0, even for the less basic chosen models, the nitrosalicylic acids. Previous study showed that phthalic and nitrophthalic, also humic and nitrohumic acids model compounds, presented complexed species with molybdenum only till pH 6.5. The calculated formation constants showed that the substitution of the nitro group in the orto position was less favoured than in the para substitution, probably due to a steric hindrance in the former, which was clearly seen in the double substituted salicylic nitro derivative. The cyclic voltammetry as well as the Ultraviolet-Visible obtained spectra were able to show that the chemistry of molybdenum in aqueous solutions as the pH is increased is very complex, and the molybdate stops acting as an anion only after pH around 4, when it finally becomes a cation MoO2(2+) (M).

Model compounds of humic acid and oxovanadium cations. Potentiometric titration and EPR spectroscopy studies

The stability constants and the isotropic EPR parameters Ao (hyperfine splitting constant) and go (g value) were obtained by potentiometric titrations and EPR spectroscopy, respectively, of 85%v/v aqueous solutions of model compounds of humic acids - salicylic acid (SALA) - and both nitrohumic acids, a laboratory artifact - nitrosalicylic acids, 3-nitrosalicylic acid (3-NSA), 5-nitrosalicylic acid (5-NSA) and 3,5-dinitrosalicylic acid (3,5-DNSA) and oxovanadium cations. It was possible to record EPR spectra of those model compounds and the ion VO2+ (V(IV)), and the stability constants were obtained from a solution of VO3+ (V(V)), the values for the logarithms of the stability constants ranging from 12.77 ± 0.04 to 7.06 ± 0.05 for the species ML, and from 9.90 ±0.04 to 4.06 ± 0.05 for the species ML2 according to the decrease in the acidity of the carboxylic and the hydroxyl groups in the aromatic ring of the model compounds studied as the -NO2 substituents were added. Species distribution diagrams were also obtained for the equilibria studied. The EPR parameters showed that as the logarithm of the overall stability constants increase, go values also increase, while Ao values show a tendency to decrease.

Electrochemical and spectroscopic studies of the complexed species of models of nitrohumic acids derived from phthalic acid

The study of model compounds is necessary in order to obtain information about complex organic substances as in the case of humic substances (HS). These substances are potential organic fertilizers and have other important functions in soils, natural waters and organic sediments. The main chemical properties of the complexes formed from 3-nitrophthalic and 4-nitrophthalic acids and the metal ions Fe(III) and Zn(II) were studied using potentiometric titrations, ultraviolet-visible spectroscopy (UV-Vis) and cyclic voltammetry (CV). A trial potentiometric titration was done with a mixture of the models for nitrohumic acids and Cu(II). Equilibrium constants for the systems were calculated and UV-Vis and CV were employed to monitor the formation of the species. Comparative studies involving chelating centres of nitrosalicylic acids and nitrocatechols with Fe(III), Zn(II) and Cu(II) are presented. The initial studies involving the nitrohumic substances (NHS), a laboratory artifact of HS have been made and good evidence was found for the further use of NHS as a potential organic fertilizer as well as HS. In this present work one of the observed advantages of NHS over HS was that some aromatic nitro- centres can bind some metal ions at p[H] values of normal soils, near 7.0 to 7.5.