Underappreciated and Complex Role of Nitrous Acid in Aromatic Nitration under Mild Environmental Conditions: The Case of Activated Methoxyphenols

Toxicity of nitrophenolic pollutant 4-nitroguaiacol to terrestrial plants and comparison with its non-nitro analogue guaiacol (2-methoxyphenol)

Phenols, and especially their nitrated analogues, are ubiquitous pollutants and known carcinogens which have already been linked to forest decline. Although nitrophenols have been widely recognized as harmful to different aquatic and terrestrial organisms, we could not find any literature assessing their toxicity to terrestrial plants. Maize (monocot) and sunflower (dicot) were exposed to phenolic pollutants, guaiacol (GUA) and 4-nitroguaiacol (4NG), through a hydroponics system under controlled conditions in a growth chamber. Their acute physiological response was studied during a two-week root exposure to different concentrations of xenobiotics (0.1, 1.0, and 10 mM). The exposure visibly affected plant growth and the effect increased with increasing xenobiotic concentration. In general, 4NG affected plants more than GUA. Moreover, sunflower exhibited an adaptive response, especially to low and moderate GUA concentrations. The integrity of both plant species deteriorated during the exposure: biomass and photochemical pigment content were significantly reduced, which reflected in the poorer photochemical efficiency of photosystem II. Our results imply that 4NG is taken up by sunflower plants, where it could enter a lignin biosynthesis pathway.

Kinetics and product identification of water-dissolved nitroguaiacol photolysis under artificial sunlight

Nitroguaiacols are typical constituents of biomass-burning emissions, including absorbing aerosols which contribute to climate change. Although they are also harmful to humans and plants, their atmospheric fate and lifetimes are still very speculative. Therefore, in this work, the photolysis kinetics of aqueous-phase 4-nitroguaiacol (4NG) and 5-nitroguaiacol (5NG), and the resulting photo-formed products were investigated under artificial sunlight, observing also the effect of sunlight on the absorption properties of the solutions. We found the photolysis of 5NG slower than that of 4NG, whereas the absorbance in the visible range prevailed in the 5NG solutions at the end of experiments. Although we identified dinitroguaiacol as one of the 4NG photolysis products, which increased light absorption of 4NG-containing solutions, considerably more chromophores formed in the 5NG photolyzed solutions, implying its stronger potential for secondary BrC formation in the atmosphere. In general, denitration, carbon loss, hydroxylation, nitration, and carbon gain were characteristic of 4NG phototransformation, while carbon loss, hydroxylation, and carbon gain were observed in the case of 5NG. The photolysis kinetics was found of the first order at low precursor concentrations (<0.45 mM), resulting in their lifetimes in the order of days (125 and 167 h illumination for 4NG and 5NG, respectively), which suggests long-range transport of the investigated compounds in the atmosphere and proposes their use as biomass-burning aerosol tracer compounds.

Mechanism of nitro-byproducts formation during persulfate-based electrokinetic in situ oxidation for remediation of anthracene contaminated soil

Persulfate-based electrokinetic (EK) chemical oxidation appears to be a novel and viable strategy for the in situ remediation of polycyclic aromatic hydrocarbons (PAHs) polluted soil; however, the possible toxic byproducts of PAHs have been overlooked. In this study, we systematically investigated the formation mechanism of the nitro-byproducts of anthracene (ANT) during the EK process. Electrochemical experiments revealed that NH₄⁺ and NO₂^- originating from nitrate electrolyte or soil substrates were oxidized to NO₂^• and NO^• in the presence of SO₄^•-. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS/MS) analysis with ¹⁵N labeling revealed the formation of nitro-byproducts (14 kinds), including 1-hydroxy-4-nitro-anthraquinone and its similar derivatives, 4-nitrophenol, and 2,4-dinitrophenol. The nitration pathways of ANT have been proposed and described, mainly including the formation of hydroxyl-anthraquinone-oxygen and phenoxy radicals and the subsequent addition of NO₂^• and NO^•. ANT-based formation of nitro-byproducts during EK, which is usually underestimated, should be further investigated due to their enhanced acute toxicity, mutagenic effects, and potential threat to the ecosystem.

Phototransformation of Vanillin in Artificial Snow by Direct Photolysis and Mediated by Nitrite

The photodegradation of vanillin, as a proxy of methoxyphenols emitted by biomass burning, was investigated in artificial snow at 243 K and in liquid water at room temperature. Nitrite (NO₂^-) was used as a photosensitizer of reactive oxygen and nitrogen species under UVA light, because of its key photochemical role in snowpacks and atmospheric ice/waters. In snow and in the absence of NO₂^-, slow direct photolysis of vanillin was observed due to back-reactions taking place in the quasi-liquid layer at the ice-grain surface. The addition of NO₂^- made the photodegradation of vanillin faster, because of the important contribution of photoproduced reactive nitrogen species in vanillin phototransformation. These species triggered both nitration and oligomerization of vanillin in irradiated snow, as the identified vanillin by-products showed. Conversely, in liquid water, direct photolysis was the main photodegradation pathway of vanillin, even in the presence of NO₂^-, which had negligible effects on vanillin photodegradation. The results outline the different role of iced and liquid water in the photochemical fate of vanillin in different environmental compartments.

Catalytic behavior of nitrous acid for acetaminophen transformation during the freezing process

This study demonstrates the transformation of acetaminophen by reactive nitrous acid in a frozen solution and its abnormal stoichiometry. The chemical reaction between acetaminophen and nitrous acid (AAP/NO₂^- system) was negligible in the aqueous solution; however, the reaction rapidly progressed if the solution started to freeze. The ultrahigh performance liquid chromatography-electrospray ionization tandem mass spectrometry measurements showed that polymerized acetaminophen and nitrated acetaminophen were formed in the proceeding reaction. Electron paramagnetic resonance spectroscopy measurements showed that nitrous acid oxidized acetaminophen via a one-electron transfer reaction producing acetaminophen-derived radical species, which is the cause of acetaminophen polymerization. We demonstrated that a relatively smaller dose of nitrite than acetaminophen caused significant acetaminophen degradation in the frozen AAP/NO₂^- system and revealed that the dissolved oxygen content notably affected acetaminophen degradation. We showed that the reaction occurs in a natural Arctic lake matrix (nitrite and acetaminophen spiked). Considering that the freezing phenomenon is common in the natural environment, our research provides a possible scenario for the freezing chemistry of nitrite and pharmaceuticals in environmental chemistry.

Yields and molecular composition of gas phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitroaromatic compounds

Recently, volatile chemical products (VCPs) have been increasingly recognized as important precursors for secondary organic aerosol (SOA) and ozone in urban areas. However, their atmospheric chemistry, physical transformation, and their impact on climate, environment and human health remain poorly understood. Here, the yields and chemical composition at the molecular level of gas and particle phase products originating from the photooxidation of one of these VCPs, benzyl alcohol (BnOH), is reported. The SOA was generated in the presence of seed aerosol from nebulized ammonium sulfate solution in a 14.5 m3 smog chamber operated in flow mode. More than 50 organic compounds containing nitrogen and/or up to seven oxygen atoms were identified by mass spectrometry. While a detailed non-targeted analysis has been made, our primary focus has been to examine highly oxygenated and nitro-aromatic compounds. The major components include ring-opening products with high oxygen to carbon ratio (e.g., malic acid, tartaric acids, arabic acid, trihydroxy-oxo-pentanoic acids, and pentaric acid), and ring-retaining products (e.g., benzaldehyde, benzoic acid, catechol, 3-nitrobenzyl alcohol, 4-nitrocatechol, 2-hydroxy-5-nitrobenzyl alcohol, 2-nitrophloroglucinol, 3,4-dihydroxy-5-nitrobenzyl alcohol). The presence of some of these products in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. These oxygenated oxidation products made dominant contributions to the SOA particle composition in both low and high NOx systems. Yields, organic mass to organic carbon ratio, and proposed reaction schemes for selected compounds are provided. The aerosol yield was 5.2% for BnOH/H2O2 at SOA concentration of 52.9 µg m-3 and ranged between 1.7-8.1 % for BnOH/NOx at SOA concentration of 40.0-119.5 µg m-3.

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.

pH affects the aqueous-phase nitrate-mediated photooxidation of phenolic compounds: implications for brown carbon formation and evolution

Brown carbon (BrC) is known to have important impacts on atmospheric chemistry and climate. Phenolic compounds are a prominent class of BrC precursors that are emitted in large quantities from biomass burning and fossil fuel combustion. Inorganic nitrate is a ubiquitous component of atmospheric aqueous phases such as cloudwater, fog, and aqueous aerosols. The photolysis of inorganic nitrate can lead to BrC formation via the photonitration of phenolic compounds in the aqueous phase. However, the acidity of the atmospheric aqueous phase adds complexity to these photonitration processes and needs to be considered when investigating BrC formation from the nitrate-mediated photooxidation of phenolic compounds. In this study, we investigated the influence of pH on the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol, catechol, 5-nitroguaiacol, and 4-nitrocatechol initiated by inorganic nitrate photolysis. The reaction rates, BrC composition and quantities were found to depend on the aqueous phase pH. Guaiacol, catechol, and 5-nitroguaiacol reacted substantially faster at lower pH. In contrast, 4-nitrocatechol reacted at slower rates at lower pH. For all four phenolic compounds, the initial stages of photooxidation resulted in an increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range due to the formation of light absorbing products formed via the addition of nitro and/or hydroxyl groups to the phenolic compound. Greater photo-enhancement was observed at lower pH during the nitrate-mediated photooxidation of guaiacol and catechol. In contrast, greater photo-enhancement was observed at higher pH during the nitrate-mediated photooxidation of 5-nitroguaiacol and 4-nitrocatechol. This indicated that the effect that the aqueous phase pH has on the composition and yields of BrC formed is not universal, and will depend on the initial phenolic compound. These results provide new insights into how the atmospheric aqueous phase acidity influences the reactivities of different phenolic compounds and BrC formation/evolution during photooxidation initiated by inorganic nitrate photolysis, which will have significant implications for how the atmospheric fates of phenolic compounds and BrC formation/evolution are modeled for areas with high levels of inorganic nitrate.

Is a thin mechanism appropriate for aromatic nitration?

The mechanism of toluene nitration by NO₂BF₄ in dichloromethane solution is investigated by performing advanced ab initio MD simulations of the reaction trajectories, including at full quantum mechanical level the effects of both the solvent and of the counterion. The time evolution of the encounter complex, as well as that of the associated electronic structure, for different trajectories reveals that a single electron transfer step fastly occurs after reactants are accommodated in a common solvation shell, always preceding the formation of the σ-complex. The present results strongly suggest that the regioselectivity of the reaction is spin-density driven and that a thin mechanism, one based on reaction intermediates and transition states, can be appropriate to describe aromatic nitration.

Seasonal variability of nitroaromatic compounds in ambient aerosols: Mass size distribution, possible sources and contribution to water-soluble brown carbon light absorption

Nitroaromatic compounds (NACs) as important constituents of atmospheric humic-like substances (HULIS) and brown carbon (BrC) affect the Earth's climate and pose a serious environmental hazard. We investigated seasonal size-segregated NACs in aerosol samples from the urban background environment in Ljubljana, Slovenia. Total concentrations of twenty NACs in PM_15.6 were on average from 0.51 ng m^-3 (summer) to 109 ng m^-3 (winter), and contributed the most to submicron aerosols (more than 74%). Besides 4-nitrocatechol (4NC) as the prevailing species, methylnitrocatechols (MNCs) and nitrophenols (NPs), we reported on some very rarely mentioned, but also on five novel NACs (i.e., 3H4NBA: 3-hydroxy-4-nitrobenzoic acid, 3MeO4NP: 3-methoxy-4-nitrophenol, 4Et5NC: 4-ethyl-5-nitrocatechol, 3Et5NC: 3-ethyl-5-nitrocatechol and 3MeO5NC: 3-methoxy-5-nitrocatechol). Concentrations of 3MeO5NC, 4Et5NC and 3Et5NC were enhanced during cold seasons, contributing up to 11% to total NAC in winter. In cold season, NAC size distributions were characterized with the peaks in the broader size range of 0.305-1.01 μm (accumulation mode), with 4NC and alkyl-nitrocatechols (∑(M/Et)NC) as the most abundant, followed by 4-nitrosyringol, nitrophenols and nitroguaiacols. In spring, a pronounced peak of ∑(M/Et)NC was observed in the accumulation mode (0.305-0.56 μm) as well as in the coarse one. A strong correlation of all NACs with ∑(M/Et)NC and levoglucosan indicates that primary emissions of wood burning were the most important source of NACs, but their secondary formation (e.g., aqueous-phase at higher ambient RH) in cold season could also be a significant one. In warmer season, NACs may be mostly derived from traffic-related aromatic VOCs. The contribution of NACs to the light absorption of the aqueous extracts was up to 10-times higher (contribution to Abs₃₆₅ up to 31%) than their mass contributions to WSOC (up to 3%) of corresponding size-segregated aerosols, confirming that most of the identified NACs are strong BrC chromophores.

2-Methoxyethyl Nitrite as a Reagent for Chemoselective On-Water Nitration

An on-water approach has been developed that allows a nitration of tyrosines and phenols under mild conditions. We envisioned that the assembly of tyrosine/tyrosyl radical couples with interfacial water molecules would realize a biomimetic stacking hydrogen atom transfer (HAT) transition state to facilitate the electron-transfer process. The optimal organic nitrite, 2-methoxyethyl nitrite, resulted in rapid coupling of the tyrosyl radicals with •NO 2 at the oil-water interface to afford the nitrated phenols. Many characteristics found in our on-water strategy are distinct from other complementary systems that include radical nitration. These enticing roles of water in the reaction process introduce new avenues to explore in the design of synthetic organic chemistry systems.

Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice

We investigated benzoic acid oxidation via the reaction of hydrogen peroxide (H₂O₂) and nitrite (NO₂^-). The oxidation of benzoic acid by reactive nitrous acid (HONO) was negligible, and the reactivity of the H₂O₂/NO₂^- system decreased with a decrease in temperature under aqueous conditions. However, freezing markedly accelerated the chemical reaction. Based on Raman microscope measurements, concentrated species were confirmed in certain regions of the ice. We proposed that the change in nitrite speciation (accordingly, a decrease in the pH below pK_a), derived from the freezing concentration effect, was the reason for the accelerated reactions. The oxidation characteristics of the system were monitored under varying conditions, such as initial pH, dosage ratio, benzoic acid concentration, and reaction with various benzene derivatives. The ultrahigh-performance liquid chromatography/electrospray ionization/mass spectrometry (UHPLC/ESI/MS) measurement showed that peroxynitrous acid (HOONO)-mediated oxidation generated hydroxylated and nitrated byproducts. Additionally, decarboxylated products were detected, indicating direct electron transfer from the organic compounds to HOONO. As freezing is a global phenomenon, and H₂O₂ and NO₂^- are ubiquitous in the environment, the transformation of aromatic compounds with H₂O₂/NO₂^- in cold environments must be considered in environmental chemistry.

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics

A prominent source of hydroxyl radicals (^•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO₂^-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as ^•OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO₂^-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO₂^-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO₂ for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Is Aromatic Nitration Spin Density Driven?

The mechanism of aromatic nitration is critically reviewed with particular emphasis on the paradox of the high positional selectivity of substitution in spite of low substrate selectivity. Early quantum chemical computations in the gas phase have suggested that the retention of positional selectivity at encounter-limited rates could be ascribed to the formation of a radical pair via an electron transfer step occurring before the formation of the Wheland intermediate, but calculations which account for the effects of solvent polarization and the presence of counterion do not support that point of view. Here we report a brief survey of the available experimental and theoretical data, adding a few more computations for better clarifying the role of electron transfer for regioselectivity.

Pollution levels and deposition processes of airborne organic pollutants over the central Adriatic area: Temporal variabilities and source identification

First data on polycyclic aromatic hydrocarbons (PAHs) and nitro-aromatic compounds (NACs) in aerosols as well as of PAHs, polychlorinated biphenyls (PCBs) and NACs in bulk and wet atmospheric deposition samples were simultaneously obtained during 6-month-long field campaign at the costal central Adriatic area. Special attention was given to open-fire biomass burning episodes as extreme events common for the overall Mediterranean coastal area in order to gain a better understanding of the atmospheric variabilities and potential sources of trace organic pollutants in coastal environments. Diesel and gasoline combustion related to land and maritime traffic as well as occasional open-fire episodes (forest fires) were found to be the dominant pollution sources of PAHs in PM₁₀ particles. NACs were determined almost exclusively in samples affected by biomass burning episodes. Open-fire episodes had a strong contribution to the overall NACs atmospheric deposition fluxes. Several chlorinated congeners of PCBs were predominantly contributed in deposition samples.

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 PM_0.5 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 K_OW > 4.5).

Trace level nitrite sensitized photolysis of the antimicrobial agents parachlormetaxylenol and chlorophene in water

Nitrite (NO₂^-)-sensitized photolysis plays an important role in the attenuation of effluent-derived trace organic contaminants (e.g., anilines, phenolic compounds, etc.) in surface waters. However, the kinetics, mechanisms, and influencing factors of photolysis of many emerging contaminants sensitized by NO₂^- still remain largely unknown. Herein, we report that NO₂^--sensitized photolysis of the antimicrobial agents parachlormetaxylenol (PCMX) and chlorophene (CP) in aqueous solution under ultraviolet 365 nm (UV₃₆₅) radiation. A nonlinear increase in photolysis rate constants of PCMX and CP was observed with increasing NO₂^- concentration. Radical quenching studies and kinetic modeling revealed that hydroxyl radical (HO•) and nitrogen dioxide radicals (NO₂^•) contributed dominantly to the removal of PCMX and CP. Solid phase extraction (SPE) combined with high resolution-mass spectrometry (HR-MS) analysis identified a series of intermediate products including hydroxylated, nitrated, nitrosated, and dimerized derivatives. Experiments with isotopically labelled nitrite (¹⁵NO₂^-) showed that the nitro- and nitroso-substituents of intermediate products were derived from the nitrite nitrogen. Based on the identified products and theoretical computations, the mechanisms and pathways of NO₂^--sensitized photolysis of PCMX and CP are elucidated. Deoxygenation partially inhibited the formation of 4-chloro-3,5-dimethyl-2-nitrophenol (nitro-PCMX) while the presence of HO• scavenger such as isopropanol (i-PrOH) suppressed the further transformation of nitro-PCMX. The presence of Mississippi River natural organic matter (MRNOM) inhibited the removal of PCMX and CP, likely due to light screening and radical quenching. However, appreciable degradation of PCMX and CP was still observed in wastewater and wetland water matrices. Results of this study shed some light on the transformation and fate of PCMX and CP in NO₂^--rich wastewater effluents or effluent-impacted surface waters under solar radiation.

Guaiacol Nitration in a Simulated Atmospheric Aerosol with an Emphasis on Atmospheric Nitrophenol Formation Mechanisms

Atmospheric nitrophenols are pollutants of concern due to their toxicity and light-absorption characteristics and their low reactivity resulting in relatively long residence times in the environment. We investigate multiphase nitrophenol formation from guaiacol in a simulated atmospheric aerosol and support observations with the corresponding chemical mechanisms. The maximal secondary organic aerosol (SOA) yield (42%) is obtained under illumination at 80% relative humidity. Among the identified nitrophenols, 4-nitrocatechol (3.6% yield) is the prevailing species in the particulate phase. The results point to the role of water in catechol and further 4-nitrocatechol formation from guaiacol. In addition, a new pathway of dark nitrophenol formation is suggested, which prevailed in dry air and roughly yielded 1% nitroguaiacols. Furthermore, the proposed mechanism possibly leads to oligomer formation via a phenoxy radical formation by oxidation with HONO.

Secondary Formation of Aromatic Nitroderivatives of Environmental Concern: Photonitration Processes Triggered by the Photolysis of Nitrate and Nitrite Ions in Aqueous Solution

Aromatic nitroderivatives are compounds of considerable environmental concern, because some of them are phytotoxic (especially the nitrophenols, and particularly 2,4-dinitrophenol), others are mutagenic and potentially carcinogenic (e.g., the nitroderivatives of polycyclic aromatic hydrocarbons, such as 1-nitropyrene), and all of them absorb sunlight as components of the brown carbon. The latter has the potential to affect the climatic feedback of atmospheric aerosols. Most nitroderivatives are secondarily formed in the environment and, among their possible formation processes, photonitration upon irradiation of nitrate or nitrite is an important pathway that has periodically gained considerable attention. However, photonitration triggered by nitrate and nitrite is a very complex process, because the two ionic species under irradiation produce a wide range of nitrating agents (such as •NO2, HNO2, HOONO, and H2OONO+), which are affected by pH and the presence of organic compounds and, in turn, deeply affect the nitration of aromatic precursors. Moreover, aromatic substrates can highly differ in their reactivity towards the various photogenerated species, thereby providing different behaviours towards photonitration. Despite the high complexity, it is possible to rationalise the different photonitration pathways in a coherent framework. In this context, this review paper has the goal of providing the reader with a guide on what to expect from the photonitration process under different conditions, how to study it, and how to determine which pathway(s) are prevailing in the formation of the observed nitroderivatives.

Gas-phase treatment of polyethylene terephthalate waste in nitrating atmosphere for recycling purposes

The paper describes a new method of waste polyethylene terephthalate (PET) recycling based on gas-phase treatment of the material in nitrating atmosphere. It was found that PET samples kept in the nitrating atmosphere (obtained by vaporizing 12 mol L^-1 HNO₃) at a temperature of 403-463 K (130-190 °C) for 5-24 h dissolve in 0.5 mol L^-1 NaOH, in contrast to the untreated PET samples. Ozonation of the obtained alkaline solutions leads to a complete decomposition of the organic compounds present in the solutions. The resulting PET decomposition degree is higher than 99.9 % irrespective of the plastic color. In addition, the possibility of terephthalic acid recovery from the alkaline solutions of the conversion products was shown.

Aqueous-Phase Brown Carbon Formation from Aromatic Precursors under Sunlight Conditions

At present, there are still numerous unresolved questions concerning the mechanisms of light-absorbing organic aerosol (brown carbon, BrC) formation in the atmosphere. Moreover, there is growing evidence that chemical processes in the atmospheric aqueous phase can be important. In this work, we investigate the aqueous-phase formation of BrC from 3-methylcatechol (3MC) under simulated sunlight conditions. The influence of different HNO2/NO2− concentrations on the kinetics of 3MC degradation and BrC formation was investigated. Under illumination, the degradation of 3MC is faster (k2nd(global) = 0.075 M−1·s−1) in comparison to its degradation in the dark under the same solution conditions (k2nd = 0.032 M−1·s−1). On the other hand, the yield of the main two products of the dark reaction (3-methyl-5-nitrocatechol, 3M5NC, and 3-methyl-4-nitrocatechol, 3M4NC) is low, suggesting different degradation pathways of 3MC in the sunlight. Besides the known primary reaction products with distinct absorption at 350 nm, second-generation products responsible for the absorption above 400 nm (e.g., hydroxy-3-methyl-5-nitrocatechol, 3M5NC-OH, and the oxidative cleavage products of 3M4NC) were also confirmed in the reaction mixture. The characteristic mass absorption coefficient (MAC) values were found to increase with the increase of NO2−/3MC concentration ratio (at the concentration ratio of 50, MAC is greater than 4 m2·g−1 at 350 nm) and decrease with the increasing wavelength, which is characteristic for BrC. Yet, in the dark, roughly 50% more BrC is produced at comparable solution conditions (in terms of MAC values). Our findings reveal that the aqueous-phase processing of 3MC in the presence of HNO2/NO2−, both under the sunlight and in the dark, may significantly contribute to secondary organic aerosol (SOA) light absorption.

OH radicals reactivity towards phenol-related pollutants in water: temperature dependence of the rate constants and novel insights into the [OH-phenol]˙ adduct formation

Substituted phenols are known to readily react with the hydroxyl radical (OH˙), which is the most powerful atmospheric oxidant and is also most often used in advanced oxidation processes (AOP) for wastewater treatment. We report temperature-dependent (278.15-318.15 K) second order kinetic rate constants for the aqueous-phase reactions of OH˙ with phenol and four substituted phenols: catechol, phloroglucinol, pyrogallol and 3-methylcatechol, with the last two measured for the first time. The constructed Hammett plots for mono- and di-substituted phenols have the potential to be further applied for predicting the reaction rate constants of other substituted phenols at 298.15 K. This will significantly facilitate the optimization of AOP and improve the predictive capabilities of atmospheric multiphase models in the future. Moreover, an advancement in the understanding of the underlying mechanism, i.e. OH˙ addition to the aromatic ring is made by theoretical calculations at the M06-2X level. We demonstrate that the position of substituents on the aromatic ring is important for the [OH-phenol]˙ adduct formation, which is supported by the experiment and theoretical calculations. Adjacent and nonadjacent electron donor/acceptor substituents differently impact the interplay between the activation energy and entropy. We also show that explicit solvation has to be accounted for in theoretical models in order to explicitly describe the formation of the transition state.

Nitrite-Mediated Photooxidation of Vanillin in the Atmospheric Aqueous Phase

Nitrite (NO₂^-) and its conjugate acid, nitrous acid (HNO₂), have long been recognized as a ubiquitous atmospheric pollutant as well as an important photochemical source of hydroxyl radicals (·OH) and reactive nitrogen species (·NO, ·NO₂, ·N₂O₃, etc.) in both the gas phase and aqueous phase. Although NO₂^-/HNO₂ plays an important role in atmospheric chemistry, our understanding on its role in the chemical evolution of organic components in atmospheric waters is rather incomplete and is still in dispute. In this study, the nitrite-mediated photooxidation of vanillin (VL), a phenolic compound abundant in biomass burning emissions, was investigated under pH conditions relevant for atmospheric waters. The influence of solution pH, dissolved oxygen, and ·OH scavengers on the nitrite-mediated photooxidation of VL was discussed in detail. Our study reveals that the molecular composition of the products is dependent on the molar ratio of NO₂^-/VL in the solution and that nitrophenols are the major reaction products. We also found that the light absorbance of the oxidative products increases with increasing pH in the visible region, which can be attributed to the deprotonation of the nitrophenols formed. These results contribute to a better understanding of methoxyphenol photooxidation mediated by nitrite as a source of toxic nitrophenols and climatically important brown carbon in atmospheric waters.

Electrochemistry as a Tool for Studies of Complex Reaction Mechanisms: The Case of the Atmospheric Aqueous-Phase Aging of Catechols

The ultimate goal in the understanding of complex chemical processes is a complete description of the underlying reaction mechanism. In the present study and for this purpose, a novel experimental platform is introduced that builds upon electrochemistry capable of generating reactive intermediate species at the electrode surface. The atmospherically relevant nitration of catechols is taken as a case example. First, we confirm the recently proposed nitration mechanism, advancing the understanding of atmospheric brown carbon formation in the dark. We are able to selectively quantify aromatic isomers, which is beyond the limits of conventional electroanalysis. Second, we identify a new pathway of nitrocatechol hydroxylation, which proceeds simply by oxidation and the addition of water. This pathway can be environmentally significant in the dark aqueous-phase formation of secondary organic aerosols. Third, the developed methodology is capable of selectively detecting a wide range of nitroaromatics; a possible application in environmental monitoring is proposed.

Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds

Methoxyphenols, which are emitted through biomass burning, are an important species in atmospheric chemistry. In the present study, temperature-dependent aqueous-phase OH radical reactions of six methoxyphenols and two related phenols have been investigated through laser flash photolysis and the density functional theory. The rate constants obtained were in a range of (1.1-1.9) × 10¹⁰ L mol^-1 s^-1 with k(3-MC) > k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP) > k(2-EP) ≈ k(2-MP). We derived the parameters of these reactions from the obtained T-dependent rate constants and found a mean Arrhenius activation energy of 16.9 kJ mol^-1. The diffusion rate constants were calculated for each case and compared to the measured ones. Generally, the rate constants are found to be close to fully diffusion-controlled (k_diff = (1.4-1.5) × 10¹⁰ L mol^-1 s^-1 for all reactions). A structure-function relationship was established through the measurement result, which could be used for predicting unknown rate constants of other phenolic compounds. All of these findings are expected to enhance the predictive capabilities of models, such as the chemical aqueous-phase radical mechanism.