Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches

Investigating the gas-phase reaction mechanism of catechol with ozone: Product analysis and insights

Volatile aromatic compounds (VOCs) are ubiquitous in the environment, they can be emitted from biogenic and anthropogenic sources. They can contribute to the formation of many products leading to the formation of secondary organic aerosols (SOA). The products of the gas phase reaction of 1,2-benzenediol (catechol) with ozone were studied in a simulation chamber at atmospheric pressure and 294 ± 2 K in presence of different levels of relative humidity (0-60%). The gas phase products were monitored continuously by a PTR-ToF-MS for 2 h, whereas filters samples were collected directly from the reaction chamber and analyzed by thermo-desorption gas chromatography; TD-GC-MS. This study shows the different potential chemical pathways that catechol could follow to form a variety of products under dry, low and high humidity conditions. The molecular mass 98 was found to be distinctive and appears in the gas phase when humidity in the reaction chamber is between 20 and 60%. Other new masses (m/z) such as 176, 154, 116, 68, 72, 80, 96, 108, and 124 were also detected under different experimental conditions. Furthermore, the catechol concentration has been monitored continuously by a PTR-ToF-MS from low to high humidity conditions (RH = 7.5-78.8%). The purpose of the latter is to suggest that the formation of catechol-H2O clusters occurs in the gas phase of the reaction chamber causing a decrease in catechol reactivity towards other gases and subsequently a decrease in the rate constant.

Secondary organic aerosol formation from atmospheric reactions of anisole and associated health effects

Anisole (methoxybenzene) represents an important marker compound of lignin pyrolysis and a starting material for many chemical products. In this study, secondary organic aerosols (SOA) formed by anisole via various atmospheric processes, including homogeneous photooxidation with varying levels of OH• and NO_x and subsequent heterogeneous NO₃• dark reactions, were investigated. The yields of anisole SOA, particle-bound organoperoxides, particle-induced oxidative potential (OP), and cytotoxicity were characterized in view of the atmospheric fate of the anisole precursor. Anisole SOA yields ranged between 0.12 and 0.35, depending on the reaction pathways and aging degrees. Chemical analysis of the SOA suggests that cleavage of the benzene ring is the main reaction channel in the photooxidation of anisole to produce low-volatility, highly oxygenated small molecules. Fresh anisole SOA from OH• photooxidation are more light-absorbing and have higher OP and organoperoxide content. The high correlation between SOA OP and organoperoxide content decreases exponentially with the degree of OH• aging. However, the contribution of organoperoxides to OP is minor (<4%), suggesting that other, non-peroxide oxidizers play a central role in anisole SOA OP. The particle-induced OP and particulate organoperoxides yield both reach a maximum value after ∼2 days' of photooxidation, implicating the potential long impact of anisole during atmospheric transport. NO_x-involved photooxidation and nighttime NO₃• reactions facilitate organic nitrate formation and enhance particle light absorption. High NO_x levels suppress anisole SOA formation and organoperoxides yield in photooxidation, with decreased aerosol OP and cellular oxidative stress. In contrast, nighttime aging significantly increases the SOA toxicity and reactive oxygen species (ROS) generation in lung cells. These dynamic properties and the toxicity of anisole SOA advocate consideration of the complicated and consecutive aging processes in depicting the fate of VOCs and assessing the related effects in the atmosphere.

OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4-chlorobenzotrifluoride (p-ClC6H4CF3)

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride (p-chlorobenzotrifluoride, p-ClC₆H₄CF₃, PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k₁, for the OH + PCBTF reaction were measured over the temperature ranges 275-340 and 385-940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k₁(298-353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k₁(T) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k₁(275-385 K) = (1.50 ± 0.15) × 10^-14 exp((705 ± 30)/T) cm³ molecule^-1 s^-1, where k₁(298 K) = (1.63 ± 0.03) × 10^-13 cm³ molecule^-1 s^-1 and a positive temperature dependence at elevated temperatures given by k₁(470-950 K) = (5.42 ± 0.40) × 10^-12 exp(-(2507 ± 45) /T) cm³ molecule^-1 s^-1. The present k₁(298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k₂, was also measured in THALAMOS using a relative rate technique that yielded k₂(298 K) = (7.8 ± 2) × 10^-16 cm³ molecule^-1 s^-1. As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3-4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH-PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available.

Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy

We report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at ∼662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 μm (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO₃-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10^-11 to 10^-16 cm³ molecule^-1 s^-1). Rate constants of (6.5 ± 0.5) × 10^-15, (7.0 ± 0.4) × 10^-13, and (5.8 ± 0.5) × 10^-16 cm³ molecule^-1 s^-1 for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration-time profiles of NO₃ and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 ± 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 ± 0.30) × 10^-11 cm³ molecule^-1 s^-1 for the oxidation of 2-methoxyphenol by NO₃ radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from ∼20% to ∼12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants.

Atmospheric Reactivity of Methoxyphenols: A Review

Methoxyphenols emitted from lignin pyrolysis are widely used as potential tracers for biomass burning, especially for wood burning. In the past ten years, their atmospheric reactivity has attracted increasing attention from the academic community. Thus, this work provides an extensive review of the atmospheric reactivity of methoxyphenols, including their gas-phase, particle-phase, and aqueous-phase reactions, as well as secondary organic aerosol (SOA) formation. Emphasis was placed on kinetics, mechanisms, and SOA formation. The reactions of methoxyphenols with OH and NO₃ radicals were the predominant degradation pathways, which also had significant SOA formation potentials. The reaction mechanism of methoxyphenols with O₃ is the cycloaddition of O₃ to the benzene ring or unsaturated C═C bond, while H-abstraction and radical adduct formation are the main degradation channels of methoxyphenols by OH and NO₃ radicals. Based on the published studies, knowledge gaps were pointed out. Future studies including experimental simulations and theoretical calculations of other representative kinds of methoxyphenols should be systematically carried out under complex pollution conditions. In addition, the ecotoxicity of their degradation products and their contribution to SOA formation from the atmospheric aging of biomass-burning plumes should be seriously assessed.

Theoretical investigation on the mechanisms and kinetics of OH/NO3-initiated atmospheric oxidation of vanillin and vanillic acid

Vanillin and vanillic acid are two kinds of lignin pyrolysis products that are generated by biomass combustion. The gas-phase oxidation mechanisms of vanillin and vanillic acid initiated by OH/NO₃ radicals were investigated by using density functional theory (DFT) at M06-2X/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) level. The initial reactions of vanillin and vanillic acid with OH/NO₃ radicals can be divided into two patterns: OH/NO₃ addition and H-atom abstraction. For vanillin reacted with OH radical, the OH addition mainly occurs at C2-position to produce highly chemically activated intermediate (IM2). The oxidation products 3,4-dihydroxy benzaldehyde, malealdehyde, methyl hydrogen oxalate, methylenemalonaldehyde, carbonyl and carbonyl compounds are formed by the subsequent reactions of IM2. H-atom abstracting from aldehyde group occurs more easily than from the other positions. In addition, vanillin reacting with NO₃ radicals principally proceeds via NO₃-addition at C1 sites and H-atom abstracting from OH group (C1) to generate HNO₃. The primary reaction mechanisms of vanillic acid with OH/NO₃ radicals were similar to vanillin. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was performed to calculate the rate constants of the significant elementary reactions. The total rate constants for OH-initiated oxidation of vanillin and vanillic acid are 5.72 × 10^-12 and 5.40 × 10^-12 cm³ molecule^-1 s^-1 at 298 K and 1 atm. The atmospheric lifetimes were predicted to be 48.56 h and 51.44 h, respectively. As a supplement, the kinetic calculations of NO₃ radicals with two reactants were also discussed. This work investigates the atmospheric oxidation processes of vanillin and vanillic acid, and hopes to provide useful information for further experimental research.

Reactivity of aromatic contaminants towards nitrate radical in tropospheric gas and aqueous phase

Aromatic compounds (ACs) give a substantial contribution to the anthropogenic emissions of volatile organic compounds. Nitrate radicals (NO₃) are significant oxidants in the lower troposphere during nighttime, with concentrations of (2-20) × 10⁸ molecules cm^-3. In this study, the tropospheric gas and liquid phase reactions of ACs with nitrate radical are investigated using theoretical computational methods, which can give a deep insight into the reaction mechanisms and kinetics. Results show that the reactivity of ACs with nitrate radicals decreases as the electron donating characteristics of the functional group on the ACs decrease, as ΔG^≠ of the reaction with NO₃ increasing from -1.17 to 17.84 kcal mol^-1. The reaction of NO₃ towards ACs in the aqueous phase is more preferable, with the atmospheric lifetime 0.07-1281 min. An assessment of the aquatic toxicity of ACs and their degradation products indicated that the risk of their degradation products remains and should be given more attention.

Effect of pH on ·OH-induced degradation progress of syringol/syringaldehyde and health effect

Syringol and syringaldehyde are widely present pollutants in atmosphere and wastewater due to lignin pyrolysis and draining of pulp mill effluents. The hydroxylation degradation mechanisms and kinetics and health effect assessment of them under high and low-NO_x regimes in atmosphere and wastewater have been studied theoretically. The effect of pH on reaction mechanisms and rate constants in their ·OH-initiated degradation processes has been fully investigated. Results have suggested that aqueous solution played a positive role in the ·OH-initiated degradation reactions by decreasing the energy barriers of most reactions and changing the reactivity order of initial reactions. For Sy^- and Sya^- (anionic species of syringol and syringaldehyde), most initial reaction routes were more likely to occur than that of HSy and Hsya (neutral species of syringol and syringaldehyde). As the pH increased from 1 to 14, the overall rate constants (at 298 K) of syringol and syringaldehyde with ·OH in wastewater increased from 5.43 × 10¹⁰ to 9.87 × 10¹⁰ M^-1 s^-1 and from 3.70 × 10¹⁰ to 1.14 × 10¹¹ M^-1 s^-1, respectively. In the NO_x-rich environment, 4-nitrosyringol was the most favorable product, while ring-opening oxygenated chemicals were the most favorable products in the NO_x-poor environment. On the whole, the NO_x-poor environment could decrease the toxicities during the hydroxylation processes of syringol and syringaldehyde, which was the opposite in a NO_x-rich environment. ·OH played an important role in the methoxyphenols degradation and its conversion into harmless compounds in the NO_x-poor environment.

Kinetic study on the heterogeneous degradation of coniferyl alcohol by OH radicals

Coniferyl alcohol derived from lignin pyrolysis, is a potential tracer for wood burning emissions, but its atmospheric stability toward OH radicals is not well known. In this work, the degradation kinetics of coniferyl alcohol by OH radicals was studied using a flow reactor at different OH concentrations, temperatures, and relative humidity (RH). The results showed that coniferyl alcohol could be degraded effectively by OH radials, and the average second-order rate constant (k₂) was (11.6 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1 at the temperature and RH of 25 °C and 40%, respectively. Additionally, increasing temperature facilitated the degradation of coniferyl alcohol and the Arrhenius equation could be expressed as k₂ = (1.7 ± 0.3) × 10^-9exp [-(1480.2 ± 55.6)/T] at 40% RH. Meanwhile, increasing RH had a negative impact on the degradation of coniferyl alcohol. According to the k₂ obtained under different conditions, the atmospheric lifetime of coniferyl alcohol was in the range of 13.5 ± 0.4 h to 22.9 ± 1.4 h. The results suggested that the atmosphere lifetime of coniferyl alcohol was predominantly controlled by OH radicals.

Nighttime Chemical Transformation in Biomass Burning Plumes: A Box Model Analysis Initialized with Aircraft Observations

Biomass burning (BB) is a large source of reactive compounds in the atmosphere. While the daytime photochemistry of BB emissions has been studied in some detail, there has been little focus on nighttime reactions despite the potential for substantial oxidative and heterogeneous chemistry. Here, we present the first analysis of nighttime aircraft intercepts of agricultural BB plumes using observations from the NOAA WP-3D aircraft during the 2013 Southeast Nexus (SENEX) campaign. We use these observations in conjunction with detailed chemical box modeling to investigate the formation and fate of oxidants (NO₃, N₂O₅, O₃, and OH) and BB volatile organic compounds (BBVOCs), using emissions representative of agricultural burns (rice straw) and western wildfires (ponderosa pine). Field observations suggest NO₃ production was approximately 1 ppbv hr^-1, while NO₃ and N₂O₅ were at or below 3 pptv, indicating rapid NO₃/N₂O₅ reactivity. Model analysis shows that >99% of NO₃/N₂O₅ loss is due to BBVOC + NO₃ reactions rather than aerosol uptake of N₂O₅. Nighttime BBVOC oxidation for rice straw and ponderosa pine fires is dominated by NO₃ (72, 53%, respectively) but O₃ oxidation is significant (25, 43%), leading to roughly 55% overnight depletion of the most reactive BBVOCs and NO₂.

Heterogeneous kinetics of methoxyphenols in the OH-initiated reactions under different experimental conditions

Methoxyphenols as the potential tracers for wood smoke emissions, are emitted into the atmosphere in large quantities but their atmospheric chemical behaviors have not been well characterized. In this work, heterogeneous kinetics of methoxyphenols in the OH-initiated reactions was investigated using a flow reactor under different experimental conditions. The average second-order rate constants (k₂) of vanillic acid (VA), coniferyl aldehyde (CA), and syringaldehyde (SA) were (4.72 ± 0.51) × 10^-12, (10.59 ± 0.50) × 10^-12, (12.25 ± 0.60) × 10^-12 cm³ molecule^-1 s^-1, respectively, obtained at relative humidity (RH) and temperature of 40% and 25 °C. In addition, the results showed that high temperature played a positive role in promoting these reactions while high RH had an inhibiting impact. The k₂ values of VA, CA, and SA at 40% RH and different temperature followed the Arrhenius expressions, i.e., k₂ = (2.45 ± 0.40) × 10^-10exp [-(1170.73 ± 47.35)/T], k₂ = (6.40 ± 0.26) × 10^-10exp [-(1516.16 ± 13.71)/T], and k₂ = (1.02 ± 0.13) × 10^-9exp [-(1310.79 ± 36.75)/T], respectively. Based on the determined rate constants, the atmospheric lifetimes of these three methoxyphenols ranged from 0.54 to 2.18 d under different conditions. The experimental results indicate that OH radicals might play an important role in controlling the atmospheric lifetimes of methoxyphenols, and also help to further cognize the chemical behaviors of methoxyphenols in the atmosphere.

Infrared spectroscopy of secondary organic aerosol precursors and investigation of the hygroscopicity of SOA formed from the OH reaction with guaiacol and syringol

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) synchrotron analyses supplemented by density functional theory (DFT) anharmonic calculations have been undertaken to study the fundamental vibrational signatures of guaiacol and syringol, two methoxyphenol compounds found at the highest concentrations in fresh wood smoke and precursors of secondary organic aerosols (SOA) affecting the radiative balance and chemistry of the atmosphere. Nitroderivatives of these two compounds have also been studied experimentally for nitroguaiacol and theoretically for nitrosyringol. All the active fundamental vibrational bands have been assigned and compared to available gas phase measurements, providing a vibrational database of the main precursors for the analysis of SOA produced by atmospheric oxidation of methoxyphenols. In addition, the SOA formed in an atmospheric simulation chamber from the OH reaction with guaiacol and syringol were analyzed using the ATR-FTIR synchrotron spectroscopy and their hygroscopic properties were also investigated. The vibrational study confirms that nitroguaiacol and nitrosyringol are the main oxidation products of methoxyphenols by OH and are key intermediates in SOA production. The hydration experiments highlight the hydrophilic and hydrophobic characters of nitrosyringol and nitroguaiacol, respectively.

Nitrosonium-Mediated Phenol–Arene Cross-Coupling Involving Direct C–H Activation

The nitrosonium ion (NO+) is a highly versatile nitration and nitrosation reagent, as well as a strong one-electron oxidant. Herein, we describe an environmentally benign and mild method for the in situ formation of NO+ from readily available inorganic nitrate salts, i.e. lithium nitrate, through a finely tuned interplay between formic acid and MeOH, which are used as the solvent system. This methodology was applied to the NO+-induced oxidative C–H activation of methoxy-substituted phenols, which are versatile lignin-derived aromatic feedstocks, to achieve C–C cross-coupling reactions with arenes. The regeneration of NO+ by atmospheric molecular oxygen enables substoichiometric use of the nitrate.

Theoretical study on the mechanism of the gas phase reaction of methoxybenzene with ozone

Methoxybenzene (MB), is seen as a potential air pollutant which may cause environmental issues in the troposphere.