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

Secondary Organic Aerosol Generated from Biomass Burning Emitted Phenolic Compounds: Oxidative Potential, Reactive Oxygen Species, and Cytotoxicity

Phenolic compounds are largely emitted from biomass burning (BB) and have a significant potential to form SOA (Phc-SOA). However, the toxicological properties of Phc-SOA remain unclear. In this study, phenol and guaiacol were chosen as two representative phenolic gases in BB plumes, and the toxicological properties of water-soluble components of their SOA generated under different photochemical ages and NOx levels were investigated. Phenolic compounds contribute greatly to the oxidative potential (OP) of biomass-burning SOA. OH-adducts of guaiacol (e.g., 2-methoxyhydroquinone) were identified as components of guaiacol SOA (GSOA) with high OP. The addition of nitro groups to 2,5-dimethyl-1,4-benzoquinone, a surrogate quinone compound in Phc-SOA, increased its OP. The toxicity of both phenol SOA (PSOA) and GSOA in vitro in human alveolar epithelial cells decreased with aging in terms of both cell death and cellular reactive oxygen species (ROS), possibly due to more ring-opening products with relatively low toxicity. The influence of NOx was consistent between cell death and cellular ROS for GSOA but not for PSOA, indicating that cellular ROS production does not necessarily represent all processes contributing to cell death caused by PSOA. Combining different acellular and cellular assays can provide a comprehensive understanding of aerosol toxicological properties.

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.

The hydration of an oxy-polycyclic aromatic compound: the case of naphthaldehyde

The study of the intermolecular interactions of polycyclic aromatic compounds, considered as important pollutants of the Earth's atmosphere since they are emitted by the partial combustion of fuels, is essential to understand the formation and aging of their aerosols. In this study, the hydration of α-naphthaldehyde and β-naphthaldehyde isomers was investigated through a combination of Fourier transform microwave spectroscopy and quantum chemical calculations. Monohydrate structures were observed experimentally for both isomers, with two hydrate structures observed for β-naphthaldehyde and only one for α-naphthaldehyde, consistent with computational predictions. Analysis of the monohydrate structures indicated that the β-isomer exhibits higher hydrophilicity compared to the α-isomer, supported by electronic densities, hydration energies, and structural considerations. Further computational calculations were conducted to explore the planarity of the naphthaldehyde hydrates. Different levels of theory were employed, some of these revealing slight deviations from planarity in the hydrate structures. Low-frequency out-of-plane vibrational modes were examined, and the inertial defect was used to assess the planarity of the hydrates. The results suggested that the hydrates possess a predominantly planar structure, in agreement with the highest level of computational calculations and the absence of c-type transitions in the experimental spectra. Additionally, calculations were extended to dihydrate structures by attaching two water molecules to the naphthaldehyde isomers. The most stable dihydrate structures were predicted to be combinations of the observed monohydrate positions. However, experimental observation of the most stable dihydrate structures was challenging due to their very low vapour pressure, calling for complementary experiments using laser ablation nozzles.

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.

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.

Experimental and Theoretical Studies of Trans-2-Pentenal Atmospheric Ozonolysis

We investigated the kinetics, mechanism and secondary organic aerosols formation of the ozonolysis of trans-2-pentenal (T2P) using four different reactors with Fourier Transform InfraRed (FTIR) spectroscopy and Gas Chromatography (GC) techniques at T = 298 ± 2 K and 760 Torr in dry conditions. The rate coefficients and branching ratios were also evaluated using the canonical variational transition (CVT) state theory coupled with small curvature tunneling (CVT/SCT) in the range 278–350 K. The experimental rate coefficient at 298 K was (1.46 ± 0.17) × 10−18 cm3 molecule−1 s−1, in good agreement with the theoretical rate. The two primary carbonyls formation yields, glyoxal and propanal, were 57 ± 10% and 42 ± 12%, respectively, with OH scavenger compared to 38 ± 8% for glyoxal and 26 ± 5% for propanal without OH scavenger. Acetaldehyde and 2-hydroxypropanal were also identified and quantified with yields of 9 ± 3% and 5 ± 2%, respectively, in the presence of OH scavenger. For the OH production, an upper limit of 24% was estimated using mesitylene as OH tracer. Combining experimental and theoretical findings enabled the establishment of a chemical mechanism. Finally, the SOA formation was observed with mass yields of about 1.5%. This work provides additional information on the effect of the aldehyde functional group on the fragmentation of the primary ozonide.

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.

Theoretical study on the mechanisms, kinetics and ecotoxicity assessment of OH-initiated reactions of guaiacol in atmosphere and wastewater

The OH-initiated transformation mechanisms, kinetics and ecotoxicity assessment of guaiacol (2-methoxyphenol) in the presence of O₂/NO_x were investigated both in atmosphere and wastewater. The solvent effect lowers the energy barriers of initial OH-addition reactions more than H-abstraction reactions, leading to much higher addition branching ratio (Γ_add) of 0.92 in aqueous solution than that of 0.42 in gas-phase. At 298 K, the overall rate constants of the title reactions in atmosphere and wastewater are 5.56 × 10^-12 and 1.41 × 10^-11 cm³ molecule^-1 s^-1 with corresponding half-lives of 34.6 h and 0.82 s, respectively. In atmosphere, all the proposed favorable products including nitroguaiacols, methoxybenzoquinone, 2-hydroxyphenyl formate, 2-methoxybenzene-1, 3-diol and dialdehyde could contribute to secondary organic aerosols (SOAs). In wastewater, NO₂ addition reactions lead to higher toxicity of products (nitroguaiacols and 2-methoxybenzene-1, 4-diol) than that of parental guaiacol. However, O₂/NO addition pathways may generate less harmful products except for methoxybenzoquinone (P3) which is with higher toxicity than guaiacol. Therefore, more attention should be focused on the products formed from OH-initiated reactions of guaiacol both in atmosphere and wastewater.

Investigating adsorption performance of heavy metals onto humic acid from sludge using Fourier-transform infrared combined with two-dimensional correlation spectroscopy

Efforts to improve sludge resource utilization have become increasingly important. In this study, humic acid (HA) was extracted from sludge samples collected from a sewage treatment plant, and then used for the adsorption of heavy metals. We used two-dimensional correlation spectroscopy (2D-COS) integrated with Fourier-transform infrared spectroscopy (FTIR) to explore the adsorption between sludge HA (HA) and three metal ions (Cu, Ni, and Pb). The resulting adsorbing data conformed to the isotherm of Langmuir adsorption. The maximum capacity values (q_m) were 5.34, 1.49, and 26.29.8 mg/g for Cu, Ni, and Pb, respectively. The data from 2D-FTIR-COS analysis showed that the susceptibility of the functional group followed the order 2300 → 1130 → 1330 → 1480 → 1580 cm^-1 for Cu(II) and Ni(II), and 2300 → 1130 → 1330 → 1480 → 1200 → 1580 cm^-1 for Pb(II). The sludge HA with Pb(II) showed more adsorption sites than sludge HA with Cu(II) and Ni(II), and these adsorption sites could preferentially bond with Pb(II) at × 1 compared with Cu(II) and Ni(II). Our findings indicate that 2D-FTIR-COS technology has great potential for application as a useful tool for understanding the adsorption mechanism between adsorbents with heavy metals.

Conformational landscape and inertial defect of methoxyphenol isomers studied by mm-wave spectroscopy and quantum chemistry calculations

Because methoxyphenols (MP) are emitted in significant quantities during biomass fires and contribute to the secondary organic aerosols formation which impacts the climate, their gas phase monitoring in the atmosphere is crucial and requires accurate rovibrational cross sections determined with a good knowledge of their ground state (GS) and vibrationally excited state (ES) molecular parameters. Therefore, the rotational spectra of the two isomers, 2-MP (guaïacol) and 4-MP (mequinol), have been measured in absorption and in emission at room temperature using a frequency multiplication chain and a mm-wave Fourier transform chirped-pulse spectrometer, respectively. Guided by quantum chemistry calculations, the conformational landscape has been characterised and the observation of only one rotamer in the spectra of 2-MP and 4-MP has been explained. For 2-MP, the most stable conformation is justified by an intramolecular O-H⋯OCH₃ hydrogen-bond which has been characterised by a topology analysis of the electron density. In a global fit including more than 30 000 line assignments, rotational and quartic centrifugal constants of the GS and the three lowest energy ES have been determined allowing to reproduce the millimeter-wave spectra at the experimental accuracy. The same work has been performed on the cis-rotamer of 4-MP highlighting some perturbations marring the fit quality for two vibrationally ES. Finally, the isomeric dependence of the negative inertial defect ΔI agrees with that of the lowest energy out of plane mode ν₄₅, and the variation of ΔI with the degree of vibrational excitation allows a fine estimation of v₄₅ = 1 vibrational wavenumber.

Full Conformational Landscape of 3-Methoxyphenol Revealed by Room Temperature mm-wave Rotational Spectroscopy Supported by Quantum Chemical Calculations

Room temperature millimeter-wave rotational spectroscopy supported by high level of theory calculations have been employed to fully characterise the conformational landscape of 3-Methoxyphenol, a semi-volatile polar oxygenated aromatic compound precursor of secondary organic aerosols in the atmosphere arising from biomass combustion. While previous rotationally-resolved spectroscopic studies in the microwave and in the UV domains failed to observe the complete conformational landscape, the 70-330 GHz rotational spectrum measured in this study reveals the ground state rotational signatures of the four stable conformations theoretically predicted. Moreover, rotational transitions in the lowest energy vibrationally excited states were assigned for two conformers. While the inertial defect of methoxyphenol does not significantly change between conformers and isomers, the excitation of the methoxy out-of-plane bending is the main contribution to the non-planarity of the molecule.