Photolysis and Heterogeneous Reaction of Coniferyl Aldehyde Adsorbed on Silica Particles with Ozone

Heterogeneous chemistry of ozone with floor cleaning agent: Implications of secondary VOCs in the indoor environment

Human daily activities such as cooking, and cleaning can affect the indoor air quality by releasing primary emitted volatile organic compounds (VOCs), as well as by the secondary product compounds formed through reactions with ozone (O₃) and hydroxyl radicals (OH). However, our knowledge about the formation processes of the secondary VOCs is still incomplete. We performed real-time measurements of primary VOCs released by commercial floor-cleaning detergent and the secondary product compounds formed by heterogeneous reaction of O₃ with the constituents of the cleaning agent by use of high-resolution mass spectrometry. We measured the uptake coefficients of O₃ on the cleaning detergent at different relative humidities in dark and under different light intensities (320 nm < λ < 400 nm) relevant for the indoor environment. On the basis of the detected compounds we developed tentative reaction mechanisms describing the formation of the secondary VOCs. Intriguingly, under light irradiation the formation of valeraldehyde was observed based on the photosensitized chemistry of acetophenone which is a constituent of the cleaning agent. Finally, we modeled the observed mixing ratios of three aldehydes, glyoxal, methylglyoxal, and 4-oxopentanal with respect to real-life indoor environment. The results suggest that secondary VOCs initiated by ozone chemistry can additionally impact the indoor air pollution.

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.

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.

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.

Heterogeneous reactions of particulate methoxyphenols with NO₃ radicals: kinetics, products, and mechanisms

Methoxyphenols, tracers for wood smoke, are emitted into the atmosphere in large quantities, but their chemical degradation in the atmosphere has not been well characterized. In this study, heterogeneous kinetics of particulate syringaldehyde (SA), vanillic acid (VA), and coniferyl aldehyde (CA) with NO₃ radicals is investigated with a mixed-phase relative rate method. A vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer and an atmospheric gas analysis mass spectrometer are used to monitor online the decays of particulate methoxyphenols and gas-phase isoprene synchronously. The reactive uptake coefficients of NO₃ radicals on SA, VA, and CA particles are calculated to be 0.33, 0.31, and 0.28, respectively, according to the measured methoxyphenol loss ratios and the average NO₃ concentrations. The effective rate constants for heterogeneous reactions of particulate SA, VA, and CA with NO₃ radicals measured under experimental conditions are 5.7 × 10⁻¹², 5.2 × 10⁻¹², and 3.5 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively. In addition, oxalic acid, 2,6-dimethoxybenzoquinone, 5-nitro-VA, 4,6-dinitrogaiacol, protocatechuic acid, vanillin, 5-nitrovanillin, VA, and 5-nitro-CA are identified as the reaction products by gas chromatography-mass spectrometry analysis. On the basis of the identified products, the reaction mechanisms of methoxyphenols with NO₃ radicals are proposed. The main transformation pathway of methoxyphenols is the NO₃ electrophilic addition, followed by H-abstraction and nitro-substituted processes. The experimental results might shed light on the chemical behaviors of methoxyphenols at night.