Heterogeneous kinetics of the OH-initiated degradation of fenthion and parathion

Fenthion and parathion are two representative kinds of organophosphorus pesticides and widely used in agriculture. They are directly or indirectly released into the atmosphere by spraying or volatilization processes. However, their heterogeneous reactivity toward OH radicals has not yet been well understood. Therefore, this work investigated the heterogeneous kinetics of the OH-initiated degradation of surface-bound fenthion and parathion using a flow reactor. The results showed that OH radicals played an important role in the atmospheric degradation of fenthion and parathion. Their average rate constants were (7.20 ± 0.77) × 10^-12 and (10.40 ± 0.60) × 10^-12 cm³/(mol· sec) at a relative humidity (RH) and temperature of 35% and 20 °C, respectively, suggesting that they have relatively short lifetimes in the atmosphere. In addition, a negative RH dependence and a positive temperature dependence of the rate constants were observed. The Arrhenius expressions of fenthion and parathion were k₂ = (1.34 ± 0.48) × 10^-9exp[-(1432.59 ± 105.29)/T] and k₂ = (1.96 ± 1.38) × 10^-9exp[-(1619.98 ± 222.02)/T], respectively, and their overall activation energy was estimated to be (11.88 ± 0.87) and (13.48 ± 1.83) kJ/mol. The experimental results will update the kinetic data of fenthion and parathion in the atmosphere and be helpful to further understand their atmospheric transportation processes.

Insight into the Primary and Secondary Particle-Bound Methoxyphenols and Nitroaromatic Compound Emissions from Solid Fuel Combustion and the Updated Source Tracers

Methoxyphenols and nitroaromatic compounds (NACs) have strong atmospheric radiative forcing effects and adverse effects on human health. They are emitted from the incomplete combustion of solid fuels and are secondarily formed through photochemical reactions. Here, an on-site study was conducted to determine the primary emission and secondary formation of particulate phase products from a variety of solid fuels through a potential aerosol mass-oxidation flow reactor. Emission factors for total quantified methoxyphenols and NACs (i.e., EF∑Methoxyphenols and EF∑NACs) varied by 2 orders of magnitude among different fuels, which were greatly influenced by volatile matter, incomplete combustibility, flame intensity, and combustion temperature. Guaiacol and 4-nitro-2-vinylphenol were used as tracers for primary organic aerosol due to the low aged-to-fresh ratios (0.21-0.97), while 4-methyl-guaiacol, 4-ethyl-guaiacol, eugenol, 4-methyl-syringol, isoeugenol, acetovanillone, syringaldehyde, homovanillin acid, vanillin acid, and syringic acid were identified as secondary organic aerosol (SOA) (aged-to-fresh ratios between 1.90 and 4.20). During simulated aging, the -CHO group reacted with the hydroxyl radical (•OH) to form the -COOH group, but there was no correlation between syringol and 4-nitrosyringol, implying that •OH is the main reactant rather than the nitriate radical (•NO3) in the atmospheric aging processes of methoxyphenols. Aging caused substantially different emission profiles due to variable photochemical reaction properties. The fresh EFs for guaiacol emitted from the biomass burning ranged from 3.80 ± 0.44 to 26.2 ± 5.40 mg·kg-1, which were much higher than those in coal combustions (of 0.03 ± 0.01 to 1.42 ± 0.28 mg·kg-1). However, the aged EFs (EFaged) for guaiacol was 1.02 ± 0.06 to 1.61 ± 0.11 mg·kg-1 in most biomass combustions, which were comparable with those of the bituminous chunk (1.20 ± 0.16 mg·kg-1). Therefore, guaiacol, a traditional biomass marker, is not an ideal tracer for aged PM2.5 emitted from biomass burning. Indeed, the syringol/guaiacol and syringol/4-nitrosyringol ratios were found to be more suitable and efficient to be used in source characterization.

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.

Relative abundance of saccharides, free amino acids, and other compounds in specific pollen species for source profiling of atmospheric aerosol

Though studies in bioaerosols are being conducted with increasing frequency over the past decade, the total breadth of knowledge on bioaerosols and their role in atmospheric processes is still minimal. In order to better characterize the chemical composition of fresh biological aerosol for purposes of source apportionment and tracing in the atmosphere, several plant pollen species were selected for detailed chemical analyses. For this purpose, different pollen species were purchased and collected around Reno, Nevada, USA, for further extraction and detailed chemical analysis. These species included aspen, corn, pecan, ragweed, eastern cottonwood, paper mulberry, rabbitbrush, bitterbrush, lodgepole pine, and Jeffrey pine. Saccharides, free amino acids, and various other polar compounds (e.g., anhydrosugars and resin acids) were quantitatively analyzed using gas chromatography and ultra-high performance liquid chromatography coupled with mass spectrometry techniques (GC-MS and UPLC-MS), with the purpose to identify differences and nuances in chemical composition of specific pollen species. The saccharides β-d-fructose, α-d-glucose, and β-d-glucose were ubiquitously found across all pollen samples (10), and sucrose was found in five samples. d-galactose was also found in pine species. Total saccharides were 4.0 to 29% of total dry weight across all samples. Total free amino acids were 0.29% to 15% of total dry weight across all samples, with the most common amino acid being proline. Chemical profiles (including both saccharides and amino acids) of surface-deposited aerosol in the Lake Tahoe area correlated most closely with pine pollen than other analyzed pollen species, indicating that chemical profiles of pollen can be used to infer its contribution to local aerosols.

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.

Understanding the Impact of Relative Humidity and Coexisting Soluble Iron on the OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants

The current uncertainties in the reactivity and atmospheric persistence of particle-associated chemicals present a challenge for the prediction of long-range transport and deposition of emerging chemicals such as organophosphate flame retardants, which are ubiquitous in the global environment. Here, the OH-initiated heterogeneous oxidation kinetics of organophosphate flame retardants (OPFRs) coated on inert (NH₄)₂SO₄ and redox-active FeSO₄ particles were systematically determined as a function of relative humidity (RH). The derived reaction rate constants for the heterogeneous loss of tricresyl phosphate (TCP; k_TCP) and tris(2-butoxyethyl) phosphate (TBEP; k_TBEP) were in the range of (2.69-3.57) × 10^-12 and (3.06-5.55) × 10^-12 cm³ molecules^-1 s^-1, respectively, depending on the RH and coexisting Fe(II) content. The k_TCP (coated on (NH₄)₂SO₄) was relatively constant over the investigated RH range while k_TBEP was enhanced by up to 19% with increasing RH. For both OPFRs, the presence of Fe(II) enhanced their k by up to 53% over inert (NH₄)₂SO₄. These enhancement effects (RH and Fe(II)) were attributed to fundamental changes in the organic phase state (higher RH lowered particle viscosity) and Fenton-type chemistry which resulted in the formation of reactive oxygen species, respectively. Such findings serve to emphasize the importance of ambient RH, the phase state of particle-bound organics in general, and the presence of coexisting metallic species for an accurate description of the degradation kinetics and aging of particulate OPFRs in models used to evaluate their atmospheric persistence.

Aromatic acids as biomass-burning tracers in atmospheric aerosols and ice cores: A review

Biomass burning (BB) is one of the largest sources of carbonaceous aerosols with adverse impacts on air quality, visibility, health and climate. BB emits a few specific aromatic acids (p-hydroxybenzoic, vanillic, syringic and dehydroabietic acids) which have been widely used as key indicators for source identification of BB-derived carbonaceous aerosols in various environmental matrices. In addition, measurement of p-hydroxybenzoic and vanillic acids in snow and ice cores have revealed the historical records of the fire emissions. Despite their uniqueness and importance as tracers, our current understanding of analytical methods, concentrations, diagnostic ratios and degradation processes are rather limited and scattered in literature. In this review paper, firstly we have summarized the most established methods and protocols for the measurement of these aromatic acids in aerosols and ice cores. Secondly, we have highlighted the geographical variability in the abundances of these acids, their diagnostic ratios and degradation processes in the environments. The review of the existing data indicates that the concentrations of aromatic acids in aerosols vary greatly with locations worldwide, typically more abundant in urban atmosphere where biomass fuels are commonly used for residential heating and/or cooking purposes. In contrast, their concentrations are lowest in the polar regions which are avoid of localized emissions and largely influenced by long-range transport. The diagnostic ratios among aromatic acids can be used as good indicators for the relative amounts and types of biomass (e.g. hardwood, softwood and herbaceous plants) as well as photochemical oxidation processes. Although studies suggest that the degradation processes of the aromatic acids may be controlled by light, pH and hygroscopicity, a more careful investigation, including closed chamber studies, is highly appreciated.

Determination of short-term changes in levoglucosan and dehydroabietic acid in aerosols with Condensation Growth Unit - Aerosol Counterflow Two-Jets Unit - LC-MS

Residential areas in urban agglomerations and also in the countryside are often burdened with high concentrations of aerosol in winter, this originating from local combustion sources. Aerosol sources can be identified by a monitoring of organic markers of biomass burning. Abundant markers of biomass and softwood burning are levoglucosan and dehydroabietic acid, respectively. The aim of this research was to develop an analytical method for the determination of levoglucosan and dehydroabietic acid in aerosol over short time periods involving aerosol sampling into liquid samples, quantitative pre-concentration of analytes, and their determination by liquid chromatography - mass spectrometry. A Condensation Growth Unit - Aerosol Counterflow Two-Jets Unit (CGU-ACTJU) sampler was used for the quantitative collection of aerosol directly into water. Dehydroabietic acid was pre-concentrated from the aqueous phase by solid phase extraction (C-18). Afterwards, levoglucosan in water samples was concentrated on a vacuum evaporator. The detection limits of levoglucosan and dehydroabietic acid were 28 ng m^-3 and 5.5 ng m^-3, respectively. The results obtained by the developed method were compared with an independent determination of both markers in aerosol by means of the sampling of aerosols on a filter and subsequent analysis by GC-MS. The developed method demonstrated sufficient agreement with the independent determination for generated standard aerosol as well as for urban aerosol over an eight-day winter campaign. The presented method allows the monitoring of concentration changes in biomass burning markers in 2-h intervals.

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 chemistry and reaction dynamics of the atmospheric oxidants, O3, NO3, and OH, on organic surfaces

Heterogeneous chemistry of the most important atmospheric oxidants, O₃, NO₃, and OH, plays a central role in regulating atmospheric gas concentrations, processing aerosols, and aging materials.