Atmospheric degradation of lindane and 1,3-dichloroacetone in the gas phase. Studies at the EUPHORE simulation chamber

A combined experimental and computational investigation on the OH radical and Cl atom-initiated reaction of 2,3-dichloropropene in troposphere

Temperature-dependent kinetics of OH radical and Cl atom-initiated reaction of an important halogenated alkene, 2,3-Dichloropropene (23DCP), were investigated using absolute and relative methods over 278-363 K. Pulsed laser photolysis - laser induced fluorescence technique and relative rate method using gas chromatography with flame ionization detector were employed for studying the kinetics of 23DCP with OH radical and Cl atom, respectively. The obtained Arrhenius expressions were kOH(expt)=(4.08 ± 1.63) × 10-13exp{(1043 ± 124)/T} cm3 molecule-1 s-1 and kCl(expt)=(1.54 ± 0.24) × 10-11exp{(705 ± 48)/T} cm3 molecule-1 s-1. Computational calculations were conducted to validate our experimental kinetic results and provide new insights into the importance of a particular pathway among all based on thermodynamic parameters. The addition of OH/Cl to the terminal carbon of the double bond present in 23DCP proved to be the predominant pathway across the selected temperature range for the present study (200-400 K). The degradation mechanism of these reactions was proposed by analyzing the products with the aid of gas chromatography with mass spectrometry. Calculating various atmospheric implication parameters can help to understand how the release of 23DCP may affect the troposphere.

Photolytic degradation of commonly used pesticides adsorbed on silica particles

The currently used pesticides are mostly semi-volatile organic compounds. As a result, a fraction of them can be adsorbed on atmospheric aerosol surface. Their atmospheric photolysis is poorly documented, and gaps persist in understanding their reactivity in the particle phase. Laboratory experiments were conducted to determine the photolysis rates of eight commonly used pesticides (i.e., cyprodinil, deltamethrin, difenoconazole, fipronil, oxadiazon, pendimethalin, permethrin, and tetraconazole) using a flow reactor. These pesticides were individually adsorbed on hydrophobic silica particles and exposed to a filtered xenon lamp to mimic atmospheric aerosols and sunlight irradiation, respectively. The estimated photolysis rate constants ranged from less than (3.4 ± 0.3) × 10-7 s-1 (permethrin; >47.2 days) to (3.8 ± 0.2) × 10-5 s-1 (Fipronil; 0.4 days), depending on the considered compound. Moreover, this study assessed the influence of pesticide mixtures on their photolysis rates, revealing that certain pesticides can act as photosensitizers, thereby enhancing the reactivity of permethrin and tetraconazole. This study underscores the importance of considering photolysis degradation when evaluating pesticide fate and reactivity, as it can be a predominant degradation pathway for some pesticides. This contributes to an enhanced understanding of their behavior in the atmosphere and their impact on air quality.

Influence of pesticide mixture on their heterogeneous atmospheric degradation by ozone and OH radicals

Pesticides in the atmosphere can exist in both gaseous and particulate phases due to their semi-volatile properties. They can undergo degradation when exposed to atmospheric oxidants like ozone and hydroxyl radicals. The majority of studies on the atmospheric reactivity of pesticides study them in combination, without considering potential mixture effects that could induce uncertainties in the results. Therefore, this study aims to address this gap, through laboratory studies using a flow reactor, and by evaluating the degradation kinetics of pendimethalin mixed with folpet, tebuconazole, and S-metolachlor, which were simultaneously adsorbed on hydrophobic silica particles that mimic atmospheric aerosols. The comparison with other mixtures, including pendimethalin, from the literature has shown similar reactivity with ozone and hydroxyl radicals, indicating that the degradation kinetics of pesticides is independent of the mixture. Moreover, the degradation rates of the four pesticides under study indicate that they are not or slightly degraded by ozone, with half-lives ranging from 29 days to over 800 days. In contrast, when exposed to hydroxyl radicals, tebuconazole exhibited the fastest reactivity, with a half-life of 4 days, while pendimethalin had a half-life of 17 days.

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.

Assessing the atmospheric fate of pesticides used to control mosquito populations in Houston, TX

During the summer months, urban areas are literal hot spots of mosquito-borne disease transmission and air pollution. Public health authorities release aerosolized pesticides directly into the atmosphere to help control adult mosquito populations and thereby reduce the threat of diseases, such as Zika Virus. The primary adulticides (i.e. pesticides used to control adult mosquito populations) in Houston, TX are permethrin and malathion. These adulticides are typically sprayed at night using ultra-low volume sprayers. Particulate matter (PM) samples including total suspended and fine PM (PM < 2.5 μm in aerodynamic diameter) were collected at four ground-based sites across Houston in 2013 and include daytime, nighttime, and 24 h samples. Malathion is initially sprayed as coarse aerosol (5-25 μm), but is measured in fine aerosol (<2.5 μm) and coarse aerosol in the urban atmosphere. Particle size is relevant both for deposition velocities and for human exposure. Atmospheric permethrin concentrations measured in nighttime samples peak at 60 ng m^-3, while malathion nighttime concentrations peak near 40 ng m^-3. Malaoxon, an oxidation product of malathion, was also frequently detected at concentrations >10 ng m^-3, indicating significant nighttime oxidation. Based on the loss of malathion and the increase in malaoxon, the atmospheric half-life of malathion in Houston was estimated at <12 h, which was significantly shorter than previous half-life estimates (∼days). Importantly, malaoxon is estimated to be 22-33 times more toxic to humans than malathion. Both the aerosol size and the half-life are critical for mosquito control, human exposure, and risk assessment of these routine pesticides.

Atmospheric oxidation of Folpet initiated by OH radicals, NO3 radicals, and O3

The fate of Folpet is dictated by oxidation initiated by atmospheric oxidants such as O₃, OH, and NO₃ radicals. Considering the oxidant concentration in the atmosphere, Folpet seems to be mainly scavenged by NO₃ radicals, especially at night.

Predicting the rate constants of semivolatile organic compounds with hydroxyl radicals and ozone in indoor air

Semivolatile organic compounds (SVOCs) in air can react with hydroxyl radicals (OH), nitrate radicals (NO₃) and ozone (O₃). Two questions regarding SVOC reactivity with OH, NO₃ and O₃ in the gas and particle phases remain to be addressed: according to the existing measurements in the literature, which are the most reactive SVOCs in air, and how can the SVOC reactivity in the gas and particle phases be predicted? In the present study, a literature review of the second-order rate constant (k) was carried out to determine the SVOC reactivity with OH, NO₃ and O₃ in the gas and particle phases in ambient and indoor air at room temperature. Measured k values were available in the literature for 90 polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organophosphates, dioxins, di(2-ethylhexyl)phthalate (DEHP) and pesticides including pyrifenox, carbamates and terbuthylazine. PAHs and organophosphates were found to be more reactive than dioxins and PCBs. Based on the obtained data, quantitative structure-activity relationship (QSAR) models were developed to predict the k value using quantum chemical, molecular, physical property and environmental descriptors. Eight linear and nonlinear statistical models were employed, including regression models, bagging, random forest and gradient boosting. QSAR models were developed for SVOC/OH reactions in the gas and particle phases and SVOC/O₃ reactions in the particle phase. Models for SVOC/NO₃ and SVOC/O₃ reactions in the gas phase could not be developed due to the lack of measured k values for model training. The least absolute shrinkage and selection operator (LASSO) regression and random forest models were identified as the most effective models for SVOC reactivity prediction according to a comparison of model performance metrics.

Heterogeneous degradation of pesticides by OH radicals in the atmosphere: Influence of humidity and particle type on the kinetics

Pesticides can be adsorbed on the surface of atmospheric aerosol, depending on their physicochemical properties. They can be degraded by atmospheric oxidants such as OH radicals but the influence of some environmental parameters on the degradation kinetics, especially relative humidity and particle surface type, is not well understood. Heterogeneous degradation by OH radicals of eight commonly used pesticides (i.e., difenoconazole, tetraconazole, cyprodinil, fipronil, oxadiazon, pendimethalin, deltamethrin, and permethrin) adsorbed on hydrophobic and hydrophilic silicas at a relative humidity ranging from 0% to 70% was studied. Under experimental conditions, only cyprodinil, deltamethrin, permethrin, and pendimethalin were degraded by OH radical in atmospheric relevant concentration. Second-order kinetic constants calculated for the pesticides degraded by OH radicals ranged from (1.93 ± 0.61) × 10^-13 cm³ molecule^-1 s^-1 (permethrin, hydrophobic silica, 30% RH) to (4.08 ± 0.27) × 10^-12 cm³ molecule^-1 s^-1 (pendimethalin, hydrophilic silica, 0% RH). Results obtained can contribute to improve the understanding of the atmospheric fate of pesticides and other semi-volatile organic compounds in the particulate phase and they highlight the importance of taking humidity and particle type into account for the determination of pesticides atmospheric half-lives.

Atmospheric Oxidation of a Thiocarbamate Herbicide Used in Winter Cereals

The gas-phase atmospheric degradation of prosulfocarb (a widely used thiocarbamate herbicide in winter cereals) at different NOx concentrations was investigated at the large outdoor European PHOtoREactor (EUPHORE) in Valencia, Spain. Photolysis under sunlight conditions and reaction with ozone were shown as unimportant. The rate constant for the reaction of prosulfocarb with OH radicals was determined as k = (2.9 ± 0.5) × 10^-11 cm³ molecule^-1 s^-1 at 288 ± 10 K and atmospheric pressure by a conventional relative rate method. Significant ozone and aerosol formation was observed following the reaction of prosulfocarb with OH radicals, and the main detected carbon-containing gas-phase products were benzaldehyde, S-benzyl formyl(propyl)carbamothioate, and S-benzyl propanoyl(propyl)carbamothioate.

Solar radiation as a swift pathway for PAH photodegradation: A field study

The photodegradation of polycyclic aromatic hydrocarbons (PAHs) may be an important degradation pathway of PAHs in regions with a high solar radiation. The present investigation was aimed at studying the photodegradation of PAHs after their deposition on surface soils with different textures. Photodegradation by-products were also identified and semi-quantified, as well as correlated with the decrease of parent compounds. The experiment was performed by deploying soil samples spiked with a mixture of the 16 US EPA priority PAHs in a methacrylate box, exposed to solar radiation for 7days, meaning a solar energy of 102.6MJm^-2. As hypothesized, the individual PAHs were volatilized, sorbed and/or photodegraded, depending on their physicochemical properties, as well as the soil characteristics. Low and medium molecular weight PAHs were more sorbed and photodegraded in fine-textured Regosol soil, while a higher volatilization was observed in the coarse-textured Arenosol soil. In contrast, high molecular weight PAHs were more photodegraded in Arenosol soil. Specially low half-lives were noted for anthracene and benzo(a)pyrene, agreeing with previous findings at laboratory scale. Nine by-products were identified, including oxy-, nitro- and hydro-PAHs, whose toxic and mutagenic potential might be higher than the 16 priority PAHs.

Atmospheric degradation of the organothiophosphate insecticide - Pirimiphos-methyl

The gas phase atmospheric degradation of pirimiphos-methyl (a widely used organophosphate insecticide and acaricide in many European regions) has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. Its photolysis has been studied under sunlight conditions and its reaction rate constant with OH radicals was measured by the relative rate method. The reaction with ozone was also investigated. The tropospheric degradation of pirimiphos-methyl is controlled mainly by the OH radical reaction. The rate coefficient of the OH reaction with pirimiphos-methyl, k, was measured by a conventional relative rate technique, where aniline was taken as a reference. The resulting value of the OH reaction rate constant with pirimiphos-methyl was k=(1.14±0.2)×10^-10cm³molecule^-1s^-1. The tropospheric lifetime of pirimiphos-methyl with respect to the reaction with OH radicals was estimated to be around 1.6h (283±10) K and atmospheric pressure. Significant aerosol formation was observed in the OH reaction with yields that ranged from 25 to 37%, and with particle diameters below 550nm. This therefore reveals a high human risk due to PM<1, without taking into account the chemical composition of the degradation products. SO₂, glyoxal and other oxygenated and nitrogenated compounds were the main degradation products detected.