Liquid phase transformation mechanism of ß-caryophyllonic acid initiated by hydroxyl radicals and ozone in atmosphere.

ß-caryophyllonic acid (BCA), as an important precursor of aqueous secondary organic aerosols (aqSOA), has adverse effects on the atmospheric environment and human health. However, the key atmospheric chemical reaction process in which BCA participates in the formation of aqueous secondary organic aerosols is still unclear. In this study, the reaction mechanism and kinetics of BCA with ·OH and O3 were investigated by quantum chemical calculations. The initiation reactions between BCA and ·OH include addition and H-abstraction reaction pathways, subsequent intermediates will also react with O2, ultimately undergo a cracking reaction to generate small molecular substances. The reaction of BCA with O3 can generate primary ozone oxides and the Criegee Intermediates oIM3, subsequent main reaction products include keto-BCA, as well as other small molecule aqSOA precursors. The entire reaction process increases the O/C ratio of aqSOA in the aqueous phase and generates products of small molecules such as 4-formylpropionic acid, which plays an important role in the formation of aqSOA. At 298K, the transformation rate constants of BCA initiated by ·OH and O3 are 1.47 × 1010 M-1 s-1 and 3.16 × 105 M-1 s-1, respectively, the atmospheric lifetimes of BCA reacting with ·OH range from 0.86 h-5.40 h, while the lifetimes of BCA reacting with O3 range from 0.44 h-10.04 years. This suggests that BCA primarily reacts with ·OH. However, under higher O3 concentrations, its ozonolysis becomes significant, promoting the formation of aqSOA. According to the risk assessment, the toxicity of most transformation products (TPs) gradually decreased, but the residual developmental toxicity could not be ignored. In this paper, the atmospheric liquid phase oxidation mechanisms of sesquiterpene unsaturated derived acid were studied from the microscopic level, which has guiding significance for the formation and transformation of aqSOA in atmosphere.

Contrasting molecular characteristics and formation mechanisms of biogenic and anthropogenic secondary organic aerosols at the summit and foot of Mt. Huang, East China.

Secondary organic aerosol (SOA) exerts a considerable influence on atmospheric chemistry. However, little information about the vertical distribution of SOA in the alpine setting is available, which limited the simulation of SOA using chemical transport models. Here, a total of 15 biogenic and anthropogenic SOA tracers were measured in PM2.5 aerosols at both the summit (1840 m a.s.l.) and foot (480 m a.s.l.) of Mt. Huang during the winter of 2020 to explore their vertical distribution and formation mechanism. Most of the determined chemical species (e.g., BSOA and ASOA tracers, carbonaceous components, major inorganic ions) and gaseous pollutants at the foot of Mt. Huang were 1.7-3.2 times higher concentrations than those at the summit, suggesting the relatively more significant effect of anthropogenic emissions at the ground level. The ISORROPIA-II model showed that aerosol acidity increases as altitude decreases. Air mass trajectories, potential source contribution function (PSCF), and correlation analysis of BSOA tracers with temperature revealed that SOA at the foot of Mt. Huang was mostly derived from the local oxidation of volatile organic compounds (VOCs), while SOA at the summit was mainly influenced by long-distance transport. The robust correlations of BSOA tracers with anthropogenic pollutants (e.g., NH3, NO2, and SO2) (r = 0.54-0.91, p < 0.05) indicated that anthropogenic emissions could promote BSOA productions in the mountainous background atmosphere. Moreover, most of SOA tracers (r = 0.63-0.96, p < 0.01) and carbonaceous species (r = 0.58-0.81, p < 0.01) were correlated well with levoglucosan in all samples, suggesting that biomass burning played an important role in the mountain troposphere. This work demonstrated that daytime SOA at the summit of Mt. Huang was significantly influenced by the valley breeze in winter. Our results provide new insights into the vertical distributions and provenance of SOA in the free troposphere over East China.

Mechanisms, kinetics and eco-toxicity assessment of singlet oxygen, sulfate and hydroxyl radicals-initiated degradation of fenpiclonil in aquatic environments.

Fenpiclonil is an agricultural phenylpyrrole fungicide, which raise the concern about its ecotoxicological effects. In this paper, we investigate the indirect photochemical transformation mechanisms, environmental persistence and eco-toxicity of fenpiclonil initiated by various active oxidants (1O2, •OH and SO4•‾) in aquatic environments. The results shown that 1O2 can react with pyrrole ring by cycloaddition pathways to form the endo-peroxides. In addition, •OH and SO4•‾ initial mechanisms are calculated, suggesting that •OH-initiated mechanisms play a dominant role in the degradation process of fenpiclonil at high rate constants (2.26 ×109 M-1 s-1, at 298 K). The kinetic calculation results indicate that high temperature is more favorable for the degradation of fenpiclonil. To better understand the adverse effects of the transformation products formed during the subsequent reaction of •OH-adduct IM10, the computational toxicology has been used for the toxicity estimation. The results show that aquatic toxicity of these products decrease with degradation process, especially the decomposition products (TP3 and TP4). However, TP1 and TP2 are still toxic and developmental toxicant. The study provides guidance for further experimental research and industrial application of fungicide degradation from the perspective of theoretical calculation.

Indirect Photodegradation of Sulfamethoxazole and Trimethoprim by Hydroxyl Radicals in Aquatic Environment: Mechanisms, Transformation Products and Eco-Toxicity Evaluation.

The bacteriostatic antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), have frequently been found in wastewater and surface water, which raises the concerns about their ecotoxicological effects. The indirect photochemical transformation has been proven to be an efficient way to degrade SMX and TMP. In this study, the reaction mechanisms of the degradation by SMX and TMF by OH radicals were investigated by theoretical calculations. Corresponding rate constants were determined and the eco-toxicity of SMX and TMP and its degradations products were predicted using theoretical models. The results indicate that the most favorable pathways for the transformation of SMX and TMP are both •OH-addition reaction of benzene ring site with lowest Gibbs free energy barriers (6.86 and 6.21 kcal mol-1). It was found that the overall reaction rate constants of •OH-initial reaction of SMX and TMP are 1.28 × 108 M-1 s-1 and 6.21 × 108 M-1 s-1 at 298 K, respectively. When comparing the eco-toxicity of transformation products with parent SMX and TMP, it can be concluded that the acute and chronic toxicities of the degraded products are reduced, but some products remain harmful for organisms, especially for daphnid (toxic or very toxic level). This study can give greater insight into the degradation of SMX and TMP by •OH through theoretical calculations in aquatic environment.

Indirect photodegradation of fludioxonil by hydroxyl radical and singlet oxygen in aquatic environment: Mechanism, photoproducts formation and eco-toxicity assessment.

Fludioxonil has been proven valuable as a broad-spectrum fungicide. However, there are concerns about its risk posed to non-target organisms in aquatic environments. In this paper, the mechanism, photoproducts transformation and eco-toxicity of fludioxonil during •OH/1O2-initiated process were systematically studied using quantum chemistry and computational toxicology. The results indicate that the two favorable pathways of •OH/1O2-initiated reactions are both occurred in pyrrole ring. It can conclude that the rate constants of •OH and 1O2 are 1.23 × 1010 and 3.69 × 107 M-1 s-1 at 298K, respectively, which results in half-lives of <2 days in surface waters under sunlit near-surface conditions. Based on toxicity assessments, these photoproducts showed a decreased aquatic toxicity but the majority products are still toxic. This study gives more insight into the chemical transformation mechanism of fludioxonil in aquatic environments.

Mechanism and kinetic study of the reaction of benzoic acid with OH, NO3 and SO4 - radicals in the atmosphere.

Benzoic acid (BA) is one of the most common organic acids in the Earth's atmosphere and an important component of atmospheric aerosol particles. The reaction mechanism of OH, NO3 and SO4 - radicals with BA in atmospheric water droplets and that of OH radicals with BA in the atmosphere were studied in this paper. The results show that in atmospheric water droplets the potential barriers of the elementary addition reactions of BA with OH radicals are lower than those of elementary abstraction reactions, and the potential barriers of OH-initiated reactions are less than for NO3 and SO4 - reactions. The initiation reactions of OH radicals and BA are exothermic, but the abstraction reactions of NO3 and SO4 - are endothermic processes. Among the products, 6-hydroxybenzoic acid (6-HBA) and 4,6-dihydroxybenzoic acid (4,6-DHBA) are the most stable, while 3-hydroxybenzoic acid (3-HBA) and 3,5-dihydroxybenzoic acid (3,5-DHBA) are much less stable and, thus, much less abundant compared to 6-HBA and 4,6-DHBA. The initiation and subsequent degradation of BA with OH radicals in the gas phase were calculated. The products of addition and abstraction reactions of BA with OH radicals can be further oxidized and degraded by O2/NO. According to the results of kinetic calculations, the total reaction rate constant of OH radicals with BA at 298.15 K in atmospheric water droplets is 2.35 × 10-11 cm3 per molecule per s. The relationship between reaction rate constants, temperature and altitude were also investigated and discussed in the present study.