Field measurement of the organic peroxy radicals by the low-pressure reactor plus laser-induced fluorescence spectroscopy

Dimeric Product of Peroxy Radical Self-Reaction Probed with VUV Photoionization Mass Spectrometry and Theoretical Calculations: The Case of C2H5OOC2H5

Organic peroxy radicals (RO₂) as key intermediates in tropospheric chemistry exert a controlling influence on the cycling of atmospheric reactive radicals and the production of secondary pollutants, such as ozone and secondary organic aerosols (SOA). Herein, we present a comprehensive study of the self-reaction of ethyl peroxy radicals (C₂H₅O₂) by using advanced vacuum ultraviolet (VUV) photoionization mass spectrometry in combination with theoretical calculations. A VUV discharge lamp in Hefei and synchrotron radiation at the Swiss Light Source (SLS) are employed as the photoionization light sources, combined with a microwave discharge fast flow reactor in Hefei and a laser photolysis reactor at the SLS. The dimeric product, C₂H₅OOC₂H₅, as well as other products, CH₃CHO, C₂H₅OH and C₂H₅O, formed from the self-reaction of C₂H₅O₂ are clearly observed in the photoionization mass spectra. Two kinds of kinetic experiments have been performed in Hefei by either changing the reaction time or the initial concentration of C₂H₅O₂ radicals to confirm the origins of the products and to validate the reaction mechanisms. Based on the fitting of the kinetic data with the theoretically calculated results and the peak area ratios in the photoionization mass spectra, a branching ratio of 10 ± 5% for the pathway leading to the dimeric product C₂H₅OOC₂H₅ is measured. In addition, the adiabatic ionization energy (AIE) of C₂H₅OOC₂H₅ is determined at 8.75 ± 0.05 eV in the photoionization spectrum with the aid of Franck-Condon calculations and its structure is revealed here for the first time. The potential energy surface of the C₂H₅O₂ self-reaction has also been theoretically calculated with a high-level of theory to understand the reaction processes in detail. This study provides a new insight into the direct measurement of the elusive dimeric product ROOR and demonstrates its non-negligible branching ratio in the self-reaction of small RO₂ radicals.

Observation and modeling of organic nitrates on a suburban site in southwest China

Here we report the measurements of two types of organic nitrates (ONs), peroxy nitrates (PNs) and alkyl nitrates (ANs), in Chengdu, China, during summer 2019. The average concentrations of PNs and ANs were 1.3 ± 1.1 ppbv and 0.5 ± 0.3 ppbv during the day, with peaks of 7.7 ppbv and 1.9 ppbv, respectively, which were in the middle and upper end of the reported levels in China. Much higher PNs and ANs concentrations were found during the photochemical pollution period than during the clean period. Box model simulation was capable of reproducing PNs during photochemical pollution episodes but showed overestimation in other periods, which was likely caused by the simplification of PNs sinks. The OH oxidation of aldehydes and ketones was the most important source of the PNs precursors, PAs (peroxyacyl radicals), except for the thermal decomposition of PNs, which was further confirmed by the relative incremental reactivity (RIR) analysis. The model basically reproduced the observed ANs by the refinement of related mechanisms, with isoprene contributing to its formation by 29.2 %. The observed PNs and total oxidants (O_x = NO₂ + O₃) showed a good positive correlation, with a ratio of PNs to O_x of 0.079, indicating a strong suppression of PNs chemistry to ozone formation. The model quantified the suppression of PNs chemistry on the peak ozone production rate by 21.3 % on average and inhibited ozone formation up to 20 ppbv in total. The RIR analysis suggests that the production of both O₃ and ANs was in the VOC-limited regime and highlights the importance of VOC control (especially aromatics) to mitigate photochemical pollution in Chengdu. The study deepens the understanding of photochemical pollution in urban areas of western China and further emphasizes the impacts of ONs chemistry on ozone pollution.

Atmospheric environment monitoring technology and equipment in China: A review and outlook

Accurate monitoring of the atmospheric environment and its evolution are important for understanding the sources, chemical mechanisms, and transport processes of air pollution and carbon emissions in China, and for regulatory and control purposes. This study gives an overview of atmospheric environment monitoring technology and equipment in China and summarizes the major achievements obtained in recent years. China has made great progress in the development of atmospheric environment monitoring technology and equipment with decades of effort. The manufacturing level of atmospheric environment monitoring equipment and the quality of products have steadily improved, and a technical & production system that can meet the requirements of routine monitoring activities has been initiated. It is expected that domestic atmospheric environment monitoring technology and equipment will be able to meet future demands for routine monitoring activities in China and provide scientific assistance for addressing air pollution problems.

Review of technologies and their applications for the speciated detection of RO2 radicals

Peroxy radicals (RO₂), which are formed during the oxidation of volatile organic compounds, play an important role in atmospheric oxidation reactions. Therefore, the measurement of RO₂, especially distinct species of RO₂ radicals, is important and greatly helps the exploration of atmospheric chemistry mechanisms. Although the speciated detection of RO₂ radicals remains challenging, various methods have been developed to study them in detail. These methods can be divided into spectroscopy and mass spectrometry technologies. The spectroscopy methods contain laser-induced fluorescence (LIF), UV-absorption spectroscopy, cavity ring-down spectroscopy (CRDS) and matrix isolation and electron spin resonance (MIESR). The mass spectrometry methods contain chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF), chemical ionization mass spectrometry (CIMS), CI-Orbitrap-MS and the third-generation proton transfer reaction-time-of-flight mass spectrometer (PTR3). This article reviews technologies for the speciated detection of RO₂ radicals and the applications of these methods. In addition, a comparison of these techniques and the reaction mechanisms of some key species are discussed. Finally, possible gaps are proposed that could be filled by future research into speciated RO₂ radicals.

Progress in quantitative research on the relationship between atmospheric oxidation and air quality

Atmospheric oxidizing capacity (AOC) is an essential driving force of troposphere chemistry and self-cleaning, but the definition of AOC and its quantitative representation remain uncertain. Driven by national demand for air pollution control in recent years, Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research. This paper will give a brief review of these developments. First, AOC indexes were established that represent apparent atmospheric oxidizing ability (AOIe) and potential atmospheric oxidizing ability (AOIp) based on aspects of macrothermodynamics and microdynamics, respectively. A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing, and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country. In addition, the detection of ground or vertical profiles for atmospheric OH·, HO₂·, NO₃· radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments. Moreover, laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O₃ and NO₂, which are typical oxidants in the surface/interface atmosphere, and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies, multiphase and multi-interface conditions were obtained. Finally, based on the GRAPES-CUACE adjoint model improved by Chinese scholars, simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized. Normalized numerical simulations of AOIe and AOIp were performed, and regional coordination of AOC was adjusted. An optimized plan for controlling O₃ and PM_2.5 was analyzed by scenario simulation.

Intercomparison of OH radical measurement in a complex atmosphere in Chengdu, China

Atmospheric oxidation is a driving force of complex air pollution, and accurate hydroxyl radical (OH) measurement is helpful in investigating the radical-cored photooxidation mechanism in the troposphere. A self-developed laser-induced fluorescence instrument by the Anhui Institute of Optics Fine Mechanics, Chinese Academy of Sciences (AIOFM-LIF), was able to measure OH concentration with high sensitivity and good time resolution, and a detection limit of 1.7 × 10⁵ cm^-3 (1σ, 30 s). A long-period, multi-level intercomparison of hydroxyl radical (OH) measurements between AIOFM-LIF and PKU-LIF (the Peking University laser-induced fluorescence system) was conducted in Chengdu, China. The measurement between two instruments was in excellent agreement in the 5-min time resolution. Linear regression analysis reported a linear slope of 0.96 with a 0.68 × 10⁶ cm^-3 offset, and the correlation coefficient R² was 0.85. The overall linearity with only a slight offset indicated a negligible influence on OH measurement. No noticeable artifacts from ozonolysis were observed under the condition of high ozone and ozonolysis-related compound concentrations. In addition to the subtraction of background signal through wavelength modulation, the dynamic correction on ozone photolysis interference ensured high intercomparison quality in both relatively constant and rapidly varying periods. Based on the reliability of OH_AIOFM and OH_PKU, comparisons under different oxidation-related species (NOx, VOCs, O₃, PM_2.5) levels and typical scenarios (rich-BVOC and high-reactivity) were carried out to evaluate the performance under complex atmospheres. A slightly higher drift was observed in a certain scenario, but the general data variability due to environmental changes did not affect the measurement accuracy. The intercomparison demonstrated that both systems are able to achieve reliable OH data under typical conditions of complex atmospheric pollution in China. Additional improvements are necessary for future intercomparisons in order to enhance the confidence in OH detection accuracy.

Atmospheric Autoxidation of Organophosphate Esters

Organophosphate esters (OPEs), widely used as flame retardants and plasticizers, have frequently been identified in the atmosphere. However, their atmospheric fate and toxicity associated with atmospheric transformations are unclear. Here, we performed quantum chemical calculations and computational toxicology to investigate the reaction mechanism of peroxy radicals of OPEs (OPEs-RO₂^•), key intermediates in determining the atmospheric chemistry of OPEs, and the toxicity of the reaction products. TMP-RO₂^• (R₁) and TCPP-RO₂^• (R₂) derived from trimethyl phosphate and tris(2-chloroisopropyl) phosphate, respectively, are selected as model systems. The results indicate that R₁ and R₂ can follow an H-shift-driven autoxidation mechanism under low NO concentration ([NO]) conditions, clarifying that RO₂^• from esters can follow an autoxidation mechanism. The unexpected autoxidation mechanism can be attributed to the distinct role of the ─(O)₃P(═O) phosphate-ester group in facilitating the H-shift of OPEs-RO₂^• from commonly encountered ─OC(═O)─ and ─ONO₂ ester groups in the atmosphere. Under high [NO] conditions, NO can mediate the autoxidation mechanism to form organonitrates and alkoxy radical-related products. The products from the autoxidation mechanism have low volatility and aquatic toxicity compared to their corresponding parent compounds. The proposed autoxidation mechanism advances our current understanding of the atmospheric RO₂^• chemistry and the environmental risk of OPEs.