Reactivity of aromatic contaminants towards nitrate radical in tropospheric gas and aqueous phase

Aromatic compounds (ACs) give a substantial contribution to the anthropogenic emissions of volatile organic compounds. Nitrate radicals (NO₃) are significant oxidants in the lower troposphere during nighttime, with concentrations of (2-20) × 10⁸ molecules cm^-3. In this study, the tropospheric gas and liquid phase reactions of ACs with nitrate radical are investigated using theoretical computational methods, which can give a deep insight into the reaction mechanisms and kinetics. Results show that the reactivity of ACs with nitrate radicals decreases as the electron donating characteristics of the functional group on the ACs decrease, as ΔG^≠ of the reaction with NO₃ increasing from -1.17 to 17.84 kcal mol^-1. The reaction of NO₃ towards ACs in the aqueous phase is more preferable, with the atmospheric lifetime 0.07-1281 min. An assessment of the aquatic toxicity of ACs and their degradation products indicated that the risk of their degradation products remains and should be given more attention.

Open path incoherent broadband cavity-enhanced measurements of NO3 radical and aerosol extinction in the North China Plain

We describe the observation of the NO₃ radical using an incoherent broadband cavity-enhanced absorption spectrometer in an open-path configuration (OP-IBBCEAS) in a polluted summer environment in continental China. The instrument was installed 17 m above the ground at the top of a residential complex near the CAREBeijing-NCP 2014 site in Wangdu, Hebei province, about 200 km southwest of Beijing over the period 28 to 30 June 2014. The separation between the transmitter and receiver components of the instrument was 335 cm and the effective pathlength in clean reference air was ~3.4 km. NO₃ was detected above the detection limit on all three nights when the instrument was operational. The maximum mixing ratio measured was ~175 pptv with a detection sensitivity of ~36 pptv for measurements with an average acquisition time of 10 min. While most extractive instruments try to avoid interferences arising from aerosol extinction, the open path configuration has advantages owing to its ability to detect trace gases even in the presence of aerosol loading. Moreover, concurrent retrieval of aerosol optical extinction is possible from analysis of the absorption magnitude of the oxygen B-band at 687 nm. The experimental setup, its calibration, data acquisition, and analysis procedure are discussed, and the results presented here demonstrate the sensitivity and specificity that can be achieved at high spatial and temporal resolution using the novel configuration of IBBCEAS in the open path.

Room-temperature and temperature-dependent QSRR modelling for predicting the nitrate radical reaction rate constants of organic chemicals using ensemble learning methods

Experimental determinations of the rate constants of the reaction of NO3 with a large number of organic chemicals are tedious, and time and resource intensive; and the development of computational methods has widely been advocated. In this study, we have developed room-temperature (298 K) and temperature-dependent quantitative structure-reactivity relationship (QSRR) models based on the ensemble learning approaches (decision tree forest (DTF) and decision treeboost (DTB)) for predicting the rate constant of the reaction of NO3 radicals with diverse organic chemicals, under OECD guidelines. Predictive powers of the developed models were established in terms of statistical coefficients. In the test phase, the QSRR models yielded a correlation (r(2)) of >0.94 between experimental and predicted rate constants. The applicability domains of the constructed models were determined. An attempt has been made to provide the mechanistic interpretation of the selected features for QSRR development. The proposed QSRR models outperformed the previous reports, and the temperature-dependent models offered a much wider applicability domain. This is the first report presenting a temperature-dependent QSRR model for predicting the nitrate radical reaction rate constant at different temperatures. The proposed models can be useful tools in predicting the reactivities of chemicals towards NO3 radicals in the atmosphere, hence, their persistence and exposure risk assessment.

Investigation of terpinolene + ozone or terpinolene + nitrate radical reaction products using denuder/filter apparatus

Terpinolene's (1-methyl-4-(propan-2-ylidene)cyclohexene) reaction with ozone or the nitrate radical was investigated using a denuder/filter apparatus in order to characterize gas-phase and particulate reaction products. Identification of the reaction products (i.e., aldehydes, ketones, dicarbonyls and carboxylic acids) was made using two derivatization methods; O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) to derivatize the carbonyl products or 3-Ethyl-1-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDC) and 2,2,2-trifluoroethylamine hydrochloride (TFEA) to derivatize the carboxylic acid products. Proposed carbonyl products for ozonolysis of terpinolene are: 4-methylcyclohex-3-en-1-one, 2-hydroxy-4-methylcyclohex-3-en-1-one, glyoxal, methyl glyoxal, 3-oxobutanal, and 6-oxo-3-(propan-2-ylidene)heptanal. Proposed carbonyl products for nitrate radical reaction of terpinolene are: 2-hydroxy-4-methylcyclohex-3-en-1-one, glyoxal, methyl glyoxal, and 4-oxopentanal. No carboxylic acid products were detected with either oxidizing reactant.