With stricter regulation of atmospheric volatile organic compounds (VOCs) originating from fossil fuel-based vehicles and industries, the use of volatile chemical products (VCPs) and the transformation mechanism of VCPs have become increasingly important to quantify air quality. Volatile methylsiloxanes (VMS) are an important class of VCPs and high-production chemicals. Using quantum chemical calculations and kinetics modeling, we investigated the reaction mechanism of peroxy radicals of VMS, which are key intermediates in determining the atmospheric chemistry of VMS. L2-RSiCH2O2 • and D3-RSiCH2O2 • derived from hexamethyldisiloxane and hexamethylcyclotrisiloxane, respectively, were selected as representative model systems. The results indicated that L2-RSiCH2O2 • and D3-RSiCH2O2 • follow a novel Si–C–O rearrangement-driven autoxidation mechanism, leading to the formation of low volatile silanols and high yield of formaldehyde at low NO/HO2 • conditions. At high NO/HO2 • conditions, L2-RSiCH2O2 • and D3-RSiCH2O2 • react with NO/HO2 • to form organic nitrate, hydroperoxide, and active alkoxy radicals. The alkoxy radicals further follow a Si–C–O rearrangement step to finally form formate esters. The novel Si–C–O rearrangement mechanism of both peroxy and alkoxy radicals are supported by available experimental studies on the oxidation of VMS. Notably, the high yield of formaldehyde is estimated to significantly contribute to formaldehyde pollution in the indoor environment, especially during indoor cleaning.

The adsorption of nitrobenzene at the surface of hexagonal ice was studied by grand canonical Monte Carlo (GCMC) simulations at 200 K by employing TIP5P water model and our modified force field for nitrobenzene. We found that the number of adsorbed nitrobenzene molecules gradually increases with relative pressure before the condensation point and the condensation precedes the monolayer adsorption saturation. The adsorption follows the Langmuir shape only up to a very low coverage. At this low coverage, the adsorption of the molecules occurs independently from each other to adsorption sites (called α sites), where adsorbed nitrobenzene molecules lie almost in parallel with the ice surface to facilitate strong electrostatic interactions with ice surface. More importantly, in the α-type adsorption, a typical O–H···π bond for the adsorption of aromatics on the ice surface is not preferable for nitrobenzene. With increasing surface coverage, additional adsorbed molecules do not take unoccupied α sites due to attractive interactions among adsorbates, inducing a deviation of the adsorption isotherm from the Langmuir shape. In addition, the calculated adsorption energy (−75.98 kJ/mol) for nitrobenzene agrees well with the value (−71.35 kJ/mol) from our validating quantum chemistry calculations, implying the reliability of the results from GCMC simulations.

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-RO 2• ), key intermediates in determining the atmospheric chemistry of OPEs, and the toxicity of the reaction products. TMP-RO 2• (R 1 ) and TCPP-RO 2 • (R 2 ) derived from trimethyl phosphate and tris(2-chloroisopropyl) phosphate, respectively, are selected as model systems. The results indicate that R 1 and R 2 can follow an H-shift-driven autoxidation mechanism under low NO concentration ([NO]) conditions, clarifying that RO 2• from esters can follow an autoxidation mechanism. The unexpected autoxidation mechanism can be attributed to the distinct role of the (O) 3 P(O) phosphate-ester group in facilitating the H-shift of OPEs-RO 2• from commonly encountered OC(O) and ONO 2 ester groups in the atmosphere. Under high [NO] conditions, NO can mediate the autoxidation mechanism to form organonitrates and alkoxy radicalrelated 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 RO 2• chemistry and the environmental risk of OPEs.

The mechanism and kinetics of OH-initiated oxidation of BTBPE, an alternative of PBDEs, were investigated.