Nitrogen-Containing Compounds Enhance Light Absorption of Aromatic-Derived Brown Carbon

The formation of secondary brown carbon (BrC) is chemically complex, leading to an unclear relationship between its molecular composition and optical properties. Here, we present an in-depth investigation of molecular-specific optical properties and aging of secondary BrC produced from the photooxidation of ethylbenzene at varied NOx levels for the first time. Due to the pronounced formation of unsaturated products, the mass absorption coefficient (MAC) of ethylbenzene secondary organic aerosols (ESOA) at 365 nm was higher than that of biogenic SOA by a factor of 10. A high NOx level ([ethylbenzene]0/[NOx]0 –1) was found to significantly increase the average MAC300–700nm of ESOA by 0.29 m2 g–1. The data from two complementary high-resolution mass spectrometers and quantum chemical calculations suggested that nitrogen-containing compounds were largely responsible for the enhanced light absorption of high-NOx ESOA, and multifunctional nitroaromatic compounds (such as C8H9NO3 and C8H9NO4) were identified as important BrC chromophores. High-NOx ESOA underwent photobleaching upon direct exposure to ultraviolet light. Photolysis did not lead to the significant decomposition of C8H9NO3 and C8H9NO4, indicating that nitroaromatic compounds may serve as relatively stable nitrogen reservoirs and would effectively absorb solar radiation during the daytime

Role of (H₂O)_<i>n</i> (<i>n</i> = 1–2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH3CH2OH + •OH reaction has been investigated at the CCSD­(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH3CH2OH, the bared reaction proceeds through three elementary paths, producing CH2CH2OH, CH3CH2O, and CH3CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH3CHOH is favored over the temperature range of 216.7–425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH3CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH2CH2OH, CH3CH2O, and CH3CHOH are computed to be 5.2 × 10–13, 8.6 × 10–14, and 9.0 × 10–11 cm3 molecule–1 s–1, respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3–5 and 6–8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH3CH2OH + •OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it