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Recent research on atmospheric particle formation has shown substantial discrepancies between observed and modeled atmospheric sulfate levels. This is because models mostly consider sulfate originating from SO2 oxidation by •OH radicals in mechanisms catalyzed by solar radiation while ignoring other pathways of non-radical SO2 oxidation that would substantially alter atmospheric sulfate levels. Herein, we use high-level quantum chemical calculations based on density functional theory and coupled cluster theory to show that monoethanolamine (MEA), a typical alkanolamine pollutant released from CO2 capture technology, can facilitate the conversion of atmospheric SO2 to sulfate in a non–•OH–radical oxidation mechanism. The initial process is the MEA-induced SO2 hydrolysis leading to the formation of HOSO2−•MEAH+. The latter entity is thereafter oxidized by ozone (O3) and nitrogen dioxide (NO2) to form HSO4−•MEAH+, which is an identified stabilizing entity in sulfate-based aerosol formation. Results show that the HOSO2−•MEAH+ reaction with O3 is kinetically and thermodynamically more feasible than the reaction with NO2. The presence of an additional water molecule further promotes the HOSO2−•MEAH+ reaction with O3, which occurs in a barrierless process, while it instead favors HONO formation in the reaction with NO2. The investigated pathway highlights the potential role alkanolamines may play in SO2 oxidation to sulfate, especially under conditions that are not favorable for •OH production, thereby providing an alternative sulfate source for aerosol modeling. The studied mechanism is not only relevant to sulfate formation and may effectively compete with reactions with sulfur dioxide and hydroxyl radicals under heavily polluted and highly humid conditions such as haze events, but also an important pathway in MEA removal processes.

Methacrolein oxide (MACR-OO), the isopropenyl substituted Criegee intermediate (CI), is one product of isoprene ozonolysis. In this work, we report MACR-OO's photo-isomerization paths with electronic structure calculation at the CASSCF and MS-CASPT2 levels and trajectory surface-hopping (TSH) nonadiabatic dynamics simulation at the CASSCF level. Our calculated results show that the ring-closure is the dominant photo-induced unimolecular isomerization of MACR-OO in the S 1 state. In addition, a new photo-induced ring-closure to heterocyclopentane dioxole in syn_syn -MACR-OO is found. The findings of MACR-OO are expected to deepen the understanding of the substituted CIs and their photochemistry.

Acetaldehyde (CH 3 CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids through reaction with nitrogen containing chemical species. Quantum chemical and reaction kinetics studies are reported for acetaldehyde formation from the chemical reaction of C( 3 P) with a methanol molecule adsorbed at the eighth position of a cubic water cluster. We present extensive quantum chemical calculations for total spin S = 1 and S = 0. The UωB97XD/6-311++G(2d,p) model chemistry is employed to optimize the structures, compute minimum energy paths and zero-point vibrational energies of all reaction steps. For the optimized structures, the calculated energies are refined by CCSD(T) single point computations. We identify four transition states on the triplet potential energy surface (PES), and one on the singlet PES. The reaction mechanism involves the intermediate formation of CH 3 OCH adsorbed on the ice cluster. The rate limiting step for forming acetaldehyde is the C-O bond breaking in CH 3 OCH to form adsorbed CH 3 and HCO. We find two positions on the reaction path where spin crossing may be possible such that acetaldehyde can form in its singlet spin state. Using variational transition-state theory with multidimensional tunnelling we provide thermal rate constants for the energetically rate limiting step for both spin states and discuss two routes to acetaldehyde formation. As expected, quantum effects are important at low temperatures., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The hydrolysis reaction of CH 2 OO with water and water clusters is believed to be a dominant sink for the CH 2 OO intermediate in the atmosphere. However, the favorable route for the hydrolysis of CH 2 OO with water clusters is still unclear. Here global minimum searching using the Tsinghua Global Minimum program has been introduced to find the most stable geometry of the CH 2 OO(H 2 O) n (n = 1-4) complex firstly. Then, based on these stable complexes, favorable hydrolysis of CH 2 OO with (H 2 O) n (n = 1-4) has been investigated using the quantum chemical method of CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2d,2p) and canonical variational transition state theory with small curvature tunneling. The calculated results have revealed that, although the contribution of CH 2 OO + (H 2 O) 2 is the most obvious in the hydrolysis of CH 2 OO with (H 2 O) n (n = 1-4), the hydrolysis of CH 2 OO with (H 2 O) 3 is not negligible in atmospheric gas-phase chemistry as its rate is close to the rate of the CH 2 OO + H 2 O reaction. The calculated results also show that, in a clean atmosphere, the CH 2 OO + (H 2 O) n (n = 1-2) reaction competes well with the CH 2 OO + SO 2 reaction at 298 K when the concentrations of (H 2 O) n (n = 1-2) range from 20% relative humidity (RH) to 100% RH, and SO 2 is 2.46 × 10 11 molecules per cm 3 . Meanwhile, when the RH is higher than 40%, it is a new prediction that the CH 2 OO + (H 2 O) 3 reaction can also compete well with the CH 2 OO + SO 2 reaction at 298 K. Besides, Born-Oppenheimer molecular dynamics simulation results show that all the favorable channels of the CH 2 OO + (H 2 O) n (n = 1-3) reaction cannot react on a time scale of 100 ps in the NVT simulation. However, the NVE simulation results show that the CH 2 OO + (H 2 O) 3 reaction can be finished well at 8.5 ps, indicating that the gas phase reaction of CH 2 OO + (H 2 O) 3 is not negligible in the atmosphere. Overall, the present results have provided a definitive example of how the favorable hydrolysis of important atmospheric species with (H 2 O) n (n = 1-4) takes place, which will stimulate one to consider the favorable hydrolysis of water and water clusters with other Criegee intermediates and other important atmospheric species.

The H 2 O 2 + HO → HO 2 + H 2 O reaction is an important reservoir for both radicals of HO and HO 2 catalyzing the destruction of O 3 . Here, this reaction assisted by NH 3 and HCOOH catalysts was explored using the CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling. Two possible sets of mechanisms, (i) one-step routes and (ii) stepwise processes, are possible. Our results show that in the presence of both NH 3 and HCOOH catalysts under relevant atmospheric temperature, mechanism (i) is favored both energetically and kinetically than the corresponding mechanism (ii). At 298 K, the relative rate for mechanism (i) in the presence of NH 3 (10, 2900 ppbv) and HCOOH (10 ppbv) is respectively 3-5 and 2-4 orders of magnitude lower than that of the water-catalyzed reaction. This is due to a comparatively lower concentration of NH 3 and HCOOH than H 2 O which indicates the positive water effect under atmospheric conditions. Although NH 3 and HCOOH catalysts play a negligible role in the reservoir for both radicals of HO and HO 2 catalyzing the destruction of O 3 , the current study provides a comprehensive example of how acidic and basic catalysts assisted the gas-phase reactions., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)