Extended mechanisms for the oxidation of aromatics in the atmosphere
Final Report Abstract
In this work, the atmospheric oxidation processes of aromatic compounds are investigated, using quantum chemical and theoretical kinetic methodologies. It was found that the OH-initiated oxidation of phenol proceeds predominantly by addition on the ortho-position, with some contribution of abstraction of the hydroxy-H-atom, leading to the stable benzoxy radical. The subsequent chemistry of the OH-phenol adducts proceeds by syn-ortho O2 addition, ring closure, and a second anti-O2-addition. Formation of HO2 is found to be minor. The formation of epoxides from the RO2 intermediates appears to be important. Dissociation reactions of the alkoxy radicals formed from the RO2 intermediates lead to non-ring-retaining, highly oxygenated products, who could play a key role in formation of secondary organic aerosols (SOA). A spin-off of our studies of phenol was the investigation of ortho-nitrophenol, which is currently thought to act as a minor source of HONO and OH in the atmosphere. Our calculations greatly extend earlier literature data, improving the accuracy of the data and characterizing several additional reaction pathways on both the lowest-lying singlet S 0 and triplet T1 states. At atmospheric pressures, it was found that the S0 ground state potential energy surface is not active in photodissociation. The lowest-lying triplet state, T1, yields OH as the main photodissociation product, with NO and HONO secondary products at about 25% of the OH yield. Formation of NO2, and H- and O-atoms remains negligible. Photodissociation yields are low at UV-A wavelengths, but UV-B energies allow for significantly enhanced product formation. o-Nitrophenol is not a strong source of HONO in the atmosphere, and is insufficient to explain the discrepancy between observed HONO and [HONO] calculated from atmospheric kinetic models. The OH-initiated chemistry of toluene is likewise a complex mechanism, comparable to that of phenol. Accurate kinetic treatment of this system requires explicit analysis of the internal rotors, and cyclic conformers of the intermediates. We found that the reaction proceeds predominantly by OH- addition in ortho-position of the methyl group, and proceeds largely similar to the phenol oxidation described above. We found no accessible pathway than enable signifant dealkylation of the adduct, contrary to some experimental work. Formation of epoxides, and chain reactions leading to highly oxydized products are the important reaction channels, and could affect the SOA formation in the poluted environments. Partial auto-oxidation reactions remain active even in high-NOx condition, though the products may not be stable enough to partition to the aqueous phase.
Publications
- “Atmospheric oxidation mechanism of phenol: a theoretical study” Symposium on Atmospheric Chemical Mechanisms, Davis (CA, USA), Dec. 2014
H.K. Chakravarty, L. Vereecken
- “Overview of recent theoretical calculations on aromatic chemistry” Symposium on Atmospheric Chemical Mechanisms, Davis (CA, USA), Dec. 2014
L. Vereecken
- “Theoretical investigation of the OH-initiated atmospheric oxidation of phenol” 23th International Symposium on Gas Kinetics, Szeged (Hungary), July 2014
H.K. Chakravarty, L. Vereecken
- Theoretical Study on the Formation of H- and O-Atoms, HONO, OH, NO, and NO2 from the Lowest Lying Singlet and Triplet States in Ortho-Nitrophenol Photolysis. Int. J. Chem. Kinet. 48, 785-795, 2016
L. Vereecken, H. K. Chakravarty, B. Bohn, J. Lelieveld
(See online at https://doi.org/10.1002/kin.21033) - Theory-based studies on reactions in the troposphere 24th International Symposium on Gas Kinetics, York (U.K.), July 2016
Luc Vereecken, I.-H. Acir, T. Brauers, H. Chakravarty, H.-P. Dorn, I. Gensch, R. Häseler, A. Hofzumahaus, M. Kaminski, X. Li, A. Lutz, S. Nehr, A. Novelli, T. Piansawan, F. Rohrer, R. Wegener, A. Kiendler-Scharr, A.Wahner
