TY - JOUR
T1 - Computational Investigation of RO2 + HO2 and RO2 + RO2 Reactions of Monoterpene Derived First-Generation Peroxy Radicals Leading to Radical Recycling
AU - Iyer, Siddharth
AU - Reiman, Heidi
AU - Møller, Kristian H.
AU - Rissanen, Matti P.
AU - Kjaergaard, Henrik G.
AU - Kurtén, Theo
PY - 2018/12/13
Y1 - 2018/12/13
N2 - The oxidation of biogenically emitted volatile organic compounds (BVOC) plays an important role in the formation of secondary organic aerosols (SOA) in the atmosphere. Peroxy radicals (RO 2 ) are central intermediates in the BVOC oxidation process. Under clean (low-NO x ) conditions, the main bimolecular sink reactions for RO 2 are with the hydroperoxy radical (HO 2 ) and with other RO 2 radicals. Especially for small RO 2 , the RO 2 + HO 2 reaction mainly leads to closed-shell hydroperoxide products. However, there exist other known RO 2 + HO 2 and RO 2 + RO 2 reaction channels that can recycle radicals and oxidants in the atmosphere, potentially leading to lower-volatility products and enhancing SOA formation. In this work, we present a thermodynamic overview of two such reactions: (a) RO 2 + HO 2 → RO + OH + O 2 and (b) R′O 2 + RO 2 → R′O + RO + O 2 for selected monoterpene + oxidant derived peroxy radicals. The monoterpenes considered are α-pinene, β-pinene, limonene, trans-β-ocimene, and Δ 3 -carene. The oxidants considered are the hydroxyl radical (OH), the nitrate radical (NO 3 ), and ozone (O 3 ). The reaction Gibbs energies were calculated at the DLPNO-CCSD(T)/def2-QZVPP//ωB97X-D/aug-cc-pVTZ level of theory. All reactions studied here were found to be exergonic in terms of Gibbs energy. On the basis of a comparison with previous mechanistic studies, we predict that reaction a and reaction b are likely to be most important for first-generation peroxy radicals from O 3 oxidation (especially for β-pinene), while being less so for most first-generation peroxy radicals from OH and NO 3 oxidation. This is because both reactions are comparatively more exergonic for the O 3 oxidized systems than their OH and NO 3 oxidized counterparts. Our results indicate that bimolecular reactions of certain complex RO 2 may contribute to an increase in radical and oxidant recycling under high HO 2 conditions in the atmosphere, which can potentially enhance SOA formation.
AB - The oxidation of biogenically emitted volatile organic compounds (BVOC) plays an important role in the formation of secondary organic aerosols (SOA) in the atmosphere. Peroxy radicals (RO 2 ) are central intermediates in the BVOC oxidation process. Under clean (low-NO x ) conditions, the main bimolecular sink reactions for RO 2 are with the hydroperoxy radical (HO 2 ) and with other RO 2 radicals. Especially for small RO 2 , the RO 2 + HO 2 reaction mainly leads to closed-shell hydroperoxide products. However, there exist other known RO 2 + HO 2 and RO 2 + RO 2 reaction channels that can recycle radicals and oxidants in the atmosphere, potentially leading to lower-volatility products and enhancing SOA formation. In this work, we present a thermodynamic overview of two such reactions: (a) RO 2 + HO 2 → RO + OH + O 2 and (b) R′O 2 + RO 2 → R′O + RO + O 2 for selected monoterpene + oxidant derived peroxy radicals. The monoterpenes considered are α-pinene, β-pinene, limonene, trans-β-ocimene, and Δ 3 -carene. The oxidants considered are the hydroxyl radical (OH), the nitrate radical (NO 3 ), and ozone (O 3 ). The reaction Gibbs energies were calculated at the DLPNO-CCSD(T)/def2-QZVPP//ωB97X-D/aug-cc-pVTZ level of theory. All reactions studied here were found to be exergonic in terms of Gibbs energy. On the basis of a comparison with previous mechanistic studies, we predict that reaction a and reaction b are likely to be most important for first-generation peroxy radicals from O 3 oxidation (especially for β-pinene), while being less so for most first-generation peroxy radicals from OH and NO 3 oxidation. This is because both reactions are comparatively more exergonic for the O 3 oxidized systems than their OH and NO 3 oxidized counterparts. Our results indicate that bimolecular reactions of certain complex RO 2 may contribute to an increase in radical and oxidant recycling under high HO 2 conditions in the atmosphere, which can potentially enhance SOA formation.
U2 - 10.1021/acs.jpca.8b09241
DO - 10.1021/acs.jpca.8b09241
M3 - Journal article
C2 - 30449100
SN - 1089-5639
VL - 122
SP - 9542
EP - 9552
JO - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
JF - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
IS - 49
ER -