Formation of Highly Oxidized Molecules from NO3 Radical Initiated Oxidation of Delta-3-Carene: A Mechanistic Study

TY - JOUR

T1 - Formation of Highly Oxidized Molecules from NO3 Radical Initiated Oxidation of Delta-3-Carene: A Mechanistic Study

AU - Draper, Danielle C.

AU - Myllys, Nanna

AU - Hyttinen, Noora

AU - Moller, Kristian H.

AU - Kjaergaard, Henrik G.

AU - Fry, Juliane L.

AU - Smith, James N.

AU - Kurten, Theo

PY - 2019/8/15

Y1 - 2019/8/15

N2 - NO3 radical oxidation of most monoterpenes is a significant source of secondary organic aerosol (SOA) in many regions influenced by both biogenic and anthropogenic emissions, but there are very few published mechanistic studies of NO3 chemistry beyond simple first generation products. Here, we present a computationally derived mechanism detailing the unimolecular pathways available to the second generation of peroxy radicals following NO3 oxidation of δ-3-carene, defining generations based on the sequence of peroxy radicals formed rather than number of oxidant attacks. We assess five different types of unimolecular reactions, including peroxy and alkoxy radical (RO2 and RO) hydrogen shifts, RO2 and RO ring closing (e.g., endoperoxide formation), and RO decomposition. Rate constants calculated using quantum chemical methods indicate that this chemical system has significant contribution from both bimolecular and unimolecular pathways. The dominant unimolecular reactions are endoperoxide formation, RO H-shifts, and RO decomposition. However, the complexity of the overall reaction is tempered as only 1 or 2 radical propagation pathways dominate the fate of each radical intermediate. Chemical ionization mass spectrometry (CIMS) measurements using the NO3 - reagent ion during δ-3-carene + NO3 chamber experiments show products consistent with each of the three types of unimolecular reactions predicted to be important from the computational mechanism. Moreover, the SIMPOL group contribution method for predicting vapor pressures suggests that a majority of the closed-shell products inferred from these unimolecular reactions are likely to have low enough vapor pressure to be able to contribute to SOA formation.

AB - NO3 radical oxidation of most monoterpenes is a significant source of secondary organic aerosol (SOA) in many regions influenced by both biogenic and anthropogenic emissions, but there are very few published mechanistic studies of NO3 chemistry beyond simple first generation products. Here, we present a computationally derived mechanism detailing the unimolecular pathways available to the second generation of peroxy radicals following NO3 oxidation of δ-3-carene, defining generations based on the sequence of peroxy radicals formed rather than number of oxidant attacks. We assess five different types of unimolecular reactions, including peroxy and alkoxy radical (RO2 and RO) hydrogen shifts, RO2 and RO ring closing (e.g., endoperoxide formation), and RO decomposition. Rate constants calculated using quantum chemical methods indicate that this chemical system has significant contribution from both bimolecular and unimolecular pathways. The dominant unimolecular reactions are endoperoxide formation, RO H-shifts, and RO decomposition. However, the complexity of the overall reaction is tempered as only 1 or 2 radical propagation pathways dominate the fate of each radical intermediate. Chemical ionization mass spectrometry (CIMS) measurements using the NO3 - reagent ion during δ-3-carene + NO3 chamber experiments show products consistent with each of the three types of unimolecular reactions predicted to be important from the computational mechanism. Moreover, the SIMPOL group contribution method for predicting vapor pressures suggests that a majority of the closed-shell products inferred from these unimolecular reactions are likely to have low enough vapor pressure to be able to contribute to SOA formation.

KW - NO3 Radical

KW - Monoterpenes

KW - Computational

KW - Mechanism

KW - SOA Formation

U2 - 10.1021/acsearthspacechem.9b00143

DO - 10.1021/acsearthspacechem.9b00143

M3 - Journal article

SN - 2472-3452

VL - 3

SP - 1460

EP - 1470

JO - ACS Earth and Space Chemistry

JF - ACS Earth and Space Chemistry

IS - 8

ER -