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Limononic acid (3-isopropenyl-6-oxoheptanoic acid, LA), as an important precursor of secondary organic aerosols (SOA), has attracted extensive attention in the field of atmospheric chemistry. However, its microscopic oxidation mechanism is still unclear. In this study, the density functional theory calculations were conducted to study the oxidation mechanism of LA by ·OH in aqueous phase. The results show that the reactions of hydroxylation are more likely to occur than the dehydrogenation reaction, because of the lower free energy barrier (2.3–5.4 kcal mol−1). At 298 K, the total rate constant of the reaction initiated by ·OH is 1.06 × 1010 M−1 s−1, which fits the experimental value well. Among all reactions, the hydroxylation reaction in C 9 site of LA is the most favorable pathway, and the corresponding hydroxylation intermediate (IM4) can react with ·OH, H 2 O, dissolved O 2 , and HO 2 ·. Three important tropospheric free radicals (R·, RO· and RO 2 ·) are generated during the subsequent reaction process. Meanwhile alcohols, ketones, aldehydes, and oxidized acids can be formed in the overall reaction scheme. These products are the precursor for the formation of SOA, and this transformation process will increase the O/C ratio of aqueous phase SOA. This study has an important significance for understanding the oxidation mechanism of monoterpenoids in the atmospheric aqueous phase. [Display omitted] • OH-addition reaction of C 9 site is more like to occur of LA. • The intermediate can react with ·OH, H 2 O, dissolved O 2 , and HO 2. • Three important tropospheric free radicals (R·, RO· and RO 2 ·) are generated during the reaction process. • The transformation process will increase the O/C ratio of aqueous phase SOA. [ABSTRACT FROM AUTHOR]

In this paper, density-functional theory (DFT) was employed to investigate the reaction mechanism of SO 2 and NO 2 and the important role of intermediate N 2 O 4 formation. In addition, the effect of pure particulate surface (TiO 2) and particulate surface containing other atmospheric components (H 2 O, NH 3 and SO 4 2−) on the conversion of SO 2 and NO 2 to sulfate also was analyzed. The detailed information that intermediate N 2 O 4 acts as the oxidant was demonstrated in this oxidation reaction of SO 2 and NO 2 (as a rate-determining step). The pure particulate surface (TiO 2) has hardly effect on the oxidation process of SO 2 and N 2 O 4. Whereas, different amounts of H 2 O and NH 3 molecules as well as SO 4 2−, contained on the particulate surface, can effectively reduce the activation energy of oxidation step. And, the optimal process is that SO 2 is oxidized by cis-ONONO 2 with the energy barrier of 4.54 kcal/mol when one NH 3 molecule and one H 2 O molecule are contained on the TiO 2 (101) surface. When more H 2 O molecules are contained on particulate surface, SO 2 tends to form HSO 3 − first. However, HSO 3 − is more difficult to be further oxidized by asy-ONONO 2 than SO 2. This study gains more insight into the contribution of SO 2 and NO 2 to haze and the potential impact of atmospheric constituents (including H 2 O/NH 3 /TiO 2 and SO 4 2−/TiO 2) on the formation sulfate. Image 1 • Asy-ONONO 2 acted as oxidant is easily contained on the particulate surface. • The oxidation of SO 2 and asy-ONONO 2 is easy than HSO 3 −. • The oxidation step of SO 2 and N 2 O 4 is the rate-determining step. • Pure particulate surface has hardly effect on the rate-determining step. • Particulate containing H 2 O, NH 3 and SO 4 2− promote the reaction of SO 2 and N 2 O 4. [ABSTRACT FROM AUTHOR]