The oxidation mechanism of gas-phase ozonolysis of limonene in the atmosphere†

Limonene with endo- and exo-double bonds is a significant monoterpene in the atmosphere and has high reactivity towards O-3. We investigated the atmospheric oxidation mechanism of limonene ozonolysis using a high level quantum chemistry calculation coupled with RRKM-ME kinetic simulation. The additions of O-3 can take place at both the endo- and exo-double bonds with a branching ratio of 0.87 : 0.13, forming four major highly energized CIs* (named Syn-2a*, Syn-2b*, Anti-2b* and Anti-2c*) with the relative higher fractions of 0.21 : 0.35 : 0.27 : 0.11. A yield of 4% for Limona-ketone was obtained as well. For the unimolecular isomerization pathways of limonene + O-3 -> POZs -> CIs* -> SOZ, VHP, or dioxirane, five, one, or none of the internal rotations are treated as hindered internal rotors for CIs*. We obtained percentages of 0.59 : 0.18 : 0.14 in total for separate isomerization routes in the formation of VHPs, dioxirane and SOZs from CIs* using the fourth-order Runge-Kutta method. Additionally, a yield of similar to 5% was acquired for stabilized CIs compiling the fractions of different addition routes. About similar to 10% of stabilized Anti-2b would isomerize to VHP and 90% would isomerize to SOZs. Isomerization to VHPs dominates the fate of stabilized Syn-2a, Syn-2b and Anti-2c. The overall yield of OH radicals was 0.61. Our study suggested a yield of 0.17 for stabilized SOZs and 0.18 for dioxirane, although both their fates are ambiguous.

Automated summary

The study found that limonene ozonolysis forms highly energized CIs* and generates Limona-ketone. The percentages of different isomerization routes from CIs* to VHPs, dioxirane, and SOZs are 0.59:0.18:0.14, with an overall yield of OH radicals at 0.61.