Gas-Phase Ozone Oxidation of Monoterpenes: Gaseous and ...

254 JIANZHEN YU ET AL.
Figure 16. Formation mechanism for A 3 and A 7 in the O 3 /α-pinene reaction.
6.3. α-PINENE/O 3 AND 3 -CARENE/O 3
The formation mechanisms of products from ozone oxidation of α-pinene have
been described in an earlier paper (Yu et al., 1998). Here we only discuss the
formation pathways for the newly identified products (Figure 16). The formation of
A 3 arises from isomerization of the Criegee biradical, known as the ester channel
(Atkinson, 1997). Hydroxy pinonic acid is postulated to result from its aldehyde
precursor, 10-hydroxy-pinonaldehyde (A 12 ). We do not yet attempt to propose
formation mechanism for A 11 ,A 13 and A 14 , as their chemical structures are not
yet certain.
The formation pathways for most of the products in the 3 -carene/O 3 reaction
(Figure 17) are similar to those in the α-pinene/O 3 reaction, as a result of the
internal C=C bond common to both reactants. Those products unique to the 3 -
carene/O 3 reaction do not have obvious routes of formation. The presence of pinic
acidinthe 3 -carene/O 3 reaction system may derive from similar routes to those
responsible for the formation of pinic acid in the sabinene/O 3 reaction system.
7. Summary and Conclusions
This study has identified a substantial fraction of the particulate products from
ozone oxidation of each of the four monoterpenes: α-pinene, β-pinene, 3 -carene,
and sabinene. β-Pinene and sabinene are structurally analogous in that both are
bicyclic and have an external unsaturated bond where ozone oxidation takes place.
α-Pinene and 3 -carene also share one common structural moiety, an internal unsaturated
bond. A number of analogous product pairs, including major products,
have been identified for each pair of monoterpenes. These products are consistent
with the established understanding of the mechanism of ozone-alkene reactions.