Development and application of a possible mechanism for the generation of cis-pinic acid from the ozonolysis of alpha- and beta-pinene

Recent experimental studies have identified cis-pinic acid (a C-9 dicarboxy lic acid) as a condensed-phase product of the ozonolysis of both alpha- and beta-pinene, and it is currently believed to be the most likely degradatio n product leading to the prompt formation of new aerosols by nucleation. Th e observed timescale of aerosol formation appears to require that cis-pinic acid is a first-generation product, and a possible mechanism for its forma tion has therefore been developed. The key step in the proposed mechanism r equires that the isomerisation of a complex C-9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO2 an d a C-8 organic radical: [GRAPHICS] Thermodynamic and kinetic arguments are presented, on the basis of semi-emp irical electronic structure calculations, which support this proposed mecha nism, and thereby the competition between the two pathways. The transfer of the labile aldehydic H atom is shown to be especially facile in this case because it occurs though an unstrained transition state; this feature can i n turn be attributed to the cis-substitution of the four-membered ring, whi ch enforces the steric proximity of the acyl-oxy and aldehyde groups. The m echanism can explain the formation of cis-pinic acid from both alpha-and be ta-pinene, because the acyl-oxy radical is likely to be formed following th e decomposition of excited Criegee biradicals formed in both systems. It is also possible that a similar isomerisation reaction of a complex C-10 alph a-carbonyl oxy radical by a 1,8 H atom shift might explain the very recentl y observed formation of cis- 10-hydroxy-pinonic acid from alpha-pinene ozon olysis, and this possibility is also explored. An existing detailed scheme describing the degradation of alpha-pinene (part of the Master Chemical Mec hanism, MCM) is updated to include the proposed cis-pinic acid and cis-10-h ydroxy-pinonic acid formation mechanisms, and the values of several uncerta in parameters are adjusted on the basis of reported yields of a series of o rganic products from the ozonolysis of alpha-pinene. The updated degradatio n scheme is incorporated into a boundary layer box model, and representativ e ambient concentrations of the organic acids and other oxygenated products are calculated for a range of representative conditions appropriate to the boundary layer over central Europe. The simulated concentrations of the or ganic acids in general, and cis-pinic acid in particular, are strongly depe ndent on the level of NOx, and suggest that new aerosol formation from the oxidation of alpha-pinene is likely to be more favoured at lower NOx levels . (C) 2000 Elsevier Science Ltd. All rights reserved.