Reaction Mechanisms for the Atmospheric Oxidation of Monocyclic Aromatic Compounds

Abstract

The atmospheric oxidation of aromatic compounds leads to highly oxidized intermediates with oxygenation ratios O:C nearing or exceeding unity, and contributes efficiently to the formation of secondary organic aerosols and ozone. The intermediates in the mechanism differ strongly from those in aliphatic or unsaturated compounds oxidation, with mul-tifunctionalization showing cyclic peroxides, epoxides, hydroperoxides, resonance-stabilized alkyl radicals, peroxy-and alkoxy radicals, and other functionalities. The multistep oxidation sequence is complex, with a highly branched mechanism where the relative importance of the competing reactions is highly site-and stereo-specific. Several of the critical steps are reversible, leading to kinetics, and product yields that depend on the reaction conditions. In this chapter, the literature data on the elementary reaction steps in this mechanism are summarized and tabulated, based mostly on the available theoretical work, with support from experimental studies. The influence of substitution on the oxidation mechanism is discussed. Significant progress was made in recent years to understand the chemistry, with several novel reaction steps shown to be of importance. At the same time, the quantification of the reaction kinetics remains limited to information on a subset of compounds, with no available structure-activity relationships predicting substitution-, site-, and stereo-specific rate coefficients.