Free Radical Stages in the Chain Mechanism of Atmospheric Photonucleation of Biogenic Aldehydes

The atmospheric transformations of vegetation-emitted organic compounds are initiated by their interaction with photolytically generated short-lived free radicals. The chain process, which is a sequence of radical stages, leads to the formation of condensable products as the nuclei of aerosol phase. The free radicals generated during the photolysis of benzaldehyde and biogenic aldehydes, including aromatic ones (salicylic, ortho- and para-anisic), were identified under laboratory conditions. Chromatographic analysis of the products formed after the introduction of additional free radicals shows that the composition and amount of condensable products change. The field measurements of the concentrations of aldehydes and the products of their photochemical decomposition under sunlight were carried out, and the compounds formed in the interaction of initial aldehydes with free radicals were detected. These products can be employed to estimate the concentrations of free radicals even below the limit of radical detection by physicochemical methods. The rates of free radical generation and sink were shown to change with altitude in the troposphere. The approach taking into account the vertical transport of air masses and the corresponding changes in photolysis rate constants has been developed. The formation of condensable products is the chain termination stage in the whole process of atmospheric photonucleation of biogenic aldehydes. The new data on the altitudinal variation in the concentrations of short-lived free radicals allow calculating photonucleation rates for biogenic aldehydes at different altitudes. The developed kinetic schemes can be used to simulate the formation of organic atmospheric aerosol in the troposphere taking into account the vertical transport of air masses.