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Determination of Photolysis Frequencies and Quantum Yields for Small Carbonyl Compounds using the EUPHORE Chamber

Small aldehydes are formed during the photochemical oxidation of many VOC’s, olefins and terpenes in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the removal of these compounds conducting to the formation of peroxy radicals and in the case of photolytic decomposition, either to stable molecules or free radicals. The photolysis frequencies for various small aldehydes were measured in the EUPHORE Smog Chambers by photolysing the aldehydes with natural sunlight. The actinic flux during the experiment was measured with a spectroradiometer. The decay of aldehydes and formation of products were analysed by FTIR spectroscopy, gas chromatography and HPLC. The major products are explained in the case of acetaldehyde, propionaldehyde and i-butyraldehyde by a mechanism involving a primary dissociation step which leads to the formation of free radicals. The product analysis for photolysis experiments of butyraldehyde, pentanal, 2-Methylbutyraldehyde and 3Methylbutyraldehyde indicates that for these molecules two primary photodissociation steps occur which gives either stable molecules or free radicals. Integrated quantum yields can be calculated from the ratio of the theoretical photolysis frequency using the measured radiation data and known absorption cross-sections by assuming a quantum yield of unity and the measured photolytic decay rate. The results obtained can be employed in numerical models which describe the tropospheric degradation of these compounds in order to assess the importance of the additional radical production on the atmospheric oxidation capacity and ozone formation potential of the precursors VOCs.