Photolysis of atmospherically important carbonyls: Quantum yield measurements using an NO radical tracer method

Carbonyls are an important class of volatile organic compounds (VOC) in the atmosphere, being both directly emitted and produced by the oxidation of other VOCs. UV-B carbonyl photolysis produces radicals and so is an important driver of atmospheric radical cycles, that are a key route for the breakdown of primary pollutants and formation of secondary pollutants such as ozone and organic aerosol.

However, photochemical data concerning atmospheric photolysis of carbonyl compounds (i.e. photolysis cross-sections and quantum yields) are limited, and air quality research tools such as the Master Chemical Mechanism (MCM; mcm.york.ac.uk) must rely on using parameters from a small number of surrogate compounds to estimate photolysis rates for a larger suite of photo-labile VOCs.

To address this, we have developed a new laboratory flow reactor that utilises UV-LED technology to study photolysis quantum yields. This uses nitric oxide as a tracer for the peroxy radical photoproducts, assisted by a zero-dimensional chemical box model of the reactor system. Preliminary results for acetaldehyde and butanone show reasonable agreement with literature values, and the technique is fast and relatively easy to apply to a range of previously understudied compounds such as longer chain and branched ketones.

A wider understanding of the structure-reactivity trends of carbonyl quantum yields will improve atmospheric modelling capabilities, including better predictions of atmospheric impacts on air quality and human health.