Quantum yields of CHDO above 300 nm

Ernst-Peter Röth and Luc Vereecken

Abstract

The photolysis of mono-deuterated formaldehyde, CHDO, is a critical process in the deuterium enrichment of stratospheric hydrogen formed from methane. In this work, a consistent description of the quantum yields of the molecular and radical channels of the CHDO photolysis is deduced from literature data. The fluorescence measurements of Miller and Lee (1978) provided a first data set to deduce the product quantum yields. An alternative analysis is based on the measured quantum-yield spectrum for the radical channel of the CD₂O photolysis by McQuigg and Calvert (1969), which is corrected for wavelength dependency and combined with the CH₂O quantum-yield spectrum to provide an approximation for CHDO. Both approaches provide consistent results. Finally, the findings of Troe (1984, 2007) enable the specification of the pressure dependence of the quantum yield for CH₂O and CD₂O and, hence, for CHDO. We find that the radical channel does not show a pressure dependence, whereas the molecular channel is dominated by tunneling and quenching processes. Simplified representations are given that are readily implemented in kinetic atmospheric models. As an example of their application, the altitude dependence of the ratio of and is provided. Also, the importance of the photolysis of formaldehyde on the yield of HD in the atmosphere is shown through the altitudinal dependence of the isotopic fractionation.