Measurement of isotope shifts in C12 and C13 glyoxal absorption at visible wavelengths, and first application to DOAS retrievals

Glyoxal (CHOCHO) is an atmospheric trace gas produced by the radical-initiated oxidation of hydrocarbons. High resolution measurements of the glyoxal absorption spectrum in the visible wavelength range are needed as reference spectra to calibrate optical spectroscopic measurements of this gas. The UV-vis absorption spectra of C13-disubstituted glyoxal and ambient (C12) glyoxal were collected at moderately high (1 cm -1 ) resolution. Isotopic shifts of up to 0.289 nm were observed at the 451-456 nm strong band, but no shifts were observed at the 438-441 nm band. The ability to differentiate between atmospheric glyoxal and 13 CHO 13 CHO (C13 glyoxal) is assessed through spectral analysis of synthetic glyoxal spectra with added noise under various conditions, including the presence/absence of other gases that overlap the glyoxal spectral features, various forms of experimental noise and measurement resolution, and by varying the spectral window used for analysis. It is found that decreasing resolution linearly increases (R 2 >0.95) deviation from known C12 and C13 glyoxal concentrations, while varying baseline correction polynomials does not produce such a trend. Furthermore, the presence of H2O has no significant effect on the glyoxal retrieval, while addition of NO2 can cause ~10% deviations. These deviations border on significance: they are slightly less than 3σ of the retrieval error. It is thus concluded that the now available C13 absorption spectrum provides a means to distinguish C12 and C13 glyoxal through multispectral techniques such as cavity enhanced DOAS.