Evidence for 13-carbon enrichment in oxalic acid via iron catalyzed photolysis in aqueous phase

To investigate the effect of photochemical aging on the stable carbon isotopic ratio (delta C-13) of oxalic acid (OxA), a dominant organic species in atmospheric aerosols, we conducted a laboratory photolysis of OxA under H2O2-Fe3+(Fe2+)-UV system in aqueous phase and measured delta C-13 of remaining OxA. Our results showed that a significant photolysis of OxA occurred with OH radical but the isotopic fractionation of OxA was insignificant. In contrast, in the presence of Fe3+ (Fe2+), we found a significant enrichment of C-13 in remaining OxA. We also found that kinetic isotope effect (KIE) of OxA largely depends on photochemical age (irradiation time) and concentration ratios of OxA to iron; 3.20 +/- 0.49 parts per thousand (2.18 +/- 1.18 parts per thousand) and 21.62 +/- 5.41 parts per thousand in 90 min and 180 min irradiation, in which OxA and Fe3+ (Fe2+) ratios were 50:1 and 200:1, respectively. The enrichment of C-13 in remaining OxA was more significant during the photolysis catalyzed by Fe3+ (7 parts per thousand) than by Fe2+ (3 parts per thousand) in 90 min irradiation when OxA and iron ratios are the same (50:1). This study provides a laboratory evidence for the isotopic enrichment of C-13 in OxA with photochemical aging. This approach is useful for better interpretation of atmospheric isotopic measurements in terms of the extent of atmospheric processing of aerosols. Citation:Pavuluri, C. M., and K. Kawamura (2012), Evidence for 13-carbon enrichment in oxalic acid via iron catalyzed photolysis in aqueous phase, Geophys. Res. Lett., 39, L03802, doi:10.1029/2011GL050398.