Wavelength-dependent isotope fractionation in visible light O3 photolysis and atmospheric implications

The O-17 and O-18 isotope fractionation associated with photolysis of O-3 in the Chappuis band was determined using a broadband light source with cutoff filters at 455, 550, and 620 nm and narrowband light sources at 530, 617, and 660 nm. The isotope effects follow a mass- dependent fractionation pattern ( delta O-17/delta O-18 = 0.53). Contrary to theoretical predictions, fractionations are negative for all wavelength ranges investigated and do not change signs at the absorption cross-section maximum. Our measurements differ from theoretical calculations by as much as 34% in (18)epsilon(O3+hv) = ((18)J/(16)J - 1). The wavelength dependence is also weaker than predicted. Photo-induced fractionation is strongest when using a low-wavelength cutoff at 620 nm with (18)epsilon(O3+hv) = -26.9(+/- 1.4)parts per thousand. With decreasing wavelength, fractionation values diminish to (18)epsilon(O3+hv) = -12.9(+/- 1.3)parts per thousand at 530 nm. Results from an atmospheric model demonstrate that visible light photolysis is the most important tropospheric sink of O-3, which thus contributes about one sixth to the ozone enrichment.