Isotope effects in visible light photolysis and chemical destruction of ozone

Abstract

Ozone (O3) plays a key role for the radiative balance of the earth and for chemical processes in the atmosphere. In addition, it has a very peculiar isotopic composition: The heavy stable isotopes of oxygen, 17O and 18O are strongly enriched in O3 and the enrichments are of almost equal magnitude, which is referred to as mass-independent fractionation. Such a fractionation effect is very uncommon on earth. Whereas the strong and unusual isotope effect in O3 formation is well established, the effect of O3 destruction reactions to its isotopic composition is not well understood. We investigated the isotope effects resulting from O3 photolysis in the Chappuis band and from chemical removal via the reaction and O+O3. To separately quantify isotope effects in O3 photolysis and O+O3 we carried out experiments with pure O3, but also with carbon monoxide (CO) as bath gas. The CO acts as a sink for O(3P) via the reaction CO + O(3P) à CO2 and thus suppresses chemical removal of O3. We obtained fractionations for O3 photolysis of 18εhν = −16.1 (±1.4)‰ and 17εhν = −8.05 (±0.7)‰ and 18εO+O3 = −11.9 (±1.4)‰ and 18εO+O3 = −5.95 (±0.7)‰ for O+O3. The largest uncertainty in these experiments arose from an unidentified additional loss of O3 inexperiments with CO as bath gas. We also determined the (low resolution) wavelength dependence of the fractionation associated with photolysis of O3 in the Chappuis band using a broadband light source with cutoff filters at 455, 550, 620nm and narrow band light sources at 530, 617 and 660nm. In contrast to theoretical calculations the leftover O3 was enriched at all wavelengths and the wavelength dependence was weaker than predicted. The photo-induced fractionation was strongest when using a cut-off filter at 620nm (18εhν = −26.9(±1.4)‰) and diminished with decreasing wavelength (18εhν = −12.9(±1.3)‰ at 530 nm). We also performed O3 photochemical recycling experiments in a large excess of O2 where O3 is in isotope equilibrium between production and destruction. Based on the knowledge of the fractionation in the visible light photolysis, we derived the isotope effect of the formation reactions only. This was done over a wide rage of temperatures (181 to 314K). The fractionation in photolysis was shown not to depend on temperature. Our results on the temperature dependence of the isotope effect in O3 formation had much smaller error that previously accepted values. Large differences were found for the mass-independent isotope signal in O3 formation. The observed isotope effects in O3 formation were further evaluated to derive relative rate coefficients for individual isotope specific O3 formation channels. Our results present strong constraints on the relative rate coefficients for the O3 formation channels in which 16O reacts with 16O18O and 16O17O, respectively, of 1.225 (±0.002) and 1.150 (±0.002). As a final result, we present an internally consistent set of relative rate coefficients that is in agreement with the most recent observations of individual rate coefficient ratios.