Abstract
Abstract. The photolysis rate constant of dichlorine peroxide (ClOOCl, ClO dimer) JClOOCl is a critical parameter in catalytic cycles destroying ozone (O3) in the polar stratosphere. In the atmospherically relevant wavelength region (λ > 310 nm), significant discrepancies between laboratory measurements of ClOOCl absorption cross sections and spectra cause a large uncertainty in JClOOCl. Previous investigations of the consistency of published JClOOCl with atmospheric observations of chlorine monoxide (ClO) and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis, and thus could only constrain the ratio of JClOOCl over the ClOOCl formation rate constant krec. Here, we constrain the atmospherically effective JClOOCl independent of krec, using ClO measured in the same air masses before and directly after sunrise during an aircraft flight that was part of the RECONCILE field campaign in the winter 2010 from Kiruna, Sweden. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the increase in ClO concentrations is significantly faster than expected from JClOOCl based on the absorption spectrum proposed by Pope et al. (2007), but does not warrant cross sections larger than recently published values by Papanastasiou et al. (2009). In particular, the existence of a significant ClOOCl absorption band longwards of 420 nm is not supported by our observations. The observed night-time ClO would not be consistent with a ClO/ClOOCl thermal equilibrium constant significantly higher than the one proposed by Plenge et al. (2005).
Highlights
Once chlorine is activated on polar stratospheric clouds (Solomon et al, 1986) or background aerosol (Drdla and Muller, 2010) in the polar stratosphere and sunlight is available, photochemical ozone loss occurs essentially via two catalytic cycles, the chlorine monoxide (ClO) dimer cycle (Molina and Molina, 1987): ClO + M krec −−−−ClOOCl kdiss (R1)ClOOCl+hν −J−C−lO−O→Cl Cl + ClOO (R2)ClOO + M → Cl + O2 + M (R3)2 × (Cl + O3 → ClO + O2) (R4) 2O3+hν→ 3O2Published by Copernicus Publications on behalf of the European Geosciences Union
Previous studies investigating the consistency of JClOOCl resulting from laboratory measurements with atmospheric observations of ClO and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis rate
From the observed [ClO] increase, JClOOCl was estimated and compared with [ClO] increases resulting from four sets of ClOOCl absorption cross sections/scaled spectra obtained for the atmospheric conditions prevailing during the flight
Summary
Once chlorine is activated on polar stratospheric clouds (Solomon et al, 1986) or background aerosol (Drdla and Muller, 2010) in the polar stratosphere and sunlight is available, photochemical ozone loss occurs essentially via two catalytic cycles, the ClO dimer cycle (Molina and Molina, 1987): ClO. The photolysis rate constants estimated for absorption cross sections and spectra i–iv and solar zenith angle (SZA) equal 91◦ (solid line) and 92◦ (dashed line) are shown in panel (c). Previous studies investigating the consistency of JClOOCl resulting from laboratory measurements with atmospheric observations of ClO and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis rate. From the observed [ClO] increase, JClOOCl was estimated and compared with [ClO] increases resulting from four sets of ClOOCl absorption cross sections/scaled spectra obtained for the atmospheric conditions prevailing during the flight.
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