A re-evaluation of the ClO/Cl2O2 equilibrium constant based on stratospheric in-situ observations

[摘要] In-situ measurements of ClO and its dimer carried out during the SOLVEII/VINTERSOL-EUPLEX and ENVISAT Validation campaigns in the Arctic winter2003 suggest that the thermal equilibrium between the dimer formation anddissociation is shifted significantly towards the monomer compared to thecurrent JPL 2002 recommendation. Detailed analysis of observations made inthermal equilibrium allowed to re-evaluate the magnitude and temperaturedependence of the equilibrium constant. A fit of the JPL format forequilibrium constants yields K_EQ=3.61x10^-27exp(8167/T), butto reconcile the observations made at low temperatures with the existinglaboratory studies at room temperature, a modified equation,K_EQ=5.47x10^-25(T/300)^-2.29exp(6969/T), is required. This formatcan be rationalised by a strong temperature dependence of the reactionenthalpy possibly induced by Cl₂O₂ isomerism effects. Atstratospheric temperatures, both equations are practically equivalent. Usingthe equilibrium constant reported here rather than the JPL 2002recommendation in atmospheric models does not have a large impact onsimulated ozone loss. Solely at large zenith angles after sunrise, a smalldecrease of the ozone loss rate due to the ClO dimer cycle and an increasedue to the ClO-BrO cycle (attributed to the enhanced equilibrium ClOconcentrations) is observed, the net effect being a slightly stronger ozoneloss rate.

[学科分类] 大气科学