Diurnal variation of stratospheric short-lived species

Licentiate thesis, 2012

Diurnal variation of stratospheric short-lived species Maryam Khosravi Chalmers University of Technology Department of Earth and Space Sciences Abstract The depletion of ozone in the stratosphere has a direct impact on the amount of ultraviolet radiation reaching the Earth’s surface. The ozone abundance and distribution is controlled by the photo-chemical reactions and catalytic cycles involving halogens (chlorine and bromine), odd hydrogen and odd nitrogen species as well as by atmospheric transport. An introduction to ozone related chemistry of the stratosphere and modelling of short-lived species using photo-chemical models is presented. A one dimensional (1D) atmospheric model is used in two distinct studies: modeling of short-lived species in the Arctic lower stratosphere (paper I) and in the tropical mid to upper stratosphere (paper II). The first part of this thesis describes the diurnal variation of chlorine monoxide, ClO, which is the most important short-lived species controlling ozone in the polar lower stratosphere during winter and early-spring. The ClO-dimer cycle, involving ClO and its nighttime reservoir Cl2O2, contributes to about 75%of the polar ozone loss. ClO measurements from an airborne submillimeter radiometer in the Arctic twilight have been compared with the results from a 1D photo-chemical model (MISU-1D), in order to validate the model and to test the kinetics of the reactions controlling the partitioning of chlorine species during the course of a day. The results show that cross sections leading to faster photolysis rates of Cl2O2 match best with the ClO observations. This is consistent with the recent version of the chemical kinetics evaluation by the Jet Propulsion Laboratory. Slower photolysis rates can not be reconciled with the observations since active chlorine higher than the total available chlorine would be required. The model reproduces higher nighttime ClO than the observations, however the nighttime ClOmodelled using recent JPL recommendations of the thermal equilibrium constant agree within the uncertainty range of the observations. The sensitivity of the model to the assumed albedo and temperature are also tested. Neither the temperature nor the albedo uncertainties allow us to reconcile the model with the lower observed nighttime ClO. Moreover, it is found that the ClO-BrO cycle decreases ClO mostly around sunrise and sunset. The second part of the thesis presents the partitioning and diurnal variation of chlorine, bromine, hydrogen, nitrogen and oxygen species in the tropics from the stratosphere to the lower mesosphere. Model results of the diurnal variation of HOCl (as one of the chlorine reservoirs), the related short-lived species ClO and HO2 and HCl (as the main chlorine reservoir) for the tropics and three altitudes (35, 45 and 55 km) are compared with measurements from five satellite instruments. The model results generally agree with the observations both in terms of the absolute values and the differences between day and night.