Atmospheric Chemistry and Physics Hydroxyl in the stratosphere and mesosphere – Part 1: Diurnal variability

Abstract. Diurnal variations in hydroxyl (OH) in the strato-sphere and mesosphere are analyzed using measurements from the Aura Microwave Limb Sounder (MLS). The pri-mary driver for OH diurnal variations is the ultraviolet actinic flux that initiates the photochemical production of reactive hydrogen species. The magnitude of this flux is governed largely by changes in solar zenith angle (SZA) through-out the day, and OH diurnal variations are well approxi-mated by an exponential function of the secant of SZA. Mea-sured OH concentrations are fit to a function of the form exp[−βsec(SZA)], where the parameter β is a function of al-titude. We examine the magnitude of β and show that it is re-lated to the optical depths of ultraviolet absorption by ozone and molecular oxygen. Values of β from SLIMCAT model simulations show the same vertical structure as those from MLS and the average level of agreement between model and measurements is 6%. The vertical profile of β from MLS can be represented by a simple analytic formulation involving the ozone and water vapor photodissociation rates. This formu-lation is used to infer the altitude dependence of the primary production mechanisms for OH: the reaction of excited-state atomic oxygen with water vapor versus the direct photodis-sociation of water vapor.