Abstract
Abstract. Diurnal variations in hydroxyl (OH) in the stratosphere and mesosphere are analyzed using measurements from the Aura Microwave Limb Sounder (MLS). The primary 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) throughout the day, and OH diurnal variations are well approximated by an exponential function of the secant of SZA. Measured OH concentrations are fit to a function of the form exp[−βsec(SZA)], where the parameter β is a function of altitude. We examine the magnitude of β and show that it is related 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 formulation 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 photodissociation of water vapor.
Highlights
Hydroxyl (OH) is a key reactive species for photochemical reactions that regulate ozone throughout most of the stratosphere (∼20–55 km altitude) and mesosphere (∼55–90 km)
Diurnal variations of OH in the stratosphere and mesosphere as observed by Microwave Limb Sounder (MLS) are well described by exponential functions of the secant of solar zenith angle (SZA)
The OH diurnal variation is fit to a function of the form [OH] = [OH]oexp[−βsec(SZA)], where [OH]o and β are altitude-dependent fit parameters
Summary
Hydroxyl (OH) is a key reactive species for photochemical reactions that regulate ozone throughout most of the stratosphere (∼20–55 km altitude) and mesosphere (∼55–90 km). Note that Eq (3) is valid only for the direct solar radiation and neglects diffuse irradiance from cloud, aerosol, or Rayleigh scattering This diffuse component is generally small above 30 km and for wavelengths shorter than 320 nm (Meier and Anderson, 1982), conditions that are relevant to the production of HOx in the mid-to-upper stratosphere and mesosphere. Photolysis calculations indicate a typical impact of 5–15% due to scattering on O(1D) production near 30 km, but this decreases rapidly with increasing altitude and is less than 1% above 50 km Based on these considerations and on the combination of Eqs. Examination of the altitude dependence in this parameterization permits the interpretation of the observations as they relate to the photodissociation rates for ozone and water vapor
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