JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. C7, PAGES 4973-4980, JUNE 20, 1982 Observationof the Diurnal Variation of Atmospheric Ozone J. L. LEAN 1 Department of Physics, University of Adelaide, Adelaide, Australia 5001 Ozonedensities in the stratosphere andmesosphere havebeen derived from broad-band photometer measurements of Hartley band absorption of middle ultraviolet radiation. Seven rockets were launchedduring October-November 1979 from Wallops Island. Six rockets, each carrying one detector comprising two UV photometers, were launched at different times of the day. A seventh rocket, with three similardetectors each havingthree UV photometers, was launchedat the time of a full moonand provided estimates of the nighttime ozonedensities. Results from these rocket flights form a basis for investigating ozone diurnal variations. The number of flights provide greater statistical reliabilityfor the ozoneprofiles than is generally afforded from in situ measurements with a single rocket. During the night, an enhancement in ozone densities occurred at altitudes above about 50 km. At 70 km, for example, the nighttime ozone wasdetermined to be a factorof 6.4 greater than at sunset. In addition, these experiments suggest thatnear 40 km themagnitude of theozone density at noon may be greater by 10-15% than the nighttime concentration. INTRODUCTION AtmoSpheric ozone is important becauseit absorbssolar ultraviolet radiation, preventingthis harmful radiation from reaching the earth's surface. That pollutantsoriginating in the troposphere and diffusingup to the stratosphere may effect irreversible depletionof ozone has promptedefforts toward quantitatively understanding its temporal and spacial behavior. Since the first theoreticalpostulation of the existenceof atmospheric ozone by Chapman [1930], the photochemistry of the stratosphere and mesosphere has been advanced and refined to include numerous complex reactions between ozone and free radicals. Ozone is formed by atomic oxygen recombining with molecular oxygen in the presence of a third body. This occurs in the mesosphere and stratosphere where atomic oxygen is producedby photodissociation of molecular oxgyen in the Schumann-Runge bandsand Herz- berg continuum, respectively. Destruction of ozone arises partly from its photodissociation by radiationat wavelengths less than 1.08/•m and partly via chemicalactivity involving oxygen, hydrogen, nitrogen, and chlorine radicals. Photodis- sociation of ozone by radiationbetween 198and 312.5 nm is especiallyimportant because it produces metastable atomic oxygen O(•D), which is responsible for theformation of OH and NO, two of the primary radicals that participate in ozone-destroyingcatalytic cycles. Explanation of the ozone distribution observed in the atmosphere demands that a complex chemical reaction schemebe considered. In the mesosphere there is strong coupling between odd oxygen(O + 03) and odd hydrogen (H + OH + HO2). Inclusion of the latter in the chemical reaction scheme acts to reduce the ozone concentration relative to that predictedby the Chapman mechanism alone. Additional catalytic destruction cycles involving odd nitro- gen must be introduced to understand the observed ozone concentrationsin the stratosphere. • Now at Cooperative Institutefor Research in Environmental Sciences,University of Colorado/NOAA, Boulder, Colorado 80309. Copyright 1982by the AmericanGeophysical Union. Paper number 2C0192. 0148-0227/82/002C-0192505.00 Atomic oxygen is the instigator of the photochemistry that produces ozone. Photodissociation of 02, and also 03, provides atomic oxygen at a rate that is determined, at a particular altitude, by the solar radiation available at that altitude. Penetration of solar radiationin the atmosphere, according to the Lambert-Beer law, dependson the solar zenith angle.Thusthe atomic oxygen production is expected to exhibit diurnal variations that may initiate associated changes in the ozone concentration. During the nighttime,dissociation rates are considerably reduced, and, in the mesosphere,below 80 km, atomic oxygen disappears rapidly via very fast recombination with molecular oxygen. This results in a nighttime enhancement of 03 that can be qualitatively represented by solving time- dependent continuity equations pertaining to the major and minor atmospheric constituents. Althoughdifferentcalcula- tions [see, for example, Shimazakiand Laird, 1970;Hunt, 1971; Thomas and Bowman, 1972; Keneshea et al., 1979] agree about the broad trend of ozone's diurnal variations in the mesosphere, there are important differences between the various modelsin the amplitudeof the ozone concentration at different times and altitudes. Reproducing detailed fea- tures of ozone's diurnal behavior is difficult because an extensivephotochemical scheme with a large rangeof time constants must be considered and because the•specific effects of transport probably require a more sophisticated parameterization than is afforded by the vertical eddy diffu- sion coefficient. Ozone in the stratosphere, at altitudes below about50 km, is generally considered to be diurnally invariant [Park and London, 1974; Logan et al., 1978; Prather, 1981]. Thi• is because the overlappingatmosphere shieldsthe ozone at these lower altitudes from dissociating solar radiation. Re- cently, however,detailed calculations by Herman [ 1979] and also Groves and Tuck [1980] have suggested that there may be significant daytime ozone variationsdown to altitudesof about30 km, and experimental observations of a 5% deple- tion in the ozone concentration at 42 km near sunrise have been reported by Aimedieu et al. [1981]. Ozone'sbehavior near40-45 km is of special importance because it is at thesealtitudes, according to the predictions of atmospheric models, that ozone is particularly sensitive to perturbations, both natural and anthropogenic. Chlorofiuo- 4973