Effect of molecular multiple scattering and surface albedo on atmospheric photodissociation rates

Abstract

Results are presented from an extensive theoretical investigation aimed at evaluating the effect of molecular multiple scattering and surface albedo on photodissociation rates. Results are compared with similar calculations typical of most atmospheric photochemical models which only describe absorption in a direct solar beam. The effect of molecular multiple scattering and surface albedo on photodissociation rates, which can be sizable, depends strongly on solar zenith angle, surface albedo, altitude, and wave‐length regime. Various atmospheric photodissociation processes are categorized by spectral type based upon the wavelength regime in which the photodissociation process occurs. Three basic wavelength regimes are noted, and results characteristic of each regime are presented. Adjustment factors are provided for generalizing the pure absorption calculations.

Measurement of the photodissociation coefficient of NO2 in the atmosphere: I. Method and surface measurements

“…If diffuse light, arising from multiple scattering is included, J N02 can increase to about 7 x lC-3 S-1 • If, in addition, all of the light which reaches the ground is reflected isotropically, J N02 values as high as 2.8 x 10-2 S-1 are calculated due, largely, to the 2 cos Xo factor associated with Lambertian reflection. Other calculations of J NOa (e.g., Luther and Gelinas, 1976) show similar albedo and scattering effects. Recent calculations of the actinic flux by Nicolet et al (1982) show that the extraterrestrial flux at 360 nrn is enhanced by a factor of 4.25 at 0 km, Xo =0 deg and albedo = 1.0.…”

“…Moreover, a calculation of the actinic flux at the altitude under consideration must include the attenuation due to absorption by atmospheric gases and particles as well as the contributions from diffuse (scattered) radiation and reflections from clouds and the surface. The importance of these effects have been amply demonstrated (e.g., Luther and Gelinas, 1976;Mugnai et al, 1979), but the computational complexity involved in treating them, especially at large zenith angles, and the uncertainties in parameterization of surface and cloud albedo and aerosol loading make a direct measurement of J N02 preferable for a test of Equation…”

Measurement of the photodissociation coefficient of NO2 in the atmosphere: II, stratospheric measurements

“…The importance of these effects has been described (e.g. Luther and Gelinas, 1976;Mugnaietal., 1979). Model calculations ofJ NOz are therefore complex and involve the uncertainties in parameterizing surface and cloud albedo and scattering due to aerosols.…”

Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations

“…In most studies the impact of tropospheric clouds modifying optical paths is discussed in terms of energy deposition [Fouquart et al, 1990, and references therein] or photolysis [e.g., Luther and Gelinas, 1976, and references therein]. Besides potentially changing the atmospheric absorber densities (by (photo-) chemistry or convective transport) the effect of clouds on spectroscopic measurements is believed to be more important in modifying the optical paths.…”