Effect of molecular anisotropy on backscattered ultraviolet radiance

Abstract

The effect of molecular anisotropy on backscattered UV (BUV) radiances is computed by accounting for it in both Rayleigh optical thickness and the scattering-phase matrix. If the effect of molecular anisotropy is included only in the optical thickness and not in the phase matrix, then for high sun (θ(0) ∼ 0°), the nadir radiance (I(0)) leaving the top of the atmosphere is approximately 1.8% higher than the radiance (I(op)) computed with the effect included in the phase matrix. For very low sun (θ(0) > 80°), I(0) is approximately 2.3% lower than I(op). For off-nadir radiances the relative increase (decrease) depends on both the local zenith angle as well as the azimuth angle. Also, an increase in the surface reflectivity decreases the effect of molecular anisotropy on the upwelling radiances. Exclusion of the anisotropy factor in the Rayleigh-phase matrix has very little effect (<1%) on ozone retrieval from the BUV-type instruments. This is because of the ratio technique used in the retrieval algorithm, which practically cancels out the anisotropy effect.