Inelastic scattering in ocean water and its impact on trace gas retrievals from satellite data

[摘要] Over clear ocean waters, photons scattered within the water bodycontribute significantly to the upwelling flux. In addition to elastic scattering, inelastic Vibrational RamanScattering (VRS) by liquid water is also playing a role and can have a strong impact on the spectral distribution of the outgoingradiance. Under clear-sky conditions,VRS has an influence on trace gas retrievals from space-borne measurements of thebackscattered radiance such as from e.g. GOME (Global Ozone Monitoring Experiment). The effect is particularly important forgeo-locations with small solar zenith angles and over waters with low chlorophyll concentration.

In this study, a simple ocean reflectance model (Sathyendranathand Platt, 1998) accounting for VRS has been incorporated into a radiative transfermodel. The model has been validated by comparison with measurements from a swimming-pool experiment dedicated to detectthe effect of scattering within water on the outgoing radiation and also with selected data sets fromGOME. The comparisons show good agreement between experimental and model data and highlightthe important role of VRS.

To evaluate the impact of VRS on trace gas retrieval, a sensitivity study was performed on synthetic data. If VRS is neglected in the dataanalysis, errors of more than 30% are introduced for the slant column (SC) ofBrO over clear ocean scenarios. Exemplarily DOAS retrievals of BrO from real GOME measurementsincluding and excluding a VRS compensation led to comparable results as in the sensitivity study, but with somewhat smaller differencesbetween the two analyses.

The results of this work suggest, that DOAS retrievals of atmospheric trace species from measurements of nadir viewingspace-borne instruments have to take VRS scattering into account over waters with low chlorophyll concentrations, and that a simplecorrection term is enough to reduce the errors to an acceptable level.

[学科分类] 大气科学