Inelastic scattering in ocean water

Over clear ocean waters, photons scattered within the water body contribute significantly to the upwelling flux. In addition to elastic scattering, inelastic Vibrational Raman Scattering (VRS) by liquid water is also playing a role and can have a strong impact on the spectral distribution of the outgoing radiance. Under clear-sky conditions, VRS has 5 an influence on trace gas retrievals from space-borne measurements of the backscat-tered radiance such as from e.g. GOME (Global Ozone Monitoring Experiment). The effect is particularly important for geo-locations with small solar zenith angles and over waters with low chlorophyll concentration. In this study, a simple ocean reflectance model (Sathendranath and Platt, 1998) ac-10 counting for VRS has been incorporated into a radiative transfer model. The model has been validated by comparison with measurements from a swimming-pool experiment dedicated to detect the effect of scattering within water on the outgoing radiation and also with selected data sets from GOME. The comparisons showed good agreement between experimental and model data and highlight the 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 data analysis, errors of about 36% are introduced for the slant column (SC) of BrO over clear ocean scenarios. The VRS-related error for the SC of HCHO is about 75%. Exemplarily DOAS retrievals of BrO from real GOME measurements including and excluding a VRS compensation led 20 to comparable results as in the sensitivity study, but with somewhat smaller differences between the two analyses. The results of this work suggest, that DOAS retrieval of atmospheric trace species from measurements of nadir viewing space-borne instruments have to take VRS scattering into account over waters with low chlorophyll concentrations, and that a simple 25 correction term is enough to reduce the errors to an acceptable level.