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. The angular distribution of the light reflected by the Earth's surfaceinfluences top-of-atmosphere (TOA) reflectance values. This surfacereflectance anisotropy has implications for UV/Vis satellite retrievals ofalbedo, clouds, and trace gases such as nitrogen dioxide ( NO2 ). Theseretrievals routinely assume the surface to reflect light isotropically. Herewe show that cloud fractions retrieved from GOME-2A and OMI with the FRESCOand OMCLDO2 algorithms have an east–west bias of 10 % to 50 %, whichare highest over vegetation and forested areas, and that this bias originatesfrom the assumption of isotropic surface reflection. To interpret theacross-track bias with the DAK radiative transfer model, we implement thebidirectional reflectance distribution function (BRDF) from the Ross–Lisemi-empirical model. Testing our implementation against state-of-the-artRTMs LIDORT and SCIATRAN, we find that simulated TOA reflectance generallyagrees to within 1 %. We replace the assumption of isotropic surfacereflection in the equations used to retrieve cloud fractions over forestedscenes with scattering kernels and corresponding BRDF parameters from a dailyhigh-resolution database derived from 16 years' worth of MODIS measurements.By doing this, the east–west bias in the simulated cloud fractions largelyvanishes. We conclude that across-track biases in cloud fractions can beexplained by cloud algorithms that do not adequately account for the effectsof surface reflectance anisotropy. The implications for NO2 air massfactor (AMF) calculations are substantial. Under moderately polluted NO2 and backward-scattering conditions, clear-sky AMFs are up to 20 %higher and cloud radiance fractions up to 40 % lower if surfaceanisotropic reflection is accounted for. The combined effect of these changesis that NO2 total AMFs increase by up to 30 % for backward-scatteringgeometries (and decrease by up to 35 % for forward-scatteringgeometries), which is stronger than the effect of either contribution alone.In an unpolluted troposphere, surface BRDF effects on cloud fractioncounteract (and largely cancel) the effect on the clear-sky AMF. Our resultsemphasise that surface reflectance anisotropy needs to be taken into accountin a coherent manner for more realistic and accurate retrievals of clouds and NO2 from UV/Vis satellite sensors. These improvements will bebeneficial for current sensors, in particular for the recently launchedTROPOMI instrument with a high spatial resolution.

The importance of surface reflectance anisotropy for cloud and NO&lt;sub&gt;2&lt;/sub&gt; retrievals from GOME-2 and OMI

The importance of surface reflectance anisotropy for cloud and NO<sub>2</sub> retrievals from GOME-2 and OMI