The TOMS aerosol retrieval is probably the most promising approach to global aerosol remote sensing, because surface contributions in the ultraviolet are small not only over oceans but also over land. However, due to unresolved discrepancies for derived surface (minimum) reflectance maps among different TOMS instruments, currently mid-visible aerosol optical depths are only accurate to within 0.1. The use of larger surface reflectance contributions in the retrieval for this comparison in part may explain the TOMS trend to smaller aerosol optical depths (than AERONET). Underestimates are especially large, when optical depths are largest. This seems to indicate that the sub-pixel cloud-screening is extremely conservative and rejects the largest aerosol optical depths. Such rejection suggests that the largest aerosol optical depths are usually found in (TOMS-pixel) regions with clouds. Another important factor in TOMS retrievals is the strength of the background molecular scattering, requiring accurate data on aerosol altitude and site altitude. Thus, without orographic adjustments, aerosol optical depth overestimates are expected for high-altitude sites (e.g. Mongu).

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