<p><strong>Abstract.</strong> Extraterrestrial spectral response calibration of a Multi-Filter Rotating Shadow band Radiometer (MFRSR) under Amazonian Forest atmosphere pristine conditions using the Langley plot method was performed and evaluated. The MFRSR is installed in central Amazonia as part of a long-term monitoring site, which was used in the context of the GoAmazon2014/5 Experiment. It has been operating continuously since 2011 without regular extraterrestrial calibration, preventing

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... s application to accurate monitoring of aerosol particles. Once calibrated, the MFRSR measurements were applied to retrieve aerosols particles columnar optical properties, specifically Aerosol Optical Depth (AOD_&amp;lambda;) and Ångström Exponent (AE), which were evaluated against retrievals from a collocated CIMEL sunphotometer belonging to the AErosol RObotic NETwork (AERONET). Results obtained revealed that Amazonian pristine conditions are able to provide MFRSR extraterrestrial spectral response with relative uncertainty lower than 1.0<span class="thinspace"></span>% at visible channels. The worst estimate (air mass<span class="thinspace"></span>=<span class="thinspace"></span>1) for absolute uncertainty in AOD_&amp;lambda; retrieval varied from ~<span class="thinspace"></span>0.02 to ~<span class="thinspace"></span>0.03, depending on the assumption regarding uncertainty for MFRSR direct-normal irradiance measured at the surface. Obtained Root Mean Square Errors (RMSE<span class="thinspace"></span>~<span class="thinspace"></span>0.025) from the evaluation of MFRSR retrievals against AERONET AOD_&amp;lambda; were, in general, lower than estimate MFRSR AOD_&amp;lambda; uncertainties, and close to AERONET field sunphotometers (~<span class="thinspace"></span>0.02).</p>