Abstract. Extraterrestrial spectral response calibration of a multi-filter rotating
shadow band radiometer (MFRSR) under pristine Amazonian Forest atmosphere
conditions was performed using the Langley plot method. 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 its application to accurate monitoring of aerosol
particles. Once calibrated, the MFRSR measurements were applied to retrieve
aerosol particle columnar optical properties, specifically aerosol optical depth (AODλ) and
Ångström exponent (AE), which were evaluated against retrievals from
a collocated Cimel Sun photometer belonging to the AErosol RObotic NETwork
(AERONET). Results obtained revealed that pristine Amazonian conditions are
able to provide MFRSR extraterrestrial spectral response with relative
uncertainty lower than 1.0 % in visible channels. The worst estimate (air
mass =1) for absolute uncertainty in AODλ retrieval varied
from ≈0.02 to ≈0.03, depending on the assumption regarding
uncertainty for MFRSR direct normal irradiance measured at the surface. The
obtained root mean square error (RMSE ≈0.025) from the evaluation of
MFRSR retrievals against AERONET AODλ was, in general, lower than
estimated MFRSR AODλ uncertainty, and close to the uncertainty of
AERONET field Sun photometers (≈0.02).