Aerosol optical depth from spectral direct normal irradiance measurements in Montevideo, Uruguay.

Abstract

Aerosols are liquid or solid particles with diameters between 2.5 and 10 µm suspended in the lower layers of the atmosphere. Aerosol Optical Depth (AOD) is a relevant parameter that quantifies their concentration in the atmosphere. It is usually estimated from sun photometer measurements at specific wavelengths. The objective of this work is to implement a simple inversion algorithm to retrieve AOD at six different wavelengths (340, 380, 440, 500, 675 and 870 nm) using solar direct normal spectral irradiance ground measurements from a relatively low cost collimated spectroradiometer (EKO MS-711) at a low-altitude site in Montevideo, Uruguay. The results obtained are compared with AERONET products for the same site, including AOD and Angström coefficient. The results of AOD for all wavelengths show a consistent negative mean bias (MBD, unitless), between −0.005 and −0.015, and dispersion (RMSD, unitless) between 0.021 and 0.015 (to be compared to a mean reference AOD of 0.097). These metrics improve considerably for very clear days, MBD up to ± 0.001 and RMSD under 0.007 (to be compared to a mean reference AOD of 0.058). These results are considered to be a first step in implementing the methodology and acquiring local knowledge about AOD retrievals using relatively simple instrumentation.