A new zenith-looking narrow-band radiometer-based system (ZEN) for dust aerosol optical depth monitoring

• Published in: Atmospheric Measurement Techniques
• Main Authors: Almansa, A. Fernando, Cuevas, Emilio, Torres, Benjamín, Barreto, África, García, Rosa D., Cachorro, Victoria E., Frutos, Ángel M., López, César, Ramos, Ramón
• Format: Text
• Language: English
• Published: 2018
• Subjects: Aerosol Robotic Network
• Online Access: https://doi.org/10.5194/amt-10-565-2017; https://amt.copernicus.org/articles/10/565/2017/

A new zenith-looking narrow-band radiometer based system (ZEN), conceived for dust aerosol optical depth (AOD) monitoring, is presented in this paper. The ZEN system comprises a new radiometer (ZEN-R41) and a methodology for AOD retrieval (ZEN-LUT). ZEN-R41 has been designed to be stand alone and without moving parts, making it a low-cost and robust instrument with low maintenance, appropriate for deployment in remote and unpopulated desert areas. The ZEN-LUT method is based on the comparison of the measured zenith sky radiance (ZSR) with a look-up table (LUT) of computed ZSRs. The LUT is generated with the LibRadtran radiative transfer code. The sensitivity study proved that the ZEN-LUT method is appropriate for inferring AOD from ZSR measurements with an AOD standard uncertainty up to 0.06 for AOD 500 nm ∼ 0.5 and up to 0.15 for AOD 500 nm ∼ 1.0, considering instrumental errors of 5 %. The validation of the ZEN-LUT technique was performed using data from AErosol RObotic NETwork (AERONET) Cimel Electronique 318 photometers (CE318). A comparison between AOD obtained by applying the ZEN-LUT method on ZSRs (inferred from CE318 diffuse-sky measurements) and AOD provided by AERONET (derived from CE318 direct-sun measurements) was carried out at three sites characterized by a regular presence of desert mineral dust aerosols: Izaña and Santa Cruz in the Canary Islands and Tamanrasset in Algeria. The results show a coefficient of determination ( R 2 ) ranging from 0.99 to 0.97, and root mean square errors (RMSE) ranging from 0.010 at Izaña to 0.032 at Tamanrasset. The comparison of ZSR values from ZEN-R41 and the CE318 showed absolute relative mean bias (RMB) < 10 %. ZEN-R41 AOD values inferred from ZEN-LUT methodology were compared with AOD provided by AERONET, showing a fairly good agreement in all wavelengths, with mean absolute AOD differences < 0.030 and R 2 higher than 0.97.