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

Abstract

Abstract.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.

Column Integrated Water Vapor and Aerosol Load Characterization with the New ZEN-R52 Radiometer

“…The ZEN-AOD-lookup table (LUT) method previously described in [37] was used to retrieve AOD from zenith sky measurements. This method compares the ZSR measured at four of the five ZEN-R52 nominal wavelengths (440, 500, 675 and 870 nm) with a LUT of precomputed ZSRs at these wavelengths.…”

“…In a sensitivity study carried out by [37], the authors showed that the uncertainty of the ZEN-AOD-LUT method increases with AOD, having a standard uncertainty ranging from 0.06 for AOD 500 ≈ 0.5 to 0.15 for AOD 500 ≈ 1.0, when an instrumental uncertainty of 5% is considered.…”

“…The ZEN-R41 is a radiometer jointly developed by Sieltec Canarias and the IARC, conceived to monitor AOD from sky radiance measurements at the zenithal direction and at different spectral bands. A complete description of the ZEN-R41 can be found in [37]. The ZEN-R52 (Figure 2) is a newer instrument version whose hardware and software have been significantly improved and optimized.…”

“…We present, in this paper, the new ZEN-R52 zenith-looking narrow-band radiometer, an improved version of the ZEN-R41 prototype previously presented in [37]. This is a radiometer specifically designed to expand ground-based aerosol load and PWV observations in remote areas, filling the current observational gaps in the current ground-based networks and therefore improving the capability for satellite product validation and aerosol model evaluation and assimilation.…”

“…As many authors have already proved ( [13,[33][34][35][36][37], sun photometric measurements at 940 nm can be used to derive PWV. In this case, the Beer-Lambert-Bouguer law, defined to be applied to regions with a smooth spectral variation in atmospheric transmittance inside the band pass, must be modified to account for the water vapor transmittance (T w,λ ):…”