Design, development and testing of a new low-cost and robust zenith-looking multi narrowband radiometer (ZEN) for AOD retrieval

A look-up table (LUT) methodology for AOD retrieval from zenith sky radiance has been developed and applied to AERONET Cimel sunphotometers from Santa Cruz de Tenerife, Izaña and Tamanrasset (Algeria) validating the results against AERONET AOD. The LUT was optimised for mineral dust aerosols. The methodology has been applied to a new low-cost and robust zenith-looking multi narrow-band radiometer developed in collaboration with SIELTEC S.L. company. Estimated AOD with the new prototypes demonstrated good results when validated against reference AOD from AERONET (Figure 1).

Figure 1.- AOD scatter plot at 440 nm between CE318-AERONET and CE318-LUT for Izaña (a), Santa Cruz (b) and Tamanrasset (c) stations in 2013 for 20º < SZA < 65º (a, b, c). The black solid lines are the least-square fits, and the dashed lines are the diagonals (y = x). The least-square fit parameters are shown in the legend (slope, intercept, correlation coefficient R and number of data N). AOD differences in 440 nm between CE318-AERONET and CE318-LUT with respect to CE318-AERONET (d, e, f) and SZA (º) (g, h, i). Reprinted from Almansa et al. (2017).

During 2018, we worked with a second improved version of the ZEN radiometer, the ZEN-R52, with a new design to minimize the reflections of sunlight inside the radiometer and that incorporates a new filter for the precipitable water vapor determination. Preliminary results show that this new prototype significantly improves AOD measurement of ZEN-R41 prooa consequence of reducing the field of view and a better stray light rejection. A new cloud screening algorithm has been also developed being validated with lidar and total-sky cameras. The agreement of PWV from ZEN-R52 with PWV from AERONET and GNSS is excellent. A detailed description of the ZEN-R52 prototype and new products evaluation results will be subject of a scientific paper to be published in 2019-2020.