Aerosol retrievals from the EKO MS-711 spectral direct irradiance measurements and corrections of the circumsolar radiation

Between April and September 2019, AOD at several wavelengths (340, 380, 440, 500, 675, and 870 nm) was determined from spectral direct UV–visible normal solar irradiance (DNI) performed with an EKO MS-711 grating spectroradiometer and compared with synchronous AOD measurements from a reference AERONET sun photometer.

The EKO MS-711 was calibrated at the Izaña Observatory by using the Langley plot method during the study period. Although this instrument has been designed for spectral solar DNI measurements, and therefore has a field of view (FOV) of 5° that is twice the recommended amount in solar photometry for AOD determination, the AOD differences compared to the AERONET– Cimel reference instrument (FOV ~ 1.2°) are fairly small. A comparison of the results from the Cimel AOD and EKO MS-711 AOD presents a root mean square (RMSE) of 0.013 (24.6 %) at 340 and 380 nm, and 0.029 (19.5 %) for longer wavelengths (440, 500, 675, and 870 nm). However, under relatively high AOD, near-forward aerosol scattering might be significant because of the relatively large circumsolar radiation (CSR) due to the large EKO MS-711 FOV, which results in a small but significant AOD underestimation in the UV range.

Figure 1.- AOD differences (EKO AOD–Cimel AOD) versus the optical air mass (ma). Black lines represent the U95 uncertainty limits. Reprinted from García et al. (2020).

The AOD differences decrease considerably when CSR corrections, estimated from LibRadtran radiative transfer model simulations, are performed. This leads to reduction of RMSE to 0.006 (14.9 %) at 340 and 380 nm, and 0.005 (11.1 %) for longer wavelengths. The EKO AOD–Cimel AOD differences within the WMO traceability limits were > 95% at 500, 675, and 870 nm with no CSR corrections (Figure 1). After applying the CSR corrections, the percentage of AOD differences within the WMO traceability limits increased for all wavelengths (see Fig. 19.1) and were > 95% for 380, 440, 500, 675, and 870 nm.

The EKO MS-711 has proven to be an instrument which, despite having been designed for solar radiation measurements, can provide high-quality AOD measurements in the VIS and near-IR ranges, with excellent results when compared to the AERONET–Cimel reference radiometer.