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

Spectral direct UV-visible normal solar irradiance (DNI) has been measured with an EKO MS-711 grating spectroradiometer, which has a spectral range of 300-1100 nm, and 0.4 nm step, at the Izana Atmospheric Observatory (IZO, Spain). It has been used to determine aerosol optical depth (AOD) at several wavelengths (340, 380, 440, 500, 675, and 870 nm) between April and September 2019, which has been compared with synchronous AOD measurements from a reference Cimel and Aerosol RObotic NETwork (AERONET) sun photometer. The EKO MS-711 has been calibrated at the Izana Atmospheric 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 degrees that is twice the recommended amount in solar photometry for AOD determination, the AOD differences compared to the AERONET-Cimel reference instrument (FOV similar to 1.2 degrees) are fairly small. A comparison of the results from the Cimel AOD and EKO MS-711 AOD presents a root mean square (rms) 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. The AOD differences decrease considerably when CSR corrections, estimated from libRadtran radiative transfer model simulations, are performed and obtain an rms of 0.006 (14.9 %) at 340 and 380 nm, and 0.005 (11.1 %) for longer wavelengths. The percentage of 2 min synchronous EKO AOD-Cimel AOD differences within the World Meteorological Organization (WMO) traceability limits were >= 96% at 500, 675, and 870 nm with no CSR corrections. After applying the CSR corrections, the percentage of AOD differences within the WMO traceability limits increased to > 95% for 380, 440, 500, 675, and 870 nm, while for 340 nm the percentage of AOD differences showed a poorer increase from 67% to a modest 86 %.