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

Abstract

Abstract. 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 Izaña 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 Izaña 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∘ 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 (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 %.

Water Vapor Retrievals from Spectral Direct Irradiance Measured with an EKO MS-711 Spectroradiometer—Intercomparison with Other Techniques

“…The modelled spectra were convolved with the slit function of the EKO MS-711. DN I o (λ) was determined using the modified Langley-plot technique [14,40] for each wavelength between 930 and 960 nm ( Figure 2). The τ a (λ) has been determined from the DNI measurements performed with the EKO MS-711 following the methodology described in [40].…”

“…DN I o (λ) was determined using the modified Langley-plot technique [14,40] for each wavelength between 930 and 960 nm ( Figure 2). The τ a (λ) has been determined from the DNI measurements performed with the EKO MS-711 following the methodology described in [40]. In that work the AOD values were evaluated at the 340, 380, 440, 500, 675 and 870 nm wavelengths, thus the spectral AOD between 930 and 960 nm has been derived by applying the Ångström formula [55].…”

“…In that work the AOD values were evaluated at the 340, 380, 440, 500, 675 and 870 nm wavelengths, thus the spectral AOD between 930 and 960 nm has been derived by applying the Ångström formula [55]. It is important to note that the FOV of the EKO spectroradiometer is 5º, whereby the AOD retrievals have been post-corrected of circumsolar radiation following the procedure presented in [40]. The effect of the large EKO MS-711 FOV on the measured solar absorption spectra was studied in [40] by the circumsolar ratio (CR) defined as:…”

“…With this configuration, the solar DNI spectrum is sampled with a temporal resolution of 1 min, with the integration time of each measurement varying between 10 ms to 5 s, depending on the light intensity. For more details refer to [40]. The PWV content in the atmosphere column is determined from the measured vertical profiles of temperature, relative humidity and atmospheric pressure acquired with the meteorological radiosondes [6].…”

Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry

“…The IZO monitoring activities routinely participate in multiple international networks and databases, highlighting its role as an absolute calibration site for two of the most important ground-based photometer aerosol networks: NASA-AERONET and GAW precision-filter radiometers (GAW-PFR). In this sense, IZO is commonly used for optimal application of the Langley-Plot technique to a large number of activities, such as absolute direct sun/moon photometric calibration [30,56,58,59] or the retrieval of reference top-of-atmosphere (TOA) solar spectrum from the ground at different spectral ranges [60,61]. Refer to [55] for more details about IZO and its atmospheric monitoring programmes.…”