The system comprises a single channel ultra-low stray-light AvaSpec-ULS2048x64-EVO (f=75 mm) spectrometer by Avantes, the entrance optics and a two-axes tracker. The spectrometer’s detector is a back-thinned Hamamatsu charge-coupled device (CCD) array of 2048 pixels with Signal to Noise Ratio (SNR) 450:1 for a single measurement at full signal. The spectrometer covers the spectral range 280–539 nm and uses a 50 μm wide entrance slit. Mercury discharge lamp spectra were recorded to determine the instrument’s slit function and the spectral resolution was found 0.55 nm full width at half maximum (FWHM) at 436 nm. The spectrometer is positioned inside a thermally isolated box, where the temperature is maintained at +10oC using a thermoelectric Peltier system. The entrance optics are mounted on a two-axes tracker with two stepper motors controlling the azimuth viewing angle and the elevation viewing angle allowing both direct sun and off-axis observations. A third motor rotates a filter-wheel of 8 positions with different optical components (diffuser, attenuation and band-pass filters), used for the measurement of direct-sun and scattered radiation spectra and an opaque position for the measurement of the dark signal. The instrument operates automatically and is controlled by a custom-made software. The entrance comprises also a telescope with a plano-convex lens that focuses the collected solar radiation onto one end of an optical fiber. Simultaneous azimuth and elevation angle calibration is regularly performed by sighting the sun, so no horizon scans are necessary for the elevation angle calibration.
Final profucts are: aerosol extinction profile, aerosol optical depth (AOD), columnar values of NO2