Spectral AOD comparisons using a multi‑instrument approach

Ultraviolet atmospheric measurements play a key role in characterizing aerosol properties, as absorption and scattering in this spectral range were highly sensitive to particle size and composition. The Izaña Observatory offers an exceptional natural laboratory for such observations due to its stable free‑tropospheric conditions and long‑term reference datasets. Within this framework, the aerosol optical depth (AOD) retrieved from a BTS spectroradiometer was evaluated by comparing its AOD measurements with those from a CIMEL CE318‑T sunphotometer (AERONET) and a other spectrometer.

Langley analysis, applied to the BTS spectral measurements, characterized the instrument under stable atmospheric conditions and provided a consistent reference framework. Daily AOD values were then obtained from the processed morning measurements using standard quality‑control procedures. The comparison with the CIMEL and Brewer instruments considered both clean free‑tropospheric days and periods affected by transported mineral dust, allowing to assess the behaviour of the BTS across a wide range of aerosol loadings and atmospheric conditions.

The BTS spectrometer reproduced the temporal evolution and spectral behaviour captured by the reference instruments, with consistent agreement at both wavelengths and slightly more stable performance at 340 and 380 nm. Differences generally remained within expected uncertainty ranges, although larger deviations appeared under higher aerosol concentrations, reflecting the increased sensitivity of UV retrievals to atmospheric variability. Overall, the comparison indicated that the BTS provided reliable UV AOD measurements under high-altitude conditions.