Sunphotometers

Sunphotometers are optical instruments used to measure direct solar radiation in multiple narrow spectral bands. They provide quantitative data on atmospheric properties such as aerosol optical depth (AOD), precipitable water, and ozone, which are important for climate research, air quality assessment, and satellite data validation. They are typically deployed on fixed sites or in mobile campaigns and are often integrated into networks for long-term monitoring.

The basic principle involves tracking the sun with a solar tracker and recording direct beam irradiance I(λ) at selected wavelengths. The measured irradiance is related to the extraterrestrial solar irradiance Io(λ), the atmospheric optical depth τ(λ), and the relative air mass m by the Beer–Lambert–Bouguer equation: I(λ) = Io(λ) exp(-τ(λ) m). By performing measurements over a range of solar elevations, the wavelength-dependent AOD can be retrieved. Io(λ) must be calibrated to establish a stable extraterrestrial reference, commonly using Langley plots at stable, high-altitude sites or through instrument intercomparisons.

Typical sunphotometers use multiple narrow-band filters or interferometric optics and detectors such as photodiodes or photomultipliers. Many instruments are part of standardized fleets, such as the Cimel sun photometer used in the AERONET network, which also supports retrieval of water vapor and other atmospheric constituents.

Limitations include dependence on clear sky and sun visibility, sensitivity to calibration drift and stray light, and routine maintenance requirements. Despite these constraints, sunphotometers provide valuable, ground-based measurements of atmospheric transmission essential for understanding aerosols and validating remote sensing data.