Comparison of aerosol size distributions, radiative properties, and optical depths determined by aircraft observations and Sun photometers during SAFARI 2000

The Meteorological Office C-130 aircraft performed a dedicated flight over the Etosha Pan surface-based Aerosol Robotic Network (AERONET) Sun photometer site on 13 September 2000 during the Southern African Aerosol Regional Science Initiative (SAFARI 2000) intensive measurement campaign. Aerosol optical depths at different wavelengths, τ_aerλ, are derived from in situ measurements of the scattering and absorption coefficients and from various radiometric measurements and compared to those derived from the Sun photometer site. The estimates of τ_aerλ from the various measurements are shown to be in good agreement. The exception to this is when τ_aerλ is derived from the Passive Cavity Aerosol Spectrometer Probe (PCASP), as this method is shown to be extremely sensitive to the pitch angle of the aircraft; therefore, τ_aerλ differs for profile ascents and profile descents. However, the aerosol size distribution measured by the PCASP and derived from the AERONET site are in excellent agreement over the 0.05-1.0 μm radius range, which contains the majority of the optically active particles. C-130-derived refractive indices and single scattering albedos are also shown to be in excellent agreement with those derived from the AERONET site. The consistency between in situ and remotely sensed data suggests that, for aerosol well mixed in the vertical, data from AERONET may be used with confidence in validating satellite measurements and modeling studies of the radiative properties and effects of aerosols.