Aerosol microphysical retrievals from Precision Filter Radiometer direct solar radiation measurements and comparison with AERONET

Synchronized sun-photometric measurements from the AERONET-CIMEL and GAW-PFR aerosol networks are used to compare retrievals of the aerosol optical depth, effective radius and volume concentration during a high temporal resolution measurement campaign at the Athens site in the Mediterranean Basin from 14–22 July 2009. During this period, direct sun AOD retrievals from both instruments exhibited small differences in the range 0.01–0.02 despite the presence of a strong dust event. In addition to AERONET-CIMEL inversion data, an independent inversion method was applied that involves expanding the particle size distribution in terms of measurement kernels so as to estimate bulk particle parameters from a linear-estimated combination of the input optical data. AOD measurements obtained from both CIMEL and PFR instruments using this method also showed reasonable agreement. For low aerosol loads (AOD < 0.2), measurements of the effective radius by the PFR were found to be −20% to +30% different from CIMEL values for both direct sun data and inversion data. At higher loads (AOD > 0.4), measurements of the effective radius by the PFR are consistently 20% lower than CIMEL for both direct sun and inversion data. Volume concentrations at low aerosol loads from the PFR are up to 80% higher than the CIMEL for direct sun data, but inversion data suggests that volume concentrations from the PFR are up to 20% lower than the CIMEL under these same conditions. At higher loads, the percentage difference in volume concentrations from the PFR and CIMEL is systematically negative with inversion data predicting differences 30% lower than those obtained from direct sun data. An assessment of the effect of errors in the AOD retrieval on the estimation of PFR bulk parameters was made using Monte Carlo simulations and demonstrated that it is possible to estimate the effective radius with an uncertainty below 60% and the volume concentration with an uncertainty below 65% even when AOD < 0.2 and when the input errors are as high as 10%. Highlights – A comparison of high temporal resolution synchronous CIMEL and PFR direct sun AOD measurement retrievals – Calculation of bulk aerosol microphysics parameters using a linear estimation inversion technique – A comparison of retrieved aerosol volume concentrations and effective radii from CIMEL and PFR inversions – An analysis of the sensitivity of PFR retrievals to random errors on the optical input data