Retrieval of aerosol microphysical properties using surface MultiFilter Rotating Shadowband Radiometer (MFRSR) data: Modeling and observations

[1] MultiFilter Rotating Shadowband Radiometers (MFRSRs) are widely deployed over the world. These radiometers measure the total, direct, and diffuse components of shortwave, narrowband irradiance at six wavelengths. For five of these wavelengths, aerosol optical depths and single scattering albedos can be retrieved. We describe here a simple retrieval technique that can significantly extend the capability of the MFRSR to study atmospheric aerosols and can provide a means for simultaneous retrieval of the mean particle radius, total number of particles ( for an assumed size distribution), and the imaginary refractive index. This technique is based on measurements of the direct irradiances at two wavelengths (0.415 mu m and 0.870 mu m) and the diffuse irradiance at 0.415 mu m. Our technique requires assumptions regarding the shape of the aerosol size distribution, and the real part of the refractive index, as well as an estimate of the surface albedo at 0.415 mu m. Given plausible values of these quantities, sensitivity tests show that successful retrievals of aerosol characteristics can be achieved. However, the retrieval does not work when the aerosol is dominated by coarse mode particles such as dust, because of the weak dependence of aerosol optical thickness on wavelength. The technique has been applied to derive time series of aerosol microphysical properties from MFRSR measurements taken during a single day, 27 April 2003, of the Mexico City Metropolitan Area field campaign. Additionally, MFRSR-derived aerosol properties are in good agreement with Aerosol Robotic Network (AERONET) retrievals made also in Mexico City.