Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements

Sensitivity studies are conducted regarding aerosol optical property retrie val from radiances measured by ground-based Sun-sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a ne w inversion concept for simultaneously retrieving aerosol size distribution , complex refractive index, and single-scattering albedo from spectral meas urements of direct and diffuse radiation. The perturbations of the inversio n resulting from random errors, instrumental offsets, and known uncertainti es in the atmospheric radiation model are analyzed. Sun or sky channel misc alibration, inaccurate azimuth angle pointing during sky radiance measureme nts, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of thre e typical and optically distinct aerosols with bimodal size distributions ( weakly absorbing water-soluble aerosol, absorbing biomass-burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the r etrieval to be polydispersed homogeneous spheres with the same complex refr active index. Therefore we also examined how inversions with such an assump tion bias the retrievals in the case of nonspherical dust aerosols and in t he case of externally or internally mixed spherical particles with differen t refractive indices. The analysis shows successful retrieval of all aeroso l characteristics (size distribution, complex refractive index, and single- scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almuc antar (with angular coverage of scattering angles up to 100 degrees or more ). The retrieval accuracy is acceptable for most remote sensing application s even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization o f low optical depth situations for all aerosol types, where high relative e rrors may occur in the direct radiation measurements of aerosol optical dep th. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75 degrees or less) in the sky radianc e results in the loss of practical information about refractive index. Accu rate azimuth angle pointing is critical for the characterization of dust. S cattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single-scatter ing albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40 degrees scattering angle only, where nonspherical effects are minor, re sults in accurate retrievals of aerosol size distributions of nonspherical particles.