Simultaneous retrieval of aerosol refractive index and particle size distribution from ground-based measurements of direct and scattered solar radiation

Ground-based sunphotometer observation of direct and scattered solar radiat ion is a traditional tool for providing data on aerosol optical properties. Spectral transmission and solar aureole measurements provide an optical so urce of aerosol information, which can be inverted for retrieval of microph ysical properties (particle size distribution and refractive index). Howeve r, to infer these aerosol properties from ground-based remote-sensing measu rements, special numerical inversion methods should be developed and applie d. We propose two improvements to the existing inversion techniques employe d to derive aerosol microphysical properties from combined atmospheric tran smission and solar aureole measurements. First, the aerosol refractive inde x is directly included in the inversion procedure and is retrieved simultan eously with the particle size spectra. Second, we allow for real or effecti ve instrumental pointing errors by including a correction factor for scatte ring angle errors as a retrieved inversion parameter. The inversion techniq ue is validated by numerical simulations and applied to field data. It is s hown that ground-based sunphotometer measurements enable one to derive the real part of the aerosol refractive index with an absolute error of 0.03-0. 05 and to distinguish roughly between weakly and strongly absorbing aerosol s. The aureole angular observation scheme can be refined with an absolute a ccuracy of 0.15-0.19 deg. Offset corrections to the scattering angle error are generally found to be small and consistently of the order of -0.17. Thi s error magnitude is deduced to be due primarily to nonlinear field-of-view averaging effects rather than to instrumental errors. (C) 1999 Optical Soc iety of America OCIS codes: 280.1100, 010.1100, 010.1110.