Retrieval of the real part of the refractive index of smoke particles fromSun/sky measurements during SCAR-B

A method is used to retrieve the real part of the refractive index of ambie nt aerosol particles in the entire vertical column using ground-based measu rements of the angular dependence of the spectral sky radiance. The method is applied to smoke aerosol particles using spectral Sun/sky data measured by the AERONET (Aerosol Robotic Network) radiometers in Cuiaba, Brazil, dur ing the SCAR-B (Smoke, Clouds, and Radiation-Brazil) experiment in 1995. Th e refractive index is retrieved from comparison between measurements taken in the solar almucantar and calculations using Mie theory. First the aeroso l size distribution is derived from sky radiance at scattering angles less then 40 degrees, then the refractive index is derived from sky radiances fo r angles of 20 degrees-100 degrees, Simulations and sensitivity studies are presented showing that the expected error is +/-0.03. Application of the m ethod to the Cuiaba region, which is dominated by smoke from cerrado vegeta tion burning, resulted in a mean value for the real part of the index of re fraction of 1.53 +/- 0.04, 1.55 +/- 0.04, 1.59 +/- 0.02, and 1.58 +/- 0.01, respectively, for wavelengths of 438, 670, 870, and 1020 nm, Though we do not have independent verification of the results, we tested the effect of w ater vapor on the refractive index. The low humidification factors measured in Brazil and the lack of high relative humidities suggested a small effec t of water vapor. In fact, as expected, a nonsignificant correlation was ob served between the retrieved values of refractive index and total precipita ble water vapor. Application to aerosol in the eastern United States (not r eported here), with high humidity and high humidification factors, did show a strong reduction of the refractive index with increase of the total prec ipitable water vapor, thus generating confidence in the methodology.