Use of the Ångstrom exponent to estimate the variability of optical and physical properties of aging smoke particles in Brazil

In situ airborne measurements from the Smoke, Clouds and Radiation‐Brazil (SCAR‐B) study show that during aging over 1–4 days the physical and optical properties of smoke particles are correlated. Consequently, if one optical or physical property of the smoke particles is determined, other properties can be derived. This methodology is validated using multiwavelength Ångstrom exponents determined from the ground‐based Sun photometer measurements in SCAR‐B. It is shown that the Ångstrom exponent determined from Sun photometers for the wavelength intervals 339–437 nm and 437–669 nm are well correlated with particle size, single‐scattering albedo, and the backscatter ratio ( r 2 >0.8). Therefore, when almucantar sky radiance data are not available and for remote sensing applications (such as MODIS), some of the uncertainties in the properties of smoke particles can be reduced by applying these relationships. Using this methodology, major oscillations were observed in smoke particle properties in Brazil on timescales of ∼5–15 days, resulting in variations of the volume median diameter and single‐scattering albedo of ±0.04 μm and ±0.05, respectively. In comparison, the mean value of the dry smoke particle volume median diameter and single‐scattering albedo over all of Brazil was 0.27 μm and 0.86, respectively. A daily cycle in smoke particle properties was also observed. The weekly and seasonal variability in the single‐scattering albedo is shown to have significant consequences for retrieving aerosol optical depths from satellite measurements.