The extinction optical thickness of particles obtained from scattering and absorption coefficients measured by an airborne integrating nephelometer and particle soot absorption photometer, respectively, is compared with the aerosol optical thickness derived from a ground-based multifilter rotating shadowband radiometer, a Sun photometer, and a Raman lidar for 9 days. These 9 days are selected from intensive operation periods of the Atmospheric Radiation Measurement in April 1997, September 1997, and August 1998 at the southern Great Plains. For April 1997 and September 1997 cases the difference between the extinction optical thickness of particles estimated from vertical profiles and the extinction optical thickness of aerosol derived from the multifilter rotating shadowband radiometer is not significant. For August 1998 cases when the boundary layer relative humidity is higher than April 1997 and September 1997 cases, the extinction optical thickness of particles is 0.03 to 0.07 less than the extinction optical thickness of aerosol. The difference corresponds to 25% to 31% of the extinction optical thickness of aerosol. Based on these comparisons, the upper and lower limits of the single-scattering albedo of particles present in the lower part of troposphere are 0.97 and 0.84, respectively. ¿ 2000 American Geophysical Union