Comparisons are presented of ambient aerosol optical depths (AOD) derived from airborne in situ measurements of aerosol properties and from Sun photometer measurements off the mideastern seaboard of the United States in July 1996. At 450 nm the Sun photometer measurements of AOD were greater than the in situ measurements by, on average, (12 ± 5)%. Possible reasons for this difference are discussed. Comparisons of Angstrom coefficients for the ambient aerosol, based on in situ and Sun photometer measurements, are presented, as well as estimates of the ambient aerosol single-scattering albedo. At a wavelength of 550 nm, the mean value of the ambient single-scattering albedo was (0.95 ± 0.03). Also compared are various optical properties of the dry aerosol derived from airborne in situ measurements with values calculated from a model of the dry aerosol. At a wavelength of 550 nm, measurements and calculations of the total light scattering and light backscattering coefficients for the dry aerosol agree, on average, to within (23 ±2)% and (8 ± 2)%. The calculated and measured dry aerosol absorption coefficients at 535 nm are in agreement for an assumed value of the mass absorption efficiency for the black carbon content of the dry aerosol of 14 m² g⁻¹.