An optical closure study was carried out on the basis of measurements taken during five research flights in October 2003 over the waters surrounding Nova Scotia. Measurements of aerosol size spectra were made using a variety of instruments, and the size-segregated chemical signature was determined with an Aerodyne Aerosol Mass Spectrometer. The aerosol scattering and backscattering coefficients were determined with an integrating nephelometer at three visible wavelengths. At a wavelength of 550 nm and at altitudes less than 1000 m, the mean total scattering coefficient of the dry in-cabin aerosol is 26 Mm−1, with a standard deviation of 10 Mm−1, while the mean backscattering coefficient is 1.7 Mm−1 with a standard deviation of 0.8 Mm−1. On the basis of data from instruments within the cabin, closure between the directly measured and calculated total scattering coefficients is attained for more than 70% of cases, but is not attained for the backscattering coefficients. Coarse particles are found to account for roughly half of the total scattering and 70% of the backscattering for altitudes up to ∼1000 m. The scattering contribution from coarse particles is found to account for approximately 65% of the total scattering and 88% of the backscattering when calculated on the basis of measurements taken outside of the aircraft, which are not subject to inlet losses for larger particles.