We report results from the 2004 New England Air Quality Study--Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) based on measurements of aerosol optical properties in the marine boundary layer of the Gulf of Maine. The in situ data collected on board the NOAA research vessel Ronald H. Brown includes extensive properties of aerosol scattering coefficient, absorption coefficient, and optical depth at multiple wavelengths in the visible. From these, intensive properties were derived, including the ¿ngstr¿m exponent of scattering, single scattering albedo, and submicrometer fraction of scattering. The optical particle properties were sorted by time and location on the basis of aerosol source regions and air mass back trajectories. The results indicate a large degree of temporal and spatial variability in the observed parameters: up to 80% in the scattering and absorption coefficients, 55% in the ¿ngstr¿m exponent, 16% in the single scattering albedo, and 25% in the submicrometer fraction of scattering. The variability is mainly due to the multitude of aerosol sources and transport pathways in the study area. During NEAQS 2004 the mean scattering coefficients were about 20 to 40% lower than those measured during NEAQS 2002 but showed a similar variability. The mean absorption coefficient for the aerosol during NEAQS 2002 was about 80% larger than that measured during NEAQS 2004. The modal parameters of the volume-size distributions and refractive index and density of the particles were consistent with the aerosol models used in the MODIS land and ocean aerosol retrieval algorithms.