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Understanding of chemical, physical, and radiative processes-emissions, transport, deposition, and modification of aerosol optical properties due to ageing-is of major importance to global and regional climate simulations and projections as well as health impairment. This paper presents aerosol optical properties retrieved with the Multifilter Rotating Shadowband Radiometers (MFRSRs) and the source attribution based on back trajectories and in situ aerosol chemical composition analysis obtained during the Aerosol Life Cycle Intensive Observational Period at Brookhaven National Laboratory on Long Island, NY, during July and August 2011. The aerosol optical properties retrieved with the MFRSR exhibit excellent agreement with those obtained with a colocated Cimel sunphotometer. Apportioning aerosol optical depths by size modes reveals several episodes of high loading of fine aerosol (diameter less than 2.5 μm). Analysis of optical and physical properties of aerosols as well as their chemical composition obtained by an in situ high-resolution time-of-flight aerosol mass spectrometer together with back trajectories indicates that the principal source of high concentrations of fine aerosols observed during July 18-24 was forest fires in western Canada, consistent with reports by the Canadian Forest Service and satellite observations by the Moderate Resolution Imaging Spectroradiometer (MODIS).