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Size‐resolved composition, water content, and mixing characteristics of fine (<2μm) particles were measured during intensive field studies in Los Angeles (Southern California Air Quality Study (SCAQS)(1987)) and at the Grand Canyon (winter 1990, summer 1992). Aerosol optical properties were calculated from these measurements using Mie theory. Calculated scattering coefficients are in reasonable agreement with directly measured values for internal and external mixing models as well as for a hybrid model based on measured mixing characteristics. Calculated dry sulfate mass scattering efficiencies cover the range reported in previous studies based on multiple linear regression analysis (MLR) (∼3 to 7 m2/g) but increase systematically with size and are sensitive to assumptions regarding the chemical mechanism of secondary aerosol formation. Calculated organic carbon (1.4*mass of carbon associated with particulate organics) scattering efficiencies (∼3 to 7 m2/g ) show no systematic dependence on median size but are somewhat higher than previously published estimates based largely on MLR. Organic carbon and sulfates were the dominant contributors to fine particle scattering in both Grand Canyon studies (50 and 38% during the 1990 Navajo Generating Station (NGS) study, respectively, and 48 and 40% at Meadview). However, large discrepancies are found for organic carbon concentrations measured with different samplers, especially in the desert Southwest where concentrations are low. These observations underline the need for improved sampling and characterizing methodologies for carbon‐containing particulate species.