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Abstract: Aerosol optical properties measured at the Pasadena, CA site during the CalNex field campaign in May–June 2010 are summarized. Average measurements of PM2.5 aerosol extinction, scattering, absorption coefficients, and single scattering albedo (b ext, b scat, b abs and SSA) at λ = 532 nm were 62 Mm−1, 58 Mm−1, 4 Mm−1, and 0.92, respectively. The aerosol optical densities were 5 times lower than during the SCAQS study in 1987, highlighting major progress in PM control in the Los Angeles area in the last two decades. The period May 30–June 8 2010 was characterized by exceptionally high aerosol loading (b ext up to 250 Mm−1). During this period, b ext, b scat, and SSA tended to peak during the mid-morning. Correlation of PM2.5 b ext, b scat with mass concentration data yielded mass scattering and mass extinction coefficients of 3.5–5.1 m2 g−1 for 532 nm. Aerosol b abs were compared directly to mass concentration of elemental carbon (EC) yielding a campaign average mass absorption cross section (M.A.C.) of 5.7 ± 1.8 m2 g−1. TEM analysis of particles suggests soot was often internally mixed or adhering to sulfate and/or organics. Total non-refractory PM1 mass was a good quantitative indicator of coated soot fraction. Alteration of M.A.C. with mixing/coating state was not detected, however, increases in M.A.C. were linked to the presence of light absorbing, water-soluble organic carbon (WSOC) suggesting a possible role of this material invisible light absorption in the LA basin. [Copyright &y& Elsevier]