Aerosol optical properties at Pasadena, CA during CalNex 2010

Aerosol optical properties measured at the Pasadena, CA site during the CalNex field campaign in May-June 2010 are summarized. Average measurements of PM _2.5 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 PM _2.5 b _ext, b _scat with mass concentration data yielded mass scattering and mass extinction coefficients of 3.5-5.1 m ² 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 m ² g ^-1. TEM analysis of particles suggests soot was often internally mixed or adhering to sulfate and/or organics. Total non-refractory PM ₁ 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.