10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Influence of Aging on Mass Absorption Coefficient and Single Scattering Albedo of SOA: Oxidation vs. Organo-nitrate Formation
STEPHEN ZIMMERMAN, Justin Dingle, Alexander Frie, Justin Min, Roya Bahreini, University of California, Riverside
Abstract Number: 910 Working Group: Carbonaceous Aerosol
Abstract Atmospheric particles directly influence the global radiative budget by absorbing and scattering solar radiation. In this study, chemical and microphysical properties of secondary organic aerosol (SOA) generated in a smog chamber are tracked to investigate effects of aging on SOA mass absorption coefficient (MAC) and single scattering albedo (SSA). SOA is formed via photooxidation of biogenic (α-pinene [AP] and longifolene [LGF]), and aromatic (1-methylnaphthalene [1-MN], Phenol [PH], and Toluene [TOL]) compounds with hydroxyl radical (OH.) in the presence of low and high concentrations of nitrogen oxides (NOx), using hydrogen peroxide (H2O2) and HONO as the OH. source. An Aerodyne mini-aerosol mass spectrometer (mAMS) is used to obtain fast, size-resolved, non-refractory chemical composition and mass concentrations of organic and nitrate submicron particles while online optical properties are measured by a Cavity Attenuated Phase-Shift (CAPS, at 632 nm) spectrometer and Photoacoustic Extinctiometer (PAX, at 375 nm). Fractional contribution of CO2+ from organic aerosol, ratios of C2H3O+/CO2+, and average carbon oxidation states (OSc) are calculated to capture the effect of aging on the bulk aerosol as oxidation occurs. Additionally, contributions of organo-nitrates are estimated by comparing the observed NO+/NO2+ ratios of SOA with that of known organo-nitrate compounds and NH4NO3. Lower NO+/NO2+ ratio values (2-5) are observed from H2O2-sourced OH. while ratios from HONO-sourced OH. range from 3-8, suggesting formation of more organo-nitrates with HONO. During HONO experiments, the NO+/NO2+ ratios are highest at the beginning of the SOA growth and decrease as oxidation proceeds. On the other hand, MAC increases monotonously with oxidation; such observations suggest chromophores other than organo-nitrates are being formed that contribute to the increase in relative absorption. MAC values calculated at 375 nm have similar ranges from 0.01-4 (m2/g) when comparing between oxidant sources. Biogenic SOA have smaller maximum MAC values ≈0.3 (m2/g) than aromatic SOA, which have a range of maximums, from 1-4 (m2/g) depending on the precursor VOC. Further investigation will be conducted to identify the major chemical components driving the observed changes in optical properties.