AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Heterogeneous Oxidation of Internally Mixed Primary and Secondary Organic Aerosol: A Case for the Importance of Secondary Chemistry
KATHERYN KOLESAR, Chris Ruehl, Gabriel Isaacman, Gina Buffaloe, Theodora Nah, Allen H. Goldstein, Kevin Wilson, Christopher Cappa, University of California, Davis
Abstract Number: 365 Working Group: Aerosol Chemistry
Abstract Primary and secondary organic aerosol (POA and SOA) can exist as internally mixed particles in the atmosphere, yet laboratory studies tend to investigate the physical and chemical properties of each separately. Consequently, interactions between POA and SOA in the particle are largely unknown which could lead to errors in predicting particle hygroscopicity, light attenuation and lifetime. The heterogeneous oxidation of squalane, a C$_(30) saturated, branched hydrocarbon found in diesel emissions, by hydroxyl radicals (in the presence of O$_2) has been previously well-characterized, and the effective uptake coefficient at [OH] ~ 10$^(10) molecules cm$^(-3) was gamma$_(eff,Sq) = 0.30 ± 0.07. Here, the influence of internal mixing of squalane with two different complex mixtures, SOA from alpha-pinene + O$_3 and motor oil, on gamma$_(eff,Sq) is investigated. Particle composition was measured with a vacuum ultraviolet aerosol mass spectrometer that uses soft ionization (10.5 eV photons), allowing for the unique characterization of squalane reactivity separate from the additional components. We found that as the SOA coating thickness increases (or squalane volume fraction decreases) the γeff,Sq also increases. Similarly, when squalane was doped into motor oil the gamma$_(eff,Sq) was increased over that of pure squalane. The largest increase of the effective uptake coefficient observed in both the motor oil + squalane and squalane + SOA systems is ~5 times that of pure squalane. Under typical 24-hour average [OH] (1 x 10$^6 molecules cm$^(-3)) the atmospheric lifetime of pure particulate squalane (D$_p = 180 nm) with respect to OH oxidation is ~13 days, whereas a particle of mixed composition has an oxidation lifetime of ~3 days.