Modeling the formation and properties of traditional and non-traditional secondary organic aerosol: problem formulation and application to aircraft exhaust
We present a methodology to model secondary organic aerosol (SOA) formation from the photo-oxidation of unspeciated low-volatility organics (semi-volatile and intermediate volatile organic compounds) emitted by combustion systems. It is formulated using the volatility basis-set approach. Unspeciated low-volatility organics are classified by volatility and then allowed to react with the hydroxyl radical. The new methodology allows for larger reductions in volatility with each oxidation step than previous volatility basis set models, which is more consistent with the addition of common functional groups and similar to those used by traditional SOA models. The methodology is illustrated using data collected during two field campaigns that characterized the atmospheric evolution of dilute gas-turbine engine emissions using a smog chamber. In those experiments, photo-oxidation formed a significant amount of SOA, much of which could not be explained based on the emissions of traditional speciated precursors; we refer to the unexplained SOA as non-traditional SOA (NT-SOA). The NT-SOA can be explained by emissions of unspeciated low-volatility organics measured using sorbents. We show that the parameterization proposed by Robinson et al. (2007) is unable to explain the timing of the NT-SOA formation in the aircraft experiments because it assumes a very modest reduction in volatility of the precursors with every oxidation reaction. In contrast the new method better reproduces the NT-SOA formation. The NT-SOA yields estimated for the unspeciated low-volatility organic emissions in aircraft exhaust are similar to literature data for large n-alkanes and other low-volatility organics. The estimated yields vary with fuel composition (Jet Propellent-8 versus Fischer-Tropsch) and engine load (ground idle versus non-ground idle). The framework developed here is suitable for modeling SOA formation from emissions from other combustion systems.