Modeling organic aerosol from the oxidation of α-pinene in a Potential Aerosol Mass (PAM) chamber

A model has been developed to simulate the formation and evolution of secondary organic aerosol (SOA) and was tested against data produced in a Potential Aerosol Mass (PAM) flow reactor and a large environmental chamber. The model framework is based on the two-dimensional volatility basis set approach (2D-VBS), in which SOA oxidation products in the model are distributed on the 2-D space of effective saturation concentration (C_i ^*) and oxygen-to-carbon ratio (O: C). The modeled organic aerosol mass concentrations (C_OA) and O: C agree with laboratory measurements within estimated uncertainties. However, while both measured and modeled O: C increase with increasing OH exposure as expected, the increase of modeled O: C is rapid at low OH exposure and then slows as OH exposure increases while the increase of measured O: C is initially slow and then accelerates as OH exposure increases. A global sensitivity analysis indicates that modeled C_OA values are most sensitive to the assumed values for the number of C_i^* bins, the heterogeneous OH reaction rate coefficient, and the yield of first-generation products. Modeled SOA O: C values are most sensitive to the assumed O: C of first-generation oxidation products, the number of C_i^* bins, the heterogeneous OH reaction rate coefficient, and the number of O: C bins. All these sensitivities vary as a function of OH exposure. The sensitivity analysis indicates that the 2D-VBS model framework may require modifications to resolve discrepancies between modeled and measured O:C as a function of OH exposure.