Modeling secondary organic aerosol formation using a simple scheme in a three-dimensional air quality model and performing systematic mechanism reduction for a detailed chemical system

Two approaches are adopted for studying the formation of secondary organic aerosols (SOA). The first approach is to develop a "simplified" SOA module within a 3-dimensional air quality model, the MC2AQ. A parameterized SOA yield method by Odum et al. [1996] is used as the framework for the SOA formation. The SOA module includes the important factors affecting SOA formation: the temperature; vaporization enthalpy; primary organic aerosol; low NOx/HC condition; compound interactions; and a newly developed simple scheme for the oligomer formation. The model predictions of the organic aerosols at rural areas are close to the observations, but the SOA in the urban site is underestimated. Overall, temperature is a dominant factor affecting the gas/particle partitioning process, and high SOA concentrations are found at nighttime with low temperature or in the cold regions. Moreover, the inclusion of the compound interaction not only affects the water uptake, but also leads to more SOA formed. For example, an additional 20% of the SOA mass is formed at a RH of 85%.; The second approach is to model the SOA formation using a box model and a subset of the explicit master chemical mechanism (MCM) describing the alpha-pinene oxidation (928 organic reactions and 310 organic compounds). The study shows that total SOA mass decreases with the NOx/HC ratio. Furthermore, aerosol fractions for the PAN-like compounds and the nitrates increase with the NOx/HC ratio. On the contrary, the aerosol fractions for the organic peroxides and organic acids decrease with the NOx/HC ratio. In addition, 28 out of 149 condensable products are identified as important compounds for the SOA formation and mechanism reduction purposes. Finally, five systematic mechanism reduction techniques are applied, in sequence, to the subset of the MCM for the SOA formation. The final reduced mechanism (348 organic reactions and 125 organic compounds) could reproduce dominant condensable compounds, four functional groups, and the total SOA mass accurately within 16% under a wide range of conditions.