Application of genetic algorithms to a detailed chemical reaction system for secondary organic aerosol formation

An explicit chemical mechanism is instrumental to modeling secondary organic aerosol (SOA) formation. It is, however, difficult to implement an explicit mechanism in a 3-dimensional air quality model because of the high computational cost caused by a large number of reactions and organic species involved. To address this issue, a new simplified chemical reaction system is first proposed for α-pinene SOA formation, which uses the volatility-basis set speciation for condensable products. The simple reaction system reflects the evolution of chemical species from a near-explicit master chemical mechanism (MCM) and it also unifies reactions between SOA precursors with different oxidants under different conditions. A total of 440 unknown parameters (product yields of parameterized products, reaction rates, etc.) from the new reaction system are estimated by using multi-objective genetic algorithms operating on the detailed mechanism. The number of species was reduced from 300 in the detailed mechanism to 30 in the simplified mechanism. Output species profiles, obtained from original subset of MCM reactions for α-pinene oxidation, are reproduced for scenarios under a wide range of HC/NOx conditions. Ultimately, the same unified simple reaction system with updated parameters could be used to describe the SOA formation from different precursors.