An observation-based model for secondary inorganic aerosol (OBM-SIA) is developed to determine the sensitivity of formation of sulfate (SO42–) and nitrate (NO3–) to changes in their precursors. The model incorporates CB05 chemical mechanism with inclusion of two recently discovered OH enhancement pathways and essential aqueous phase chemical reactions, thermodynamic equilibriums for gas-aerosol phase apportionment and size distribution of SO42– and NO3–. A sequence of present time-frame observations of precursors and particle compositions are used to drive the simulation and to determine responses to perturbed emission rates of precursors. OBM-SIA obviates the need for uncertain emission inventories and boundary layer dynamic conditions, and makes use of data that are increasingly available due to recent advances in online instruments for various gaseous and aerosol components, thereby offering a cost-effective tool for the analysis of SIA-precursor relationships in the atmosphere.
OBM-SIA is applied to hourly gaseous and particulate composition data during a wintertime pollution episode in Hong Kong. The major pathway responsible for the production of NO3– is the reaction of OH and NO2 in the gas phase, while the gas phase oxidation of SO2 by OH and aqueous phase oxidation of S(IV) by O3 contribute most significantly on SO42– production. NO3– production is more sensitive to the reduction of NOx and anthropogenic VOCs. Reduction of SO42– formation is however most sensitive to SO2 reduction and it becomes more effective as more SO2 is reduced. The work illustrates the utility of OBM-SIA in suggesting evidence-based control strategies for effective reduction of targeted SIAs.