An observation-based box model (OBM) for secondary inorganic aerosol has been developed to evaluate the sensitivity of formation of sulfate and nitrate to changes in the emissions of their precursors (i.e., NOx, SO2, and VOCs). The model incorporates CB05 chemical mechanism and essential aqueous phase chemical reactions, thermodynamic equilibriums for gas-aerosol phase apportionment and size distribution of sulfate and nitrate.  In OBM, a sequence of present time-frame observations of precursors and particle compositions and meteorological data are used to drive the simulation and to determine responses to perturbed emission rates. The use of observational data ensures that the calculations are carried out for the proper combinations of NOx and VOCs conditions. OBM has the advantage of obviating the need for uncertain emission inventories and boundary layer dynamic conditions.The OBM is applied to hourly gaseous and particulate composition data measured during a wintertime pollution episode in Hong Kong. The OBM analysis suggests that local processes contributed 78% of nitrate and 14% of sulfate in PM2.5 during the episode hours (i.e., hours with air pollution index exceeding 100). The nitrate level is found to be sensitive to the change in NOx and anthropogenic VOCs (AVOCs) emissions. Each percent reduction (near 10% reduction rate) in NOx and AVOCs results in a reduction of 0.53% and 0.66% in nitrate mass, respectively. Reduction of sulfate formation, on the other hand, is most sensitive to reduction of SO2. The OBM provides a relatively simple and cost-effective tool for analyzing the increasing database of high time resolution measurements of VOCs and major aerosol ionic species.

Observation-based box model for analyzing formation of PM<sub>2.5 </sub>sulfate and nitrate

243rd National Meeting (2012)

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Observation-based box model for analyzing formation of PM2.5 sulfate and nitrate

Observation-based box model for analyzing formation of PM_2.5sulfate and nitrate