Evaluation of the equilibrium, dynamic, and hybrid aerosol modeling approaches

The continued development of the dynamic and hybrid approaches (Pilinis et al. 2000; Capaldo et al. 2000) for the simulation of atmospheric aerosol dynamics is discussed in this paper. A linear interpolation method is proposed for the mapping of the moving aerosol size/composition distribution onto a fixed size grid. The 3 aerosol modules are incorporated into a trajectory model that includes descriptions of gas-phase chemistry, secondary organic aerosol formation, vertical dispersion, dry deposition, and emissions. The 3 approaches are evaluated against measurements from the Southern California Air Quality Study (SCAQS). All 3 models predict the 4-6 h averaged PM2.5 (particulate matter with diameter less than or equal to 2.5 microns) and PM10 (particulate matter with diameter less than or equal to 10 microns) mass concentrations of the major aerosol species with errors <30%. For the aerosol size/composition distribution, however, the dynamic and hybrid models show better agreement with measurements than the equilibrium model. The hybrid model aerosol size distribution predictions are similar to the dynamic model results. The hybrid approach in this case combines accuracy with computational efficiency. The dynamic approach is the most accurate, but at a higher computational cost. Daily average PM2.5 aerosol species predicted by the aerosol models with 16, 8, and 4 size sections all give reasonable agreement with the measurements. All 3 aerosol models show consistent sensitivities of nitrate, sulfate, and total PM2.5 to the changes of NOx, VOCs, NH3, and primary sulfate emissions.