Cloud and fog processing of aerosols: Modeling the evolution of atmospheric species in the aqueous phase

Atmospheric sulfate has been implicated in the development of adverse health effects, visibility reduction, and the production of acid rain and acid fogs. Aqueous-phase chemistry can account for most secondary sulfate formation in environments where clouds or fogs are present. Evidence has been shown that the degree of size resolution in aqueous-phase chemistry models may be important in accurately predicting aqueous-phase sulfate production.; A Variable Size Resolution Model (VSRM) is developed in an effort to combine the accuracy of a size-resolved aqueous-phase chemistry model with the efficiency of a less accurate bulk model. Based on critical input conditions, the model executes bulk or size-resolved calculations. For a range of conditions, the VSRM predicts sulfate production within 3% of a six-section size resolved model but is 15 times faster.; Secondly, the effects of size-dependent fog chemistry and physics on the pre-fog aerosol size/composition distribution are examined for a San Joaquin Valley fog. A size-resolved fog model reproduces the amounts and trends of the observed bulk fog water concentrations and size-dependent composition of major species. The sulfate predictions of the highly size resolved fog model are compared with those of the VSRM. It is shown that for lengthy fog events in relatively clean environments deposition of fog droplets is a dominant process in the evolution of the size/composition distribution of aerosols over the course of fog processing. The results indicate a need for a larger number of measurements of depositional fluxes for individual species and the need for aqueous-phase concentration measurements from early on during fog events.; Due to computational restrictions, size-resolved chemistry models heretofore have been considered infeasible for use in three dimensional chemical transport models even though the more efficient bulk models can significantly underpredict sulfate production. In the fourth chapter, the VSRM is incorporated into PMCAMx, an Eulerian photochemical grid model with detailed treatment of particulate matter. A fall air pollution episode in California's South Coast Air Basin is simulated, and model predictions are compared to observations. Without aqueous-phase chemistry, the model fails to predict observed sulfate concentrations. With the inclusion of the VSRM, the model predicts sulfate concentrations within 30% of observed values. For limited computational costs (5% overhead), the VSRM can be included in a three-dimensional chemical transport model.; PMCAMx has been applied to the Eastern United States to simulate a period during July 2001. Preliminary results show that PMCAMx can match observed sulfate levels in the Pittsburgh region and that the model computational requirements are reasonable. In an environment where clouds are present, PMCAMx with an active aqueous-phase chemistry model predicts significantly higher sulfate in cloud-covered regions than in the case without activated cloud chemistry. The VSRM introduces only a 30% increase in computational requirements over the model with equilibrium aerosol treatment and no cloud chemistry.