Séminaire du CEREA - 30 septembre 2003

Development and Application of the Variable Size Resolution Aqueous-Phase Chemistry Model

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) has been 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.
Due to computational restrictions, size-resolved aqueous-phase chemistry models heretofore have been considered infeasible for use in most three dimensional chemical transport models. This is in spite of the fact that the more efficient bulk models can significantly underpredict sulfate production. The VSRM has been incorporated into PMCAMx, an Eulerian photochemical grid model with detailed treatment of particulate matter. Results from the application of PMCAMx for two very different episodes will be discussed.
Model predictions and observations have been compared for a fall air pollution episode in California's South Coast Air Basin. 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 only limited computational costs (5% overhead), the VSRM can be included in a three-dimensional chemical transport model. PMCAMx also 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 active aqueous-phase chemistry predicts noticeably higher sulfate in cloud-covered regions than does the model without activated cloud chemistry.

Le séminaire aura lieu dans la salle de réunion du CEREA B220 à 11h00.

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