Under EPA contract, GEOMET has completed a program of air quality measurement, model development and model validation at Washington National Airport. The measurement program has generated six months of air quality, airport operational, and meteorological data, some of which were used in model development and validation. These data are available on magnetic tape. A new model was developed from the Northern Research and Engineering Corporation (NREC) model. Initially methods for calculating single values of short-term concentrations, for calculating maximum values of short-term concentrations, and for calculating long-term average values of concentrations were changed. The first change involved substituting a single direction only for the superimposed wind direction variability parameter. The next two changes were handled jointly by calculating long-term average concentrations from a sufficiently large sample of single short term values. The NREC model with improvements still consistently underpredicted. Further modifications (improved aircraft classification system, improved aircraft operational modes and pollution emission rates, improved details of airport representation, improved environ area source modeling and emission rates, more representative depiction of line sources, and inclusion of major peripheral highways) were then made. This model with above changes is called the modified (GEOMET) model.