The CALINE4 roadway dispersion model has been applied to concentrations of NO x and NO 2 measured near Gandy Boulevard in Tampa, FL (USA) during May 2002. A NO x emission factor of 0.86grmi −1 was estimated by treating NO+NO 2 (NO x ) as a conserved species and minimizing the differences between measured and calculated NO x concentrations. This emission factor was then used to calculate NO 2 concentrations using the NO/NO 2 transformation reactions built into CALINE4. A comparison of measured and calculated NO 2 concentrations indicates that for ambient O 3 concentrations less than 40ppb the model under-predicts the chemical transformation of NO. The enhanced transformation of NO may be due to reactions of NO with oxidants such as peroxy radicals that are present either in the atmosphere or in vehicle exhaust.