Application of CALINE4 to roadside NO/NO2 transformations

The CALINE4 roadway dispersion model has been applied to concentrations of NOx and NO2 measured near Gandy Boulevard in Tampa, FL (USA) during May 2002. A NOx emission factor of 0.86 gr mi−1 was estimated by treating NO+NO2 (NOx) as a conserved species and minimizing the differences between measured and calculated NOx concentrations. This emission factor was then used to calculate NO2 concentrations using the NO/NO2 transformation reactions built into CALINE4. A comparison of measured and calculated NO2 concentrations indicates that for ambient O3 concentrations less than 40 ppb 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.