The AERMOD dispersion model was evaluated for its performance in predicting 1-hour average nitrogen dioxide (NO2) concentrations in the vicinity of the largest petrochemical industrial complex in Thailand during the period between January 2012 and December 2013. Measured data from 10 ambient air monitoring stations were intensively used to compare with modeled results. Model results indicated that the tier 1 approach (full conversion of NOx to NO2) provided the most accurate results compared with other tiers. It also performed very well in predicting the extreme end of NO2 concentrations. With an absence of emission data from mobile sources, tier 1 was concluded as the most appropriate scheme for prediction of ambient NO2 ground-level concentrations in this study.