The impact of oxides of nitrogen (NOx) emissions from motor vehicles to the air quality in city-state Singapore is analyzed using AIRVIRO, a regional scale dispersion model developed by the Swedish Meteorological and Hydrological institute. In a predominantly urban location like Singapore, it is difficult to separate out the contribution of pollutants from mobile and point sources at different locations. In this work, a new approach is used by first modeling only the impact of point and area sources and then overlaying the traffic impact on air quality at different locations. Monthly scenario simulations are run with point, area and traffic sources of emissions for the Gaussian model validation. Street Canyon modeling is used for street segments surrounded by buildings on either side. A simplified photochemical model, which takes into account NOx undergoing chemical transformations in the urban atmosphere, is used to account for variations in NOx and ozone levels with respect to traffic data. The diurnal variation of NOx concentration levels is studied as a function of ozone levels at site, hourly traffic counts and meteorological parameters. The impact on ambient air quality within the breathing zone of the public from mobile sources, is found to be about 40% at urban stations although overall emissions from mobile sources is only 24%. The proposed approach appears to predict the variations in NOx as a function of traffic and meteorological conditions.