ABSTRACT
The present study was conducted to investigate the impacts of retrofitted CNG vehicles on air pollutant emissions in Tehran over a 20 years period using available statistical data, precise automobile tests, data analysis, and LEAP Software. The obtained results revealed an increase in air pollutant emissions including CO [equal to 1030.77 thousand tons], NO[x] [equal to 1030.77 thousand tons], THC [269.02 thousand tons] and CO[2] [equal to 38.43 thousand tons] resulting from utilizing retrofitted CNG vehicles within the studied period. Considering the obtained results, if the current trend of producing retrofitted CNG vehicles continues, a total cost of 435 million USD will impose on the environment, annually. According to the emission test results, by implementing retrofitted CNG vehicles policy, a higher emission rate of dual-fuel vehicles will be expected compared to their petrol-fueled counterparts. This is due to poor quality of the policy implementation such the use of substandard conversion kits and old technology of retrofit technologies