This study presents the assessment of nitrogen dioxide (NO2) pollution within the twin cities of Islamabad and Rawalpindi by using car MAX-DOAS (Multi Axis-Differential Optical Absorption Spectroscopy) instrument during two different scenarios of with and without the availability of CNG (compressed natural gas) fuel. Information perceived from this study can be used to get an idea about recent air quality conditions of twin cities and its repercussions on ecological and human health. International air quality monitoring field campaigns were conducted during November 2012 and December 2013. Results showed different concentrations of NO2 (68.2, 74.25 and 93.65 ppb) at various locations of Islamabad and Rawalpindi cities, exceeded the Pak-NEQS levels of 42.5 ppb. High NO2 concentrations can be attributed to emissions from an oil refinery, traffic congestion and solid waste dump site along IJP road, and due to non-availability of CNG during December 2013. Results compared with OMI satellite observations exhibited that NO2 columns from OMI observations are largely underestimated.