Air pollution is becoming a major environmental issue in Pakistan owing to rapid urbanization and economic growth. In order to assess the extent of air quality within the major urban environments, PM2.5 pollutant has been analyzed during the period 2007-2011 in Islamabad; and 2007 to 2008 in Lahore, Peshawar and Quetta. Seasonal and diurnal variation of PM2.5 mass concentration and meteorological factors affecting the emissions, secondary PM2.5 formation and accumulation of pollutants have been analyzed. Air quality monitoring data and meteorological data were obtained from Federal and Provincial Pakistan Environmental Protection Agencies. Ambient air quality data of Islamabad, Pakistan, for six representative air pollutants (carbon monoxide (CO), oxides of nitrogen (NO and NOy′), sulfur dioxide (SO2), ozone (O3), fine particulate matter (PM2.5), non- methane hydrocarbons (NMHCs), and meteorology was collected for five years (2007- 2011). In Islamabad, the annual average PM2.5 mass concentrations were 81.1±48.4 μg m- 3 , 93.0±49.9 μg m-3, 47.8±33.2 μg m-3, 79.0±49.2 μg m-3, 66.1±52.1 μg m-3 during 2007 to 2011, respectively; and the highest hourly values observed were 303 μg m-3 during December 2007, 495.0 μg m-3 during November 2008, 259.8 μg m-3 during September 2009, 456.0 μg m-3 during October 2010, and 379.0 μg m-3 during January 2011. Comparison of the four cities during summer 2007 to spring 2008 shows that all the four cities have PM2.5 concentration exceeding the Pakistan National Environmental Quality Standards (annual average concentration of 25 μg m-3; and 24 hourly average concentration of 40 μg m-3) for ambient air. During the same time period (i.e. summer 2007 to spring 2008), the highest seasonal PM2.5 mass concentration for Islamabad was observed as 98.5 μg m-3 during spring 2008; 150.4±87.9 μg m-3; 104.1±51.1 μg m-3 and 72.7±55.2 μg m-3 for Lahore, Peshawar, and Quetta during fall 2007 respectively. Wind speed and temperature have a negative correlation with the mass concentration of PM2.5. Moreover, the relation of vapor pressure is weak but mostly negative. Diurnal profile for all the cities suggests an association of PM2.5 with vehicular traffic. Data analysis revealed annual average mass concentration of PM2.5 (~45 to ~95 μg m-3) and NO concentration (~41 to ~120 μg m-3) exceed the Pakistan’s National Environmental Quality Standards (NEQS). The annual O3 concentration is within the permissible limits; however, some of the hourly concentration exceeds the NEQS mostly during summer months. Correlation studies suggest that carbon monoxide has as a significant (p-value ≤0.01) positive correlation with NO and NOy′; whereas, with ozone, a significant (p-value ≤0.01) negative correlation is observed. The regression analysis estimates the background CO concentration to be ~250 to ~500 ppbv in Islamabad. The higher ratio of CO/NO (~10) suggests that mobile sources are the major contributor to NO concentration. On the other hand, the ratio analysis of SO2/NO for Islamabad (~0.011) indicates that the point sources are contributing to SO2 in the city. NO and SO2 correlation indicates a direct emission sources containing high sulfur content. The correlation of PM2.5 and NO suggests that a fraction of secondary PM2.5 is produced by chemical conversion of NO into nitrates. The regional background O3 concentration for Islamabad has been determined to be ~31ppbv. The study suggests that there is an increase in O3 concentration with increases in photochemical conversion of NO to reservoir NOy′ species. In order to investigate the contribution of local or transboundary sources of air pollution towards the high ozone episodes in Islamabad, backward trajectories using NOAA HYSPLIT model were computed. Furthermore, simulations of two selected high ozone episodes were carried out by using Weather Research and Forecasting (WRF) model to assess the influence of meteorological conditions on level and variation of ozone during episode period. The HYSPLIT back trajectories have revealed that a number of back trajectories are originated from west, south-west and eastern transboundary pollution sources. It has been observed that local sources are also contributing towards pollution in Islamabad when high concentrations are observed during stagnant conditions. Furthermore, when air masses from west, south-west and south-east are advecting into the city, stagnant conditions lead to accumulation of pollutants. It has been revealed that most of the episodes occurred during stagnant conditions followed by advection from far-off regions. The study recommends that an extended air quality and climate modeling may be conducted to get an insight into the tropospheric chemistry of the area leading to many frequent high ozone episodes. There is also need to develop effective control strategies to meet the ambient air quality standards through the use of an integrated assessment model.