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In this study we present an analysis of spatio–temporal variability of monthly averaged Vertical Tropospheric Columns (VTCs) of NO2 over Pakistan using OMI (ozone monitoring instrument) dataset from December 2004 to November 2008. The results have shown significant spatial and temporal variability of NO2 column values over the study region. Four NO2 hotspots and a high density corridor were identified within the study region. The main sources of NO2 emissions in these areas were also investigated. During the study period, an average value of NO2 was observed to be 1.102 ± 0.081 × 1015 molecules/cm2, with an increasing trend of 3.29% per year. Twin cities of Islamabad/Rawalpindi, Lahore, Dera Ghazi Khan and Karachi have shown positive trends of 44.10%, 23.48%, 31.40%, and 32.32% per year respectively. Karachi has shown the highest and the lowest mean monthly average values of 11.33 × 1015 molecules/cm2 and 0.98 × 1015 molecules/cm2 respectively. Air mass trajectories for hotspot regions have been used to track possible long–range transport of NO2.
Haq et al. (Mon,) studied this question.