Detection of Trends and Seasonal Variation in Tropospheric Nitrogen Dioxide over Pakistan

In this study, spatial and temporal distributions of tropospheric NO2 vertical column densities over Pakistan during the time period of 2002-2012 are discussed. Data products from the satellite instrument SCIAMACHY are used. The results show a large NO2 growth over major cities of Pakistan, particularly the areas with rapid urbanization. Different seasonal cycles were observed in different regions of Pakistan. In the provinces of Punjab (north east), Khyber Paktunkhwa (north west) and Sindh (south east), NO2 columns are maximum in winter and minimum in summer months while a reversed seasonality was observed in the province of Baluchistan (south west). We compared the observed spatio-temporal patterns to existing emission inventories and found that for the most populated provinces the NOx emissions are clearly dominated by anthropogenic sources. In these areas also the strongest positive trends were observed. NOx released from soils and produced by lightning both together contribute about 20% for the provinces of Punjab, Sindh, and Khyber Paktunkhwa, while its contribution in Baluchistan is much stronger (similar to 50%). NOx emissions from biomass burning are negligible. This finding can also explain the observed summer maximum in Baluchistan, since the highet lightning activity occurs during the Monsoon season. Our comparison also indicates that the inventories of anthropogenic NOx emissions over Pakistan seem to underestimate the true emissions by about a factor of two.