Urban nitrogen dioxide is mainly from traffic. While exhaust catalysts have decreased total nitrogen oxides from petrol cars by around 96%, real-world tests show that pollution controls on diesel cars are not as effective. New diesels produce similar nitrogen oxides to those bought 15 years ago.
Typical modern diesel cars emit around 20 times more nitrogen oxides than petrol cars. Less traffic would be the most effective way to decrease air pollution but is difficult to achieve quickly. Electric cars require new charging points. Offering incentives for small petrol and petrol hybrid cars, as well as reversing the trend towards more diesel vehicles, could be a practical route to controlling urban nitrogen dioxide.
and for more details, lets have a look at this study from 2004:
This study concludes that the replacement of gasoline with modern diesel vehicles in the U.S. may drive up photochemical smog, including total column ozone, near surface ozone, and nitrogen-containing species, over the U.S. on average and in the Southeast in particular, unless NOx emissions and the NO2:NO ratio from diesel vehicles are reduced to or below those of gasoline vehicles. The study also finds that vehicle NOx controls may be more effective than NO2:NO ratio controls at reducing ozone.http://www.stanford.edu/group/efmh/jacobson/Articles/I/DiesNOxTextnew.pdf