The vehicles could be retro-fitted with gasoline, propane, CNG or even electric (probably powered by fuel cells).
Hobbyists (and a few companies) have been converting internal combustion cars to battery-electric for decades.
NO2 molecules are a definite problem in urban environments since UV can drive them to combine with oxygen to form ozone (O3) and nitric oxide (NO). The NO can then react with numerous other compounds to work itself back to NO2 and the cycle repeats, increasing the low altitude ozone concentration.
(ref:
https://www3.epa.gov/ttncatc1/dir1/fnoxdoc.pdf )
--dick
