What are some of the challenges to reducing air pollution?
Fossil fuels are very potent fuels, so they are convenient to use as they contain a lot of energy compared to their size. Due to these conveniences, we want to use them.
Furthermore, renewable energy cannot support our energy use yet.
Also, many engines are built around certain fuels. Diesel engines filled with biofuel would not work.
This is also true for using hydrogen, which is also very explosive, as it requires a specific type of engine.
The cost to build these things is too high when compared to the, seemingly, distant issues of global warming.