One of the largest causes of pollution in the Phoenix area is the amount of cars on the roads. Although expensive, the solution is REAL mass transit. What I mean is a commuter rail system that is fast and inexpensive. Our current bus system is way too slow. For example, it takes me between 30 and 50 minutes to drive to work (depending on traffic), but I rode the bus, it would take about 2 hours. Even if if was free I wouldn't take the bus. Commuter rail would probably drop the time down to 45 minutes to an hour. That would be worthwhile.
We could have a subway, and elevated train, or use existing track. The trick is for it to have it's own right of way (not have to wait for traffic). You could have a stop every mile or so with the buses fanning out from the rail stops. If the commuter rail hit malls and shopping centers as much as possible you would already have parking and the businesses would get a boost from the commuter traffic.
If we can pull half of the cars off of the road, we would greatly reduce the "brown cloud", make traffic run faster, and make Phoenix more accessable to everyone.