EPA regs and the decline of coal power are huge. I also think, but haven't seen good quantification yet (please link me if you have it) that we're really benefitting from two additional things in this decade. One, the rapidly increasing share of solar and other renewable power. Two, increasing share of hybrids and EVs in the aggregate vehicle fleet. Hopefully we can eventually clean up all the small engine crap out there too. I was born in the 80s and remember the 90s-early 00s haze and hope we never go back!