It's from the exhaust of all the nitros you pass each and every lap!
lol, passing the nitros isn't the problem; it's keeping my vehicles on the tires that is. I tend to overthrottle it (just because I can) and usually wayyy overshoot jumps and/or go too fast around corners.
No kidding. I swear, it's just too tempting to kill the nitros on the straight aways, you know? I love seeing that and all the looks from the sidelines. Cool until you slam into the barrier at the end of the straight.