That's a nasty photo angle, so it's hard to see details. I can't see the markings on the tank to even tell reporting marks, number, etc. to know who owns it.
It's definately not a pressurized car, so that rules out gasses like chlorine. Looks to me like a standard general service car, probably used for asphalt or fuel oil. (some nice black spill stains on the side of the car)