Engaging in any kind of reducing or concentrating of otherwise gasious pollutants would cause caustic damage to vehicles and to surrounding city or urban residential and commercial buildings, more so when mixed with water which would accelerate oxyidation.
Really? Well what about in rainy cities like Seattle and Vancouver and even New York in the fall and winter - doesn't the rain do the same thing there i.e. washes exhaust pollution out of the atmosphere?
Also whatever goes up must come down sometime...and somewhere - so its ok for L.A.'s pollution to fall inland or over Utah,Nevada etc.?
Using "water mist" to basically wet down automobile exhausts along busy freeways would help clean the breathable atmosphere and whatever is washed down could be collected and either recycled or desposed of more safely.