In a polluted urban area, which of the following constituents of photochemical smog has the least…

In the standard composition of a polluted urban atmosphere, carbon monoxide is present at the parts per million level, that is thousands of parts per billion, and non-methane hydrocarbons run to several hundred parts per billion. Ozone builds up to a few hundred parts per billion during a smog episode. Nitrogen dioxide is usually the smallest of these, typically only a few tens of parts per billion, since much of the nitrogen oxide burden exists as NO.