Which one of the following is not a constituent of photochemical smog?

Photochemical smog forms when nitrogen oxides and volatile organic compounds react in sunlight, producing ozone, peroxyacetyl nitrate and aldehydes such as formaldehyde. Dioxins are persistent organic pollutants released mainly by combustion of chlorinated waste and are not photochemical smog products. Hence dioxins is the odd one out.