Model results and measurements in the Northern Hemisphere have shown that growth in surface emissions of nitrogen oxides and hydrocarbons leads to increases in concentration of tropospheric ozone.
Tropospheric ozone is toxic to plants, humans and other organisms, and it is a greenhouse gas.
The model also indicates that the impact on global warming of increases in surface emissions of nitrogen oxides has previously been overestimated by a factor of five. Much of this variation in ozone increase is explained by differences in emission estimates. The model takes into account the emissions, chemistry, transport and deposition of tropospheric trace gases. The chemical mechanisms are summarised in the Figure, where the inner cycle involves the photo dissociation of NO 2 leading to ozone and NO formation, and the subsequent back reaction of these species to regenerate NO 2 . Competing with this back reaction is the outer cycle involving the formation of peroxy radicals from methane, nonmethane hydrocarbons and carbon monoxide.