The chemistry of ozone formation was simply too complex. Only a large and detailed research model requiring massive computing power could simulate it accurately. The story behind the model's development is interesting in itself. It also illustrates some important attributes of integrated assessment modeling, and of systems analysis generally.
Tropospheric ozone is a secondary pollutant.
It forms when nitrogen oxides, volatile organic compounds, and carbon monoxide mix in the presence of sunlight. An integrated assessment of ozone pollution must take account of the entire chain of pollution, from the emission of each chemical precursor to its final impact on human health and the environment. The illustration suggests how difficult it can be to devise measures to limit ozone. In areas that typically have comparably low nitrogen oxide concentrations, such as Scandinavia and some Mediterranean areas, the easiest way to reduce ozone is to reduce NO x even further. But in northwestern Europe and other areas where concentrations are generally high, reducing NO x levels could actually increase ozone pollution.