Chart 4 - Ambient Ozone

Ground-level ozone is the primary contributor to urban smog, although sulfur, nitrogen, carbon, and fine particulate matter contribute to smog's formation as well. Ozone is not emitted directly into the air but forms when volatile organic compounds (VOCs) combine in sunlight with NO2, dependent upon weather-related factors. This makes it difficult to accurately predict changes in ozone levels due to reductions in VOCs and NO2. VOCs evaporate into the atmosphere from motor vehicles, chemical plants, refineries, factories, consumer and commercial products such as lighter fluid, perfume, and other industrial sources. VOCs also occur naturally as a result of photosynthesis.

Even though ozone is the most pertinent air quality problem, ambient levels have declined by 30 percent between 1974 and 1997. Trends in ambient ozone concentrations are influenced by various factors: changes in meteorological conditions from year to year, population growth, VOC to nitrogen oxide ratios, and by fluctuations in emissions from ongoing control measures. Ozone problems occur most often on warm, clear, windless afternoons.

The December 1991 National Academy of Sciences report on ozone revealed that most of the variation in ozone comes from "natural fluctuations in the weather," not from "year-to-year changes in emissions." Therefore, it concludes that current ozone reduction strategies may be misguided because they do not account for naturally occurring VOCs.16