Sulfur dioxide, for example, rarely as much as 50 parts per billion of the atmosphere, even where its commissions are highest, and yet it contributes to acid deposition, to the corrosion of stone and metal and to the aesthetic nuisance of decreased visibility. The compounds, which re similarly scarce, are important in the formation of both acid deposition and what is called photochemical smog, a product of solar-driven chemical reactions in the atmosphere. The chlorofluorocarbons, which as a group account for part per billion or so of the atmosphere, are the agents primarily responsible for depleting the stratospheric ozone layer. in addition, rising levels of chlorofluorocarbons, together with methane (Cll), nitrous oxide (NTO) and carbon dioxide (CO, by far the most abundant trace gas a 350 parts per million- are enhancing the greenhouse effect.