Rainfall can influence pollution levels by cleansing the air of particles and fumes. Ambient temperature influences air quality by affecting the way engines burn fuel and the ways in which chemicals emitted into the air interact. For example, ground level ozone, a major air pollutant in urban areas, is not emitted from a particular source but is formed in the atmosphere through the interaction of nitrogen oxides and hydrocarbons in the presence of sunlight. Regional topography and wind patterns effect air quality levels by influencing the dispersion of air pollutants. Mountain ranges are an example of the type of topographic feature that can trap pollutants in an area.
Winds play an important role by dispersing pollution or moving it to downwind areas.
If stagnant winds conditions prevail, pollution will stand idle allowing the build-up of high pollution concentrations in a single area. Winds can also worsen air quality by transporting pollutants from upwind areas on to a reservation. For a detailed discussion of the criteria pollutants see Figure 1-1 . If a region is declared a non-attainment area then it must take steps to improve air quality, or risk losing federal transportation funds. Air quality planners refer to this class of chemicals as 'hazardous', because of their ability to cause cancer.