The EPA (Environmental Protection Agency) has many guidelines set in place the control the use of dangerous air contaminants such as asbestos, aerosols, carbon monoxide, chlorofluorocarbons, mercury, etc. It also discusses in great detail ground level ozone which is of particular concern, especially in the summer time when the temperature rises.
Ozone can be either good or bad. When it is low it is considered bad ozone. This bad ozone is caused by poor vehicle exhaust, gasoline vapors, pollutants from industry, etc. The EPA has guidelines that must be lawfully followed to help protect the environment from these dangerous pollutants. For example, some areas require that people take their vehicles in for emissions tests. Industry is also heavily fined for breaking rules set in place by the EPA.
I would say that car emissions have been one area where the EPA has exercised a great deal of influence. Modern manufacturers have to ensure that their automobiles pass emissions standards and qualifications. Drivers have to bring their cars to be tested to ensure that their cars are functioning well enough to meet emissions standards. In this light, the dependence on automobiles is being addressed from an environmental point of view. I would say that this is a major step in attempting to improve air quality because it seeks to take something that is quite local (driving an automobile) and apply a global understanding to it (preserving the quality of air).
The Clean Air and Water Acts were passed in the early 1970s under the Nixon Administration, and the Environmental Protection Agency was also formed at that time, so Nixon's crimes in office have overshadowed his environmental achievements, which are notable.
One of the efforts of the EPA has been to help enforce regulations on auto and industrial emissions. They monitor mining operations, large industrial and oil facilities, toxic waste dumps, etc. to make sure they are in compliance with the law. Once the Superfund program was established to clean up nuclear and hazardous waste sites around the country, EPA played a primary role in getting that underway and still continues to oversee that process.