The closure of U.S. airports following the terrorist attacks of Sept. 11, 2001, provides a rare opportunity to gauge the contributions of airports and related activities to a city's air pollution.
About 30,000 flights are completed in the U.S. each day. Some of the pollution they bring was enumerated in an August 29, 2001, article in Grist magazine. Vehicle traffic to and from airports adds more pollutants, as do related businesses such as hotels and dry cleaners.
Air quality in the vast majority of larger U.S. cities is tracked by a network of monitors that measure one or more of EPA's "criteria" pollutants -- carbon monoxide, nitrogen oxides, sulfur dioxide, particulates, ozone, and lead. Keep in mind that levels of some pollutants are known to fluctuate under normal conditions; that lag periods for formation and dissemination of pollutants need to be taken into account; and that weather patterns will affect readings.
For local data, contact the environmental or health departments of your local jurisdiction or state (STAPPA and ALAPCO members list, or EPA). Some of the raw data is online, but it likely hasn't been delivered to or quality-controlled by EPA. Monitor locations and historical data are available here.