How does AirNow define city?

AirNow uses city to describe an area containing monitoring sites that are usually in close proximity to each other. These areas, also called reporting areas, are defined by state and local air quality agencies when the agencies assign the monitoring sites to the reporting areas. A reporting area can have several monitoring sites or it may have only one. For example, the state of Indiana has 6 reporting areas. Some of these reporting areas have only one monitoring site, while others have several. Current Air Quality on AirNow comes from the highest AQI reading of all the sites in a reporting area.