Site Number


The 4-digit number used to uniquely identify the air monitoring site within a state-county combination or tribal area. The values are always numeric, but are treated as a string and padded with leading zeroes so they must always have 4 digits.

There is no requirement that Site Numbers be assigned continuously or in any particular order. Regional or local organizations are thus free to allocate Site Numbers in any way they choose, as long as there is no duplication within a county or the counties that include a common tribal area.

A specific Site Number is associated with a specific physical location (latitide and longitude). Any change in location requires a new Site Number to be assigned (unless a waiver is obtained from the apporpriate regional office). Although a location change would routinely mean a new Site Number, some changes that do not change the site’s location in respect to surrounding sources and its measurement scale may require no change. An EPA regional office should be consulted for assistance in determining whether a new site Number is required. AQS also has the ability to “link” sites. This allows the time series of data (used for NAAQS and other purposes) for a parameter to be considered continuous even if the site moved and the Number changed.

If a new Site Number is needed for a site not operated by the air pollution control agency, that agency should be contacted to assist in the Number assignment, to ensure that the Number is unique within the county and, if appropriate, the ajoining counties sharing a common tribal area. In other words, when a new Site Number is assigned, it must be different from any other Site Number already existing for that combination of State Code and County Code and Tribal Area.

Business Rules

Must be exactly 4 chracters long.

Must not duplicate another site number in the same county or tribal area (even if other site is no longer active).