Precision checks are determined by performing repeated measurements of ambient-level “calibration” gases at two-week intervals for continuous analyzers, or by obtaining duplicate results from collocated samplers for manual methods. The Precision Data record is uniquely identified by the combination of monitor and date.

Precision records have a single value for each record representing Actual and Indicated values with Indicated values coming from a co-located sampler/analyzer similar to the primary (actual reading) analyzer. The Precision Report displays a daily listing of reported precision data in a tabular format. The report displays the sample values entered into the system and then calculates a percent difference between the reported values, consistent with the requirements of OAQPS (Office of Air Quality Planning and Standards) Quality Assurance guidelines.

The PARS Precision Editor (Figure 20) lists parameters in a table along with their Duration Code, Precision Check Date, and Precision Identifier Number. The parameter that is selected (by clicking Select in the left column of the table) will be displayed below the table with the following information:

Precision Test Information

• Precision Check Date: The calendar date for which the accuracy audit information pertains.
• Precision Identifier Number: A sequentially assigned number used to identify a particular precision check from others, when multiple checks are performed on the same day
• Precision Sample Identifier: The unique identity (ID) number of the reference sample used to challenge the instrument
• Precision Type Identifier: Describes the type of precision test performed
• Audit Agency Code: Agency responsible for performing a role for the monitor

Monitor Information

• Parameter: The code assigned to the parameter measured by the monitor. (A “monitor” in AQS represents not a device but the pollutant measured by a device.) Parameters may be pollutants or non-pollutants.
• Duration Code: The length of time (interval) used to acquire raw samples that are analyzed by monitors
• Parameter Type: A description of the type of parameter (e.g., ozone)
• Unit Code: AQS code for the standard parameter unit (e.g., ppm)
• POC: Parameter Occurrence Code (or Pollutant Occurrence Code), an identifier used to distinguish between multiple monitors at the same site that are measuring the same parameter. For example, the first monitor established to measure carbon monoxide (CO) at a site could have a POC of 1. If an additional monitor is established at the same site to measure CO, that monitor could have a POC of 2. However, if a new instrument is installed to replace the original instrument used as the first monitor, that would be considered the same monitor and it would still have a POC of 1.

For criteria pollutants, data from different sampling methods should only be stored under the same POC if the sampling intervals are the same and the methods are reference or equivalent. For sites where duplicate sampling is being conducted by multiple agencies or by one agency with multiple samplers, multiple POCs must be utilized to store all samples.

For non-criteria pollutants, data from multiple sampling methods can be stored under the same POC if the sampling intervals are the same and there is only one sample for the time reported. If multiple open path monitors are reporting data for the same parameter, each open path would be assigned a different POC.

• Actual Method Code: Identifies the particular method for collecting and analyzing a precision check value
• State Code: A Federal Information Processing Standards (FIPS) code that identifies one of the 50 states, U. S. territories, Washington, DC, or foreign countries. For batch loading data formats only, it may be set to “TT” to indicate that the County Code/Tribal Code field contains a Tribal Code.
• County/Tribal Code: A Federal Information Processing Standards (FIPS) code that identifies a county, or other geo-political entity, such as tribe, parish or independent city. For foreign countries, it identifies the geo-political equivalent to U. S. states, such as Mexican states or Canadian provinces.
• Site Code: A numeric identifier (ID) that uniquely identifies each air monitoring site within a county, and if it is a tribal site, within all counties included in a tribal area. There is no requirement that Site IDs be assigned continuously or in any particular order. Regional or local organizations are thus free to allocate Site IDs 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 ID is associated with a specific physical location and address. Any change in address requires a new Site ID to be assigned. This address change could include a change from the roof of one building to another. A change in location on the same roof should not normally require a new Site ID. Although an address change would routinely mean a new Site ID, some changes that do not change the site's location in respect to surrounding sources and its measurement scale would require no change. An EPA regional office should be consulted for assistance in determining whether a new site ID is required.

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

Collocated Monitor

• Collocated Parameter: The code assigned to the duplicate sampler that is paired with the primary monitor (i.e., routine monitor) to determine precision and accuracy data
• Collocated POC: The Parameter Occurrence Code (POC) of the duplicate sampler. Only applies to collocated data where the duplicate value is a recorded daily raw data point.
• Indicated Method Code: Identifies the particular method for collecting and analyzing a precision check value.

Test Results

• Actual Value: For precision and accuracy data the actual value is the concentration produced from the primary sampler (i.e., routine monitor) in a collocated sampler pair.
• Indicated Value: The measurement recorded by a monitor for a standard gaseous concentration with which it has been challenged.