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Acid Deposition Monitoring Quality Assurance - Precision

Two precision estimates are calculated. Laboratory precision is determined by a duplicate analysis of a sample. The results of the initial analysis and the re-analysis are compared. Approximately 10% of samples are re-analyzed.

The field precision is determined by operating a second sampling device in parallel with the first. The analytical results of the official and duplicate or collocated sampler are compared. One duplicate site has been established for the network.

Percent difference is determined for each pair of results using the following equation:

equation for calculating percent difference
definition of elements in equation

The precision results are reported as 95% Confidence Limits. The mean and standard deviation of the individual percent differences are determined. The confidence limits are calculated by adding and subtracting the product of 1.96 and the standard deviation to/from the average percent difference. It should be noted that many of the analytical results are zero (below the detection limit of the analytical method), so percent difference could not be determined.

Quality Assurance - Accuracy

The accuracy is estimated from an independent challenge of the analytical process with a known concentration. New York State participates in an EPA Audit program.

Also, NYSDEC is a participant in the World Meteorological Organization (WMO) which has conducted an annual intercomparison of precipitation chemistry laboratories since 1978 that are participants in the Global Atmospheric Watch (GAW) program or its predecessor the Background Air Pollution Monitoring Network (BAPMoN). Since 1996, the coordination of the intercomparisons has been conducted by Quality Assurance/Scientific Activity Center (QA/SAC) for the Americas using test samples prepared by the Precipitation Chemistry Reference Laboratory (PCRL) at the Illinois State Water Survey (SWS) in Champaign, Illinois.

Participants in the intercomparisons are currently sent three samples of simulated rainwater which contain concentrations of ions commonly found in acidic precipitation. Additionally, two more samples may also be included which contain concentrations of trace metals found in polluted precipitation.