The U.S. Geological Survey (USGS) Precipitation Chemistry Quality Assurance Project operates the external quality assurance (QA) programs for the National Atmospheric Deposition Program (NADP). The NADP has monitored the effects of wet deposition across the United States since 1978 and provides scientific investigators with a long-term, high-quality database of atmospheric-deposition information. The NADP consists of three monitoring networks that are used to collect precipitation depth data and atmospheric deposition samples for chemical analysis: (1) National Trends Network (NTN), (2) Atmospheric Integrated Research Monitoring Network (AIRMoN), and (3) Mercury Deposition Network (MDN).

The USGS external QA programs are designed to: (1) evaluate potential contamination due to handling, processing, and shipping of samples, along with contamination introduced from field exposure of the samples; (2) estimate the variability and bias of analytical results determined by separate laboratories routinely measuring wet deposition; (3) facilitate integration of data from various monitoring networks; and (4) estimate the overall variability of the NADP data, from the point of sample collection through laboratory data quality control.

Precipitation samples are collected weekly for analysis at approximately 300 NADP sites across the country. The information gained from monitoring the chemistry of our Nation's precipitation is essential for evaluating the effectiveness of emission control efforts and in determining trends in precipitation quality. It is USGS policy to quality assure the data obtained from its monitoring programs and studies. Therefore, the USGS provides the external quality assurance project for the NADP. The information gained from monitoring the chemistry of the Nation's precipitaton is essential to integrated water quality assessments at the watershed, regional and national levels.

All data collected by the PCQA project since 1978 are available on the Sciencebase.gov: PCQA Data 1978 - 2017 , PCQA Data 2017 - 2018, and PCQA Data 2019 - 2020 .