The Quality Assurance/Science Activity Centre – Americas (QA/SAC-Americas, http://qasac-americas.org/) is one of four QA/SACs that operate to ensure data quality and support science activities in the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) programme. The QA/SAC-Americas, supported by the NOAA Air Resources Laboratory, seeks to support and improve global precipitation chemistry measurements, while other QA/SACs address measurements of gases and aerosols.

WMO is an agency of the United Nations. Its mission is to contribute to the safety, well-being, and economic benefit of people everywhere in matters related to weather, climate, water resources, and environmental issues. Discovery of a seasonal ozone hole, the realization that acid rain was damaging forests and fisheries, and the prospect of rising greenhouse gas concentrations that could effect climate change led WMO in 1989 to establish the GAW Programme with its focus on atmospheric chemistry. GAW looked to national meteorological and hydrological organizations to expand their measurement systems to include measurements of ozone, greenhouse gases, smog-producing NOx and VOCs, aerosols, and precipitation chemistry.

Standard protocols for measuring chemicals in air and precipitation were lacking as was a system for evaluating the accuracy of measurements often at trace levels. Needed was a quality assurance program that could address these shortcomings. Science advisory groups were formed to engage experts in developing standard procedures and quality assurance protocols. These groups set standards for sampling and analytical equipment, calibration, operational methods, and quality control/quality assurance procedures. The primary objective was to ensure that GAW data were of known and adequate quality to describe the spatial and temporal distributions of chemicals in air and precipitation the world over.

The Science Advisory Group for Precipitation Chemistry (SAG-PC) is a panel of scientists with expertise in atmospheric chemistry, especially precipitation scavenging, the sampling and analysis of precipitation, and wet deposition rates and loads. Chaired by Richard Artz of the NOAA Air Resources Laboratory, the SAG-PC organizes and guides the preparation of regional and global assessments (e.g. Vet. et al. http://dx.doi.org/10.1016/j.atmosenv.2013.10.060), prepares guidelines for precipitation chemistry measurements (e.g., Manual for the GAW Precipitation Chemistry Programme : WMO-GAW Report No. 160), and seeks to expand precipitation chemistry monitoring in underrepresented regions and provide training for precipitation sampling and analysis. In addition, the SAG-PC provides guidance for the QA/SAC- Americas and the World Data Centre for Precipitation Chemistry (WDCPC, http://wdcpc.org/).

The QA/SAC-Americas seeks to document and improve the quality of precipitation chemistry measurements from around the world. To do this, it conducts semi-annual inter-laboratory comparison studies. Each study consists of three “rain samples” formulated by the NADP CAL. Concentrations are generally in the range of measurements observed at regionally representative sites around the world. Between 80 and 90 laboratories receive samples with typical participation rates of ~90%. Each participant is expected to measure pH, conductivity, sulfate, nitrate, ammonium, chloride, sodium, calcium, magnesium, and potassium. Measurements of fluoride and acidity are optional. Study results are summarized and posted on the QA/SAC web site. The median measurement is taken as the “accepted true value” and non-parametric statistics are computed and presented in graphical (ring diagrams) and tabular forms. Participants are invited to view their results and take corrective actions to improve performance, as needed.