Precipitation Chemistry Quality Assurance Project (PCQA)

National Atmospheric Deposition Program
Co-located Sampler Program


Purpose of the Co-located Program


The U.S. Geological Survey (USGS) initiated the external quality assurance co-located sampler program in October 1988 to provide an estimate of the overall variability of the precipitation-monitoring system established by the National Atmospheric Deposition Program /National Trends Network (NADP/NTN), from the point of sample collection through data storage in the NADP/NTN database. Overall network variability is estimated by comparing data collected using duplicate sets of equipment installed together at NADP/NTN sites for 1 water year (October 1 - September 30 of following year). At a co-located site, the site operator processes samples from each pair of collectors (the original equipment and the co-located equipment) using standard NADP/NTN procedures. All samples are analyzed as routine weekly NADP/NTN samples by the Central Analytical Laboratory (CAL) . Precipitation chemistry varies widely among NADP/NTN sites, reflecting the diverse climatic and anthropogenic influences around the United States. Selected statistics are chosen to describe overall sampling variability, without being overly sensitive to extreme outliers. Historic data are available for the co-located-sampler program in the presentations and publications links accessable on this website.


History of the Co-located Program

During 1988 through 2004, duplicate instruments of identical make and model were compared. In October 2004, three long-term co-located sites were established at sites AZ03 , WI98 , and VT99 to continue to monitor network variability and also to collect data for network transition to updated, modern field instrumentation. Instead of co-located monitoring for 1-year durations using identical sets of instruments, the long-term co-located program used both current-day and modern instruments to generate data over a 5-year period for identification of potential "step-function" changes in the NADP/NTN data resulting from the equipment upgrades. For the first 2.5 years of the study, identical sets of instruments were used at the long-term sites to generate a baseline for comparison.

The modern instruments are now being installed across the networks. The goal is to generate data to evaluate potential shifts in trends due to the new instruments so that they will not be misinterpreted as environmental changes. Today (2012), co-located sites have two identical, modern electronic rain gages plus an Aerochem Metrics collector and N-CON NTN collector. An example is Water Year 2013 sites CO00 / 00CO in Alamosa, Colorado. This setup is used for evaluation of overall variability in electronic rain gage data and continued evaluation of bias between Aerochem Metrics and N-CON NTN collectors with respect to solute concentrations in wet-deposition samples. In the future, co-location of identical, N-CON NTN collectors is planned to evaluate overall variability in wet-deposition concentrations for samples obtained by N-CON collectors. Locations of sites that have participated in the co-located sampler program from 1988 through 2012 are shown in figure 1 .

 

 



Co-located Sampler Program Results

High-Altitude Evaluation of AeroChem Deep Bucket Sampler

 


So, is it Collocated or Co-located?

Over the years of the program's operation, editorial preferences of USGS report editors has changed. Earlier publications used the term "collocated", but more recent publications use the term "co-located". For the purposes of communication in this website, the terms are interchangeable. They both mean that two independently operated sets of monitoring instrumentation are located within approximately 5 to 30 meters of each other; essentially at the same location.



LINKS

  National Atmospheric Deposition Program National Water Quality Laboratory USGS Precipitation Chemistry Page
   

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