Effects of equipment performance on data quality from the National Atmospheric Deposition Program/National Trends Network and the Mercury Deposition Network

Abstract

The U.S. Geological Survey Branch of Quality Systems operates the Precipitation Chemistry Quality Assurance project (PCQA) to provide independent, external qualityassurance for the National Atmospheric Deposition Program (NADP). NADP is composed of five monitoring networks that measure the chemical composition of precipitation and ambient air. PCQA and the NADP Program Office completed five short-term studies to investigate the effects of equipment performance with respect to the National Trends Network (NTN) and Mercury Deposition Network (MDN) data quality: sample evaporation from NTN collectors; sample volume and mercury loss from MDN collectors; mercury adsorption to MDN collector glassware, grid-type precipitation sensors for precipitation collectors, and the effects of an NTN collector wind shield on sample catch efficiency. Sample-volume evaporation from an NTN Aerochem Metrics (ACM) collector ranged between 1.1-33 percent with a median of 4.7 percent. The results suggest that weekly NTN sample evaporation is small relative to sample volume. MDN sample evaporation occurs predominantly in western and southern regions of the United States (U.S.) and more frequently with modified ACM collectors than with N-CON Systems Inc. collectors due to differences in airflow through the collectors. Variations in mercury concentrations, measured to be as high as 47.5 percent per week with a median of 5 percent, are associated with MDN sample-volume loss. Small amounts of mercury are also lost from MDN samples by adsorption to collector glassware irrespective of collector type. MDN 11-grid sensors were found to open collectors sooner, keep them open longer, and cause fewer lid cycles than NTN 7-grid sensors. Wind shielding an NTN ACM collector resulted in collection of larger quantities of precipitation while also preserving sample integrity.

Methods for evaluating potential sources of chloride in surface waters and groundwaters of the conterminous United States

“…Atmospheric deposition encompasses a diverse set of sources, processes, and sinks; monitoring atmospheric deposition to estimate chloride loads to a watershed or contributing area is fraught with uncertainty (Thimonier, 1998;Krupa, 2002;Colman and others, 2003;Dämmgen and others, 2005;Wetherbee and Rhodes, 2013). The total atmospheric deposition, which is measured as bulk precipitation, consists of dry deposition and wet deposition ( fig.…”

“…The design and implementation of an atmospheric deposition monitoring program are complex; major errors can be introduced through suboptimal designs. The quality of data depends on the equipment design and the placement and operation of the samplers (Buso and others, 2000;Krupa, 2002;Colman and others, 2003;Dämmgen and others, 2005;Wetherbee and Rhodes, 2013). The sampler geometry, material, and placement will affect results.…”

“…Compositing samples from collocated samplers, however, is not recommended, because one contaminated sample can determine the result (Dämmgen and others, 2005). The USGS provides a QA/QC program for the NADP efforts (Nilles, 2000;others, 2005, 2011;Wetherbee, and Rhodes, 2013); many elements of this program can be applied to improve local sampling. For example, it has long been known that precipitation volume data are biased low in unshielded precipitation gages, but shields have not been used in atmospheric deposition collectors for fear of contamination; the USGS QA/QC program developed a noncontaminating shield that increased the catch efficiency of wet-deposition samplers by about 39 percent (Wetherbee and Rhodes, 2013).…”