U.S. Geological Survey external quality-assurance project report to the National Atmospheric Deposition Program / National Trends Network and Mercury Deposition Network, 2007-08

The U.S. Geological Survey (USGS) used six distinct
programs to provide external quality-assurance monitoring for the National Atmospheric Deposition Program /
National Trends Network (NTN) and Mercury Deposition
Network (MDN) during 2007-08. The field-audit program
assessed the effects of onsite exposure, sample handling,
and shipping on the chemistry of NTN samples, and a
system-blank program assessed the same effects for MDN.
Two interlaboratory-comparison programs assessed the
bias and variability of the chemical analysis data from
the Central Analytical Laboratory (CAL), Mercury (Hg)
Analytical Laboratory (HAL), and 12 other participating
laboratories. A blind-audit program was also implemented
for the MDN to evaluate analytical bias in HAL total Hg
concentration data. A co-located-sampler program was
used to identify and quantify potential shifts in NADP
data resulting from replacement of original network
instrumentation with new electronic recording rain gages
(E-gages) and prototype precipitation collectors.

The results indicate that NADP data continue to be of
sufficient quality for the analysis of spatial distributions
and time trends of chemical constituents in wet deposition
across the U.S. NADP data-quality objectives continued to
be achieved during 2007-08. Results also indicate that retrofit
of the NADP networks with the new E-gages is not likely to
create step-function type shifts in NADP precipitation-depth
records, except for sites where annual precipitation depth is
dominated by snow because the E-gages tend to catch more
snow than the original NADP rain gages. Evaluation of
prototype precipitation collectors revealed no difference in
sample volumes and analyte concentrations between the original NADP collectors and modified, deep-bucket collectors,
but the Yankee Environmental Systems, Inc. (YES) collector obtained samples of significantly higher volumes and
analyte concentrations than the standard NADP collector.