The National Atmospheric Deposition Program (NADP) approved the N-CON Systems, Inc. ADS 00-120 precipitation-sample collector (NCON) as a replacement for the Aerochem Metrics model 301 collector (ACM) for the National Trends Network (NTN) in 2010.  To date, approximately one-third of the NTN ACM collectors have been replaced with NCON collectors, primarily at sites supported by the U.S. Geological Survey (USGS).  Aerochem Metrics stopped producing collectors circa 1996. Recent USGS investigation into potential colocated (paired) collector bias indicates wet-deposition samples from NCON collectors generally have higher solute concentrations compared to samples collected in ACM equipment, thus shifts in temporal trends for retrofitted sites and changes in spatial variability of wet-deposition concentrations across the network have been anticipated.

Colocated (paired) ACM and NCON collectors were operated at 12 NTN sites located in various precipitation regimes, resulting in 15 site-years of record.   Linear regression models, which related the paired collector data for all sites combined, were calculated for each NTN analyte excluding bromide and specific conductance.  The regression equations were applied to archived NTN data, collected using ACM equipment, to obtain model-adjusted concentration values for complete records from 18 NTN sites.  These 18 sites were separate from the group of colocated (paired) sites.  Temporal trends for the original (actual) and model-adjusted data were compared for these 18 sites.  Results indicate small shifts in 10-year seasonal precipitation-weighted mean concentration (PWMC) trend slopes for overall cations (-0.001 to -0.007 mgL^-1yr^-1 ), for overall anions (-0.009 to -0.028 mgL^-1yr^-1), and for hydrogen ion (+0.689 meqL^-1yr^-1).  Shifts in NO₃^- and SO₄^-2 seasonal PWMC trend slopes were greater in the Midwest and Northeast US compared to other regions using the adjusted data.

Spatially interpolated annual PWMC maps for original and model-adjusted data were also compared using ArcMap 9.3.1.  Analysis of geospatially interpolated PWMC data illustrated that incorporation of the NCON collectors into the NTN has changed the spatial variability in NADP’s Annual Summary maps.