Chemical analyses and precipitation depth data for wet deposition samples collected as part of the National Atmospheric Deposition Program in the Colorado Front Range, 2017-2019

The USGS Precipitation Chemistry Quality Assurance Project (PCQA) conducts research using the infrastructure of the National Atmospheric Deposition Program (NADP) National Trends Network (NTN). Beginning in December 2016, the PCQA installed and began operating NTN monitoring sites in the Denver-Boulder metropolitan area to study urban reactive nitrogen wet deposition. Precipitation depth data are collected at 15-minute intervals using electronically recording precipitation gages, and weekly composite samples are obtained using automated mechanical collectors. The precipitation depth data are summarized into daily, monthly, seasonal, and annual records. The samples are analyzed by the NADP Central Analytical Laboratory per NADP Stanadard Operating Procedures and methods. The NADP Program Office delivered the sample analysis results to the USGS by email, which included parameters and censored sample data that are not available on the NADP web site for public access. Measured parameters include precipitation depth, pH, hydrogen ion concentration (from pH), specific conductance, and ion concentrations for calcium, magnesium, sodium, potassium, ammonium, chloride, nitrate, sulfate, and phosphate. Less than 5 percent of the samples are for extended durations greater than 7 days when equipment failed due to extreme weather conditions and associated power or mechanical failures, and these samples are flagged accordingly by NADP. Aliquots of some of the wet-deposition samples were analyzed for stable isotopes of nitrogen and oxygen in the nitrate molecules by the USGS Reston Stable Isotope Laboratory (2017) and the University of Pittsburgh Stable Isotope Laboratory (2019). The National Oceanic and Atmospheric Administration's Hybrid Single Particle Integrated Trajectory (HYSPLIT) Model was used to track sources of air pollution that was washed from the atmosphere and into the sample collectors. The HYSPLIT model used the EDAS meteorological data set (NOAA archives) to estimate 24-hour back trajectories at 100m, 500m, and 750m above the land surface. Azimuths of the back trajectories for the starting time of each precipitation event and subsequent times separated by at least 6 hours, were estimated manually within approximately plus or minus 10 degrees, and these were precipitation weighted to compute overall azimuths for the weekly composite samples. The stable isotope values and back trajectory azimuths are only available in this data release. This data release contains chemical analyses for composite wet-deposition samples and associated daily and weekly precipitation depth data and 24-hour air parcel back trajectory azimuth estimates for locations in the Denver-Boulder, Colorado metropolitan area and nearby Front Range foothills. These data are provided in support of planned research studies conducted by the U.S. Geological Survey, University of Colorado, Colorado State University, Texas A&M University, and other colleagues. Most of these data are for urban sites, which are not considered to be regionally representative. The NADP does not always allow open access to the urban site data, and they must be obtained by contacting the NADP with a special data request. Negative values for concentrations indicate analyses below analytical detection limits whereby the detection limits are obtained by taking the absolute values.