High Fidelity of Archived Wet Deposition Samples Across Solutes
Desneiges Murray1, Sarah Benish2, Paola Miramontes3, Jody Potter3, Adam S. Wymore3, Michael D. Bell4, David Gay2, Nichole Miller2, Katie Blaydes2, Na Zhang2, Martin Shafer2, and Pamela H. Templer1
1 Boston University, Boston, MA, 2 National Atmospheric Deposition Program, Wisconsin State Lab of Hygiene, Madison, WI, 3 University of New Hampshire, Durham, NH, 4 Air Resources Division, Department of the Interior-National Park Service, Lakeview, Colorado
The National Atmospheric Deposition Program (NADP) archives National Trend Network (NTN) wet deposition samples from active monitoring sites for potential future research. Despite their potential value to the atmospheric and biogeochemical research communities, the physical archive is underutilized by researchers for additional analyses. This is due in part to questions regarding which chemical species are stable during storage duration, how storage conditions (e.g., frozen vs. refrigerated) affect sample integrity, and what quality assurance protocols should be applied to document sample quality. Establishing evidence-based criteria for archive sample quality would increase confidence in studies using archived samples and enable their use in novel research applications. In this study, we assess the fidelity of archived wet deposition samples across storage duration, storage method, and specific analytes using concentration-specific criteria that accounts for instrument detection limits and minimizes error inflation. We first compare results of chemical analyses of ammonium (NH4+) and nitrate (NO3-) performed at two laboratories (the NADP Analytical Lab (NAL) in Madison, WI and the University of New Hampshire) on archived samples collected in 2018 from 28 sites. Across 1,058 archived samples, 95 (9%) were classified as invalid for NH4+ and 188 (18%) for NO3-, with only 18 samples (1.7%) deemed invalid for both solutes; most invalid samples occurred at low concentrations. We also examined the stability of NAL archived samples collected in 2019 from 102 sites and re-analyzed each year for a period of five years of storage under refrigerated and frozen conditions. Fridge storage resulted in 2.6x more invalid samples than freezer storage, affected a broader range of analytes, and in both treatments ammonium (NH4+) was the dominant source of invalidity. Our results are encouraging, suggesting high fidelity of archived wet deposition samples across routine measured anions and cations, and cold storage types. By systematically quantifying sample stability across these dimensions, we provide foundational information to maximize the scientific utility of the NADP sample archive and similar environmental sample repositories worldwide.