Since 1978, the NADP's National Trends Network has monitored precipitation chemistry across the United States, analyzing samples for eight major ions and acidity. This vast network also has the potential to be used to monitor trace metals in precipitation that are of concern to human and environmental health, both to evaluate long-term trends and in response to acute threats (e.g., wildfires, industrial accidents). Determining which elements can be meaningfully detected in NTN samples requires an exposure study to estimate a network limit of detection based upon blank background.

Here, we present results for a set of six field blanks plus one precipitation sample, all collected in duplicate, from October to November 2025 at NTN site WI93 in Madison, Wisconsin. Each week, two blank samples were deployed in closed collectors and retrieved after seven days of continuous exposure to the sampler interior, one from each collector type (N-CON and Aerochem). No special precautions were taken to limit systemic exposure to metals during the deployment, collection, transportation, or filtering processes outside of standard NTN procedures. Filtered and unfiltered aliquots were collected into acid-washed 50 mL polypropylene containers and acidified to 2% v/v ultrapure nitric acid.

Samples were analyzed for a suite of 55 elements using a Thermo Element XR SF-ICP-MS located in the Trace Element Clean Laboratory in the Wisconsin State Laboratory of Hygiene. Results are consistent between the two NTN collectors. Limits of detection based upon blanks will be presented for dozens of elements. We note that there are observable differences between the unfiltered and filtered portions for some key elements, and that many elements of interest to human and environmental health are resolvable in precipitation samples relative to the blanks. Proposed conditions for a more focused pilot study will also be discussed.