The National Atmospheric Deposition Program (NADP) currently measures inorganic species of nitrogen and reactive species of phosphorus in wet deposition samples collected within the National Trends Network (NTN). However, new evidence indicates that precipitation can contain both inorganic and organic forms of nitrogen and phosphorus and that the contribution of organic species of nitrogen to total nitrogen (TN: org-N + inorg-N) can be significant (20-40%). These studies have been limited in scale and scope and much uncertainty remains about the contribution of organic nitrogen species to total nitrogen deposition, and the uncertainty in organic phosphorus (org-P) contributions to total phosphorus (TP) deposition is even greater.

The two major inorganic nitrogen species, ammonium (NH₄⁺) and nitrate (NO₃^-) along with an inorganic form of phosphorus, orthophosphate (PO₄^3-) are collected and analyzed by the NADP from samples collected using the standard NADP-NTN wet-deposition samplers. For collection of total nitrogen (TN) and total phosphorus (TP), NADP developed the SNiPiT (Sampler for Nitrogen and Phosphorus in Total), that attaches to NADP's existing NCON collector, and incorporates a collection bottle, pre-charged with sulfuric acid (H₂SO₄) which stabilizes the org-N and org-P species for quantitative TN and TP determinations.

Previously, NADP investigated the performance of the SNiPiT sampler and determination of total nitrogen and total phosphorus method using a potassium persulfate digestion (HACH chemistries: 10-107-04-4-C and 10-115-01-4-C). This method utilizes an autoclave digestion wherein all forms of nitrogen and phosphorus are converted into nitrate and orthophosphate, respectively, thus allowing for spectrophotometric determination of TN and TP using NADP's dual-channel Flow Injection Analyzer (FIA).

This investigation revealed that low-volume precipitation samples were being over-acidified at the collection step, causing an interference with color development in the FIA methods. Follow-up studies confirmed a positive correlation between low volume samples (samples < 50 mL) and a negative bias in sample results. In winter of 2023, a base-titration step with sodium hydroxide (NaOH) was investigated to compensate for the over-acidification interference. While viable for TN, the NaOH addition proved unsuccessful for TP, resulting in large false positives extending far beyond the calibration range. Ultra-pure NaOH and potassium hydroxide (KOH) were also investigated, but yielded similar results. In spring of 2024, we investigated whether reducing the concentration of the H₂SO₄ preservative by 50% would eliminate the TP bias. The test resulted in less sample volume required for TN analysis (≥ 20 mL), but a minimum of 100-200 mL of sample volume were needed to prevent negatively biased TP results.

Our presentation will provide a detailed overview of the outcomes from these method evaluations, and will focus on assessing the accuracy and precision of the preservation techniques. In addition, we will summarize efforts utilizing alternate analytical methods.

¹ National Atmospheric Deposition Program (NADP), Wisconsin State Laboratory of Hygiene, University of Wisconsin-Madison, Madison, WI

* Corresponding Author: Kathryn.McKinnon@slh.wisc.edu