Evaluating Methods for NADP Phosphate Wet Deposition Measurement
Jeffrey L. Collett Jr.1, Amy P. Sullivan1, Lillian E. Naimie1, Jihee Ban1, and Bret A. Schichtel2,3
1 Atmospheric Science Department, Colorado State University, Fort Collins, CO, 2 Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO, 3 National Park Service, Air Resources Division, Lakewood, CO
Phosphorus (P) is an essential nutrient for terrestrial and aquatic ecosystems. Important sources of P to the atmosphere include marine aerosols, wildfires, soil dust, and industrial activity. Atmospheric deposition of phosphate has received little attention in the United States despite its potential impacts on ecosystem health. Consequently, the NADP community is considering ways to pilot measurement of P wet deposition in weekly precipitation samples. Given potential microbial transformation of P in precipitation samples, use of SNiPiT (Sampler for Nitrogen and Phosphorus in Total) samplers charged with sulfuric acid has been a focus of method testing. Very low concentrations of P in measured samples stimulated testing of high sensitivity analytical methods. Following initial work by the NADP Central Analytical Lab, we pursued further tests to: (1) quantify blanks in prototype SNiPiT samplers, (2) select an analytical method with sufficient sensitivity to measure trace-level phosphate concentrations in precipitation samples, and (3) test for effects of sample acidification on P measurement.
Flow injection analysis was selected as an optimal method for quantifying phosphate ion. A Skalar SAN++ classic series automated wet chemistry analyzer was chosen for P measurement. The system adds reagents to the sample to form a light-absorbing complex that is measured at high sensitivity (MDL ~ 0.1 μg P L-1) using a long-path (50 cm) flow cell. Measurements of precipitation and snowpack samples collected in Colorado and Kentucky revealed P levels ranging from <1 to approximately 20 μg P L-1. Initial SNiPiT sampler prototypes, constructed from PVC, were found to yield unacceptably high P blanks. New Delrin SNiPiT prototypes provided by the NADP Program Office yield consistent, low blank values below 0.1 μg P L-1. Preliminary SNiPiT sampling protocols call for pre-charging the sampler with sulfuric acid to yield a final collected sample pH < 2 to limit microbial growth. Over-acidification (pH<1.5), however, interferes with color development and P quantification. This limits the optimal range of sample:acid volume, creating a challenge for a network with diverse and variable weekly precipitation amounts.