In this study we compared ammonium, nitrate, chloride, and phosphate concentrations in samples from co-located NADP AIRMoN and NTN samplers. AIRMoN collections are event-based, stored at 4°C (after collection) and not filtered, while the weekly NTN samples are at ambient temperature (until received at the lab where they are stored a 4°C) and filtered in the laboratory. Our primary goals were to characterize the contribution of particulate phosphorus (comparing AIRMoN – NTN) to total phosphorus and to evaluate whether significant losses or interconversion of reactive nitrogen species is observed in the NTN collections.  

NADP data from four sites (IL11, PA15, TN00 and VT99) with co-located collectors covering the period from 2011 to 2017 were used in this study. Only samples with phosphate^-concentrations equal to or greater than the mean method detection limit for that time period (0.005 mg/L) were included. Sixty-two samples from 2011 to 2017 met this criterion. Volume-weighted average weekly concentrations from the AIRMoN events were determined and compared with the measured NTN concentrations for the same time period.

Precipitation volumes and chloride concentrations (chosen as a conservative element with only minimal expected particulate contribution) between collectors were nearly identical (r² and slope = 0.954 and 0.955 and 0.961 and 0.977 for precipitation and chloride respectively), therefore direct comparisons can be made between collectors for the primary study species. The reactive nitrogen species was determined using the sum of the volume weighted ammonium and nitrate from each sample. Reactive nitrogen was highly correlated (r² = 0.938) with a slope of 1.02 indicating that losses and interconversion of these nitrogen species are small in NTN collection.  However, phosphate levels were dramatically lower in NTN collections compared with AIRMoN samples and were poorly correlated (r² of 0.0011). AIRMoN samples were 67.7% higher in phosphate. This difference may be attributed to a large particulate bound phosphorus component that is not captured in the NTN collections due to sample filtration. Thus, total phosphorus wet deposition cannot be estimated from current NTN protocols and would require analysis of an unfiltered sub-sample or separately quantifying the phosphorus on the filter. In planned future studies we will examine the efficacy of MDN collections for phosphorus total wet deposition measurements.