Total Dissolved Nitrogen in Precipitation: Measurable or Not?

Tracy Dombek
Central Analytical Laboratory,
National Atmospheric Deposition Program,
Champaign, IL USA

Concentrations of ammonium and nitrate, the dissolved inorganic components of nitrogen, are routinely determined in wet deposition samples collected for the National Atmospheric Deposition Program (NADP), however, the dissolved organic components are not routinely evaluated. A previous study conducted by the Central Analytical Laboratory (CAL) developed and evaluated an automated in-line persulfate digestion method via Flow Injection Analysis (FIA) for the determination of total dissolved organic components of nitrogen utilizing samples from the NADP AIRMoN sites. The total dissolved fraction of nitrogen was obtained by subtracting the nitrate (measured by IC) as N and ammonium (measured by FIA ammonium method) as N from the result obtained by FIA TN method.

The previous study reported minimal differences for filtered vs. unfiltered samples: an average of < 5% organic nitrogen at Bondville, IL (IL11) over a 1 year period and as high as 11.2 % organic nitrogen in samples from the Canaan Valley Institute, WV (WV99) for a 6-week period in July-August of 2005.

The purpose of this poster is to further the approach of the previous study and to identify total dissolved organic nitrogen in monthly composites of wet deposition samples collected for NADP. Excess unfiltered NADP wet samples for 3 sites IL11, WV99, and Penn State, PA (PA15) were saved and preserved by refrigeration. Precipitation-volume-weighted monthly composites were created and analyzed for total dissolved nitrogen. All analyses were conducted on the same day for all analytes. Filtered vs. unfiltered samples were also measured to verify that the differences are minimal, to confirm the findings of the previous CAL study.

A secondary purpose of this study was to reveal if ammonium and nitrogen gained or lost in NADP samples is being converted between organic and inorganic species. Samples with measurable differences between the original and subsequent reanalysis were reanalyzed for total dissolved nitrogen, nitrate and ammonium to determine if the ammonium and nitrate lost were measurable as total dissolved nitrogen.