Seasonal Atmospheric Nitrate and Ammonium Deposition along an Elevation Gradient in the Colorado Front Range using Ion Exchange Resin Columns (2018-2019)

Atmospheric deposition of reactive nitrogen (Nr) due to human activities can have measurable effects on ecosystem processing and export of nutrients, groundwater and surface-water quality. Rates of Nr deposition to lower-elevation forests immediately adjacent to the Denver/Boulder urban area, however, have only recently been measured. The focus of this study was to determine the extent of urban and agricultural N pollution transport to the Colorado Front Range. In conjunction with the Network for Urban Atmospheric Nitrogen Chemistry (NUANC) and the National Atmospheric Deposition Program (NADP), we measured wet-plus-dry Nr deposition and evaluated spatial and seasonal variations in N deposition along an elevational gradient (1603 to 3159 meters) from plains to subalpine ecosystems west of Boulder, Colorado. Ion-exchange-resin (IER) columns with attached collector funnels were deployed at 5 locations along the elevational gradient during spring, summer and fall seasons to measure bulk (wet-plus-dry) N deposition. IER columns were collected at approximately 2-month intervals in 2018 and 2019 then extracted with KCl and analyzed for nitrate plus nitrite and ammonium concentrations. Results were blank corrected using sealed field IERs deployed to measure potential contamination due to handling. The mass of nitrogen species on the column was calculated and normalized to mass per unit area per day using the surface area of the funnel opening. Snow buckets were employed periodically during the winter to obtain estimates of deposition during snow falls. Results were normalized to mass per unit area per day using the surface area of the snow bucket opening.