Nitrogen Deposition to a Southeastern U.S. Forest

During the Duke Forest CELTIC study, we used the eddy covariance flux method to measure NOy deposition along with gradients of HNO3, NH3, NH4+ and NO3- in order to elucidate components of dry deposition of nitrogen. These measurements have been viewed in light of continuous CASTNET-based concentrations to estimate annual nitrogen deposition for the Duke Forest area. Significant findings include (1) HNO3 vapor concentrations may be overestimated in the CASTNET data set, probably due to re-volatilization of aerosol-NH4NO3 and (2) total oxidized nitrogen (NOy) deposited is significantly greater than HNO3 deposition alone (~ 30-40%). The effect of (1) is an overestimate of N-deposition when using the CASTNET data; however, this is more than offset by including the total NOy flux in (2). Scaling up to an annual budget yields a best estimate of 13.4 kg N m-2 deposited in the Duke Forest area in 2003. Approximately 48% of this was due to dry deposition. Figure 1 shows that dry deposition of NOy (which includes gas phase HNO3) is a major component of N-deposition during fall and winter periods when wet deposition is at a minimum.

photo of celtic sight

CELTIC towers imbedded into the Duke Forest

Measurement of Plant Emissions

To advance understanding of global biosphere - atmosphere interactions and to predict the response of the earth system to future perturbations. This is being accomplished through multidisciplinary field, laboratory and modeling studies of the processes controlling these interactions on various scales (e.g., leaf to canopy to landscape to global).

What is nitrogen deposition?

Nitrogen deposition consists of the input of reactive nitrogen species from the atmosphere to the biosphere (e.g. taken up by plants). Pollutants are a contributor to nitrogen deposition. These pollutants derive mainly from nitrogen oxides (NOx) and ammonia (NH3). In the atmosphere NOx is transformed into several different secondary pollutants, including nitric acid (HNO3), nitrates (NO3-) as well as organic compounds, such as peroxyacetyle nitrate (PAN). NH3 is also transformed to ammonium (NH4+). Both the primary and secondary pollutants may be removed by wet deposition (the removal of gases and aerosols in the atmosphere by rain) and by dry deposition (without rain).