Measurements, Modeling, and Analysis of Fluxes of Nitrogen Compounds

Abstract

Nitrogen compounds play a significant role in atmospheric chemistry, contributing to acute effects on human health and the environment. Oxidized and reduced forms of nitrogen are attributed to increasing concentration levels of tropospheric ozone, photochemical smog, acid rain, and eutrophication of sensitive ecosystems. Nationally and internationally, increasing interest has been given to reduced forms of nitrogen, ammonia (NH3, most abundant alkaline component in the atmosphere) and ammonium (NH4+, the primary atmospheric reaction product of NH3), with their combination represented by NHX = NH3 + NH4+. Emission and deposition of oxidized and reduced forms of nitrogen depend on several meteorological parameters: near-surface winds (wind speed at 10m), friction velocity, turbulence, atmospheric stability, air temperature, surface heat flux and relative humidity, as well as the spatial distribution of sources.

This research initiative has been prepared in response to the expressed need for a better understanding of the nitrogen budget and related ammonia flux and dry deposition velocity in North Carolina. The primary focus of this research will be on the vertical fluxes of ammonia and implied dry deposition velocities over natural surfaces downwind of some typical natural/anthropogenic sources in eastern North Carolina. This particular area of emphasis is chosen because of the lack of data and knowledge of ammonia deposition in eastern North Carolina, where its sources (e.g. swine production facilities) have increased very rapidly in recent years.

An experimental study was conducted on the emission and dry deposition fluxes of ammonia under different meteorological conditions, using a micrometeorological technique (micrometeorological gradient and modified Bowen-ratio methods in conjunction with the Monin-Obukhov similarity theory) over natural surfaces in North Carolina where intensively managed agriculture/animal farms are located. Ammonia concentrations were measured simultaneously with mean wind speeds, wind directions and temperatures during Fall 2001, Winter, Spring and Summer 2002 at two heights (2 and 6m) employing a technologically advanced mobile laboratory. Diurnal and seasonal variations of ammonia flux and dry deposition velocity were investigated under a wide range of wind and atmospheric stability conditions yielding hourly variation of NH3 flux and deposition velocity during each seasonal campaign. Greater NH3 concentrations were measured during the fall measurement campaign, which were directly related to spray-effluent irrigation practices; whereas the winter season had the lowest overall concentrations, collected during each seasonal campaign (effect of colder temperatures). The largest average NH3 deposition velocities were estimated during the summer measurement campaign, whereas the winter season estimated the lowest daytime velocities.

This evaluation of nitrogen species was extended to addressing the total nitrogen budget for North Carolina during summer season. The portion of atmospherically deposited nitrogen, which reaches either land or water bodies is highly variable depending upon meteorological and seasonal conditions. Modeled dry deposition rates of NO (nitric oxide), NO2 , HNO3 and NH3, using a third generation Eulerian grid model (the United States Environmental Protection Agency's Models-3/Community Multiscale Air Quality (CMAQ) modeling system) in conjunction with measured wet deposition rates of nitrate (NO3-) and ammonium (NH4+), were evaluated in order to characterize the factors controlling the total nitrogen budget. In addition, model assessments were made of atmospheric inputs (loading) into the Neuse River Estuary in North Carolina. In North Carolina, approximately 50% of NHX or NO3- flux occurs in the form of dry and wet deposition during the summer season. The Neuse River watershed's largest contributor to dry deposition flux of nitrogen (nitrogen loading) was determined to be NH3, making up approximately 47% of the total atmospheric deposition.