An Experimental Study of The Emissions and Dry Deposition of Ammonia Over Natural Surfaces In Eastern North Carolina

Due to rapid growth of animal (primarily, hog) agriculture in Eastern North Carolina, there is an increased interest in improving our understanding of emissions, transport, transformation, and deposition of atmospheric nitrogen compounds and their potential impacts on sensitive ecosystems. The primary focus of this study is on the emission and dry deposition of reduced nitrogen compounds, such as ammonia and ammonium aerosol, over different types of natural surface. Toward this end, we have used a 7 m portable tower to measure concentrations of ammonia gas and ammonium aerosol at the two heights (2 and 6 m)above the surface using both the traditional annular denuders and continuous-flow wet denuder systems. At the same time, accurate measurements of wind velocity, temperature and their vertical gradients between the same two heights are also measured. Micrometeorological gradient and modified Bowen-Ratio methods are used to estimate the vertical fluxes, as well as dry deposition velocities of ammonia and ammonium under different meteorological (wind temperature, humidity, cloud cover, stability, etc.) conditions. The traditional annular denuder systems are found to have some serious limitations, as they required long (3 hr or more) sampling times over which hourly estimated values of eddy diffusivities may vary substantially (an average value of K over the sampling period is needed for estimating the flox from the gradient measurement). More recently acquired continuous-flow wet denuder systems are used to measure hourly-averaged concentrations and, hence, fluxes and dry deposition velocities.

The result of this study will be used to obtain improved parameterizations of the emission and dry deposition of ammonia and ammonium in regional air quality models currently being developed for atmospheric nitrogen compounds.