An Experimental Study of the Vertical Eddy Diffusivity and Dry Deposition of Ammonia on a Natural Grassy Surface

Abstract

Until recently ammonia has been relatively ignored as a primary pollutant in the United States. Due to the rapid growth of animal (hog) farms, eastern North Carolina experiences higher levels of ambient ammonia and ammonium concentrations. The primary focus of this work was on the eddy diffusivity and the dry deposition velocity of ammonia over a natural grassy surfaces downwind of some typical natural/anthropogenic sources in eastern North Carolina.

All Data was collected on a 7m aluminum walk-up tower. Temperature, wind speed, wind direction and ammonia/ammonium concentrations were collected at two different heights (2m and 6m). Citric acid coated annular denuders with filter packs were used to measure the ammonia and ammonium concentrations. The tower was located at the NADP site NC41 in Raleigh, NC. The tower is located 300-400m from a small waste lagoon used by the NCSU Educational Swine Unit. The fetch is undisturbed in all quadrants except for the northeast. The northeast quadrant is disturbed by a group of small trailers, greenhouses and swine houses. The remaining quadrants are grassy fields used for grazing approximately 100 head of cattle

Hourly-averaged measurements of temperatures, wind speeds and concentrations of ammonia and ammonium are made. We assume a horizontally-homogeneuous atmospheric surface layer. Surface-layer similarity relations were used for estimating the vertical fluxes of momentum and heat. The modified Bowen ratio and gradient method were used for estimating the vertical flux and the deposition velocity of ammonia.