Measuring Exchange of Ammonia over Cropping Systems with the Modified Bowen Ratio Technique

Paul V. Doskey
U.S. Dairy Forage Resh. Center,
Madison, WI 53706 USA

Ammonia (NH3), the only gas-phase alkaline substance in the atmosphere, reacts rapidly with sulfuric and nitric acids that are generated primarily by fossil fuel combustion to produce haze-forming fine aerosol. The principal sources of atmospheric NH3 are animal waste, nitrogen fertilizers, decomposing plant material, biomass burning, and fossil fuel combustion. Emissions from agricultural crops are expected to originate from soil soon after fertilizer is applied and also from plants throughout the growing season. However, agricultural crops, particularly corn and soybeans, have received little attention as sources of NH3. We developed a modified Bowen ratio technique to derive exchange rates of NH3 with corn and soybean surfaces. Semi-continuous measurements of the gradient in NH3 concentrations are made in concert with continuous measurements of the gradients in water vapor density and air temperature. The sampling and analytic systems for the modified Bowen ratio technique include the following components: an energy balance Bowen ratio station, a Teflon? sampling mast that draws ambient air from 2 heights above the surface into a temperature controlled enclosure, a wet effluent diffusion denuder for separating gas-phase NH3 from particle-phase ammonium (NH4+), hybrid fluorometric flow analyzers for semi-continuous measurement of the gradient in NH3 concentration, and an automated calibration system. Development of the gradient sampling and analytic techniques is described. Preliminary measurements over a soybean surface are also presented. The presentation concludes with a brief discussion of measurement and modeling needs for examining regional-scale fate and transport of NH3.