Ammonia emissions from fertilized fields not only decrease the fertilizer value but also contribute to environmental pollution in the form of off-site deposition. Quantifying the magnitude of ammonia fluxes is therefore of great interest because of economical and environmental aspects. Two factors make ammonia flux measurements particularly challenging: (i) ammonium in particulate phase often interferes with gas phase ammonia and (ii) ammonia is easily adsorbed on surfaces. Due to the lack of a suitable fast-response ammonia sensor, we decided to explore the feasibility of using denuder tubes with the relaxed eddy-accumulation technique (REA) for measuring ammonia fluxes. In the spring of 1996, the REA technique was successfully used to measure ammonia fluxes over a corn field after urea application. The denuder tubes, coated with oxalic acid, were used at the inlet of the REA system to trap ammonia in air. During the 23 days of measurements, ammonia emission rates ranged from 1 to 1023 ng NH{sub}3 m{sup}(-2) s{sup}(-1) and deposition rates ranged from 2 to 106 ng NH{sub}3 m{sup}(-2) s{sup}(-1). As expected, daytime observations show that the ammonia fluxes are highly correlated with solar radiation.
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