The use of deep UV light was investigated for the purpose of reducing ammonia from barn exhaust. The objectives of the study included (1) determining the amount of 254 and/or 185 nm light required to remove standard fractions of ammonia from model barn air, (2) examining the feasibility of providing this dosage in a realistic treatment time consistent with air turnover in barn exhaust conditions, (3) determining the chemical fate of the ammonia, (4) understand the significance of reasonable variables, such as humidity and presence of VOCs.

The research was conducted by using a gas flow system that ran quartz-tube coil through a laboratory scale lamp setup that could irradiate the gas flowing through the tube with either 254 nm or a combination of mainly 254 light along with some 185 nm light. (They are generated in tandem by use of certain relatively inexpensive lamps.) At the exit port of the quartz coil, several analytical methods were used. A direct ammonia analyzer was used, as were mass spectrometry and infrared spectroscopy. Of these, the latter was the most sensitive, and also provided the most information on the fate of the ammonia.
Irradiation with 254 nm light has previously been shown to mitigate certain VOCs, but it was found that it has little or no effect on the concentration of ammonia in air. By adding the 185 nm light, an entirely new oxidation mechanism is introduced, in which the air itself becomes the “fuel”. Initial experiments in this study used “dry air” mixtures, meaning that the air consisted largely of nitrogen and oxygen, with water excluded. To this was added ammonia in concentrations that ranged from about 10 ppm, up to 500 ppm, where normal barn exhaust contains ammonia at concentrations of 10 – 100 ppm. Irradiation of this flowing ammonia-containing air with the combination 254/185 nm light was shown to be effective in removing NH3 below detection limits with sufficiently high dosage. The major mechanism by which this occurs is the photolysis of molecular oxygen, which produces atomic oxygen and consumes the ammonia. However, atomic oxygen in air also produces ozone and nitrogen oxide (N2O). Both of these gases remained in the exhaust.
As additional gases were added to the laboratory mixture (both hydrogen sulfide [H2S] and water), levels of the ozone and N2O were mitigated. A larger scale experiment would be required to find out whether these could be brought down to acceptable levels either “naturally” through their consumption by the various pollutants in the air, or through a simple scrubbing of the exhaust (e.g., through activated carbon or similar materials). It was found that water at reasonable relative humidities for barn exhaust would have a significant impact on the mitigation of ammonia, i.e., requiring a higher dosage. However, no major impact by VOCs is anticipated.
As anticipated, the chemical fate of the ammonia is its oxidation. Depending on the initial concentration, either ammonium nitrate or nitric acid was observed as the major nitrogen-bearing product. No evidence for intermediate nitrogen oxides (NOx) compounds was obtained, implying that they are oxidized rapidly and efficiently up to the final product of nitrate.
Because of technical limitations of the setup, it was not possible to immediately determine whether the required dosage corresponded well to realistic dosages that could be easily done in barn exhaust systems. The flow/irradiation times used in the experiments (of up to a few minutes) were longer than would normally be reasonable for gas flow through barn exhausts; however the intensity of light was also lower than ought to be feasible. This, and the question of ozone and N¬2O remain the most important obstacles in bringing such a solution to industrial use. The fundamental science of converting ammonia to its natural oxidation product by means of photochemistry is sound.