Comparison of two passive flux methods to measure ammonia volatilization

Some micrometeorological methods for measuring NH ₃ loss use passive flux samplers with oxalic acid coated glass tubes. In previous work, we observed that tubes that had been exposed in plots with broiler litter, cleaned with 0.32 mol L ^-1 HCl and recoated, had uneven oxalic acid coating, which may lead to NH ₃ bypass. One of the objectives of this study was to measure the NH ₃-trapping capacity of previously exposed tubes that were cleaned by either washing and scrubbing with 0.32 mol L ^-1 HCl or heating at 560°C for 1 h before being recoated with oxalic acid. There were no detectable differences between cleaning methods in adsorbed NH ₃, but tubes cleaned with 0.32 mol L ^-1 HCl had uneven oxalic acid coating that led to a 7.3% NH ₃ bypass. Tubes cleaned by heating at 560°C for 1 h had even coating and no NH ₃ bypass. Presumably, heating the tubes at 560°C oxidized volatile organic compounds bound to the glass, which were acting as nucleating points for oxalic acid. A second objective of this study was to compare a passive flux method with masts on the periphery of a circular plot (PM method) to a method with a central, rotating mast at the center of the plot (CM method). Results from three field studies on tall fescue (Festuca arundinacea Shreb.) pastures showed that the PM method underestimated NH ₃ losses by 31 to 37% compared with the CM method. These underestimations were probably caused by NH ₃ bypass from tubes exposed to background NH ₃. Our results suggest that the CM method may be preferable to the PM method when environmental conditions favor NH ₃ bypass.