Evaluation of passive flux samplers with different orifice sizes to measure ammonia volatilization losses

Abstract

A common passive sampler used to measure NH₃ volatilization losses in field studies has a nozzle with a 1-mm, round orifice that reduces air flow to increase NH₃ adsorption. This reduction in air flow, measured by the orifice's K value (0–1), implies a reduction in the amount of NH₃ entering the sampler per unit of time, which may require a longer exposure to accumulate measurable quantities of NH₃. Increasing the diameter of the orifice would increase the NH₃ entering the sampler and possibly allow shorter exposure times, but it may also lead to increased NH₃ bypass. The objectives of this research were to (1) determine the effect of wind speed on NH₃ bypass in samplers with 1- or 2-mm orifices, (2) determine the reduction in air speed in samplers with 1- or 2-mm orifices by determining their K values, and (3) compare field NH₃ volatilization losses measured with 1- and 2-mm orifices. A laboratory study with wind speeds ranging from 0.6 to 10 m·s⁻¹ showed that both orifice sizes had NH₃ bypass at low and high wind speeds. Results from wind tunnel studies determined that flow was reduced to 63% (K = 0.63) in 1-mm orifices and to 74% (K = 0.74) in 2-mm orifices. Field studies indicated that 1-mm orifices may measure greater NH₃ volatilization losses than 2-mm orifices at average low wind speeds when maximum wind speeds <10 m·s⁻¹, but may measure equal or lower losses than 2-mm orifices when several days occur with maximum wind speeds >10 m·s⁻¹.