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

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 & BULL;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 & BULL;s(-1), but may measure equal or lower losses than 2-mm orifices when several days occur with maximum wind speeds >10 m & BULL;s(-1).