Evaluating a Modified Passive Flux Sampler for Ammonia.

A commonly used passive flux sampler for ammonia consists of a 20-cm tube coated with oxalic acid connected to a front nozzle with a stainless steel disk having a 1-mm (diameter) hole. While this system works well, it typically requires several days (14+) of exposure to collect sufficient ammonia in the tube. Increasing the size of the nozzle hole to 2-mm would increase the amount of ammonia collected in a given time, and therefore allow measurements of ammonia lost at shorter time intervals. Information on the wind speed reduction caused by the 2-mm hole, however, is not currently available. To develop that information, first we calibrated the drop in pressure across the stainless steel disk with a 2-mm hole against air speed inside the tube. Then we placed the system inside a wind tunnel and measured the drop in pressure across the stainless steel disk at tunnel wind speeds ranging from 1 to 10 m/s. The same approach was followed with a 1-mm hole in the nozzle for comparison purposes. When compared to the wind tunnel wind speed, the wind speed inside the tube was reduced to 63% for the 1-mm hole and to 77% for the 2-mm hole.