Advantages and Disadvantages of Passives
Although the ability to measure ammonia in the atmosphere is important, measuring ammonia concentrations present challenges. Specifically, ammonia concentrations in the free atmosphere are generally very low (part per billion level) and samples can be easily contaminated.
Typical ammonia measurements are made by pumping ambient air through annular denuders at a controlled flow rate. These denuders are coated with an acidic substrate. As air flows through, the acid reacts with the ammonia forming a solid that can be collected, removed, and analyzed. This method is well established and yields accurate 24-hour-average values. The drawbacks of this approach is that sampling pumps require stable power, and the fragile glass denuders are expensive and vulnerable to breakage during shipping. All of this results in an expensive mode of sampling.
In contrast, passive diffusion samplers can be readily shipped, they do not require pumps or power, and are very inexpensive to purchase and operate. However, to accurately and reliably measure ambient ammonia at concentrations in the 1 to 20 ppbv range, passive diffusion samplers are typically deployed for 14 or more days.