Inventiveness in the Air

AAES Researchers Develop
Improved Method for Measuring
Ammonia Volatilization

 Wes Wood, Sam Marshall, Charles Meadows, and Brenda Wood

Anyone who has driven past a farmer spreading broiler litter, an irrigation gun spreading dairy waste, or a hog lagoon on a windy day knows that odors are emitted from these sources. Part of this odor is ammonia, a volatile nitrogen gas that rises from the manure. AAES researchers have developed a way to monitor the volatization of this gas, which could result in safer and more efficient use of animal waste as a land-applied fertilizer.

As more and more land owners and farmers use animal waste to fertilize their land, it is vital to evaluate the amount of ammonia volatilized during and after land application. This gas is not only pungent, but it also represents a potential environmental hazard. And, as the gas is released, it also depletes the fertilizer value of land-applied manure. Measuring the amount of gas volatilized from animal waste is important in determining the nutritional and environmental impacts of land-applied animal waste.

Until recently, however, methods for determining ammonia volatilization under field conditions have been either expensive, labor intensive, or both. AAES researchers have developed a method for field-scale determination of ammonia volatilization that reduces initial costs and labor requirements without sacrificing accuracy.

The idea for an improved method for field-scale measurement of ammonia volatilization sprang from a 1995 visit to New Zealand. Researchers from New Zealand, Denmark, Australia, and Alabama gathered there to compare two field-scale methods—one developed in Denmark and one developed in Australia—to a long-time standard micrometeorological method for measuring ammonia volatilization. Liquid swine waste was land-applied, and apparatuses associated with the three methods began measuring ammonia volatilization.

The Australian method uses a device that requires no power source or special equipment, such as flow meters or anemometers. This device, referred to as an ammonia sampler, looks like a miniature space shuttle and determines ammonia loss with minimal labor. It consists of an entrance nozzle and an outer cylinder fitted with mounting pivots and fins to keep the device aligned with the wind. Inside the device are absorbing surfaces made of a spiral of waffled stainless steel that are coated with a thin layer of oxalic acid (3% weight per volume in acetone), which collects the ammonia as it flows through the air. To measure ammonia in the field, a pole (mast) is placed in the center of a 60-foot radius plot and ammonia samplers are placed at five heights on the mast (up to 10 feet) to measure the level of ammonia at each height. A second mast with samplers at the same heights is placed on the upwind edge of the circular plot to measure background ammonia. A radial plot is used to ensure that air flow to the sampler covers the same area of the plot from all directions. Because the centered sampler is equal distance from any edge of the plot, a uniform measurement is ensured.

The main disadvantage of this method is that each collection device costs about $250 to build. With 10 devices required per plot for one measurement time, and a minimum of one replacement set of devices required for subsequent measurement times, the minimum cost for ammonia measurement devices on one plot is $5,000. With three replications this figure becomes $15,000, which is cost prohibitive for many research programs.

The method from Denmark uses four-inch long glass tubes coated with oxalic acid on their inner surfaces. Two tubes are connected with silicon tubing. To each pair of glass tubes a stainless steel disk with a one twenty fifth-inch hole in the center is connected to decrease air speed inside the tube and increase ammonia collection efficiency. The tubes are mounted at five heights on four masts placed at right angles on the periphery of a 60-foot radius circular plot. At each height, two glass tube sampler units are mounted, one unit having the stainless steel disk facing the plot, and the other unit facing away from the plot. Ammonia coming from the plot is thus collected through the stainless steel disk of the first unit and the open end of the second. Conversely, background ammonia is collected through the open end of the first unit and through the disk of the second.

This method is initially cheaper than the Australian method (glass tubes are much more affordable than the “space shuttle” measurement devices). But, over the long-term the Danish method is also expensive because it has large analytical and labor requirements. One measurement time on one plot requires 80 ammonia analyses (four masts × five heights × four tubes per height). With three replications the number of ammonia analyses becomes 240. This system also requires multiple samplings after the manure is applied to monitor the release over time of atmospheric ammonia (typically a two-week period), so the number of required ammonia analyses can run into the thousands. These many analyses and the time required to change tubes between measurement periods results in a large investment in labor requirement, which makes the Danish method prohibitive for many research programs.

Figure 1. (Below) Rotating mast developed by AAES researchers being loaded with glass samples in preparation for ammonia measurement.  Figure 2. (Left) Mounting base         for the AAES rotating mast.

AAES researchers combined the best components of the two systems by mounting glass tubes on a rotating mast that sits in the center of a measurement plot. The researchers designed and built rotating masts from light-weight aircraft aluminum (Figure 1). These masts have a wind vane mounted at the top so the glass tubes always face the prevailing wind. The masts have holes drilled perpendicular to their height to accept the glass tubes. The masts slide onto a base made of a steel rod on which bearings are mounted to ensure that the mast rotates into the slightest prevailing wind (Figure 2).

Glass tubes were cut to eight inches long, and have an inner and outer diameter of one fourth- and two fifths-inch, respectively. A thin stainless steel disk with a one twenty fifth-inch hole in the center is connected to the upwind end of each glass tube, and the inner surface of each tube is coated with oxalic acid for ammonia collection (Figures 3 and 4).

Figure 3. AAES glass tube samplers are much simpler than the Australian “space shuttles.”

Figure 4. Internal steel collection coils of the Australian sampler.

The new method was tested against the Australian method, which is now considered to be a proven method for field-scale measurement of ammonia volatilization, during 1998 at the AAES Turfgrass Unit in Auburn. The test was conducted using urea as a nitrogen source, applied at a rate of 180 pounds nitrogen per acre in a one-time application that was guaranteed to promote ammonia volatilization.

Figure 5. The new AAES method (glass tubes) is highly correlated with the proven Australian method (NH3 samples).

The experiment showed that the AAES method was highly correlated with the Australian method (Figure 5), and that the AAES method absorbed more (18.1%) of the nitrogen applied as urea than did the Australian method (15.1%). Past research has shown that micrometeorological methods can underestimate true ammonia volatilization. Given this, the AAES method, which recorded higher ammonia losses, more accurately measured ammonia volatilization compared to the Australian method.

The new AAES method is an improvement over the Australian and Danish methods in terms of initial cost and analytical and labor requirements. Each of the collection devices (glass tubes and stainless steel disk) costs about $1.35 to fabricate, which is 185 times less than the cost of the Australian device. Also, the rotating mast costs no more to build than the masts constructed for the other methods. The AAES method also results in eight times less ammonia analyses per plot per measurement time than the Danish method [i.e. 10 per plot per measurement time (five each for plot and background) versus 80 per plot per measurement time]. This new method reduces these cost and labor requirements without sacrificing accuracy of field-scale ammonia volatilization estimates. In fact, the method yields a more accurate measure of ammonia volatilization because it is less likely to underestimate true volatilization.

This new device for measuring field-scale ammonia volatilization from land-applied nitrogen is less expensive, less complex, and more accurate than methods previously available. The method should aid researchers in developing systems for land application of animal manures that result in less ammonia volatilization.