DOAS Measurement of Atmospheric Ammonia Emissions at a Dairy

A new instrument for measurement of atmospheric ammonia based on short path ultraviolet spectroscopic absorption is now operational at the WSU Knott dairy farm near Pullman, WA. The instrument has a sensitivity of approximately 1 ppbv for NH3 in an integration time of less than one minute with absolute accuracy approximately +20%. Concentration measurements have been made over the various settling lagoons, downwind from the lagoons, in the operations area of the dairy, and during the application of lagoon slurry to fields. An SF6 tracer ratio technique has been used to obtain the NH3 fluxes (+30% accuracy) and to verify an atmospheric dispersion model. Determinations have been made in spring, summer and fall 2000. Preliminary summer ammonia fluxes ranged from 30 to 75μg/m s from one of the settling lagoons for the 390 cow dairy. INTRODUCTION Ammonia emissions from livestock operations are a major environmental concern because 85% of ammonia emissions in the United States arise from agricultural sources (CENR, 2000). Noxious odor from ammonia and associated reduced sulfur compounds are an acute problem for dairy farmers making working conditions unpleasant and leading to lawsuits from neighbors (Riskowski et al., 1991). This is a particular problem where urbanization has encroached on traditional farmlands. However, ammonia emissions from dairies are more than just an odor problem. Agriculturally emitted ammonia is a major source of secondary particulate matter, PM2.5, which is regulated under the new EPA National Ambient Air Quality Standard (ARS, 2000) as a human health hazard; the potential for additional air quality regulation accelerates the need for accurate estimates and mitigation of ammonia emissions (Meyer et al., 1999). The PM Measurements Workshop held in 1998 (EPA, 1998) recommends quantification of ammonia as an aerosol precursor measurement for PM2.5 source attribution. “Current estimates of ammonia emissions to the atmosphere are characterized by a high degree of uncertainty” (CENR, 2000) since most estimates from livestock operations are calculated from the difference between known N imports and exports with unaccounted N assumed to be lost as ammonia. Because of the uncertainties in this indirect approach, it is very important to obtain better estimates of NH3 emissions using more explicit, direct measurements of NH3 emissions. Livestock ammonia emissions arise from animal production units, from manure storage and lagoons, manure applied to land, urine, and manure deposition (Pain and Misslebrook, 1991). Manure collected and stored in lagoons undergoes natural decomposition and quickly consumes available oxygen, becomes anaerobic, and emits odorous gases, including ammonia. Lagoon management and alterations in livestock feed programs can significantly reduce ammonia release (Smits et al., 1997) through reducing the concentration of urea in urine or acidifying the lagoon slurry. Additional management practices can include lagoon mixing or aeration to prevent the anaerobic conditions that result in the formation of odorous compounds (Westerman and Zhang, 1997). Published work on ammonia emissions is based on a small number of short-term measurements that involve considerable uncertainty (Demmers et al., 1999), and are mostly concentrated in Europe where farming practices differ significantly from the U.S. Consequently it is difficult to evaluate the impact of management practices on ammonia emissions in this country. In this paper we report preliminary concentration and flux results from the primary sewage settling lagoon and concentration results from the application of sewage slurry to grass fields. AMMONIA INSTRUMENT Accurate measurement of ammonia is not easy because ammonia adheres strongly to inlet lines and collection surfaces rendering in situ methods unreliable (Williams, 1992). Fehsenfeld (1995) has recommended that direct spectroscopic detection of NH3 without the intervention of a collecting medium would provide continuous, unequivocal, and calibrated measurements. We have developed, and now have extensive experience with, an open path measurement system which utilizes well tested spectroscopic techniques developed by the authors and others (e.g. Mount and Harder, 1995; Galle et al., 2000). This method has been used by our group extensively for over 10 years in difficult field conditions and has proven accurate and reliable (e.g. Harder et al., 1997; Plane and Smith, 1994). We are measuring ammonia at the WSU dairy farm by short path spectroscopic absorption in the mid-ultraviolet spectral region. This method is particularly clean, involving operation in the open air with no walls or inlets for ammonia to adhere. In addition, the method is self-calibrating, being dependent only on the precision of the measurement, the absolute cross section of the gas being measured, and easily determined geometric factors. Ammonia is measured in the ultraviolet bands near 210nm wavelength with a lower sensitivity limit of several parts per billion in an integration time of several seconds to an accuracy of approximately +20%. The system we have developed and tested is conceptually simple, consisting of 1) a bright and spectrally broad ultraviolet light (UV) source, 2) a telescope to beam the UV light into the atmosphere, 3) a mirror system to reflect the light back towards the light source, 4) a receiver telescope to focus the light spectrally absorbed by the atmosphere onto a dispersing spectrograph, 5) a multi-element multiplexing digital detector, and 6) a data analysis system. Figure 1 shows a basic schematic of the system. The ultraviolet light source is a xenon high pressure emission lamp. The telescope is a 40 cm diameter parabolic mirror operated in Newtonian configuration. The mirror reflector system consists of three large (127 mm) retroreflector arrays, which turn the light beam around on itself. The beam absorbed in the atmosphere is focused onto a custommade double spectrograph for spectral analysis. The detector for the experiment is a state of the art homebuilt Reticon silicon photodiode array 1 5 0 W X e la m p 4 0 c m p r im a r y f / 5 m ir ro r r e t r o r e f le c t o r