Airborne reduced nitrogen: ammonia emissions from agriculture and other sources
Introduction
Ammonia is the most prevalent basic gas in the atmosphere, and therefore it plays a major role in the neutralization of precipitation, cloudwater and aerosols Aneja et al., 2000a, Aneja et al., 2000b. Deposition of ammonia and ammonium contributes to water and soil acidification and may cause forest damage Bouwman and Van Der Hoek, 1997, Bouwman et al., 1997, Lee and Dollard, 1994. Also, increased nitrogen supply to terrestrial and aquatic ecosystems can cause eutrophication (Walker et al., 2000). Ammonia gas has a relatively short lifetime in the atmosphere of a few hours to a few days Warneck, 1988, Dentener and Crutzen, 1994. In contrast, the ammonium ion, as an aerosol, may have a lifetime on the order of 1–15 days (Aneja et al., 1998). Gaseous ammonia typically reacts with oxides of nitrogen and sulfur to form ammonium sulfate and ammonium nitrate particles, as shown in , (Seinfeld and Pandis, 1998).
Ammonia comprises a significant portion of the PM2.5 mass; in the eastern United States, 47% of the PM2.5 mass is ammonium sulfate according to an extensive set of monitoring data (EPA, 1995) (Fig. 1).
Results from recent studies of PM2.5 show that these fine particles can deposit deep in the lungs, which may lead to increased morbidity and/or mortality (EPA, 1996). Also, these particles are in the size range that will degrade visibility (Seinfeld and Pandis, 1998). With impending changes in particulate matter air quality standards, states must develop or adjust their existing state implementation plans (SIPs) to demonstrate compliance with the standards. In order to compose acceptable SIPs, computer models of atmospheric chemistry are being improved to predict concentrations of several pollutants. These models use meteorological data and emission inventories of chemical species as input. Since ammonia is such a significant portion of particulate mass, an accurate emission inventory is necessary for input to air quality models to predict concentrations.
To estimate the total emission rate of a compound within a region (e.g., in mass/time), an emission factor, or mass of the compound emitted per unit of activity, is multiplied by an activity level. Typically, activity levels are expressed as volume of fuel burned/time, mass of material produced/time, kilometers traveled/time, or other dimensions that define the size of a source. An inventory may thus be composed of emission rates for each type of source in a region. Because ammonia is not federally regulated in the United States, there have been only a few attempts to compile emission inventories for this compound. Results of an effort by E. H.Pechan and Associates (Roe et al., 1998) are shown in Fig. 2. More than half of the emissions in the United States are from livestock, estimated using emission factors (mass of ammonia/time emitted from one animal) multiplied by activity level (number of animals).
A more recent inventory prepared by Carnegie Mellon University (CMU) shows that some sources may contribute more ammonia than previously thought (Strader et al., 2001). For example, emissions calculated for the United States in 1995 for the most important categories include: 3.4×109 kg from livestock, 7.7×108 kg from fertilizer application, 1.5×108 kg from domestic animals, 1.3×108 kg from wild animals, 1.1×108 kg from humans, 7.0×107 kg from industry, 4.7×107 kg from mobile sources, and 6.9×104 kg from publicly owned treatment works (POTWs). There may also be significant emissions from soil but values are highly uncertain. In this paper, we compare and discuss published emission factors and activity levels for ammonia in the United States. We focus on livestock as the most important category but also consider fertilizer, soil, and several minor categories.
Section snippets
Ammonia emissions from livestock
Nitrogen in livestock food sources that does not end up in a product (e.g., milk or eggs) or that does not get absorbed by the body, is excreted by the animal. Nitrogen excreted in animal feces is typically bound up in organic compounds; limited data suggest that only 1–5% of this nitrogen volatilizes as ammonia (Lockyer and Whitehead, 1990). However, some studies have shown steady year-round emissions from stored slurry that may be due to slow release of ammonia from the feces (Patni and Jui,
Ammonia emissions from fertilizer application
Fertilizer application is typically considered the second or third most important source of ammonia on a national level, depending on whether the inventory includes soil emissions. Existing inventories usually estimate the contribution from fertilizer application to be 10–20% of the national total Roe et al., 1998, Strader et al., 2001. Activity level data for fertilizer application can be obtained from the Association of American Plant and Food Control Officials (AAPFCO) that reports
Ammonia emissions from soil
Emission from soil is the most uncertain source category in an ammonia emission inventory, but soil has the potential to be a major source. A 1990 inventory for the San Joaquin Valley in California estimated soil emissions to be 40% of the total (Sonoma Technology, 1998). High quality emission factors for soil types are not available, and the physics of ammonia-surface exchange is not well understood. A soil–plant canopy system can be a source of ammonia emissions under certain conditions and a
Mobile sources
Although mobile sources are important for other airborne contaminants, they are minor sources of ammonia on a national scale, typically comprising only a few percent of the total. To estimate mobile source emissions in ammonia inventories, data have been obtained from state transportation departments that give vehicle miles traveled per year for each county in the nation. Corresponding emission factors have been obtained from Battye et al. (1994).
Industry
Industry also plays a small role in ammonia
Summary and conclusions
Ammonia is a known precursor to atmospheric aerosols. Since the accuracy of the predictions of particulate matter concentrations from an air quality model is only as good as the quality of the input data, there is a need for high quality emission inventories of important atmospheric compounds, including ammonia. There is much room for improvement in the current ammonia emission inventories, in resolution as well as the accuracy of the data. Livestock waste is the largest source of atmospheric
Acknowledgements
The authors would like to acknowledge financial support from the US Environmental Protection Agency grant number R-826371-01-0 through a subcontract with the California Institute of Technology, and financial support from the Mid-Atlantic Regional Air Management Association (MIRAMA) and the Northeast States for Coordinated Air Use Management (NESCAUM). The views reported here do not necessarily represent the views of these funding agencies. This paper has not been subject to EPA's required peer
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