Livestock Contribution to Fine Particulate Matter (PM2.5) in the US
Posted: February 16, 2009
Under the current (January 20, 2009) Environmental Protection Agency (EPA; www.epa.gov) ruling, all large confined animal feeding operations (including operations with 700 or more mature dairy cows) must notify state and local emergency response officials about ammonia or hydrogen sulfide emissions from dairy operations, if they emit 100 pounds or more of these substances in any 24-hour period. Therefore, the time is right to review the concerns with ammonia emissions and the particular role of livestock in the global context of anthropogenic air pollution.
Agricultural emissions of ammonia are primarily from fertilized land and domestic and farm animal waste. The process of ammonia formation from animal manure is well studied. Ammonia concentration in animal excreta is low. Although nitrogen concentration of feces is relatively high, it is primarily in organic and non-volatile form. The main source of ammonia emissions in manure is urinary urea, which could make up to 90% of the total urinary nitrogen. Urea per se is non-volatile, but is rapidly hydrolyzed to ammonia on contact with feces by the abundant urease activity in fecal matter. The rate of the hydrolysis process can vary significantly and depends on environmental factors such as temperature and wind velocity and on animal factors such as urine (and fecal) pH and urinary urea concentration. The rate of ammonia emissions form manure is also influenced by the type of animal housing and manure management.
The main environmental concern with ammonia emissions in the U.S. is formation of aerosols such as sulfate and nitrate (considered fine particulate matter) as result of atmospheric reactions (EPA, 2004; Air Quality Criteria for Particulate Matter). Sulfuric and nitric acids can react with atmospheric ammonia to form particulate ammonium nitrate, ammonium bisulfate, and ammonium sulfate (Robert Pinder, EPA, personal communication). When deposited in sensitive ecosystems, ammonia can cause soil acidification or eutrophication.
Air pollution (particulate matter, ozone, nitrogen dioxide, sulfur dioxide) is considered a major environmental risk to human health and according to the World Health Organization (WHO; www.who.int/mediacentre/factsheets/fs313/en) is estimated to cause approximately 2 million premature deaths worldwide per year. According to the WHO report, particulate matter (PM) affects more people than any other air pollutant. Particles are identified according to their aerodynamic diameter, as either PM10 (particles with an aerodynamic diameter smaller than 10 μm) or PM2.5 (aerodynamic diameter smaller than 2.5 μm). Even low concentrations of air pollutants have been related to a range of adverse health effects. Fine particulate matter (PM2.5) is considered more dangerous since, when inhaled, PM2.5 may reach the peripheral regions of the bronchioles, and interfere with gas exchange inside the lungs (WHO, 2005). Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer. The impact of particulate matter on human health is not without controversy; some studies did not find any relationship between fine particle pollution and human health (microvascular function and biomarkers related to inflammation, haemostasis and lipid and protein oxidation; Bräuner et al., 2008, Particle and 2 Fibre Toxicology, 5:13). Currently, the U.S. EPA is proposing to update its Air Quality Index (AQI) to reflect the latest standards for fine particle pollution (www.epa.gov/pm/actions.html). Under the proposed changes, the AQI would reach “code orange” – unhealthy for sensitive groups – when particle pollution levels reach 35.5 micrograms per cubic meter of air (μg/m3).
The direct contribution of livestock to PM2.5 and PM10 emissions is insignificant. Based on 2002 estimates (EPA, 2002; www.epa.gov/air/emissions/pm.htm), direct PM2.5 emissions from fertilizer and livestock were 1,500 tons, or a mere 0.03% of the total 4.5 million tons of PM2.5 emissions, with the biggest pollution being from fires (27% of the total emissions). Again based on EPA estimates, PM10 emissions from fertilizer and livestock were 3,100 tons, or 0.02% of the total 21 million tons of PM10 emissions (the biggest contribution was from road dust, 50% of the total).
Most animal husbandry practices have low emissions of particles. However, agricultural ammonia emissions contribute to the formation of ammonium nitrate and sulfates, which are considered PM2.5. Agriculture, and particularly farm animals, is by far the largest contributor to ammonia emissions in the U.S. Most recent estimates are for 2.1 million tons (Roy Huntley, EPA, personal communication), which is about 51% of the total ammonia emissions in the U.S. (4.1 million tons). Another 1.1 million tons of ammonia are estimated to originate from fertilizer application. Within farm animals, the contribution of ruminants (dairy and beef cattle) is approximately 52% of the total ammonia emissions (EPA, 2004; National Emission Inventory—Ammonia Emissions from Animal Husbandry Operations).
To our knowledge, the fraction of PM2.5 attributable to ammonia emitted from animal farming operations has not been quantified and thus, the true impact of farm animals on PM2.5 emissions cannot be evaluated. In the current analysis, we estimate this contribution using chemically speciated PM2.5 measurements (Robert Pinder, EPA, personal communication), as described in the National Air Quality Status and Trends Report (www.epa.gov/air/airtrends/2008). We first calculated the fraction of PM2.5 composed of ammonium and nitrate. The remainder of the PM2.5 (sulfate, crustal elements, and organic matter in the National Air Quality report) are not dependant on emissions from livestock; therefore, they were not attributed to livestock in our calculation. The fraction of ammonium and nitrate PM2.5 originating from livestock was assumed to be equal to the fraction of ammonia emissions attributable to livestock (51%).
These calculations (based on EPA’s National Air Quality Status and Trends Report and presented in Figs. 1 and 2) suggest that the farm animal contribution to PM2.5 concentrations may be significant in certain areas and climatic conditions. In winter months in the Northern Midwest, this contribution may be as large as 20%. In the cooler months, the formation of ammonium nitrate is favorable, and hence the presence of ammonia can significantly increase PM2.5 concentrations. In warm weather, the proportion of PM2.5 concentrations due to livestock is low (3-5% for most regions) and may not exceed 10%. These results are consistent with month-long air quality simulations for the Eastern United States (Tsimpidi et al., 2007; Journal of Air and Waste Management Association, 57:1489-98 and Pinder et al., 2007; Environmental Science and Technology, 41:380-386). Across different regions and weather conditions, PM2.5 formed from ammonia emitted from livestock operations were estimated to contribute on average from 9 (if ammonium bisulfate is formed) to 11% (if ammonium sulfate is formed) of the total PM2.5 concentrations in the U.S.
Figure 1. Estimated contribution of livestock to total PM2.5 concentrations for various regions of the U.S. in cool weather conditions (October-April). White bars represent PM2.5attributable to livestock (actual concentrations in μg/m3 shown above the bars). Shaded bars represent PM2.5 nonattributable to livestock.
Figure 2. Estimated contribution of livestock to total PM2.5 concentrations for various regions of the U.S. in warm weather conditions (May-September). White bars represent PM2.5 attributable to livestock (actual concentrations in μg/m3 shown above the bars). Shaded bars represent PM2.5 non-attributable to livestock.
Alexander N. Hristov, Associate Professor of Dairy Nutrition, Department of Dairy and Animal Science