for transforming the nonvolatile compounds in feed, water, and manure into volatile compounds such as ammonia (NH3), nitrous oxide (N2O), nitric oxide (NO), methane (CH4), and volatile organic compounds (VOCs), as well as hydrogen sulfide (H2S) and other odor-causing compounds.

A complication in measuring emissions from AFOs is that most emissions are released from area sources such as cattle feedlots, wastewater lagoons, or the land to which manure or lagoon liquid is applied, rather than from a few discrete point sources (e.g., animal house exhaust fans). Spatial and temporal variation exists. Measuring area emission rates often depends on measuring atmospheric concentrations and characterizing the micrometeorology or using atmospheric dispersion models to back-calculate the emission rates that gave the concentrations observed. Measuring emission rates from animal housing with forced ventilation is relatively easier; one measures concentrations and ventilation airflow rates. The variability in atmospheric concentrations possible near an area source is illustrated by measurements of ammonia shown in Figure 4-1. Over a period of about half an hour, the average NH3 concentration near a dairy wastewater lagoon varied from about 10 to 700 ppb (parts per billion). This variability (a factor of 70) was due primarily to variable wind speed and direction during the measurement period.

Although direct measurement of off-property impacts of the various emissions from every AFO is not practical, there is a need for an approach that can be used by local, state, or federal agencies to estimate emissions from individual AFOs. The overall air quality management goal is to limit emissions to concentrations that will not lead to exceedances of the NAAQS for criteria pollutants or other regulatory limits described in Chapter 6.

AFO emissions have impacts on several spatial and temporal scales. Greenhouse gases such as nitrous oxide and methane, which have long atmospheric half-lives and are transported for long distances, have global rather than local or regional effects; their annual emissions from U.S. agriculture are important but their local or regional concentrations are not. The fraction of these gases from AFOs is of some concern because some kinds of controls may be applied more efficiently to large sources than to smaller ones.

Primary particulate matter and odors are of concern mostly to individuals near the emission sources. What are important for them are not annual totals, but ambient concentrations averaged over short periods of time (typically 1 to 24 hours). These concentrations depend not only on short-term emission rates, but also on meteorological conditions at the time, including wind speed and direction, atmospheric stability, and precipitation. Some pollutants act at a variety of scales. Ammonia and H2S contribute to short-range odor and toxicity, but react in the atmosphere to form secondary fine particulate matter (PM2.5) dispersed over a regional scale; VOCs contribute to odor and also react with nitrogen oxides (NOx) in the presence of heat and sunlight to form tropospheric ozone (O3), another regional problem. For air pollutants that can have adverse human health

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