Agriculture comprises many types of activities, from crop production to raising animals for food. Agricultural activities contribute directly to emissions of air pollutants through a variety of processes: enteric fermentation in domestic livestock (i.e., methane gas produced during digestion), manure management, rice cultivation, agricultural soil management, machines used in agricultural operation (e.g., diesel irrigation pumps) and burning of agricultural residues.
There are approximately 2.2 million farms in the United States. Farms that raise animals in confinement are called animal feeding operations (AFOs). AFOs generally congregate animals, feed, manure, dead animals and production operations on a small land area. Feed is brought to the animals, rather than the animals grazing or otherwise seeking feed in pastures. AFOs are a source of a number of air pollutants of concern, including particulate matter, ammonia, hydrogen sulfide, greenhouse gases (e.g., methane and nitrous oxide), odor and volatile organic compounds. In large enough concentrations, these pollutants can cause adverse human health or ecological effects. Approximately 15 percent of AFOs qualify as Concentrated Animal Feeding Operations (CAFOs). CAFOs are AFOs that confine more than a certain number of animals or, even if they confine fewer animals, have been identified as a significant contributor of pollutants.
Agricultural burning is another important agricultural activity that can affect air quality. It is the practice of using fire to reduce or dispose of vegetative debris. Some common practices include:
field burning large areas of crop residue after harvest to reduce excess plant material, control crop diseases, weeds or pests, or maintain crop yields;
disposing of piles of agricultural debris, such as orchard trees, limbs or haystacks; and
clearing vegetation out of irrigation ditches and canals.
Agricultural burning can produce a large amount of smoke in a short amount of time. To reduce impacts, permits are usually required before burns can be conducted. These permits can restrict the type and amount of agricultural materials to be burned, and limit burning to those times when air quality and meteorological conditions are most suitable for reducing potential impacts.