Ammonia: Ammonia is a colorless, pungent, nitrogenous gas. It volatilizes from a solid or liquid material when the ammonium ion is present and other physical conditions exist. Ammonia gas can react in the atmosphere with gaseous acidic species to form fine particulates (ammonium [NH4+] aerosols), which are a health concern. Atmospheric NH3 can be deposited during rain events and lead to soil acidification and increased concentrations of nitrogen in surface waters, potentially contributing to eutrophication.

Hydrogen sulfide: Hydrogen sulfide is a colorless, pungent gas best known for its characteristic rotten-egg odor. At high concentrations, hydrogen sulfide can be toxic (silo gas), and even at low concentrations it is a respiratory irritant. Although hydrogen sulfide is not transported great distances, at the farm it can mix with other compounds to contribute to odor.

Dust and Particulate matter (PM): Particulate matter, or dust, varies in size on the basis of source and formation. The primary concerns related to airborne particles are haze/visibility and health effects. Dust emitted from farms is highly complex in size, physical properties and composition. For regulatory purposes, airborne particulates are commonly classified into PM10 (≤10 µm in aerodynamic diameter) and PM2.5 (≤2.5 µm in aerodynamic diameter). Coarse particles (2.5 to10 µm in diameter) tend to be deposited in the upper airways of the respiratory tract; fine particles (PM2.5) can reach and be deposited in the smallest airways (alveoli) in the lungs. Farms can contribute coarse particles directly through animal activity, feed preparation, animal housing ventilation units and vehicular traffic. They can also contribute fine particles as the result of a secondary formation process (gas-to-particle conversion; see section on ammonia).

Odor: Odors from livestock farms can be made up of hundreds of compounds (odorants). How these odorants interact with one another contributes to the specific character of an odor. Odorous compounds tend to be carried on dust particles, and, therefore, strategies to reduce odors from animal agriculture often include strategies to reduce dust.

Volatile organic compounds (VOCs): VOCs are a large group of organic chemicals that include an atom of carbon (excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate) and that participate in atmospheric photochemical reactions. Some of these reactions may lead to increased concentration of tropospheric ozone (a criteria pollutant) at ground level, thereby contributing to levels exceeding the National Ambient Air Quality Standard. VOCs can be odorous or contribute to farm odor.

Greenhouse Gases: Greenhouse gases are those that can absorb and emit thermal radiation (or heat) that would otherwise be lost to space.  Many greenhouse gases occur naturally in the atmosphere, such as water vapor, carbon dioxide, methane, ozone and nitrous oxide, while others are synthetic, such as chlorofluorocarbons, hydrofluorocarbons, per fluorocarbons, and sulfur hexafluoride.

The definition provided for ammonia, hydrogen sulfide, dust and particulates, odor and volatile organic compounds were adapted from the National Air Quality Site Assessment Tool (NAQSAT), a companion piece to the AMPAT tool.

The definition of greenhouse gases was adapted from Animal Agriculture and Climate Change “Contributions of Greenhouse Gas Emissions: Animal Agriculture in Perspective”, 2014. This project was funded by the Agricultural and Food Research Initiative Competitive Grants No. 2011-67003-30206 from the USDA Institute of Food and Agriculture.