Sometimes it's a mystery trying to decipher emissions control terminology. It sounds like a lot of letters strung together, rather than actual words. What is a DPF, DOC or TWC? What does PM, HC, and CO stand for? Yes, it's a world of acronyms.

The following short list of terms and definitions is a good starting point for understanding the puzzling language related to air pollution control.

DOC - Diesel Oxidation Catalyst:

Used on diesel engines, a flow-through emissions control unit (or substrate), having large open channels that allow exhaust gases to pass through unobstructed. Emissions reduction is achieved when exhaust gases interact with the catalyst coating on the channel walls. DOCs are used to reduce carbon monoxide, hydrocarbons and, to a lesser degree, particulate matter (soot). Other names used to describe a DOC are "scrubber", or "oxi-cat". DOC can also refer to a diesel oxidation converter which is a catalyzed flow-through substrate that has been placed inside of a steel body and is ready to be attached to an engine.

TWC - Three-Way Catalyst: An emissions control unit used to reduce emissions from spark-ignited (natural gas) engines, utilizing the same flow-through ceramic substrate as a DOC. A three-way catalyst is coated with a unique catalyst that significantly reduces carbon monoxide, hydrocarbons and nitrogen oxides at stoichiometric conditions. Therefore the reduction process is called "three-way" conversion.

OC - Oxidation Catalyst: Also known as a two-way catalyst, is an emissions control unit used to reduce emissions on spark-ignited (natural gas) engines, utilizing the same flow-through ceramic substrate as a DOC. A two-way catalyst is coated with a unique catalyst that significantly reduces carbon monoxide and hydrocarbons. This reduction process is called "two-way" conversion.

DPF - Diesel Particulate Filter:

Used on diesel engines, it is a wall-flow filter, containing alternating blocked channels that force exhaust gas through its porous walls, capturing soot particles. Diesel particulate filters are used for the reduction of particulate matter, carbon monoxide and hydrocarbons. Diesel particulate filters are efficient at collecting PM from diesel exhaust, resulting in a considerable amount of build-up within a few hours of operation. To clean the filter or reduce build-up, it is necessary to oxidize the solid carbon-based PM into gaseous carbon dioxide. This process is called regeneration. It is primarily dependent upon the type of catalyst applied to the diesel particulate filter, exhaust gas temperature and fuel sulfur content. DPFs are also referred to as "soot-traps" or "soot filters".

SCR - Selective Catalytic Reduction:

Used on lean-burn diesel and natural gas engines to reduce NOx emissions. The process involves a reductant such as ammonia or urea to be mixed with NOx in the exhaust, which then passes through a special flow-through catalyst to create the reduction process. During the process, the ammonia-NOx mixture is converted into harmless nitrogen and water. SCR catalysts can be either vanadium-based or iron/copper zeolite-based. Vanadium is classified as toxic. Iron zeolite catalysts are non-toxic and considered a safer alternative for SCR systems.

PM - Particulate Matter: A complex mixture made up of carbon particles, hydrocarbons and inorganics. Also referred to as soot, black carbon, black smoke and fine particle pollution, PM is a component of diesel exhaust emitted by on-road and off-road diesel vehicles such as trucks, buses, automobiles, construction equipment, ships, trains and mining vehicles. Other sources of diesel PM are stationary power generators, water pumps, and portable gen sets. PM is a result of incomplete combustion during the burning of diesel fuel.

HC - Hydrocarbons: are released into the atmosphere as a result of incomplete combustion of fossil fuels, as well as fuel evaporation. According to the Environmental Protection Agency, 47% of hydrocarbon emissions in the atmosphere can be attributed to on-road and off-road vehicles. The strong odor associated with diesel emissions is due to the presence of hydrocarbons. When hydrocarbons combine with NOx and sunlight, ozone is formed. This is a serious form of air pollution and a key component of smog.

CO - Carbon Monoxide: is produced due to the incomplete oxidation of carbon during the process of combustion when any fuel is burned. Diesel, biodiesel, gasoline, propane, natural gas, oil, wood and coal all produce carbon monoxide when burned. In the United States, 60% of CO emissions are from vehicle exhaust. In congested urban areas, up to 95% of CO emissions can be attributed to on-road and off-road vehicles. Other sources of CO emissions are industrial boilers and incinerators, unflued or defective gas heaters and heating appliances. Breathing in carbon monoxide in high concentrations is deadly to humans and animals.

NOx - Nitrogen Oxides or Oxides of Nitrogen: are formed when nitrogen (N2) and oxygen (O2) are combined at high temperatures and pressure during the combustion of fuel. All fuels, such as gasoline, diesel, biodiesel, propane, coal, and ethanol, emit NOx when burned.

The Environment Protection Agency estimates that 49% of NOx emissions come from on-road and off-road vehicles, 27% from power generation (electric utilities) and the remaining 24% from industrial, commercial and residential sources. Due to the many compounds that are a part of NOx (predominantly nitrogen dioxide and nitric oxide), the pollutant contributes to a wide variety of health and environmental problems. NOx is also a precursor of ground-level ozone and contributes to global warming.

EPA - United States Environmental Protection Agency: Federal agency charged with protecting human health and the environment. Through federal laws and regulations, the EPA sets national standards for emissions reduction levels. Visit the EPA at http://www.epa.gov.

CARB (or ARB) - California Air Resources Board: A California state agency with the mission of attaining and maintaining healthy air quality, protecting the public from toxic air contaminants and establishing laws and regulations to set emissions standards within the state of California. Established in 1967, CARB air quality regulations predate EPA regulations and usually maintain more stringent emissions rulings than federal standards.

For an extensive directory of acronyms and abbreviations click here.