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Mobile Sources and Fuels

Mobile sources are any vehicle or equipment, including automobiles, trucks, aircraft, locomotives, watercraft, construction equipment, lawnmowers and others that generate air emissions. Mobile source emissions account for a majority of all human-caused pollution emitted in Wisconsin and the Midwest.



Mobile sources include a variety of vehicles, engines and equipment and can be classified into two major categories:

On-Road Sources

On-road sources are vehicles that are normally operated on roadways. These sources include passenger cars and trucks, buses, commercial trucks and motorcycles. On-road sources are further categorized by gross vehicle weight rating (GVWR):

  • Light duty on-road vehicles are those with a GVWR of 8,500 pounds or less.
  • Heavy duty on-road vehicles are those with a GVWR over 8,500 pounds.
Non-Road Sources

Non-road sources include a wide variety of equipment and vehicles that are normally not operated on roadways. These include aircraft, construction equipment, marine vessels, locomotives, lawn and garden equipment, and vehicles used for recreation (like snowmobiles, all-terrain vehicles and recreational vessels).


Mobile source emissions account for a significant amount of all human-caused air pollution emitted in Wisconsin and throughout the Midwest. Mobile sources pollute the air through both combustion and fuel evaporation. These emissions include many air pollutants that affect human health and the environment, including carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons, nitrogen oxides (NOx), particulate matter (PM) and a variety of air toxics.

Ozone Impacts

The mobile source sector is the largest contributor to the formation of ground-level ozone. According to the 2020 National Emissions Inventory, in Wisconsin, mobile sources contribute 24% of total volatile organic compound (VOC) emissions and 55% of total emissions of oxides of nitrogen (NOx), the pollutants that form ozone. The impacts of these emissions on Wisconsin’s ozone levels are greater than those from power plants and industrial sources combined.

Elevated ozone levels can trigger a variety of health problems, particularly for children, older adults and people who have lung ailments such as asthma. Ozone can also damage trees, crops and other vegetation.

Particulate Matter Impacts

Mobile sources are significant contributors to particulate matter (PM), not only through fuel combustion (PM can be secondarily formed from emissions of VOC and NOx), but also road dust, brake wear and tire wear. Particulate emissions consist mainly of tiny, fine particles, also known as PM2.5, that are less than 2.5 microns in diameter. Unhealthy levels of PM2.5 can increase the risk of health problems like heart disease, asthma, reduced lung function and low birth weight. Fine particle pollution also contributes to regional haze (reduced visibility).

Air Toxics Impacts

Mobile source emissions contribute to ambient levels of air toxics known or suspected as human or animal carcinogens. Exposure to air toxics can also cause noncancerous health effects, such as neurological, cardiovascular, respiratory, reproductive and/or immune system damage.

Greenhouse Gas Impacts

Mobile sources are a large contributor to Wisconsin’s greenhouse gas levels, accounting for 28% of the state’s total greenhouse gas inventory in 2018. These gases trap heat in the atmosphere, affecting the climate.


There are many ways to reduce the impact of mobile sources on air quality. The DNR implements and supports many programs to address emissions from mobile sources and fuels. There are also numerous federal requirements that limit the impacts of mobile source emissions.

In addition, individual actions can help improve air quality. These actions may seem small, but together, they add up to cleaner air. Taking mass transit or carpooling, even if it is just once or twice a week, can reduce traffic congestion and pollution and save money. Biking and walking, instead of driving, can help you and the air stay healthy. Purchasing zero-emission lawn and garden equipment (like snowblowers and lawnmowers) also helps.

See the “Reducing Emissions” tab for more information on how to reduce on-road vehicle emissions.



Mobile sources typically burn gasoline or diesel fuel, but advances in technology increasingly allow vehicles to operate with zero or low emissions by utilizing electricity or renewable fuels to power the vehicle or equipment.

Under the Clean Air Act, the U.S. EPA has the authority to set national fuel emissions standards and various other requirements for fuel, such as additives. Through regulation, the impact of fuel-burning emissions on air quality both in Wisconsin and nationally can be minimized.

The most commonly used mobile source fuels are discussed below.


Gasoline is the most prevalent fuel used in mobile sources. As a result of the EPA’s regulatory programs and various other regulations, gasoline sold today in the U.S. is far cleaner than gasoline produced in previous decades. However, it is still a major source of ozone-forming compounds, greenhouse gases and air toxics.


Reformulated gasoline (RFG) is gasoline blended to burn more cleanly than conventional gasoline and to reduce ozone-forming and toxic pollutants in the air we breathe. The RFG program was mandated by Congress in the 1990 Clean Air Act Amendments and is required to be used in specific metro areas that have challenges meeting the federal air quality standards for ozone. About 25% of gasoline sold in the U.S. is reformulated.

In Wisconsin, reformulated gasoline is required to be used in the six-county Milwaukee area (Kenosha, Milwaukee, Ozaukee, Racine, Washington and Waukesha counties) to comply with the Clean Air Act.

For more information, visit the EPA’s Reformulated Gasoline website.


Diesel fuel is often used to power heavy-duty on-road vehicles (such as long-haul combination trucks), as well as non-road vehicles and equipment such as locomotives and ships. Federal diesel fuel standards have reduced harmful emissions from diesel fuel used in transportation.

Most diesel fuel is now ultra-low sulfur diesel, a cleaner burning blend that contains a maximum of 15 parts per million (ppm) of sulfur. As of 2010, all highway diesel vehicles must use ultra-low sulfur diesel. As of 2014, most non-road diesel vehicles and equipment must use ultra-low sulfur diesel. Some older locomotives and marine engines still use an older blend of low-sulfur diesel.


Fuel blended with ethanol can help reduce emissions. Ethanol is available in several different blends for use in conventional and flexible fuel vehicles. Regular unleaded gasoline contains 10% ethanol.

E15 is a blend of gasoline that contains greater than 10% ethanol and up to 15% ethanol. Under current Wisconsin law, E15 must be sold as an alternative fuel and cannot be labeled as gasoline. Model year 2001 and newer cars, light-duty trucks and flex-fuel vehicles are allowed by the EPA to use E15. Use of E15 in other vehicles or equipment may cause damage and is prohibited by federal law.

E85 (or flex fuel) is a term that refers to high-level ethanol-gasoline blends containing 51% to 83% ethanol. E85 should only be used in vehicles marked “flex fuel vehicle." It is not approved for use in gasoline-only vehicles and equipment.

For more information on ethanol-blended fuels, refer to:


Electricity is increasingly used to power both on-road and non-road vehicles and equipment. While battery electric vehicles, plug-in hybrid electric vehicles and hybrid electric vehicles have existed for years, the increased emphasis on this technology is expected to have significant impacts on Wisconsin's air quality in the years to come. Greater electrification of non-road equipment, such as outdoor recreational vehicles and lawn and garden equipment, will also contribute to clearer air.

For more information about Wisconsin’s vehicle electrification efforts, visit the Wisconsin Department of Transportation’s Wisconsin Electrification Initiative webpage.


Also known as liquefied petroleum gas or propane autogas, propane can be used to power light-, medium- and heavy-duty vehicles. Interest in propane as an alternative transportation fuel stems from its domestic availability, high energy density, clean-burning qualities and relatively low cost. It is the world's third most common transportation fuel, behind gasoline and diesel.

For more information, visit the Department of Energy’s Alternative Fuels Data Center propane webpage.


Compressed natural gas is produced by compressing natural gas to less than 1% of its volume at standard atmospheric pressure. To provide adequate driving range, compressed natural gas is stored onboard a vehicle in a compressed gaseous state. A compressed natural gas-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle. Natural gas vehicles are good choices for high-mileage, centrally fueled fleets that operate within a region supported by reliable compressed natural gas fueling.

For more information, visit the Department of Energy’s Alternative Fuels Data Center natural gas webpage.


Reducing Emissions

Despite improvements in technology and cleaner fuels, on-road vehicles remain a significant source of air pollution. To limit emissions, vehicle manufacturers employ a range of control methods, including filters, catalysts and electronic control systems. Both state and federal law require the installed emissions control equipment on vehicles to operate as designed. The programs described below help ensure that vehicle emissions are minimized.

There are also voluntary actions drivers can take to help further reduce emissions, such as minimizing idling and ensuring the proper care and operation of their vehicles.


Wisconsin's vehicle inspection and maintenance program helps identify cars and trucks with high emissions by requiring regular emissions tests. Often, a vehicle that is emitting excessively has a problem with its installed emissions control system. Once that system is repaired, the vehicle will again be emitting within legal limits.

Vehicle emissions testing is required in seven counties in Southeast Wisconsin: Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha. This Clean Air Act program is implemented specifically to help those counties attain and maintain the National Ambient Air Quality Standards for ozone but it has the additional benefit of reducing other harmful air pollutants in those areas.

Wisconsin's testing program has a long history of effectively minimizing vehicle emissions. These emission reductions are an integral part of the state’s air quality attainment efforts and are important as part of a balanced strategy that includes reductions from stationary, area and mobile source sectors.

Wisconsin’s vehicle inspection and maintenance program is administered by the Wisconsin Department of Transportation (DOT) and managed by Opus Inspections, Inc. For more information, visit:


Tampering with a vehicle's emissions control system is illegal under the federal Clean Air Act, as well as Wisconsin law. The Clean Air Act also prohibits manufacturing, selling and installing aftermarket devices that disable, bypass or reduce the effectiveness of these controls.

Tampering results in excess emissions of nitrogen oxides (NOx), particulate matter (PM) and other pollutants. Tampering can cause a vehicle to emit hundreds to thousands of times more pollution than it otherwise would.

Tampering can take two basic forms: (1) removing hardware, filters and catalysts from the installed emission control system, and (2) replacing or altering the software or calibrations that control engine operation, sometimes referred to as "tuning.” Both methods have the effect of degrading or disabling vehicle emissions controls.

The EPA has taken many enforcement actions against companies and individuals, including some located in Wisconsin, that manufactured, sold or installed aftermarket hardware or software designed to defeat pollution controls.

For more information, including fact sheets and enforcement alerts, visit the EPA’s National Enforcement and Compliance Initiative: Stopping Aftermarket Defeat Devices for Vehicle and Engines website.

Reporting Suspected Tampering

The DNR and the U.S. EPA work cooperatively to investigate reports of vehicle tampering. Anonymous reports of suspected tampering activity can be made to 1-800-847-9367 or submitted online. For more information, visit the Report a Violation webpage.

Alternately, tampering concerns may be reported directly to the EPA using the agency’s Report Environmental Violations website.


A smoking vehicle may be due to a failure or malfunction of the emissions control system or tampering. To help ensure proper operation of vehicle emission controls, Wisconsin regulations (Chapter NR 485.05, Wis. Adm. Code ) prohibit visible emissions from vehicles, specifically:

  • Gasoline-powered motor vehicles are prohibited from exhibiting visible emissions for longer than five consecutive seconds.
  • Diesel-powered vehicles are prohibited from exhibiting excessive visible emissions for longer than ten consecutive seconds.

If a vehicle is smoking, it should be promptly serviced to ensure the emissions controls are working correctly. If the vehicle is in a county that operates a vehicle emissions testing program, problems with the emissions control system could prevent a successful test and delay the registration of the vehicle.

Reporting A Smoking Vehicle

A Wisconsin-registered vehicle observed to be emitting excessive emissions can be reported to the DNR for follow-up. Please include the vehicle make/model, license plate number, and date/time and location of observation. Anonymous reports can be made to 1-800-847-9367 or submitted online. For more information, visit the Report a Violation webpage.


Unnecessary idling of cars, trucks and buses pollutes the air, wastes fuel and causes excess engine wear. Modern vehicles do not require “warming up” to operate properly, so there is no need to turn on the engine until the vehicle is ready to be driven.

Wisconsin does not have a statewide law prohibiting idling. However, individual municipalities in the state (such as Madison) may have such restrictions. The DNR encourages all vehicle operators, both personal and commercial, to reduce or eliminate idling whenever possible.


A vehicle is a large investment. Ensuring it is properly maintained not only saves money and ensures a long vehicle life but reduces harmful air emissions. Vehicle use and how it is operated will affect its impact on air quality. Check out the DNR’s guide to Vehicle Care For Clean Air to learn more.

Vapor Recovery


Gasoline contains volatile organic compounds (VOCs) and hazardous air pollutants such as benzene and toluene. VOCs, along with oxides of nitrogen, react in the atmosphere to form ground-level ozone. State and federal laws require the control of gasoline vapors to prevent their escape into the atmosphere.


Stage I vapor recovery is a control strategy that captures gasoline vapors released when a tank truck delivers gasoline to a storage tank at a gasoline dispensing facility. As the tank truck fills the gasoline storage tank, the vapors present in the tank are returned to the truck rather than released to the ambient air. The vapors are then brought back to the terminal where they are processed and turned back into liquid gasoline.

Stage I vapor recovery systems are required at gasoline dispensing facilities located in Brown, Calumet, Dane, Dodge, Door, Fond du Lac, Jefferson, Kenosha, Kewaunee, Manitowoc, Milwaukee, Outagamie, Ozaukee, Racine, Rock, Sheboygan, Walworth, Washington, Waukesha and Winnebago counties.


State and federal regulations require gasoline tank trucks to have vapor collection equipment if the trucks deliver to a gasoline dispensing facility equipped with a Stage 1 vapor recovery system. These trucks must annually test their vapor collection equipment to verify vapor tightness and report those results to the DNR to obtain the tank truck’s certification sticker. For more information on these testing requirements, refer to:

Notice: To ensure proper functionality, always download Air Management Program PDF forms to the computer before opening and entering information. To download, right-click on the document link, choose "Save link as," and save the file to a location on the computer. Open the downloaded file using Adobe Reader. For more information, visit the PDF help page.

The DNR's Annual Pressure/Vacuum Certification For Gasoline Tank Trucks (Form 4500-197) can be used to demonstrate compliance with this testing requirement. Use of this form is voluntary. Test results can be submitted using another format as long as all required information is included.


Stage II vapor recovery captures gasoline vapors when a vehicle is fueled at the pump. The vapors are returned through the pump hose to the underground petroleum storage tank instead of being released into the air. However, newer vehicles are equipped with their own vapor recovery systems called onboard refueling vapor recovery. In 2012, the U.S. EPA determined that onboard refueling vapor recovery reached widespread use and that continued use of Stage II systems is redundant. For this reason, Stage II systems are no longer required.

Stage II vapor recovery systems were required in Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties. However, facilities in these counties may voluntarily remove or decommission their Stage II systems.

For more information about changes to Stage II vapor recovery requirements in Wisconsin, refer to the following factsheets:

For more information about regulations affecting gasoline dispensing facilities, visit the DNR's Small Business Environmental Assistance Program gas stations webpage. The program's compliance calendar for gasoline dispensing facilities contains factsheets summarizing vapor recovery requirements and other relevant regulations.



Other Federal Requirements

In addition to the programs described on this webpage, other federal requirements exist to reduce the impact of mobile source emissions. These include federal emissions and fuel economy standards, as well as requirements that help ensure transportation emissions do not conflict with state air quality goals.


The U.S. EPA limits the amount of air pollution a vehicle or engine can emit by setting emission standards. The EPA has established progressively more stringent emission standards for carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter, starting in the mid-1970s for on-road vehicles and in the early 1990s for non-road engines and equipment. The EPA also sets emission standards for greenhouse gases. These standards have been critical to reducing overall emissions from mobile sources over the past few decades, as older vehicles and equipment have been replaced by newer, cleaner-burning models.

In addition to setting mobile source emission standards, the EPA regulates the fuels and fuel additives used by these sources to limit harmful emissions (see the “Fuels” tab for more information).

For more information about on-road and non-road emission standards, visit the EPA’s Regulations for Emissions from Vehicles and Engines webpage.


The National Highway Traffic Safety Administration (NHTSA), part of the federal Department of Transportation, sets Corporate Average Fuel Economy standards for light-duty vehicles and fuel consumption standards for medium- and heavy-duty trucks and engines. These standards regulate how far on-road vehicles must travel on a gallon of fuel and work in tandem with the EPA’s emission standards to reduce overall emissions from these vehicles. NHTSA also regulates the fuel-economy window stickers on new vehicles.

For more information about fuel economy standards, visit the National Highway Traffic Safety Administration’s Corporate Average Fuel Economy webpage.


Transportation conformity is a Clean Air Act requirement that connects air quality and transportation planning activities. The goal of transportation conformity is to ensure that Federal Highway Administration and Federal Transit Administration funding and approvals are given to highway and transit activities that are consistent with (“conform to”) state air quality goals. Conformity ensures that transportation activities will not cause new air quality violations, worsen existing violations or delay timely attainment of the national ambient air quality standards.

Transportation conformity in Wisconsin is governed by a multi-party agreement that includes the DNR, Wisconsin Department of Transportation, EPA, Federal Highway Administration, Federal Transit Administration, Southeastern Wisconsin Regional Planning Commission and Bay-Lake Regional Planning Commission. This interagency consultative group meets regularly to ensure transportation conformity requirements are being met.

For more information, review:

Clean Diesel Programs

Because of the public health hazards associated with diesel exhaust, the DNR operates or supports several voluntary programs to reduce emissions from diesel-fueled vehicles and equipment beyond regulatory requirements. Some programs that are primarily implemented to support other air quality goals (like greenhouse gas reduction programs) also can have the important co-benefit of reducing diesel emissions.


The DNR operates the Wisconsin Clean Diesel Grant Program to help reduce diesel emissions from vehicles and equipment across the state. This program primarily implements state rebate programs funded by the federal Diesel Emissions Reduction Act (DERA). The program also supports other federal funding opportunities to reduce diesel emissions when those are available.

For more information, visit the DNR’s Clean Diesel Grant Program webpage.


The DNR is a member of the Midwest Clean Diesel Initiative, a collaboration of federal, state and local agencies, along with communities, non-profit organizations and private companies, all working together to reduce community exposure to emissions from diesel engines and equipment. The initiative coordinates voluntary diesel emissions reduction activities in Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin.

For more information, visit the EPA’s Midwest Clean Diesel Initiative website.