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Transportation Air Quality Selected Facts and Figures

Vehicle Emissions

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What Are the Sources of Vehicle Emissions?

The power to move a motor vehicle comes from burning fuel in an engine. Emissions from vehicles are the by-products of this combustion process. In addition, VOCs escape through fuel evaporation. As vehicle exhaust systems have improved, evaporative emissions have become a larger component of total-vehicle VOC emissions.

What Emissions Come from Vehicle Exhaust?

The combustion process results in emissions of VOC, NOx, PM, and CO, which are released from the tailpipe while a vehicle is operating. Exhaust emissions occur during two modes:

What VOC Emissions Come from Fuel Evaporation?

VOCs also escape into the air through fuel evaporation. Despite evaporative emissions controls, evaporative losses can still account, on hot days, for a majority of the total VOC pollution from current-model cars. Evaporative emissions occur in several ways:

Source: U.S. Environmental Protection Agency, Automobile Emissions: An Overview, Fact Sheet OMS-5, August 1994, https://www3.epa.gov/otaq/consumer/05-autos.pdf

What Are the Emissions from a Typical Drive in My Car?

Average Emissions of a Typical Car on the Road in 2015

Title: Series of three pie charts showing the average emissions of a typical car on the road in 2015 - Description: The three pie charts show the average running exhaust, start, evaporative, and running loss emissions for VOC, CO, and NOx, respectively, of a typical car on the road in 2015. For VOC, running exhaust is 23%, start is 25%, evaporative is 41%, and running loss is 10%. For CO, running exhaust is 72% and start is 28%. For NOx, running exhaust is 76%, and start is 24%.

Source: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for VOC and NOx, winter weekday for CO, June 2015

Starting a car cold increases trip emissions compared to starting an engine that is already warm. On a given weekday, cold starts of a typical vehicle produce 2.4 grams of VOCs (25 percent of the typical daily emissions), 24.2 grams of CO (28 percent of the typical daily emissions), and 2.4 grams of NOx (24 percent of the typical daily emissions) per vehicle start. Running exhaust accounts for another 2.2 grams of VOCs, 61.9 grams of CO, and 7.6 grams of NOx per mile traveled.

VOCs are also emitted through fuel evaporation. For example, parking your car all day produces about 3.8 grams of VOCs.

Source: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for VOC and NOx, winter weekday for CO, June 2015, https://www3.epa.gov/otaq/models/moves/index.htm

Do Vehicle Emissions Rates Change with Vehicle Speed?

Emissions rates vary based on the speed a vehicle is traveling. EPA's model for highway vehicle emissions, Motor Vehicle Emission Simulator (MOVES), shows how speed affects emissions rates. VOC, NOx, CO, and PM2.5 emissions rates are typically higher at lower speeds (0-20 mph).

These curves do not represent the full range of effects associated with travel at different speeds. Emissions rates are higher during stop-and-go, congested traffic conditions than free-flow conditions operating at the same average speed.

Emissions Rates at Various Speeds

Title: Line chart comparing emissions rates in grams per mile at various speeds for VOC, NOx, CO, and PM-2.5 - Description: The line chart compares emissions rates in grams per mile at various speeds for VOC, NOx, CO, and PM-2.5 in 2015. The rates are generated by U.S. EPA's MOVES emissions model. The speeds compared are 10, 20, 30, 40, 50, and 60 miles per hour. The CO line begins at 3.0, drop to 1.9 at 30 mph, climbs to about 2.4 at 50 mph, and drops back to 2.3 at 60 mph. The NOx line begin at about 0.45 at 10 mph, drops to about 0.25 at 20 mph, and remains relatively flat for the rest of the speeds. The PM2.5 line begins at about 0.2 at 10 mph, drops to about 0.1 at 20 mph, and remains relatively flat for the rest of the speeds. The VOC line begins just above 0.0 at 10 mph, drops closer to 0.0 at 20 mph, and remains relatively flat for the rest of the speeds

Sources: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for VOC and NOx, winter weekday for CO, June 2015, https://www3.epa.gov/otaq/models/moves/index.htm

How Have Car and Truck Emissions Changed since the Clean Air Act?

The comparisons below show estimated in-use emissions rates, in grams per mile, for cars and heavy-duty diesel trucks with 2015 control technology versus 1967 vehicles (before significant emissions controls). The 1967 rates are based on EPA's previous emissions model, MOBILE6.2, while the 2015 rates are derived from EPA's newer MOVES model.

Car and Truck Emissions

Title: Bar graphs comparing the emissions rates for passenger vehicles and heavy-duty trucks in 1967 and 2015 - Description: The bar charts compare the emissions rates for VOC, NOx, CO, and PM-2.5 in grams per mile for cars and trucks in 1967 and 2015. The 1967 rates were generated by U.S. EPA's previous emissions model MOBILE6.2. The 2015 rates were generated by the current EPA model, MOVES. There is no PM-2.5 data for 1967. The chart shows very significant decreases in emissions rates for both vehicle types. For cars, the VOC value is 14.8 in 1967 and 0.1 in 2015, the NOx value is 4.4 in 1967 and 0.4 in 2015, the CO value is 87.6 in 1967 and 2.9 in 2015, and the PM-2.5 value is 0.02 in 2015. For trucks, the VOC value is 5.0 in 1967 and 0.8 in 2015, the NOx> value is 35.8 in 1967 and 10.7 in 2015, the CO value is 13.6 in 1967 and 3.3 in 2015, and the PM-2.5 value is 1.3 in 2015.

Car emissions rates have declined by more than 90 percent depending on the pollutant, while heavy-duty diesel truck emissions rates have declined by 84 percent for VOCs, 70 percent for NOx, and 76 percent for CO. Newer cars and trucks burn fuel much more efficiently now.

Sources: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for VOC and NOx, winter weekday for CO, June 2015, https://www3.epa.gov/otaq/models/moves/index.htm

U.S. Environmental Protection Agency, MOBILE6.2 model run, March 2005

Updated: 5/3/2016
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