Vehicle Emissions Primer

Sylvie Trottier
Readability
6.95

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This backgrounder describes how vehicle emissions work and how they affect air quality, including their contribution to smog.

Vehicles are pretty useful. They get you from point A to point B. They can carry stuff. They can carry people. They can go further more quickly than your own two feet can.

But they are also quite good at things that aren’t so useful. They are good at contributing to smog and climate change. If you've ever been stuck in traffic behind a big heavy truck, you've no doubt smelled the noxious gases spewing out. Even when you can't smell anything, vehicles emit all sorts of nasty stuff. This stuff can have negative effects on people and the planet.

How does an internal combustion engine work?

We all know that most cars and trucks run on fuel. Some vehicles now also run on electricity. But more often than not, most vehicles run on fossil fuels, like gasoline or Diesel. These fuels are burned inside an engine. That's why it’s called an internal combustion engine. Combustion is a type of reaction that gives off heat. You may be more familiar with the term “burning.” 

In a car's engine, fuel is put into a small, enclosed space. Then, it is ignited. The ignited fuel releases a large amount of energy as it burns the rapidly expanding gases. Think of an explosion.  By setting off hundreds of these tiny explosions every minute, a car's engine is able to move pistons up and down. This is converted into rotational motion by the crankshaft that moves the car. 

 

This animation shows the four steps that occur in a combustion engine.
This animation shows the four steps that occur in a combustion engine. The fuel enters at the left and the waste gases exit at the right (Source: UtzOnBike (derivative work) via Wikimedia Commons).

 

The gases then leave the engine through the car's exhaust pipe (tailpipe). We call these gases exhaust. Air quality is a measure of the amount of pollutants in the air. These vehicle exhaust emissions can affect air quality in both near the Earth's surface, where we live and breathe, as well as high in the atmosphere

What is smog?

Smog is a type of short-term air pollution. It happens near ground level and is localized. You can recognize a very smoggy day by the haze that hangs in the air. Historically, the word "smog" was used to describe the thick, grayish haze that occurred when smoke and fog mixed together. Today, the word smog is used to describe a type of air pollution made up of a mix of different gases and particles in the air. When the air is smoggy, it can be difficult to see places you might see on a clearer day. You might also find it a little harder to breathe. The air might also have a bit of a smell. Smog is mainly made up of particulate matter (PM) and ground level ozone (O3)

How do vehicle emissions contribute to smog?

Vehicle emissions are one of the main contributors to smog. When fuel combusts, it emits PM. PM is composed of tiny bits of soot.You may recognize it as the black gunk that accumulates on snow in winter. PM mainly comes from incomplete combustion in car engines. But it can also come from burning coal and wood. Incomplete combustion releases carbon monoxide (CO) as well as unburned fuel, which can become part of smog.

For the complete combustion of methane CH4 plus 2O2 becomes CO2 plus 2H20. For the incomplete combustion of methane 4CH4 plus 5O2 becomes 2CO plus 8H2O plus 2C.
Balanced chemical equations for the complete and incomplete combustion of methane (© 2019 Let’s Talk Science).

 

Graphic - Text Version

For the complete combustion of methane CH4 plus 2O2 becomes CO2 plus 2H20. For the incomplete combustion of methane 4CH4 plus 5O2 becomes 2CO plus 8H2O plus 2C.

So far, you’ve read about the direct products of combustion. But there are also indirect products of combustion such as nitrogen monoxide and nitrogen dioxide. Both of these are nitrous oxides (NOx). They form when nitrogen and oxygen in the air combine because of the heat released during combustion. Other indirect products include volatile organic compounds (VOCs), such as benzene and formaldehyde. These are found naturally in crude oil and gasoline. Another indirect product is sulfur dioxide (SO2), which comes from the combustion of Diesel fuel.

What is ground level ozone?

The other important ingredient in smog is ground level ozone. You may be familiar with ozone as something that is found in the upper atmosphere. Ozone in the upper atmosphere protects us from harmful ultraviolet (UV) radiation. This is ‘good’ ozone. Unfortunately, this 'good' ozone is slowly being depleted by human-made chemicals. 

Ground level ozone is not 'good' ozone. The ozone at ground level is formed when NOx and VOCs react together in still air the presence of sunlight. This is why we tend to have smog on hot summer days. But smog can also occur in winter, especially when there are a lot of vehicle emissions.

What are the consequences of smog?

On smoggy days, it can seem a lot harder to breathe properly. That's not your imagination. Smog has been linked with increased hospital visits due to reduced lung function, aggravated asthma, and other respiratory illnesses. Smog has also been linked to thousands of premature deaths every year in Canada. Children and the elderly are most at risk from smog-related health effects. 

Smog can make it difficult to breathe. In some cities, people even wear masks when outside
Smog can make it difficult to breathe. In some cities, people even wear masks when outside (Source: ilyaliren via iStockphoto).

Nature isn't immune either. When NOx and SO2 react with water in the atmosphere, they form acid precipitation in the form of rain and snow. Acid precipitation can affect the productivity of plants. It can also cause lakes and rivers to become acidic. This can kill or prevent the reproduction of some species. This, in turn, affects whole ecosystems.

References

Brain, M., & Geisler, K. H. (2018, August 16). How car engines work. HowStuffWorks.

Environment & Climate Change Canada. (2013, July 17). Particulate matter.

Government of Canada. (2016, May 19). Common air pollutants: Ground-level ozone.

Government of Canada. (2018, January 31). Smog and your health.

NOAA. (2018, August 1). Climate change: Atmospheric carbon dioxide.