Mobile Emissions

Types of Vehicle Emissions

The main component of mobile emissions is carbon dioxide (CO2), the transport sector's main contributor to climate change. Unlike all other mobile emission components, though, CO2 is not directly toxic to humans or ecosystems. It is thus generally considered separately and is not covered by this article.

The major components of tailpipe emissions that are of local or regional concern are the following:

Carbon monoxide (CO). CO forms through incomplete combustion of carbon-containing fuels. Introduction and enhancement of vehicle emission standards, catalytic converters and better fuel quality have significantly lowered CO emissions in developed countries. CO can be used as a tracer gas for the dispersion of vehicle emissions because it is comparatively inert, that is, it does not chemically interact with other substances in the atmosphere.

Nitrous oxides (NOx). NOx consist of nitrogen dioxide (NO2) and nitric oxide (NO), both by-products of combustion. Nitric oxide is generally quickly oxidized to form NO2 after it is released into the atmosphere. NO2 reacts with both oxygen and volatile organic compounds to form ozone.

Particulate matter (PM). PM does not consist of a single chemical substance but is a collection of both solid and liquid aerosols with different chemical and toxicological qualities. The primary components of PM in mobile emissions are a direct result of fuel combustion, including residues from the incomplete burning of carbon compounds. Secondary PM is formed through chemical reactions from engine emissions after their release from the combustion chamber, and contains nitrogen and sulfur compounds. PM is generally classified by particle size rather than composition. PM10 describes particles with a diameter of 10 µm and below (1 µm = 1 micrometer = 1 millionth of a meter). PM2.5 denotes all particles equal to or below 2.5 µm in diameter. Thus PM2.5 is a subclass of PM10. The two size classes are often dealt with separately because PM's effect on human health relates not only to its composition but also to the particles' size. The smaller they are, the farther the particles can penetrate into the human body, and the worse are the health effects associated with them. The combustion of diesel fuel is associated with greater PM emissions than gasoline.

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