Automobile Emissions Increase Ozone Concentrations

Two common emissions products from our automobiles contribute to the formation of ozone: volatile organic hydrocarbons and nitrogen oxides. This section will explore how these ozone precursors are produced and how they lead to the formation of ozone.

Gasoline and other fossil fuels are a mixture of hydrocarbons (compounds containing carbon and hydrogen). Complete combustion of hydrocarbons produces water and carbon dioxide. The balanced equation for combustion of octane (one component of gasoline) appears below in Equation 2.

2 C₈H₁₈ + 17 O₂ → 18 H₂O + 16 CO₂

(2)

Incomplete combustion releases volatile organic hydrocarbons (VOCs), because the hydrocarbons are not completely converted to CO₂. The combustion products we are most concerned with in this tutorial are VOCs and nitrogen oxides (recall Table 1). Nitrogen oxides are produced during combustion even when the nitrogen content of the fuel is low, because nitrogen is always present in the air used for combustion (see Box 1, below).

Box 1

Nitrogen Oxide Emissions from Combustion of Fossil Fuels

Fossil fuels vary in their nitrogen content. Fuels that have a high nitrogen content produce more NO and NO₂ when they are burned than fuels with low nitrogen content. This accounts for some of the variation of nitrogen-oxide emissions in different regions of the country. (Click here, for an Environmental Protection Agency map of nitrogen-oxide emissions for coal-fired utilities throughout the U.S.)

You might expect methane, which contains essentially no nitrogen in the fuel itself, to have no nitrogen-oxide emissions, but this is not the case. In fact, some NO is formed any time nitrogen and oxygen are in contact at the high temperature of an engine. (This is because air is about 78% nitrogen and 21% oxygen by volume.) Equations 3 and 4, below show the reaction between nitrogen and oxygen in the air, and the equilibrium expression for the reaction.

N₂ + O₂ ↔ 2NO

(3)

(4)

Recall that the value of the equilibrium constant, K_eq, varies with temperature.For the reaction in Equation 3, K_eq = 5 x10^-7 at 298 K; hence, the equilbrium lies to the left. However, as temperature increases, K_eq changes and the equilibrium shifts to the right. Thus, the equilibrium concentration of NO increases. At 675 K (a realistic temperature for the exhaust manifold in a car), K_eq = 0.01 and the ratio of NO to O₂ and N₂, while still small, is no longer negligible. In fact, at 675 K, about 3% of the nitrogen in the air will be converted to NO.

We must conclude that any time we have nitrogen and oxygen in contact at high temperatures, some NO will be produced as a result. Since nitrogen and oxygen are always present together in the air we use to burn gasoline or coal, some NO emissions are inevitable.

In the U.S., fossil fuels are mostly burned to operate motor vehicles and to produce electricity. As a result, car emissions are a substantial source of VOCs and nitrogen oxides across the nation. In parts of the midwest, most of our electricity comes from burning coal, which results in relatively high nitrogen-oxide emissions. When concentrations of nitrogen oxides are large, noncombustion VOCs can also make a noticeable contribution to total ozone levels. Noncombustion VOCs include gasoline released into the air during refueling and solvents such as paint thinner or charcoal lighter fluid that are allowed to evaporate. Therefore, there are always nitrogen oxides and VOCs present in our atmosphere today. Before strategies can be developed to reduce ozone concentration in the lower atmosphere, we must first understand how VOCs and nitrogen oxides react to form ozone.