Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University

In the troposphere, ozone is naturally formed when sunlight splits nitrogen dioxide (NO₂):

NO₂ + sunlight → NO + O

O + O₂ → O₃

Ozone (O₃) can go on to react with nitric oxide (NO) to form nitrogen dioxide (NO₂) and oxygen:

NO + O₃ → NO₂ + O₂

This results in no net gain of ozone (O₃). However, with the anthropogenic production of ozone forming precursors (NO, NO₂, and volatile organic compounds) through the combustion of fossil fuels, elevated levels of ozone in the troposphere have been found. Motor vehicle exhaust is a significant source of these ozone forming precursors: NO, NO₂, and volatile organic compounds (VOCs). For example, mobile sources make up nearly 60% of NO + NO₂ emissions.

At the high temperatures found in a car’s combustion chamber, nitrogen and oxygen from the air react to form nitric oxide (NO) and nitrogen dioxide (NO₂):

N_2(g) + O_{2 (g)}→ 2 NO_(g)

2 NO_(g) + O_2(g)→ 2 NO_2(g)

The nitric oxide (NO) emitted in the car exhaust is gradually oxidized to nitrogen dioxide (NO₂) in ambient air. This mixture of NO and NO₂ is often referred to as NO_x. When NO_x reacts with volatile organic compounds in the atmosphere in the presence of sunlight, tropospheric ozone forms, as seen in this simplified chemical reaction:

NO_x + VOCs + sunlight → O₃ + other products

This noxious mixture of air pollution, which can include aldehydes, peroxyacetyl nitrates, ozone, VOCs, and NO_x, is called photochemical smog. Ozone is the largest component of photochemical smog. This smog is found in all modern cities, but it’s found especially in cities with sunny, warm, dry climates and large numbers of motor vehicles. The yellow-brown color of smog in the air is due in part to the nitrogen dioxide present, since this gas absorbs visible light near 400 nm (Figure 1).

Short-term NO₂ exposure (30 min to 1 day) leads to adverse respiratory effects in healthy people and increased respiratory symptoms in people with asthma. NO_x reacts with ammonia and other compounds to form particulates. These small particles can penetrate the lungs and cause respiratory problems, including emphysema and bronchitis. Individuals who spend a lot of time on the road or who live near a roadway experience considerably higher exposure to NO₂.

Due to the impact it has on human health and the environment, the U.S. Environmental Protection Agency (EPA) has classified NO₂ as a criteria pollutant and has set the primary standard at 100 ppb (98th percentile of 1-h daily maximum concentrations, averaged over 3 years) and 53 ppb (annual mean). Considering that on-road vehicles account for approximately 1/3 of NO_x emissions in the U.S., automobile emissions are therefore regulated through the Clean Air Act. The U.S. EPA established emission standards that automobile manufacturers must follow when producing cars. Currently, Tier 2 emission standards set that manufacturers must have fleet average NO_x emissions of no more than 0.07 g/mile.

One way manufacturers meet this standard is by using catalytic converters on their cars. This device is placed between the engine and the tailpipe. The exhaust stream passes through the catalytic converter and is exposed to a catalyst. A reduction catalyst of platinum and rhodium is used to reduce the NO_x concentration in the exhaust. When an NO or NO₂ molecule in the exhaust contacts the catalyst, the nitrogen atom is grabbed off the molecule and held onto by the catalyst. The oxygen is freed and forms O₂. The nitrogen atom on the catalyst binds with another nitrogen atom held on the catalyst to form N₂.

Catalytic converters have greatly reduced the emissions of NO_x from car exhaust – up to 80% reduction, when performing properly. However, they only work when they have reached a fairly high temperature. Therefore, when doing a cold start of a car, the catalytic converter is removing virtually no NO_x. It isn’t until the catalytic converter reaches higher temperatures that it effectively removes the NO_x from the exhaust stream. Catalytic converters do not work on diesel passenger cars due to the lean conditions under which they operate. In addition, the sulfur in diesel fuel also deactivates the catalyst. The NO_x in diesel engines are reduced mainly through the exhaust gas recirculation (EGR) valve, which cools the temperature of the combustion gases. As a result, diesel cars generally emit more NO_x than gasoline cars.

Figure 1. Characteristic coloration for smog in California in the beige cloud bank behind the Golden Gate Bridge. The brown coloration is due to the NO_x in the photochemical smog.