Renewable Energy

Pollutants from Car Exhaust

Carbon dioxide (CO ₂) is a naturally present gas in the atmosphere. The burning of fossil fuels in an internal combustion engine is a significant contribution to the increasing concentrations of this principal greenhouse gas. Currently, according to NOAA and the Scripps Institution, the Earth's atmosphere has 384 ppm (parts per million) of CO _{2 ,} a rise of almost 10 ppm in four years from 375 ppm in 2002, and a rise of almost 70 ppm in the last fifty years from 315 ppm in 1958 when the recording started at the Mauna Loa Observatory in Hawaii [7].

Carbon monoxide (CO) is produced by incomplete combustion when carbon in the fuel is partially oxidized rather than fully oxidized into carbon dioxide. The Earth's atmosphere contains only 0.1 ppm of carbon monoxide. This concentration can be much higher in an urban setting with the large number of vehicles. Concentrations can reach 50 or even 100 ppm, which is well over safe levels [8]. Catalytic converters were installed in cars after 1975 with the primary function being to convert CO to CO ₂ in the presence of air and a metal catalyst. This has been a great environmental achievement. Since this time, the number of cars has doubled, and the amount of CO in the air has decreased by 40% [8].

Volatile organic compounds (VOCs) are emitted when fuel molecules in the engine do not burn or are only partially combusted. VOCs are organic compounds that contain hydrogen and carbon atoms. They are a major contributor to ground level ozone. Newer technologies in catalytic converters include a catalyst to oxidize these partially burnt hydrocarbons completely to CO ₂. Changes to fuel caps, vapor recovery devices on fuel pumps, and improvements of fuel lines have reduced the evaporative emissions of hydrocarbons associated with automobiles. Polar VOC molecules called aldehydes cling together in the atmosphere to form tiny liquid particles called aerosols. These aerosols in conjunction with soot are responsible for the haze of smog [8].

Nitrogen oxides (NOx) are primarily produced when elemental nitrogen in the air is broken down and oxidized inside the internal combustion engine. Nitrogen oxides play a role in the formation of ground level ozone and acid rain. Atmospheric oxidation of NO leads to different oxides of nitrogen and eventually to nitric acid, a significant source of acid rain. The sunlight catalyzed reaction in the atmosphere between unburnt hydrocarbon molecules and NOx leads to photochemical smog. A product of this reaction is ozone. Catalytic converters do include a catalyst for reducing NO back to N ₂ and O _{2 ;} this does have some problems, though, in the presence of the oxidation catalyst used to remove CO. Even though the number of automobiles has doubled, the NOx levels have remained almost constant. This is also attributed to the improved engine design and improved gasoline formulations [8].

To better understand the magnitude of the amount of these pollutants that are being produced in the local community, have students complete the Determining the Amount of Emissions Emitted by Local Commuter Vehicles activity at the end of this unit. The data used to make this activity are the actual data from my county and reflect the actual values of pollutants mentioned above in this section emitted by motor vehicles. You can get the data for your city or county from the city data website [9]. These calculations will be revisited later in the unit when we look at the amount of carbon that can be sequestered by our terrestrial carbon sinks through management of forest resources.