Catalytic converters, automobile emission

Although the naturally occurring concentration of ozone at the earth s surface is low, the distribution has been altered by the emission of pollutants, primarily by automobiles but also from industrial sources which lead to the formation of ozone. The strategy for controlling ambient ozone concentrations arising from automobile exhaust emissions is based on the control of hydrocarbons, CO, and NO via catalytic converters. As a result, peak ozone levels in Los Angeles, for instance, have decreased from 0.58 ppm in 1970 to 0.33 ppm in 1990, despite a 66% increase in the number of vehicles. 

Automotive Catalytic Converter Catalysts. California environmental legislation in the early 1960s stimulated the development of automobile engines with reduced emissions by the mid-1960s, led to enactment of the Federal Clean Air Act of 1970, and resulted in a new industry, the design and manufacture of the automotive catalytic converter (50). Between 1974 and 1989, exhaust hydrocarbons were reduced by 87% and nitrogen oxides by 24%. 

Emission Control Technologies. The California low emission vehicle (LEV) standards has spawned iavestigations iato new technologies and methods for further reducing automobile exhaust emissions. The target is to reduce emissions, especially HC emissions, which occur during the two minutes after a vehicle has been started (53). It is estimated that 70 to 80% of nonmethane HCs that escape conversion by the catalytic converter do so during this time before the catalyst is fully functional. 

Beginning with the 1975 U.S. automobiles, catalytic converters were added to nearly all models to meet the more restrictive emission standards. Since the lead used in gasoline is a poison to the catalyst used in the converter, a scheduled introduction of unleaded gasoline was also required. The U.S. petroleum industry simultaneously introduced unleaded gasoline into the marketplace. 

Photochemical smogs arise worldwide because of the action of sunlight on emissions from gasoline-powered vehicles. Decreased visibility, increased morbidity, and crop damage as a result of photochemical smogs led to introduction of the catalytic converter on automobiles in the United States. This has had only a small impact on the occurrence of photochemical smogs in the United States. 

Unlike carbon dioxide and water that are the inevitable by products of complete combustion of hydrocarbons, species such as carbon monoxide, ethene, toluene, and formaldehyde can be emitted because combustion has been interrupted before completion. Many factors lead to emissions from incomplete combustion. Emitted unburned hydrocarbons and carbon monoxide are regulated pollutants that must be eliminated. In automobiles with spark ignited engines, these emissions are almost entirely removed by the catalytic converter. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

Cyanides have been detected in automobile exhaust. The average emission rate was 11-14 mg/mile for cars not equipped with catalytic converters and 1 mg/mile for cars with catalytic converters operating under optimum conditions. Cars with malfunctioning catalytic converters may emit as much or more hydrogen cyanide than cars without such equipment (Fiksel et al. 1981). 

Gas-phase tailpipe emission rates from gasoline-powered automobiles with and without catalytic converters were 6.55 and 177 pg/km, respectively (Schauer et al, 2002). 

The gas-phase tailpipe emission rate from California Phase II reformulated gasoline-powered automobile equipped with a catalytic converter was 124 pg/km (Schauer et al., 2002). 

Schauer et al. (1999) reported ethylbenzene in a diesel-powered medium-duty truck exhaust at an emission rate of 470 pg/km. California Phase II reformulated gasoline contained ethylbenzene at a concentration of 12,800 mg/kg. Gas-phase tailpipe emission rates from gasoline-powered automobiles with and without catalytic converters were 4.18 and 434.0 mg/km, respectively (Schauer et al, 2002). 

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