What is Smog?

The chemical composition of gasoline is mostly in the form of hydrocarbon molecules of the form C_nH_2n+2 where C represents carbon atoms, H represents hydrogen atoms, and n is an integer with a mean value of approximately seven. In the case of propane, n=3. Because carbon has an atomic mass of 12 and hydrogen has an atomic mass of 1, 84% of the weight of gasoline is the carbon. Similar, natural gas is mainly composed of methane (CH₄), wherein 75% of its weight is the carbon.

When gasoline and methane burn completely in air, the two dominant emissions are carbon dioxide (CO₂) and water vapor (H₂O). In addition, there are also other vapors emitted out of the exhaust of an internal combution engine. One type is incompletely combusted hydrocarbon vapors (HC), which are credited as smog-producing emissions. A second is carbon monoxide (CO), which along with its role in the creation of smog, is also biologically harmful to the point of causing poisoning (in fact, according to the paper Omaye ST. (2002). "Metabolic modulation of carbon monoxide toxicity". Toxicology 180 (2): 139-50, more than 50% of all human poisoning cases in the world are caused by carbon monoxide). A third is NOx, which covers the various types of nitrogen oxides which are considered smog-producers, formed as the otherwise inert nitrogen gas flows through the hot engine and interacts with oxygen.

When an engine is operated rich, a condition in which there is more fuel compared to the amount of oxygen needed for complete combustion, NOx generation is depressed while HC and CO emissions are highest. When the engine is operated too lean, the engine temperature drops and HC, CO, and NOx are all reduced, unfortunately as does engine power. NOx production is maximum just lean of the stoichometric fuel/air ratio (the exact ratio of fuel and air needed for complete combustion). This is because at stoichometric balance the flame temperature is reduced by the heat of vaporization of the gasoline.

The control of engine emissions began in 1970 with the passage of the Clean Air Act. Under the act, requirements changed with technology. For example, the 1975 and 1976 federal standards (enforced in 49 states) limits were 1.5 g/mile for uncombusted hydrocarbons (HC), 15.0 g/mile for carbon monoxide (CO), and 3.1 g/mile for nitrogen oxide and nitrogen dioxide (cumulatively described as NOx). The EPA established the Federal Test Procedure (FTP) defining the conditions under which these emissions were measured. These limits required greater than 90% conversion of HC and CO for a duration of 50,000 miles. By 1996, the exhaust control duration requirement was raised to 100,000 miles. In addition, the emission limits were reduced to 0.125 g/mile NMHC (non-methane hydrocarbons) by 2004, 1.7 g/mile CO, and 0.2 g/mile NOx.

A major impetus for these standards is the desire to reduce smog in most United States cities. According to the EPA, the pollutant break-points in the Air Quality Index (AQI) that triggers air quality warnings of "Very Unhealthy", AQI score 201, are listed below.

A new type of smog known as photochemical smog was first described in the 1950’s. This form of smog is created when sunlight hits certain pollutants in the air. Usually, photochemical smog is formed by the chemical reaction of sunlight with NOx and HC in the atmosphere, which leaves airborne particulate matter (PM) and ground-level ozone (O3).