7.6 - Reduction of Air Pollutants

Learning Objectives

I. Reducing Emissions Through Action and Policy

A. Individual and Common Sense Steps

• Choosing alternatives to driving, like riding bikes, walking, or taking public transit.

• Conserving electricity, for example by turning off lights or using smart appliances. This matters because electricity in developed nations often comes from coal fired power plants or other fossil fuels, whose combustion releases pollutants like NOx and SOx.

• Eating less meat, which reduces the total energy required for food production and agriculture and puts fewer particulates into the air.

• Transitioning to non-pollution emitting renewable energy sources like hydroelectric, solar, and wind power.

• Switching to alternative fuels that burn cleaner or don't burn at all: natural gas instead of coal or diesel, ultra-low sulfur diesel, biofuels like ethanol, electric vehicles, and hydrogen fuel cells.
• Example: after a 1998 order from India's Supreme Court, Delhi switched its buses, auto-rickshaws, and taxis to compressed natural gas (CNG) in the early 2000s, cutting soot and $\text{SO}_2$ from those vehicles.

B. Laws and Regulations

• The Clean Air Act (CAA): Gives the EPA the ability to set acceptable levels for six different criteria air pollutants. The EPA can monitor those levels and levy penalties against corporations or factories that release unacceptable emissions.

• CAFE vehicle standards: The Corporate Average Fuel Economy standards set the average fuel economy, in miles per gallon, that the U.S. fleet of vehicles must meet. Raising those standards means vehicles burn less gasoline, so people travel the same distance while releasing fewer air pollutants.
• As of 2026: Congress set the fine for missing CAFE standards to $0 in July 2025, so the standards still exist on paper but can't be enforced with penalties. The EPA also repealed its separate greenhouse gas standards for vehicles in February 2026 (7.1). In September 2026 NHTSA finalized lower fuel economy standards, which a group of states is challenging in court (6.13).

• Pollution credits (cap and trade): Similar to individual transferable quotas (ITQs) used in fishing. The government sets a cap on total emissions and hands out permits (allowances) that add up to the cap. Companies that cut their emissions below their allowance can sell the extras, and companies that pollute more have to buy them. That makes polluting cost money and rewards the cheapest cuts first.
• Example: the 1990 Clean Air Act Amendments created a cap and trade program for $\text{SO}_2$ from power plants (the Acid Rain Program, 7.7). From 1995 to 2023, $\text{SO}_2$ emissions from U.S. power plants fell 95%.
• Vehicle credits: automakers can also trade credits under fuel economy and emissions rules. In 2020, Tesla earned $1.58 billion selling these credits to other automakers, more than its entire $721 million profit that year.

II. Technology for Reducing Vehicle Air Pollutants

• Vapor recovery nozzle: An air pollution control device on a gasoline pump (often with a rubber boot around the nozzle) that captures gasoline vapors (VOCs) during refueling and sends them back to the underground tank. This matters because VOCs contribute to photochemical smog and are respiratory irritants. It also reduces exposure to benzene, a known carcinogen found in gasoline.
• Today (as of 2026): most cars now have their own onboard vapor recovery (a charcoal canister that traps refueling vapors), so in 2012 the EPA let states phase out vapor recovery at the pump. California still requires it.

• Catalytic converter: A device in a car's exhaust system, used on most new U.S. cars starting with the 1975 models to meet Clean Air Act limits. Exhaust passes over a honeycomb coated with metal catalysts (platinum, palladium, and rhodium) that speed up reactions turning three pollutants into less harmful molecules:
• Carbon monoxide becomes carbon dioxide: $2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2$
• Nitrogen oxides become nitrogen and oxygen gas: $2\text{NO} \rightarrow \text{N}_2 + \text{O}_2$
• Unburned hydrocarbons (like octane in gasoline) become carbon dioxide and water: $2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}$
• Why it matters: CO is an asphyxiant, and NOx and hydrocarbons are the two ingredients of photochemical smog, so one device cuts both problems. $\text{CO}_2$ still adds to climate change, but it isn't a direct health threat the way CO is. Lead coats and ruins the catalyst, so cars with catalytic converters needed unleaded gasoline, which sped up the end of leaded gas (7.1).

III. Technology for Reducing $\text{SO}_2$ and NOx (Coal Plants)

• Crushed limestone: Limestone, largely calcium carbonate, is mixed with coal before burning. When the coal burns, the $\text{SO}_2$ produced reacts with the calcium carbonate and gets trapped as a solid called calcium sulfate. This reduces $\text{SO}_2$ emissions, and the resulting calcium sulfate can be used productively to create gypsum wall boards.

• Fluidized bed combustion (FBC): This method uses jets of air to create a bubbling fluid bed of coal and limestone. The increased efficiency lets combustion occur at lower temperatures. Reducing combustion temperatures is a great way to also reduce NOx emissions. FBC also makes the crushed limestone method more effective for $\text{SO}_2$ capture.

• Selective catalytic reduction (SCR): the main way coal and natural gas plants cut NOx. Ammonia is sprayed into the exhaust, and a catalyst helps it turn NOx into nitrogen gas and water: $4\text{NO} + 4\text{NH}_3 + \text{O}_2 \rightarrow 4\text{N}_2 + 6\text{H}_2\text{O}$.

• Wet and dry scrubbers: These large columns clean exhaust before it leaves the smokestack. They mainly remove $\text{SO}_2$ (and other acid gases) and particulates.
• Dry scrubbers: Packed with a chemical agent like calcium oxide (lime) that reacts with air pollutants. For example, $\text{SO}_2$ reacts to form calcium sulfite, converting it into a solid for disposal: $\text{CaO} + \text{SO}_2 \rightarrow \text{CaSO}_3$.
• Wet scrubbers: Feature a misting apparatus that releases droplets which trap particulate matter. The particles and the mist, which often grabs pollutants attached to the PM, fall into a sludge collection tank for disposal. At coal plants, the spray is usually a slurry of crushed limestone and water that also captures $\text{SO}_2$: $\text{CaCO}_3 + \text{SO}_2 \rightarrow \text{CaSO}_3 + \text{CO}_2$. Blowing air through the slurry turns the calcium sulfite into calcium sulfate (gypsum), which can be used for wallboard.

IV. Technology for Reducing Particulate Matter (PM)

• Electrostatic precipitator: The waste stream passes through this device, where a negatively charged electrode gives all passing particles a negative charge. Positively charged plates then grab onto those negatively charged particles, so they stick to the plates instead of leaving in the exhaust stream. The particles are periodically collected in a hopper for disposal.
• ESPs at coal plants typically remove more than 99% of the fly ash in the exhaust.

• Baghouse filter: Works like a giant vacuum cleaner bag. Exhaust passes through rows of long fabric bags that trap particles. The bags are shaken or pulsed with air now and then, so the trapped dust falls into a collection hopper for disposal in a landfill.

Big picture: Air pollutant emissions get reduced at the source through a combination of common sense conservation, government regulation like the Clean Air Act and CAFE standards, and specific technologies. For vehicles, the catalytic converter changes toxic CO and NOx into less harmful gases. At power plants, methods like mixing coal with crushed limestone (to capture $\text{SO}_2$) and using fluidized bed combustion (to reduce NOx through lower temperatures) are essential. Large filters like electrostatic precipitators and wet scrubbers make sure hazardous particulate matter and attached pollutants are removed before reaching the atmosphere.