Explain air quality using the Air Quality Index, tropospheric ozone, particulates, acid rain and indoor pollutants, and design a scrubber to test

Learning goal: Read the Air Quality Index and the criteria pollutants behind it; explain how tropospheric ozone and particulate matter form and why concentrations differ between locations; illustrate how acid rain is created and describe its effects on water, soil, organisms and structures; explain how a scrubber removes sulfur from coal combustion gases; identify indoor pollutants, their sources and impacts; and analyze a city's air-pollution history and real-time monitoring data to find root causes.

What "air quality" measures. Under the Clean Air Act the EPA sets national standards for six criteria pollutants: ground-level ozone (O₃), particulate matter (PM), carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂) and lead. Monitors across the country measure them, and the Air Quality Index (AQI) converts the concentrations of five of them (ozone, particulate matter, carbon monoxide, sulfur dioxide and nitrogen dioxide; lead has no AQI) into a single scale from 0 to 500 so the public can read it. The categories are 0–50 good (green), 51–100 moderate (yellow), 101–150 unhealthy for sensitive groups (orange), 151–200 unhealthy (red), 201–300 very unhealthy (purple) and 301–500 hazardous (maroon). The reported AQI for a day is the highest of the individual pollutant indexes, so "AQI 135, ozone" means ozone was the pollutant driving the number. An AQI is not a concentration: two pollutants at the same AQI have very different concentrations in micrograms per cubic meter or parts per billion, and the index for a pollutant is calculated over the averaging period of its standard (8 hours for ozone, 24 hours for fine particles), so a one-hour reading cannot be compared directly to it.

Tropospheric ozone. Ozone high in the stratosphere shields the surface from ultraviolet light; the same molecule near the ground is a pollutant that inflames airways and damages leaves. Ground-level ozone is not emitted directly. It forms when nitrogen oxides (NOₓ, mainly from vehicle engines and power plants) and volatile organic compounds (VOCs, from fuel vapor, solvents, paints and also from trees) react in sunlight. Because it needs sunshine and time, ozone peaks on hot, still summer afternoons and often downwind of a city rather than in it: the precursors are emitted in the morning rush and cook into ozone as the air drifts. A community investigation comparing ozone at several locations therefore expects the highest values on the downwind edge of town on a sunny afternoon and the lowest at dawn or in a shaded, wooded site; differences are explained by precursor sources, sunlight, wind direction and time of day. The 8-hour ozone standard set in 2015 is 70 parts per billion.

Particulate matter. PM is a mixture of solid and liquid particles suspended in air. PM₁₀ (particles under 10 micrometers) includes dust and pollen; PM₂.₅ (under 2.5 micrometers) comes from combustion (vehicles, power plants, wood smoke, wildfires) and from chemical reactions of SO₂ and NOₓ in the air. Fine particles matter most for health because they penetrate deep into the lungs and enter the bloodstream; long-term exposure is linked to heart and lung disease. Wildfire smoke carried from Canada in June 2023 pushed AQI values in Ohio into the unhealthy range for days, an illustration that air quality is regional and that a source hundreds of kilometers away can dominate.

Acid rain. Ordinary rain is slightly acidic, about pH 5.6, because CO₂ dissolves in it to form weak carbonic acid. Acid rain (more precisely acid deposition, since it includes snow, fog and dry particles) has a pH below about 5.0. It is created in three steps: (1) burning coal and other fuels releases SO₂ and NOₓ; (2) in the atmosphere these are oxidized and react with water to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃); (3) winds carry the acids hundreds of kilometers before rain, snow or dry particles deposit them. Ohio Valley coal-fired power plants were historically a major source of the SO₂ that acidified lakes in the Adirondacks and New England. Effects on each component of the environment: water, lakes and streams acidify, and in acid water aluminum dissolves from soil and damages fish gills, so fish and amphibians disappear (lakes on limestone bedrock resist this because the rock neutralizes acid); soil, calcium and magnesium that trees need are washed out; plants, weakened trees at high elevation, where acid fog is frequent, suffer needle loss and die-back; animals, the loss of prey and of calcium for snails and birds' eggshells; structures, limestone and marble buildings and statues dissolve, and metal corrodes. The 1990 Clean Air Act amendments created a cap-and-trade program for SO₂ from power plants, and national SO₂ emissions from that sector have since fallen by the large majority, with rainfall pH recovering in the Northeast.

How a scrubber works. A scrubber, or flue-gas desulfurization unit, removes SO₂ before it leaves the smokestack. In the common wet limestone design, exhaust gas passes up through a spray of limestone (calcium carbonate) slurry. SO₂ dissolves and reacts: SO₂ + CaCO₃ → CaSO₃ (calcium sulfite) + CO₂. Blowing air through the slurry oxidizes the sulfite to calcium sulfate, CaSO₄·2H₂O, which is gypsum, sold for wallboard. A scrubber's effectiveness is assessed by comparing the SO₂ leaving with the SO₂ entering, or by weighing the calcium sulfite or sulfate collected against a control run without limestone. Modern scrubbers remove over 90% of SO₂; the trade-offs are cost, energy use and the CO₂ released by the reaction.

Indoor pollutants. People spend most of their time indoors, where some pollutants are more concentrated than outside. Radon is a radioactive gas produced by uranium decaying in rock and soil; it seeps into basements, and long-term exposure is the second leading cause of lung cancer in the United States after smoking; many Ohio counties have elevated levels, the EPA recommends action at 4 picocuries per liter, and the fix is a vent pipe with a fan under the slab. Carbon monoxide comes from any fuel-burning appliance that is faulty or unvented and from cars idling in attached garages; it binds hemoglobin and can kill in hours, which is why detectors are required. Volatile organic compounds off-gas from paints, new furniture, cleaners and formaldehyde-bonded wood products, causing headaches and irritation. Tobacco smoke, mold from damp walls (a trigger for asthma), asbestos fibers in old insulation and combustion particles from gas stoves and wood stoves complete the list. Ventilation, source removal and detectors are the controls.

Cities with a pollution history. Detroit's industrial and automotive history and Beijing's coal heating and traffic each produced episodes of severe pollution; analyzing a case study means identifying which sources dominated, which actions worked (fuel switching, emission controls, relocating industry) and which were problematic (moving pollution elsewhere, shutting factories without replacement jobs, measures whose effect was not monitored). The same method applies to real-time data for your own community: find the current AQI and the driving pollutant on the EPA's monitoring site, look at a week's history, relate peaks to weather and sources, and present quantitative evidence for the root cause.

Misconception. "Ozone is good, so an ozone reading of 90 ppb is fine." The location decides: stratospheric ozone protects, ground-level ozone harms, and 90 ppb over 8 hours exceeds the health standard.

Worked AQI reading (hypothetical): A monitoring site reports an ozone AQI of 112 and a PM₂.₅ AQI of 58. The site's AQI for the day is 112 (the higher), reported as "unhealthy for sensitive groups" with ozone as the driving pollutant. Meaning: people with asthma, children and older adults should limit prolonged outdoor exertion in the afternoon; the general public is unlikely to be affected. The PM₂.₅ value is "moderate". Neither number is a concentration; to compare with the 70 ppb standard you would need the 8-hour ozone concentration, which at AQI 112 is somewhat above 70 ppb.

Worked ozone location comparison (hypothetical): Afternoon 8-hour ozone at four community sites on one sunny July day: downtown 62 ppb, highway interchange 58 ppb, suburb 15 km downwind 78 ppb, forest park upwind 45 ppb. Explanation: NOₓ is highest at the interchange, but fresh NO actually destroys ozone locally, and the reactions need time in sunlight, so the peak is downwind where the morning's precursors have had hours to react. The upwind park has the least precursor input. The downwind suburb exceeds the 70 ppb standard even though it has few sources of its own; the root cause is upwind traffic and industry plus the weather.

Worked acid-rain chemistry: Coal containing 2% sulfur by mass is burned: 1,000 kg of coal contains 20 kg of sulfur. Sulfur (atomic mass 32) burns to SO₂ (molar mass 64), so 20 kg of S makes 40 kg of SO₂. Without a scrubber, all 40 kg leaves the stack; with a 92% efficient scrubber, 0.08 × 40 = 3.2 kg leaves and 36.8 kg is captured. The captured SO₂ reacts with limestone: each mole of SO₂ (64 g) consumes one mole of CaCO₃ (100 g) and forms one mole of CaSO₃ (120 g); 36.8 kg of SO₂ is 36,800 g ÷ 64 g per mol = 575 mol, consuming about 57.5 kg of limestone and producing about 69 kg of calcium sulfite before oxidation to gypsum.

Worked seed-sprouting analysis (hypothetical): Radish seeds watered with solutions of pH 6.5 (tap water control), 5.0, 4.0 and 3.0, 20 seeds each, after 7 days: germinated 18, 17, 12 and 4. Germination rates 90%, 85%, 60%, 20%. The pH 5.0 result differs from the control by one seed, too small to show an effect in a single dish of 20; pH 4.0 and 3.0 show clear harm. Conclusion: in this trial, harm increased as pH fell, with the change appearing between pH 5 and 4; replicate dishes and intermediate pH values would be needed to locate a threshold, and vinegar is only a stand-in for the sulfuric and nitric acids in acid rain; the model cannot show effects on established trees, soil chemistry or aluminum release, which act over years.

Representation task: Draw the three-step creation of acid rain (emission of SO₂ and NOₓ, conversion to sulfuric and nitric acid in the atmosphere, deposition as rain, snow, fog or dry particles) and, under it, a five-row table of effects on water, soil, plants, animals and structures with one mechanism each. Add a labeled diagram of a wet limestone scrubber showing gas in, slurry spray, gypsum out and the two reactions.

Safe investigations: (a) Acid rain and seeds: prepare solutions of pH about 6.5 (tap water), 5, 4 and 3 by diluting white vinegar and checking with pH paper; water 20 radish seeds per dish for 7 days; record germination and root length. Wear eye protection; vinegar solutions only. (b) Scrubber model: to a "flue-gas condensate" of vinegar diluted to pH 3, add crushed limestone or chalk (calcium carbonate) and stir for 10 minutes; measure pH before and after and compare with an identical jar with no limestone (the control). The rise in pH and the fizzing (CO₂) model the neutralization reaction; the calcium product here, calcium acetate, stays dissolved, so any solid left behind is unreacted limestone, not the collected salt. Say what the model cannot show: it uses acetic acid rather than sulfur dioxide gas, so it does not produce calcium sulfite or sulfate, cannot be used to weigh a collected product, and does not test gas-liquid contact; a real scrubber assessment would use supplied data, never burning coal in class.

Independent performance: Using real-time and recent data from the EPA's air-quality monitoring site for your community, Ohio, the United States or another region, identify one air-pollution issue and its root cause. Success criteria: (1) at least seven days of AQI values with the driving pollutant for each, presented in a table or graph; (2) peaks related to weather (temperature, wind direction, sunshine) and to plausible sources; (3) one root cause identified with quantitative support; (4) one solution proposed, with the mechanism by which it lowers the pollutant and one drawback; (5) presentation as a poster, talk or short scientific paper, with data sources named.