Distinguish primary and secondary air contaminants, explain photochemical smog and acid rain, and explain greenhouse gases, feedback loops and how rock and ocean exchange carbon with the air

Learning goal: Distinguish primary and secondary contaminants in air and trace how secondary contaminants such as photochemical smog, ozone and acid rain form; explain how greenhouse gases warm Earth's surface and describe positive and negative feedback loops; describe the characteristics of each layer of the atmosphere and its uses to humans; and explain, through the lens of geology, how the atmosphere exchanges gases with rock and with the oceans.

The atmosphere as a resource. Air is a mixture of about 78 percent nitrogen, 21 percent oxygen and just under 1 percent argon, with small but important amounts of water vapor and carbon dioxide. It is a resource because people breathe it, it shields the surface from harmful radiation, it carries weather and it holds heat. It is shared: a contaminant released in one place travels with the wind, which is why air quality is judged at local, state and global scales.

Primary and secondary contaminants. A primary contaminant is emitted directly into the air in harmful form. Examples: carbon monoxide from incomplete burning in engines; sulfur dioxide from burning sulfur-bearing coal and from volcanoes; nitrogen oxides from high-temperature combustion in engines and power plants; volatile organic compounds (VOCs) from fuels, solvents and paints; lead; and primary particulate matter such as soot, ash and dust. A secondary contaminant is not emitted but forms in the air when primary contaminants react with each other, with water, or with sunlight. Examples: ground-level ozone, most of the components of photochemical smog, sulfuric and nitric acid in acid rain, and fine sulfate and nitrate particles. The distinction matters for control: a primary contaminant is reduced by cutting it at the source, but a secondary one is reduced only by cutting its ingredients, which may come from different sources.

Particulate matter. Particles are classed by size: PM10 are 10 micrometers or smaller, and PM2.5 are 2.5 micrometers or smaller, about one thirtieth the width of a human hair. The smaller the particle, the deeper it penetrates into the lungs, which is why fine particles are linked to asthma attacks, emphysema and other lung and heart disease. The daily Air Quality Index turns measured concentrations into a single scale on which 0 to 50 is good, 51 to 100 moderate, and above 100 unhealthy, first for sensitive groups.

How photochemical smog forms. On a hot, sunny, still day, traffic and industry supply nitrogen oxides and VOCs. Sunlight splits nitrogen dioxide into nitric oxide and a free oxygen atom; the oxygen atom joins an oxygen molecule to make ozone, O₃. On its own this is a balanced cycle, because nitric oxide reacts with ozone to remake nitrogen dioxide. Other gases break the balance. In city smog the main ones are VOCs: their breakdown products convert nitric oxide back to nitrogen dioxide without using up ozone, so ozone accumulates through the day, peaking in the afternoon. Carbon monoxide and methane can do the same job, which matters most in cleaner air far from cities. Ground-level ozone irritates lungs and damages crops. The same molecule high in the stratosphere is beneficial, which is why the location of a gas, not just its identity, decides whether it is a contaminant.

Acid rain, a geology problem at both ends. Sulfur dioxide from burning high-sulfur coal, including much of the coal in the Ohio Valley, is oxidized in the air to sulfuric acid, and nitrogen oxides to nitric acid, making rain more acidic than the natural value of about pH 5.6. Acid rain dissolves limestone and marble buildings and acidifies thin soils and lakes on poorly buffered rock. Geology also supplies the fix: power plants spray crushed limestone slurry through the exhaust in scrubbers, where calcium carbonate reacts with sulfur dioxide and captures it as a solid, releasing carbon dioxide in exchange. Since the Clean Air Act amendments of 1990, sulfur dioxide emissions in the United States have fallen sharply.

Greenhouse gases. Sunlight arrives mostly as visible and near-visible radiation, which passes through the air and warms the surface. The warmed surface emits infrared radiation. Greenhouse gases, chiefly water vapor, carbon dioxide, methane and nitrous oxide, absorb infrared and re-emit it in all directions, including back down, so the surface is warmer than it would otherwise be. The natural greenhouse effect is essential: without it Earth's average surface temperature would be roughly −18 °C instead of about 15 °C. The concern is the addition. Carbon dioxide has risen from about 280 ppm before industrialization to above 420 ppm in the mid-2020s, mainly from burning fossil fuels, and methane has more than doubled. Per metric ton, methane traps roughly 25 to 30 times as much heat as carbon dioxide over a century, though it stays in the air for only about a decade.

Feedback loops. A positive feedback amplifies a change. Warming melts snow and sea ice, exposing darker land and water that absorb more sunlight, causing more warming (ice-albedo feedback). Warmer air holds more water vapor, itself a greenhouse gas. Thawing permafrost releases carbon dioxide and methane. A negative feedback opposes a change. A warmer surface radiates more energy to space. On geological timescales, warmer, wetter climates speed the chemical weathering of silicate rock, which consumes carbon dioxide and eventually locks the carbon into limestone on the seafloor, slowly lowering carbon dioxide again. That rock thermostat works over hundreds of thousands of years, far too slowly to offset changes in a human lifetime.

The atmosphere, oceans and rock exchange carbon. Carbon dioxide dissolves in seawater, forming carbonic acid; the oceans have absorbed roughly a quarter of the carbon dioxide released by people, which makes seawater slightly more acidic. Marine organisms build shells of calcium carbonate that accumulate as limestone, sequestering carbon for millions of years. Ohio's thick limestones and dolomites are stored atmospheric carbon from ancient seas, and its coal is stored carbon from ancient swamps; burning coal returns that carbon to the air in decades. Volcanoes supply carbon dioxide and sulfur dioxide; a large eruption can inject sulfate particles into the stratosphere that reflect sunlight and cool the globe slightly for a year or two.

Layers of the atmosphere and their uses. The troposphere, from the surface to about 6 km at the poles and 18 to 20 km at the equator, holds most of the air and nearly all weather; temperature falls with height, and contaminants mix within it. The stratosphere, to about 50 km, warms with height because its ozone layer absorbs ultraviolet radiation, shielding life; long-distance jets cruise near its base. In the mesosphere, to about 85 km, temperature falls again to the coldest in the atmosphere, and most meteors burn up there. The thermosphere above is extremely thin but very hot, contains the ionosphere that reflects some radio waves, hosts the aurora and is where the International Space Station orbits, at about 400 km.

The misconception. Students often blame the ozone hole for global warming. They are different problems: ozone depletion is thinning of stratospheric ozone by chlorine compounds, which lets in more ultraviolet; global warming is added greenhouse gases trapping more infrared. Neither causes the other in any major way.

Worked concentration change: Carbon dioxide rose from about 280 ppm to about 420 ppm. Increase = 420 − 280 = 140 ppm; percentage increase = 140 ÷ 280 × 100 = 50 percent. As a fraction of air, 420 ppm = 420 ÷ 1,000,000 = 0.042 percent. The step skipped most often is dividing by the starting value, not the final value; 140 ÷ 420 would give 33 percent, which answers a different question.

Worked scrubber stoichiometry: CaCO₃ + SO₂ → CaSO₃ + CO₂. Molar masses: CaCO₃ 100 g/mol, SO₂ 64 g/mol, CO₂ 44 g/mol. To capture 1.0 metric ton of SO₂, a plant needs 100 ÷ 64 × 1.0 = 1.56 metric tons of limestone and releases 44 ÷ 64 × 1.0 = 0.69 metric tons of CO₂. The trade-off: the scrubber removes a contaminant that forms acid rain, but it requires mining limestone and adds a small amount of greenhouse gas.

Worked particulate count (hypothetical class data): Two sticky cards were exposed for 7 days, one by a school bus loading zone and one in a courtyard. Under a hand lens, five 1 mm² squares on each card held (bus zone) 38, 42, 35, 45, 40 and (courtyard) 12, 9, 15, 11, 13 particles. Means: 200 ÷ 5 = 40 per mm² and 60 ÷ 5 = 12 per mm². Per cm² per day: 40 × 100 ÷ 7 = 571 and 12 × 100 ÷ 7 = 171. The bus zone collected about 3.3 times as many particles. The conclusion is limited: the cards count visible particles, which are mostly larger than PM2.5, so they compare sites but do not measure health-relevant fine particles.

Worked primary or secondary chain: A coal-burning plant emits SO₂ and NO (primary). Downwind, SO₂ is oxidized to sulfate and sulfuric acid, and NO to NO₂ and nitric acid (secondary); on a sunny afternoon NO₂ and VOCs from nearby traffic produce ozone (secondary). A city downwind that has no plant of its own can therefore violate an ozone or acid-deposition limit; only cutting the primary emissions upwind fixes it.

Representation task: Draw a vertical profile of the atmosphere from 0 to 100 km with temperature on the horizontal axis, marking where temperature falls and rises, and label the troposphere, stratosphere with its ozone layer, mesosphere and the base of the thermosphere with one human use of each. Beside it draw a flow chart from primary to secondary contaminants for ozone and for acid rain, and a loop diagram of one positive and one negative feedback.

Safe investigation, particulate and ozone survey: (a) Spread a thin layer of petroleum jelly inside a 1 cm grid drawn on each of several index cards and fix them at chosen sites (parking lot, courtyard, classroom, bus zone) at the same height for 7 days. Under a hand lens, count particles in five 1 mm² squares marked inside the grid on each card, and record site, days, counts and mean per cm² per day (1 cm² = 100 mm²). (b) Your teacher prepares ozone test strips (Schoenbein paper) from cornstarch, water and a little potassium iodide. Wearing gloves, hang a moistened strip at each site for 8 hours out of direct sun, then compare its color with a scale: ozone oxidizes iodide to iodine, which darkens the starch. Record relative humidity, because it affects the color. Wash hands afterwards. Compare your results with the state's published daily air-quality data for your county. What the models cannot show: neither measures PM2.5 or gives ozone in ppm; they compare places, not safety.

Independent performance: Write a 300-word report to your school administrators on one local air-quality issue. Success criteria: it presents your class data in a table with means; it identifies the contaminants involved as primary or secondary and explains how the secondary ones form; it connects at least one contaminant to a health effect; it explains one link between air and rock or ocean (acid rain on limestone, scrubbers, or carbon sequestration); and it recommends two practical actions supported by the data, with one limitation of the data stated.