Every breath holds about 1022 molecules: mostly nitrogen and oxygen, a little argon and a pinch of carbon dioxide. Sometimes chimney dust, exhaust fumes and ozone join them. Fill a jar with air, smoke up a valley town and then clear it, tune a catalytic converter, save a limestone statue from acid rain, release a single chlorine atom into the ozone layer and watch CO2 change the pH of the ocean.
Composition → sources → reactions → effects → solutions. We have already solved many air problems; some are still waiting.
Sources: CRC Handbook / NOAA (nitrogen, oxygen, argon); CO₂ — global average about 423 ppm in 2024 (NOAA GML), 424.6 ppm at Mauna Loa. Before the industrial era it was about 280 ppm (IPCC AR6).
A calm adult breath is about 0.5 L, the deepest one 3–5 L. Exhaled air is about 16% oxygen and about 4% CO₂ — roughly 100 times more than inhaled air.
Soot from a kopciuch (Polish slang for an old, smoky coal stove) is made of carbon lumps with molecules of benzo[a]pyrene stuck to them — five fused carbon rings (orange drawing). B(a)P forms in incomplete combustion and travels with the dust deep into the lungs; it causes cancer. The EU target value: 1 ng/m³ as an annual average. In many Polish cities annual averages in recent years have been several times higher (GIOŚ — Poland's Chief Inspectorate of Environmental Protection, annual reports 2020–2023).
WHO 2021 guidelines for PM2.5: 15 µg/m³ as a daily average, 5 µg/m³ as an annual average. The badge color follows the thresholds of the Polish air quality index (GIOŚ) for PM2.5.
Switch the heating in all houses at once or click individual houses in the picture:
A simplified model (a “box” over the valley): concentration ≈ background + emission / (wind speed × mixing-layer height). Dust emission factors per unit of heat — order of magnitude from the EMEP/EEA Guidebook (2019/2023). The numbers on the display are model output, not measurements.
Normally air near the ground is warmer than the air above it. Warm air rises, so smoke mixes into a thick layer of air. During an inversion (cold, windless nights, high pressure) a warmer layer sits on top of a cold layer near the ground — it acts like a lid. Smoke from low chimneys stays trapped beneath it, while the tall stack of the CHP plant releases its fumes above it.
In winter, with fog, frost and an inversion: coal smoke (soot) and SO₂. In fog droplets SO₂ oxidizes to sulfuric acid. From December 5 to 9, 1952, a yellow-gray fog hung over London. At the time it was blamed for about 4,000 deaths; later analyses put the toll as high as about 10,000–12,000. This disaster led to Britain's Clean Air Act (1956). Polish winter smog belongs to the same family, although today it contains less SO₂ and more dust from home stoves.
Particles smaller than 2.5 µm reach all the way to the alveoli in the lungs, and the smallest ones pass into the blood. Breathing polluted air for a long time raises the risk of heart disease, stroke, lung disease and lung cancer. Short smog episodes are felt most by children, older people, and people with asthma or heart disease.
Platinum and palladium help oxidize CO and hydrocarbons, and rhodium helps reduce nitrogen oxides. With a rich mixture there isn't enough oxygen for oxidation; with a lean one, oxygen “beats” NO to the CO molecules, so NOₓ slip through the converter. Cerium oxide stores a little oxygen and smooths out the fluctuations.
Model: a 10 m³ bathroom, a faulty gas water heater, 1 s of simulation = 2 min. Hemoglobin binds CO about 200–250 times more strongly than oxygen (the Haldane equation). Every heating season Polish fire services record several thousand carbon monoxide incidents and dozens of deaths (PSP — Polish State Fire Service, seasons 2018–2024).
An electric car has no tailpipe, so in the city it adds no NOₓ, CO or soot right where we breathe. Dust from brakes, tires and the road surface remains, though. In Europe these sources already account for more road-traffic dust than exhaust does (EEA, 2022). Regenerative braking reduces brake-pad wear, but EVs are usually heavier, so tire wear is similar or greater. How much CO₂ charging produces depends on the energy mix: in Poland in 2023–2024 about 55–60% of electricity came from coal (Ember, ARE), and that share is falling. Even with such a mix, most life-cycle analyses find an EV comparable to or better than a combustion car, and the advantage grows with every new wind and solar farm. The cleanest kilometer is still the one traveled by tram, by bike or on foot.
In the model, 100% emissions is the typical level for Central Europe in the 1980s. Simplified model: pH is computed from the equilibrium of CO₂ with water and from the amount of acid from SO₂ and NOₓ; the rate of damage to the statue and forest is schematic.
A raindrop in clean air dissolves a little CO₂ (Henry's law), forming very weak carbonic acid:
“Acid rain” usually means precipitation with a pH below about 5. In Central Europe in the 1980s, the average pH of precipitation was about 4–4.3 in places. The pH scale is logarithmic: pH 4 means 10 times more H⁺ ions than pH 5.
Sulfur in coal burns to SO₂, and in a hot furnace nitrogen from the air combines with oxygen to form NOₓ. In clouds both gases oxidize and dissolve in the droplets:
Limestone and marble are calcium carbonate. Acid turns it into gypsum, which rain washes away (or which forms a black crust with soot), and into CO₂:
In acidified soil, aluminum is released from minerals and damages roots, while magnesium and calcium are washed out. In lakes with a pH below about 5, fish eggs and many fish species die.
The Black Triangle — the border region of Poland, Czechoslovakia and East Germany — had power plants burning high-sulfur lignite in the 1970s and 80s. Spruce forests in the Jizera and Karkonosze (Giant) Mountains died back over tens of thousands of hectares (estimates vary by source), killed by acid rain, acid fog and droughts.
Flue-gas desulfurization: the flue gas passes through a slurry of ground limestone, and the sulfur turns into construction gypsum:
Today the pH of precipitation in Poland is usually close to 5–5.6, and the Jizera forests have grown back — although the soils are still recovering.
Simplified model: a two-dimensional box, sped-up chemistry and far more oxygen atoms than in the real stratosphere — so that the cycles become visible within a minute or so. In reality a single chlorine atom, before it gets locked away in HCl or ClONO₂, destroys on the order of up to 100,000 ozone molecules (US EPA estimate, depending on conditions).
Schematic curves (effective stratospheric chlorine, 1980 = 1) based on WMO/UNEP assessments (2022) and “world avoided” analyses (NASA, 2009). Ozone over Antarctica returns to its 1980 level around 2066, as on the Atmosphere page.
High up (15–35 km) ozone is a shield: it absorbs UV-B and UV-C radiation, which damages DNA. The basics — the layers of the atmosphere and the thickness of this shield (about 3 mm if compressed) — are on the Atmosphere page.
Near the ground the same O₃ is a pollutant. It irritates the airways and harms plants. It forms on sunny summer afternoons from NOₓ and volatile organic compounds — that's photochemical smog. WHO 2021 guideline: 100 µg/m³ (8-hour average).
Why is the hole over Antarctica? In winter the polar vortex gets colder than about −78 °C and polar stratospheric clouds form. On their crystals, chlorine from the “reservoirs” (HCl, ClONO₂) turns into Cl₂. When the Sun returns in spring (September–October), Cl₂ splits into atoms and rapid ozone destruction begins. Turn on the clouds in the simulation and see what happens to HCl.
Annual averages: NOAA GML / Scripps (Mauna Loa), rounded; measurements since March 1958. The wiggle within each year is a model (two harmonics, about ±3 ppm). In spring and summer, Northern Hemisphere forests absorb CO₂; in fall and winter, decaying leaves give it back. A shorter version of this chart and the greenhouse effect are on the Atmosphere page.
Model: seawater (salinity 35 ‰, alkalinity 2300 µmol/kg) in full equilibrium with the air; equilibrium constants after Weiss (1974), Lueker et al. (2000) and Dickson (1990). The real ocean lags behind the atmosphere, so the measured drop in surface pH is somewhat smaller: from about 8.2 to about 8.1 since preindustrial times (IPCC AR6). The 2100 scenarios are SSP2-4.5 and SSP5-8.5 from IPCC AR6.
The ocean absorbs roughly a quarter of the CO₂ we emit (IPCC AR6, Global Carbon Budget). That slows warming, but it comes at a price: a pH drop of 0.1 means 26% more H⁺ ions (100.1 ≈ 1.26), because the scale is logarithmic. To build shells and skeletons, corals, clams and pteropods need carbonate ions CO₃²⁻ — and those are running low.
Cold water dissolves more CO₂ (like a cold fizzy drink), which is why polar oceans acidify fastest.
Dry air is 78% nitrogen, 21% oxygen and almost 1% argon. The rest are trace gases, measured in ppm (parts per million) and ppb (parts per billion): 1 ppm is one molecule in a million. The ideal gas equation gives the number of molecules in a given volume:
Dust concentration in a city depends on how much of it we release and on how many cubic meters of air it gets diluted in. In a simple “box” model:
During an inversion the mixing layer shrinks from several hundred meters to about 100–200 m, and in calm weather the wind carries almost nothing away. The same emissions then give a concentration several times higher.
A gas dissolves in water in proportion to its partial pressure (Henry's law: c = KH · p). That's why more CO₂ in the air means more carbonic acid in raindrops and in the ocean.
A catalyst speeds up a reaction without being used up. In a car it's our ally: on platinum, palladium and rhodium, poisonous CO and NOₓ turn into CO₂ and N₂. In the stratosphere, the chlorine atom from CFCs turned out to be a catalyst too — except that it destroys ozone. It leaves every cycle unchanged, so it can repeat it thousands of times.
Simplifications: all simulations are models with approximate numbers. The town is a “box” with a single layer of air, photochemical smog is a few reactions instead of hundreds, and the stratosphere has sped-up chemistry. Measured numbers are labeled with their source and year.