Smog, Acid Rain and Air Pollution Control
On a hot July afternoon in Los Angeles, the mountains on the horizon vanish behind a brownish haze. Meanwhile, a lake in the Adirondacks, hundreds of miles from any big smokestack, has lost its fish. What do a hazy skyline and a fishless lake have in common? Both come from gases that leave tailpipes and smokestacks and then change in the air. By the end of this page you will be able to explain both, and to judge how we can stop them.
Photochemical smog
Air pollutants come in two kinds. A primary pollutant is released directly from a source, like carbon monoxide from a tailpipe. A secondary pollutant forms later, when primary pollutants react in the air.
Where do the ingredients come from? Burning fuel in car engines at high temperature makes NOx. Unburned gasoline vapors, paint solvents and some trees release VOCs. Sunlight supplies the energy for the reactions. That is why smog is worst in summer, in sunny, car-heavy cities.
Ground-level ozone irritates the lungs, triggers asthma, and damages crop leaves. Don't confuse it with the good ozone high in the stratosphere, which blocks UV-B light.
The figure shows a typical summer weekday. Notice that NOx peaks first and ozone peaks later.
Why the delay? Morning traffic pumps out NOx. Sunlight then needs hours to drive the reactions, so ozone builds up and peaks around 1 p.m. NOx is used up in making ozone, so it falls.
Name the two primary pollutants that react in sunlight to form photochemical smog, and name the key secondary pollutant that results.
Name the two primary pollutants that react in sunlight to form photochemical smog, and name the key secondary pollutant that results.
Thermal inversions
Normally air is warmest near the ground and cools with height. Warm air rises, carrying pollutants up and away. In a thermal (temperature) inversion, a layer of warm air sits above cooler air near the ground. The cool air is denser, so it can't rise through the warm "lid," and pollutants pile up underneath.
Los Angeles is a worst-case city. It sits in a basin ringed by mountains that block winds, the cool Pacific air often slides under warm air above (a frequent inversion), summer sun drives the chemistry, and millions of cars supply NOx and VOCs.
Los Angeles often has severe smog. Explain how a thermal inversion traps pollutants near the ground, and why the city's mountains and sunny, car-heavy conditions make it…
Los Angeles often has severe smog. Explain how a thermal inversion traps pollutants near the ground, and why the city's mountains and sunny, car-heavy conditions make it worse.
Acid rain
Burning coal and oil releases sulfur dioxide (SO₂). Hot engines and power plants also release nitrogen oxides (NOx). In the air they react with water and oxygen to form sulfuric acid and nitric acid. These fall as acid deposition: acid rain, snow, fog, or dry particles.
Normal, unpolluted rain is slightly acidic, with a pH of about 5.6, because dissolved CO₂ forms weak carbonic acid. Acid rain generally has a pH below 5.6 (often 4–4.5).
Effects of acid deposition:
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Lakes: acidic water harms fish eggs and young, and dissolves aluminum from soil that poisons fish gills. Many Adirondack lakes lost fish this way.
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Forests: acid leaches nutrients like calcium and magnesium out of soil, and releases toxic aluminum that damages roots. Trees weakened this way suffer more from drought, insects and cold.
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Buildings: acid dissolves limestone and marble statues and corrodes metal.
Winds carry SO₂ and NOx hundreds of miles, so acid rain often falls downwind of the source, even in another state. That's why a smokestack in the Ohio Valley can affect a lake in New York.
Which two pollutants emitted by burning fossil fuels are the main causes of acid rain? State the approximate pH of normal, unpolluted rain.
Which two pollutants emitted by burning fossil fuels are the main causes of acid rain? State the approximate pH of normal, unpolluted rain.
Rain in a lake region in the northeastern U.S. has a pH of 4.6, while normal rain has a pH of about 5.6. (a) By what factor is the rain more acidic than normal rain?…
Rain in a lake region in the northeastern U.S. has a pH of 4.6, while normal rain has a pH of about 5.6.
(a) By what factor is the rain more acidic than normal rain? Remember that each pH unit is a tenfold change.
(b) Explain two effects of acid deposition on forests or lakes.
Controlling air pollution
It's best to stop pollution at the source, but we also have devices that clean exhaust before it leaves.
| Device | Where | What it removes | How it works |
|---|---|---|---|
| Scrubber (flue-gas desulfurization) | Coal power plant smokestack | SO₂ | Sprays a wet limestone (calcium carbonate) slurry that reacts with SO₂ and captures it as solid sludge |
| Electrostatic precipitator | Coal power plant smokestack | Particulates (soot, ash) | Charges particles so they stick to oppositely charged plates |
| Catalytic converter | Car and truck exhaust | CO, NOx, unburned VOCs | Metal catalysts (platinum, palladium, rhodium) convert them to CO₂, N₂ and H₂O |
Other strategies:
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Switch to low-sulfur coal or natural gas. Less sulfur in means less SO₂ out. Limitation: low-sulfur coal can cost more or be far away, and it still emits CO₂ and NOx.
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Scrubbers remove most SO₂ even from high-sulfur coal. Limitations: they are expensive, use lots of water, and produce sludge that must be disposed of.
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Policy: the Clean Air Act sets limits on criteria pollutants, and its cap-and-trade program for SO₂ sharply cut emissions. Other policies include emissions testing for cars, mass transit, and carpool lanes.
Identify the device used in coal power plants to remove sulfur dioxide from exhaust gases and the device used to remove particulates.
Identify the device used in coal power plants to remove sulfur dioxide from exhaust gases and the device used to remove particulates.
Compare two ways to reduce air pollution from a coal-fired power plant: (a) switching to low-sulfur coal and (b) installing a scrubber. Explain how each lowers emissions…
Compare two ways to reduce air pollution from a coal-fired power plant: (a) switching to low-sulfur coal and (b) installing a scrubber. Explain how each lowers emissions and name one limitation of each.
The table shows average daily ozone and nitrogen oxide (NOx) levels in a large city on a summer weekday. | Time | NOx (ppb) | Ozone (ppb) | |---|---|---| | 6 a.m. | 60 |…
The table shows average daily ozone and nitrogen oxide (NOx) levels in a large city on a summer weekday.
| Time | NOx (ppb) | Ozone (ppb) |
|---|---|---|
| 6 a.m. | 60 | 10 |
| 9 a.m. | 90 | 25 |
| 1 p.m. | 40 | 85 |
| 6 p.m. | 55 | 60 |
(a) Explain why NOx peaks in the morning.
(b) Explain why ozone peaks in early afternoon rather than at the morning rush hour.
(c) Propose two strategies, one technological and one policy-based, to reduce peak ozone, and explain how each works.
A state is considering a rule that requires all coal plants to install scrubbers and catalytic converters on diesel trucks. Plant owners say it will raise electricity…
A state is considering a rule that requires all coal plants to install scrubbers and catalytic converters on diesel trucks. Plant owners say it will raise electricity prices by 4%.
(a) Describe how each technology reduces a pollutant that causes acid rain or smog.
(b) Explain how lowering SO₂ and NOx would help forests and lakes downwind of the state.
(c) Evaluate the rule, considering costs, health benefits, and the fact that pollution crosses state lines.
Back to the start: the haze over Los Angeles is photochemical smog trapped by a thermal inversion, and the fishless Adirondack lake was acidified by SO₂ and NOx carried downwind from distant power plants and traffic.