Atmospheric Pollution

Air pollution 空气污染 is harmful substances in the atmosphere. Primary pollutants 一次污染物 are emitted directly (carbon monoxide, nitrogen oxides, sulfur dioxide, particulates); secondary pollutants 二次污染物 form in the air from reactions (like ozone in smog). Most come from burning fossil fuels in vehicles, power plants, and industry.

Photochemical smog 光化学烟雾 forms when sunlight drives reactions among nitrogen oxides and volatile organic compounds from car exhaust, producing ground-level ozone 臭氧. It is worst in sunny, car-heavy cities and harms lungs. (Ground-level ozone is a pollutant, unlike the protective ozone layer high up.)

Photochemical smog over Los Angeles: on a sunny day, car exhaust reacts in the light to make a brown haze that hides the skyline

A thermal inversion 逆温 traps pollution near the ground. Normally warm air rises and carries pollutants away, but in an inversion a layer of warm air sits above cooler air, capping it. Pollutants accumulate below – causing dangerous smog episodes, especially in valleys.

Normally warm air rises and disperses pollution; an inversion caps it near the ground

Rising atmospheric $\text{CO}_2$ from fossil fuels enhances the greenhouse effect and warms the climate. Particulate matter 颗粒物 (tiny solid/liquid particles, PM2.5 and PM10) penetrates deep into lungs, causing respiratory and heart disease, and reduces visibility.

Indoor air can be more polluted than outdoor. Common indoor pollutants include radon 氡 (radioactive gas from the ground, a lung-cancer risk), carbon monoxide, asbestos 石棉, tobacco smoke, mold, and VOCs from paints and furnishings. Ventilation and source control reduce exposure.

Air pollution is cut by technology and policy: catalytic converters 催化转化器 on cars, scrubbers 洗涤器 and electrostatic precipitators 静电除尘器 on smokestacks, cleaner fuels, and laws like the Clean Air Act setting emission limits. Reducing fossil-fuel use addresses the root cause.

A catalytic converter turns harmful NO and CO into harmless N2 and CO2

Acid rain 酸雨 forms when sulfur dioxide and nitrogen oxides react with water in the air to make sulfuric and nitric acids, which fall far downwind. It acidifies lakes and soils (harming fish and trees), leaches nutrients, and corrodes buildings. Reducing $\text{SO}_2$ and $\text{NO}_x$ emissions is the fix.

Nitrogen and sulfur oxides form nitric and sulfuric acid in rain

Noise pollution 噪声污染 is unwanted or harmful sound in the environment. Major sources are road traffic, aircraft, trains, industry, and construction. In people, long or loud exposure causes hearing loss 听力损失, stress 压力, sleep loss, and higher blood pressure. It also disturbs wildlife 野生动物: it masks the sounds animals use to communicate, navigate, and find mates, and underwater noise from ships is especially harmful to whales and dolphins. It is reduced by sound barriers, quieter engines and mufflers, and land-use planning such as buffer zones around airports.

Worked example. Normal rain has a pH of about 5.6; an acid-rain sample measures pH 3.6. Because pH is a base-10 logarithmic scale, each unit is a $10\times$ change in hydrogen-ion concentration, so pH 3.6 is $10^{(5.6-3.6)}=10^{2}=100$ times more acidic than normal rain. That hundred-fold jump is why acid rain damages leaves, soils, and limestone so quickly.

• Distinguish primary pollutants (emitted directly: CO, NOₓ, SO₂, particulates) from secondary (form in the air: ground-level ozone).
• Ground-level ozone is a pollutant, but the ozone layer high up is protective — do not confuse them.
• Explain how a thermal inversion traps pollution near the ground.
• Acid rain (from SO₂ and NOₓ) often falls far downwind, making it an international problem.
• Link control methods to pollutants (catalytic converters, scrubbers, cleaner fuels, emission laws).