Air Pollution & Smog

🏭 Air Pollution & Smog

Part 1 of 7 — Pollutants, Sources & the Primary/Secondary Distinction

Topics in This Part

🔑 Key Concept: Air pollution is any gas or particle added to the atmosphere in high enough concentration to harm health, ecosystems, or materials. The exam rewards you for knowing where each pollutant comes from and what it does — not just its name.

The Six Criteria Pollutants

Under the U.S. Clean Air Act, the EPA sets National Ambient Air Quality Standards (NAAQS) for six "criteria" pollutants. Memorize this table — it is the backbone of the entire unit.

💡 Why these six? They are common, well-studied, and regulated nationwide. Pollutants like $CO_{2}$ (a greenhouse gas) and VOCs are important but are not criteria pollutants — a classic exam trap.

Concept Check 🎯

Primary vs. Secondary Pollutants

The key word is formed. A secondary pollutant did not come out of any tailpipe — it was built in the air from primary precursors plus energy (sunlight) or other reactants.

$\text{NO}_x + \text{VOCs} + \text{sunlight} \;\longrightarrow\; \text{O}_3 \text{ (ground-level ozone)}$

⚠️ The single most-tested idea in this unit: ground-level ozone is a secondary pollutant. It is never emitted directly. If a question says "emitted from car exhaust," the direct emissions are $NO_{x}$ and VOCs — the ozone forms afterward.

Classify Each Pollutant 🔽

Decide whether each is a primary pollutant (emitted directly) or a secondary pollutant (formed in the atmosphere).

Point vs. Nonpoint Sources

This distinction matters for regulation: a single smokestack is easy to monitor and require scrubbers on. Nonpoint pollution (vehicle exhaust spread over a whole metro area) is harder to control and usually addressed with fleet-wide rules like catalytic converters and fuel standards.

🔑 Setting up the arc: You now know what the pollutants are and where they come from. Part 2 dives into the chemistry of the most exam-heavy secondary pollutant of all — photochemical smog.

Concept Check 🎯

🏭 Air Pollution & Smog

Part 2 of 7 — Photochemical Smog

🔑 The Idea: "Photo" = light. Photochemical smog is the brown, hazy air over sunny cities like Los Angeles, Beijing, and Mexico City. It is built by sunlight acting on car-exhaust chemicals, and it peaks on hot, sunny afternoons.

The Recipe for Photochemical Smog

Three ingredients, plus heat and light:

$\underbrace{\text{NO}_x}_{\text{engines}} \;+\; \underbrace{\text{VOCs}}_{\text{fuel, solvents}} \;+\; \underbrace{\text{sunlight (UV)}}_{\text{energy}} \;\longrightarrow\; \text{O}_3 \;+\; \text{PANs} \;+\; \text{aldehydes}$

A simplified look at the core reactions:

1. Combustion makes nitric oxide: $\text{N}_2 + \text{O}_2 \rightarrow 2\,\text{NO}$
2. NO oxidizes to brown nitrogen dioxide: $2\,\text{NO} + \text{O}_2 \rightarrow 2\,\text{NO}_2$
3. Sunlight splits $NO_{2}$, freeing an oxygen atom: $\text{NO}_2 + \text{UV} \rightarrow \text{NO} + \text{O}$
4. That free atom makes ozone: $\text{O} + \text{O}_2 \rightarrow \text{O}_3$

The brown color of smog comes mainly from $NO_{2}$. The respiratory damage comes mainly from $O_{3}$ and PANs (peroxyacetyl nitrates).

💡 Memory hook: Sun + cars = ozone. No sunlight, no photochemical smog — which is why it builds through the morning rush hour and peaks in the mid-afternoon, not at night.

Concept Check 🎯

A Day in the Life of Smog

Tracking the chemistry over a single day makes the cause-and-effect obvious:

💡 Because the sun is the switch, ozone is a daytime pollutant that lags the morning rush by a few hours — the precursors are emitted early, but the product builds as the sun climbs.

Order the Smog Chemistry 🔽

Walk through how ground-level ozone forms on a sunny afternoon.

The Ozone Paradox: "Good Up High, Bad Nearby"

The same molecule $(O_{3})$ is helpful or harmful depending on altitude:

⚠️ Exam trap: Do not confuse the two. The hole in the stratospheric ozone layer (from CFCs) is a different problem from ground-level ozone (from smog). Fixing one does nothing for the other.

Concept Check 🎯

🏭 Air Pollution & Smog

Part 3 of 7 — Industrial Smog & Acid Deposition

🔑 The Idea: Before cars dominated, the deadly smogs were industrial (sulfurous) smog from burning coal. This is the gray, sulfur-and-soot smog of old London — and it connects directly to acid rain.

Industrial (Sulfurous) Smog

The 1952 Great Smog of London killed thousands when coal smoke + fog (a cold, stagnant air mass) trapped $SO_{2}$ and soot for days.

💡 Contrast hook: Photochemical smog loves the sun; industrial smog loves the cold and coal. They are opposites in almost every way except that both harm the lungs.

Concept Check 🎯

From $SO_{2}$ and $NO_{x}$ to Acid Deposition

The same $SO_{2}$ and $NO_{x}$ that cause smog also become acid deposition ("acid rain"). In the atmosphere they oxidize and dissolve in water to form strong acids:

$\text{SO}_2 \;\longrightarrow\; \text{H}_2\text{SO}_4 \quad (\text{sulfuric acid})$ $\text{NO}_x \;\longrightarrow\; \text{HNO}_3 \quad (\text{nitric acid})$

Normal rain is slightly acidic (about pH 5.6) because of dissolved $CO_{2}$. Acid rain is rain with a pH below 5.6 — often 4 to 4.5, and sometimes lower.

⚠️ pH is logarithmic. A drop from pH 5 to pH 4 is a 10× increase in acidity, not a 20% change. From pH 6 to pH 4 is 100×.

Acidity Is Logarithmic 🧮

The pH scale is base-10: each whole-number drop in pH means 10× more hydrogen-ion $(H^{+})$ concentration.

1) A lake's rainwater goes from pH 6 to pH 4. Acidity increased by a factor of how much? (enter the number) 2) Acid rain at pH 3 is how many times more acidic than normal rainwater at pH 6? (enter the number)

Acid Rain Travels Downwind

Because $SO_{2}$ and $NO_{x}$ ride the wind while they slowly convert to acids, the damage often lands far from the source. Tall smokestacks built to improve local air quality actually push emissions higher into the wind stream, exporting acid deposition across state and national borders.

⚠️ This makes acid deposition a transboundary problem: the polluter and the harmed ecosystem are often in different jurisdictions, which is why it required regional and international agreements to address.

Match the Chemistry 🔽

🏭 Air Pollution & Smog

Part 4 of 7 — Thermal Inversions & the Role of Weather

🔑 The Idea: Pollution is about dilution. Normally, warm air rises and carries pollutants up and away. A thermal inversion flips this, trapping pollutants near the ground and turning a normal day into a deadly smog event.

Normal Conditions vs. Inversion

Normal atmosphere: the air is warmest at the ground and cools with altitude. Warm surface air is buoyant, so it rises (convection), carrying pollutants upward where they disperse.

Thermal inversion: a layer of warm air sits on top of cooler air near the ground. Cool air is denser and cannot rise through the warm "lid." Pollutants are trapped underneath and accumulate.

$\text{Normal: warm below} \Rightarrow \text{air rises} \Rightarrow \text{pollutants disperse}$ $\text{Inversion: warm above} \Rightarrow \text{cool air trapped} \Rightarrow \text{pollutants build up}$

💡 Geography matters: Cities in basins ringed by mountains (LA, Mexico City, Beijing) suffer the worst smog because both the inversion and the terrain trap the air.

Concept Check 🎯

When Inversions Turn Deadly

Inversions have caused some of history's worst air-pollution disasters:

In each case the emissions were normal — it was the trapped, undiluted air that turned routine pollution lethal. This is why meteorologists issue air-quality alerts when an inversion is forecast.

Build the Inversion 🔽

Describe what happens during a pollution-trapping thermal inversion.

Quantifying Dilution

When wind and mixing decrease, the same emissions are diluted into a smaller volume of air, so concentration rises. A rough model:

$\text{Concentration} \;\approx\; \frac{\text{Emission rate}}{\text{Wind speed} \times \text{Mixing height}}$

Cut the mixing height in half (a low inversion lid), and the concentration roughly doubles for the same emissions. This is why a still, inversion-capped morning can spike pollution even when factories emit no more than usual.

🔑 Takeaway: Pollution levels depend on emissions ÷ dispersion. Weather (wind, inversions) controls the dispersion half of that fraction.

Dilution Math 🧮

Use $\text{Concentration} \approx \dfrac{\text{Emission rate}}{\text{Wind speed} \times \text{Mixing height}}$.

1) Emissions are constant, but an inversion drops the mixing height from 1000 m to 250 m (wind unchanged). The pollutant concentration becomes how many times larger? (enter the number) 2) If, in addition, the wind speed also falls to half its previous value, the total concentration is now how many times the original? (enter the number)

🏭 Air Pollution & Smog

Part 5 of 7 — Health Effects & the Air Quality Index (AQI)

🔑 The Idea: Air pollution is a leading global health risk. The Air Quality Index (AQI) is how agencies translate pollutant concentrations into a single 0–500 color-coded number the public can act on.

Who and What Gets Hurt

Most sensitive groups: children, the elderly, pregnant people, and those with asthma or heart/lung disease.

💡 Why $PM_{2}._{5}$ is special: Its diameter is ≤ 2.5 micrometers — small enough to slip past the body's defenses, lodge deep in the alveoli, and even cross into the blood. $PM_{10}$ (coarser) is filtered out higher in the airway.

Concept Check 🎯

Reading the Air Quality Index (AQI)

The AQI runs 0–500. Higher = worse. Each band has a color and a health message:

The reported AQI for an area is set by whichever pollutant is worst at that time (the "dominant pollutant").

⚠️ Don't average pollutants. If $PM_{2}._{5}$ gives AQI 70 but ozone gives AQI 160, the reported AQI is 160 (the maximum), and the dominant pollutant is ozone.

Interpret the AQI 🔽

The "Dominant Pollutant" Rule

A monitoring station measures several pollutants at once and computes a separate AQI sub-index for each. The number reported to the public is the single highest sub-index — and the pollutant that produced it is called the dominant pollutant.

$\text{Reported AQI} = \max(\text{AQI}_{\text{PM}}, \;\text{AQI}_{\text{O}_3}, \;\text{AQI}_{\text{CO}}, \dots)$

🔑 Use the worst pollutant, never the average. Averaging would hide a single dangerous pollutant behind several clean ones — defeating the purpose of a public health warning.

Dominant Pollutant 🧮

The reported AQI for a city is the maximum of the individual pollutant AQIs (never the average).

A monitoring station reports these sub-index values for one afternoon:

• $PM_{2}._{5}$: AQI 88
• Ozone $(O_{3})$: AQI 142
• CO: AQI 35

1) What single AQI value should the city report? (enter the number)

🏭 Air Pollution & Smog

Part 6 of 7 — Indoor Air Pollution

🔑 The Idea: People spend ~90% of their time indoors, where some pollutants reach concentrations higher than outdoors. The exam loves three indoor culprits in particular: radon, carbon monoxide, and combustion smoke — plus the developing-world problem of indoor cooking fires.

Major Indoor Air Pollutants

💡 Developing vs. developed split: In developing nations the biggest indoor threat is smoke from indoor cooking and heating fires (wood, charcoal, dung) with poor ventilation. In developed nations the headline indoor risks are radon, CO, and VOCs/formaldehyde in tightly sealed, energy-efficient buildings.

Concept Check 🎯

Why Indoor Air Can Beat Outdoor Air for Hazard

Two factors make indoor exposure outsized:

1. Time. People spend roughly 90% of their lives indoors, so even moderate indoor concentrations add up to large total exposure.
2. Confinement. A room is a small, poorly mixed volume. A pollutant released inside (radon, CO, VOCs) is not diluted by the whole atmosphere — it concentrates.

💡 The same dilution logic from Part 4 applies indoors: a small enclosed volume with limited air exchange behaves like a permanent mini-inversion, letting pollutants build up.

Match Indoor Pollutant to Source 🔽

Reducing Indoor Air Pollution

⚠️ Energy-efficiency trade-off: Tightly sealed, well-insulated buildings save energy but reduce air exchange, so indoor pollutants can build up. Good ventilation is the fix — you don't have to choose between efficiency and clean indoor air.

Concept Check 🎯

🏭 Air Pollution & Smog

Part 7 of 7 — Control Technology, Policy & Mastery Check

You now know the pollutants, the two smogs, acid deposition, inversions, the AQI, and indoor air. This final part covers how we control pollution and the laws that drive it — then a mixed review and an Exit Quiz.

Control Technologies

The cleanest strategies, though, are prevention at the source: switching coal → natural gas or renewables, improving efficiency, and reformulating fuels (e.g., removing lead and sulfur).

💡 Best example of source control: Phasing out leaded gasoline removed Pb from the air almost entirely in developed nations — one of the great public-health wins, dropping children's blood-lead levels dramatically.

Key U.S. Policy: The Clean Air Act

The 1990 $SO_{2}$ cap-and-trade program is the classic success story: it set a national cap, let plants buy/sell allowances, and cut $SO_{2}$ emissions faster and cheaper than expected — sharply reducing acid rain.

Internationally, the Montreal Protocol (1987) phased out CFCs to heal the stratospheric ozone layer — a separate issue from ground-level smog, but a model of effective global cooperation.

🔑 Don't mix them up: Clean Air Act / NAAQS = U.S. criteria pollutants and smog. Montreal Protocol = global CFC phase-out for the stratospheric ozone hole.

Emission-Reduction Math 🧮

1) A coal plant emits 8000 tons of $SO_{2}$ per year. After installing a scrubber that removes 90% of $SO_{2}$, how many tons per year are still emitted? (enter the number) 2) A city's vehicle fleet emits 50,000 tons of $NO_{x}$ per year. A new standard cuts emissions to 35,000 tons. What is the percent reduction? (enter just the number, e.g. 30 for 30%)

Control vs. Prevention

The math above shows control technology works, but each device only catches a fraction of one pollutant and costs money to run. The most powerful long-term strategy is prevention — emitting less in the first place:

💡 Prevention also avoids the "co-pollutant" problem: switching a coal plant to wind power eliminates $SO_{2}$, $NO_{x}$, PM, and $CO_{2}$ all at once, whereas a scrubber only removes $SO_{2}$.

Match Control to Pollutant 🔽

Quick Reference

⚠️ Top traps: ozone is secondary; "good ozone" (stratosphere) ≠ "bad ozone" (ground); pH is logarithmic; report the maximum AQI, never the average.

Mixed Practice 🎯

Exit Quiz ✅

Answer all three to finish the lesson.