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 CO₂ (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.

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

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:

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

🏭 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 SO₂ and soot for days.

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

🏭 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.

$\text{Normal: warm below} \Rightarrow \text{air rises} \Rightarrow \text{pollutants disperse}$

🏭 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 PM₂.₅ 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. PM₁₀ (coarser) is filtered out higher in the airway.

🏭 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

🏭 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 removed Pb from the air almost entirely in developed nations — one of the great public-health wins, dropping children's blood-lead levels dramatically.