Air pollution Study Guide

📖 Core Concepts Air pollution – presence of harmful gases, particles, or chemicals in outdoor or indoor air. Primary pollutant – emitted directly in its harmful form (e.g., CO, SO₂). Secondary pollutant – forms in the atmosphere from reactions of primary pollutants (e.g., ground‑level O₃). Particulate Matter (PM) – solid/liquid particles suspended in air; PM₁₀ ≤ 10 µm, PM₂.₅ ≤ 2.5 µm, ultrafine ≤ 0.1 µm. Fine‑particle toxicity – smaller particles penetrate deeper into the lungs and can enter the bloodstream, causing the greatest health risk. Major sources – fossil‑fuel combustion (power, industry, transport), agriculture (NH₃, CH₄), household solid‑fuel use, natural events (wildfires, volcanoes). Health burden – 7 million premature deaths/year; PM₂.₅, O₃, NO₂, CO, SO₂, and certain VOCs are the deadliest. Air‑quality standards – WHO guideline for annual PM₂.₅ ≤ 5 µg m⁻³; most of the world exceeds it. 📌 Must Remember Primary vs. secondary: CO, SO₂, NOₓ = primary; O₃ = secondary. PM size matters: PM₂.₅ > PM₁₀ > coarse dust in health impact. Top mortality contributors: PM₂.₅ (≈ 4.7 M deaths), indoor PM₂.₅ (≈ 3.1 M), O₃ (≈ 0.5 M). Key emission sectors: Power & industry – largest global PM₂.₅ source. Transportation – ⅓–½ of NO₂ emissions; both exhaust & non‑exhaust (tire/brake wear). Household solid fuels – major indoor PM₂.₅ & CO source for >2 B people. Acid‑rain precursors: SO₂ → H₂SO₄; NOₓ → HNO₃. Ammonia pathway: NH₃ + H₂SO₄ → (NH₄)₂SO₄ (fine nitrate‑sulfate PM). Policy milestones: 1956 UK Clean Air Act, 1963 US Clean Air Act, 1987 Montreal Protocol (CFCs). 🔄 Key Processes Formation of ground‑level ozone NOₓ + VOCs + sunlight → O₃. Secondary PM formation SO₂ → SO₃ → H₂SO₄ → sulfate aerosol. NOₓ → HNO₃ → nitrate aerosol. NH₃ + H₂SO₄/HNO₃ → ammonium sulfate/nitrate. Acid rain cycle SO₂/NOₓ → H₂SO₄/HNO₃ (gas) → deposition → soil/water acidification. Particle deposition in the lung > 10 µm → upper airway (cough, clearance). 2.5–10 µm → bronchi, cleared slowly. < 2.5 µm → alveoli, can cross into bloodstream. 🔍 Key Comparisons PM₂.₅ vs. PM₁₀ – PM₂.₅ penetrates deepest, higher cardiovascular/respiratory risk. Primary vs. secondary pollutants – Primary emitted directly; secondary formed in situ (often more widespread). Exhaust vs. non‑exhaust vehicle emissions – Exhaust: NOₓ, CO, VOCs; Non‑exhaust: brake/tire wear (PM₂.₅, metal particles). Indoor vs. outdoor pollution – Indoor: solid‑fuel smoke, CO, VOCs; Outdoor: traffic NO₂, industrial SO₂, regional PM₂.₅. Electric vehicle (EV) tailpipe – Zero exhaust emissions but still generates non‑exhaust PM. ⚠️ Common Misunderstandings “EVs are pollution‑free.” – EVs eliminate tailpipe gases but still produce brake/tire wear particles and upstream electricity emissions. “CO₂ is not a health pollutant.” – CO₂ is a greenhouse gas, not directly toxic at ambient levels, but classified as an air pollutant under the U.S. Clean Air Act (2022). “All VOCs are harmless.” – Some VOCs (benzene, butadiene) are carcinogenic; all VOCs contribute to ozone formation. “Indoor air is always cleaner.” – In low‑income settings, indoor solid‑fuel use creates hazardous PM₂.₅ and CO levels exceeding outdoor concentrations. 🧠 Mental Models / Intuition “Pollutant cascade” – Imagine a factory line: primary emissions → atmospheric chemistry → secondary pollutants → deposition → health effects. “Size‑penetration rule” – Smaller → deeper lung → greater systemic impact. “Source‑proximity effect” – Communities near power plants, busy roads, or waste sites experience higher exposure (environmental justice). 🚩 Exceptions & Edge Cases Methane – Primary greenhouse gas; also a VOC precursor for ozone in sunny conditions. CFCs – Ozone‑depleting despite being inert at ground level; banned globally but persist in the atmosphere. Radon – Natural radioactive gas; indoor accumulation can cause lung cancer, especially for smokers. Non‑exhaust PM – Becomes dominant in future EV fleets; policy must address brake/tyre wear standards. 📍 When to Use Which Assessing health risk: Use PM₂.₅ concentrations (µg m⁻³) for mortality estimates; use NO₂ and O₃ for respiratory irritation. Choosing mitigation: Industrial point sources → scrubbers, catalytic converters. Transport → avoid‑shift‑improve framework (reduce travel, shift to public transit/biking, improve vehicle tech). Household → clean cookstoves, LPG/electric cooking, ventilation. Agriculture → limit N‑fertilizer rates, manure management to cut NH₃. Policy instrument selection: Emission standards for new equipment → immediate reductions. Carbon pricing / cap‑and‑trade → incentivizes across sectors. Public‑information (AQI) → behavior change during high‑pollution days. 👀 Patterns to Recognize Hot‑sunny day + high NOₓ/VOCs → ozone spikes. Winter inversion + high traffic → elevated PM₂.₅ and CO. Rapid urbanization in LMICs → surge in indoor solid‑fuel use and outdoor traffic PM. Sudden fire events → sharp, short‑term increase in PM₂.₅ and CO. 🗂️ Exam Traps Confusing primary vs. secondary: “Ozone is emitted directly from cars” – false; it’s secondary. Mixing units: WHO guideline is 5 µg m⁻³ (annual PM₂.₅); some questions give µg L⁻¹ – watch the unit. Assuming “all VOCs are benign” – many are toxic and ozone precursors. Attributing all health loss to CO₂: CO₂ drives climate change but is not the direct cause of acute mortality; PM₂.₅ is. Over‑looking non‑exhaust emissions in EV scenarios: EVs still contribute PM via brake/tire wear. --- Use this guide for rapid review before your exam – focus on the bullet‑point facts, the size‑penetration intuition, and the typical “source → process → impact” chain.