1. Definition

Air pollution refers to the presence of substances in the atmosphere that are harmful to living organisms or cause damage to the environment or materials. These substances, called pollutants, can be solid particles, liquid droplets, or gases.

2. Types of Air Pollutants

2.1 Primary Pollutants

  • Emitted directly from a source.
  • Examples: Carbon monoxide (CO), nitrogen oxides (NOₓ), sulfur dioxide (SO₂), particulate matter (PM), volatile organic compounds (VOCs).

2.2 Secondary Pollutants

  • Formed in the atmosphere through chemical reactions.
  • Examples: Ozone (O₃), secondary particulate matter, peroxyacetyl nitrate (PAN).

3. Sources of Air Pollution

3.1 Natural Sources

  • Volcanic eruptions
  • Forest fires
  • Dust storms
  • Pollen

3.2 Anthropogenic (Human-Made) Sources

  • Vehicle emissions
  • Industrial processes
  • Power generation (coal, oil, natural gas)
  • Agricultural activities (fertilizers, livestock)
  • Waste burning

4. Major Air Pollutants and Effects

Pollutant Source Health Effects Environmental Effects
Particulate Matter (PM2.5, PM10) Combustion, dust Respiratory, cardiovascular Reduces visibility, soil/water contamination
Nitrogen Oxides (NOₓ) Vehicles, power plants Lung irritation, asthma Acid rain, ozone formation
Sulfur Dioxide (SO₂) Fossil fuels Respiratory problems Acid rain, plant damage
Carbon Monoxide (CO) Incomplete combustion Headaches, dizziness Affects oxygen transport
Ozone (O₃) Secondary pollutant Lung function, chest pain Damages crops, materials
Lead (Pb) Industry, old fuels Neurological damage Soil/water contamination

5. Mechanisms of Air Pollution

  • Dispersion: Pollutants spread via wind and atmospheric mixing.
  • Transformation: Chemical reactions (e.g., photochemical smog).
  • Deposition: Pollutants settle on surfaces (wet and dry deposition).

6. Key Equations

6.1 Air Quality Index (AQI)

AQI is a dimensionless number used to communicate air quality.

AQI = (I_high - I_low)/(C_high - C_low) × (C - C_low) + I_low

Where:

  • C = concentration of pollutant
  • C_low, C_high = concentration breakpoints
  • I_low, I_high = AQI breakpoints

6.2 Gaussian Plume Model

Used to estimate pollutant dispersion from a point source.

C(x, y, z) = (Q / (2πuσ_yσ_z)) × exp(-y²/2σ_y²) × [exp(-(z-H)²/2σ_z²) + exp(-(z+H)²/2σ_z²)]

Where:

  • C = concentration at point (x, y, z)
  • Q = emission rate
  • u = wind speed
  • σ_y, σ_z = dispersion coefficients
  • H = effective stack height

7. Diagrams

Air Pollution Overview
Air Pollution Overview

Particulate Matter Size Comparison
Particulate Matter Size

8. Case Studies

8.1 Delhi, India (2021)

  • Recorded PM2.5 levels above 500 µg/m³, far exceeding WHO guidelines.
  • Causes: Crop burning, vehicular emissions, construction dust.
  • Impact: Schools closed, health emergency declared, increased respiratory illnesses.

8.2 California Wildfires (2020)

  • Massive wildfires led to hazardous air quality across the West Coast.
  • PM2.5 concentrations reached up to 500 µg/m³ in some areas.
  • Short-term spike in hospital admissions for asthma and heart conditions.

8.3 Beijing’s “Airpocalypse” (2020)

  • Severe smog episodes due to coal burning and traffic.
  • Government response: Temporary factory shutdowns, vehicle restrictions.
  • Long-term: Shift towards renewable energy and improved air quality monitoring.

9. Surprising Facts

  1. Indoor air pollution can be up to 5 times worse than outdoor air pollution, especially in poorly ventilated spaces using solid fuels for cooking.
  2. Air pollution is linked to cognitive decline and mental health issues, including increased risk of depression and dementia (Lancet Planetary Health, 2022).
  3. Microplastics are now detected in the air we breathe, transported by wind and deposited far from their original sources (Science, 2021).

10. Recent Research

A 2022 study published in Nature Communications found that long-term exposure to PM2.5 increases the risk of cardiovascular disease even at concentrations below current WHO guidelines. The study suggests stricter standards may be needed to protect public health.
Source: Nature Communications, 2022

11. Future Trends

  • AI and IoT in Air Quality Monitoring: Real-time, high-resolution pollution mapping using sensor networks and machine learning.
  • Green Infrastructure: Urban forests and green roofs to absorb pollutants.
  • Electrification of Transport: Shift to electric vehicles to reduce emissions.
  • Policy Innovations: Carbon pricing, stricter emission standards, and cross-border pollution agreements.
  • Personal Exposure Monitoring: Wearable devices to track individual pollutant exposure.

12. Summary Table

Aspect Key Points
Types Primary, Secondary Pollutants
Main Sources Natural & Anthropogenic
Health Impacts Respiratory, cardiovascular, neurological
Environmental Impacts Acid rain, smog, crop damage
Recent Trends AI, electrification, stricter standards

13. References

Note: Air pollution remains a critical global challenge. Future research and policy must address both outdoor and indoor sources, with a focus on vulnerable populations and emerging pollutants such as microplastics.