Outdoor air pollution originates from a combination of human activities and natural processes. Understanding these sources helps identify where pollution comes from and how it behaves in the environment. 

Main sources include: 

• Traffic emissions – cars, trucks, and other vehicles release pollutants such as nitrogen oxides (NOₓ), particulate matter (PM), and carbon monoxide (CO)  

• Industrial activities – factories, power plants, and refineries emit various gases and particles, including sulphur dioxide (SO₂) and VOCs  

• Residential heating – burning wood, coal, or other fuels contributes to PM and other pollutants, especially during colder months  

• Agriculture – livestock and fertilizer use release ammonia (NH₃), which can contribute to the formation of fine particles  

• Construction and road dust – mechanical processes generate coarse particles (PM₁₀)  

• Natural sources – wildfires, dust storms, sea spray, and pollen can significantly affect air quality  

Outdoor pollution levels are influenced by weather conditions such as wind, temperature, and rainfall, which affect how pollutants disperse or accumulate.