Introduction
Particulate matter, barely visible to the naked eye, can have devastating effects on the human body and environment. Around 800,000 people worldwide die each year from urban particulate matter exposure, while another 1.8 million die prematurely from indoor smoke produced by solid fuels. But what exactly are these particles and how do they form in our atmosphere? Understanding the chemistry behind particulate matter is crucial for addressing one of the most significant air quality challenges we face today.
Primary particulate matter
Particulate matter consists of microscopic solid or liquid particles that are incredibly diverse. They are formed in two ways: primary and secondary processes.
Primary particulate matter is emitted directly from sources like construction sites, wildfires, wood burning, factories, etc. Natural sources such as volcanoes, dust storms, forest fires also contribute to their production.
Soot from incomplete combustion; trace metals like iron, nickel, and vanadium from fuels; and mineral particles such as silicates or aluminum from soil and construction materials all contribute to primary particulate matter. Because these particles are released directly into the air, they don't need to undergo further chemical reactions to form, yet they remain highly harmful. Ultrafine carbon particles carry toxic organic compounds, metals can catalyze reactive oxygen species in the body, and mineral dust can irritate the lungs.
Secondary particulate matter
Secondary particulate matter is more complex and creates most of the ultrafine particles that reach the lungs. These particles are formed through chemical reactions in the atmosphere themselves. Secondary PM mainly include sulfates, nitrates, and organic aerosols. In some studies, they are found to contribute more to total particulate matter concentrations than primary sources of PM.
Sulfur dioxide (SO₂) from power plants reacts with water vapor to form sulfuric acid (H₂SO₄) through oxidation reactions. Through the ammonia (NH₃) concentration in the air, ammonium sulfate is formed:
H₂SO₄ + 2NH₃ → (NH₄)₂SO₄
These sulfate particles stick around in the air for days, traveling hundreds of miles from their site of formation.
Nitrogen oxides (NOₓ) from cars and trucks oxidize to form nitric acid (HNO₃), which then reacts with ammonia to create ammonium nitrate (NH₄NO₃):
HNO₃ + NH₃ ⇌ NH₄NO₃
This reaction is temperature-dependent, so at higher temperatures, the equilibrium shifts and nitrate particles evaporate. This is the reason why nitrate pollution tends to be worse in the winter.
VOC + OH/O₃/NO₃ → RO₂ (peroxy radical) → oxidized low-volatility compounds → condensation → SOA (secondary PM)
Volatile organic compounds (VOCs) from vehicle exhaust, industrial emissions, or biogenic sources (plants) react with oxidants from the air to form low-volatility organic compounds. These compounds condense onto existing particles forming ultrafine organic aerosols that contribute to PM2-5.
Conclusion
Particulate matter may be small, but its impact is immense. From soot and metals to chemical reactions high in the atmosphere, every particle tells a story of how human activity and natural processes intertwine. Understanding the chemical composition and scientific foundations behind the creation of these particles is the first step towards cleaner air and a healthier future.
References
Russell, Armistead G., and Bert Brunekreef. “A Focus on Particulate Matter and Health.” Environmental Science & Technology, vol. 43, no. 13, American Chemical Society (ACS), June 2009, pp. 4620–25, https://doi.org/10.1021/es9005459. Accessed 13 Nov. 2025.
“Getting Particular about Particulate Matter - Air King.” Air King, 10 Mar. 2021, www.airkinglimited.com/blog/getting-particular-about-particulate-matter/. Accessed 13 Nov. 2025.