Particulate Matter

Particulate matter (PM) refers to tiny solid or liquid particles suspended in the air. These particles vary in size, composition, and origin, and they can have significant impacts on human health, visibility, climate, and ecosystems.

1. Primary Particulate Matter

   • Primary PM is directly emitted into the atmosphere from various sources without undergoing chemical reactions.
   • Sources of primary PM include:
     • Combustion processes: such as vehicle engines, power plants, industrial boilers, and residential heating systems emit particles directly into the air through incomplete combustion of fossil fuels or biomass.
     • Mechanical processes: such as construction activities, mining operations, and road dust resuspension generate particles from mechanical abrasion and erosion of surfaces.
     • Natural sources: such as wildfires, volcanic eruptions, sea spray, and windblown dust contribute to primary PM emissions.

2. Secondary Particulate Matter

   • Secondary PM forms in the atmosphere through chemical reactions involving gaseous precursors, which undergo nucleation, condensation, and/or coagulation to form particles.
   • Common gaseous precursors include sulfur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3), volatile organic compounds (VOCs), and atmospheric oxidants such as ozone (O3) and hydroxyl radicals (•OH).
   • Secondary PM formation pathways include:
     • Sulfate Formation: SO2 emitted from combustion processes reacts with atmospheric oxidants and ammonia to form sulfate particles, contributing to fine particulate matter (PM2.5) formation.
     • Nitrate Formation: NOx emissions from combustion sources react with ammonia and organic compounds to form nitrate particles, also contributing to PM2.5.
     • Organic Aerosol Formation: VOCs emitted from vehicles, industrial processes, and vegetation undergo atmospheric oxidation and condensation to form organic aerosols.
     • Secondary Inorganic Aerosols: Other secondary inorganic aerosols, such as ammonium sulfate and ammonium nitrate, form through neutralization reactions involving ammonia and sulfuric/nitric acids.

3. Heterogeneous Reactions

   • Particulate matter formation can also involve heterogeneous reactions occurring on the surfaces of existing particles, such as the uptake of gases onto aerosols or the reaction of gases with liquid droplets.
   • These reactions can lead to the formation of secondary organic aerosols, the aging of particles, and changes in particle composition and properties.

Figure 1 and Figure 2 illustrates the formation of particulate matter