National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Physical And Chemical Characteristics

Particulate matter refers to all airborne solid and liquid particles, except pure water, that are microscopic in size. Particle size may range from approximately 0.005 µm to 100 µm in diameter, although the suspended portion is generally less than 40 µm. Particulate matter is unique among atmospheric constituents in that it is not defined on the basis of its chemical composition. It may include a broad range of chemical species, including: elemental and organic carbon compounds; oxides of silicon, aluminum and iron; trace metals; sulphates; nitrates and ammonia.

PM₁₀ refers to particulate matter that is 10 µm or less in diameter. PM₁₀ is generally subdivided into a fine fraction of particles 2.5 µm or less (PM_2.5), and a coarse fraction of particles larger than 2.5 µm. It is further classified as primary (emitted directly into the atmosphere) or secondary (formed in the atmosphere through chemical and physical transformations). The principal gases involved in secondary particulate formation are sulphur dioxide (SO₂), nitrogen oxides (NOx), volatile organic carbons (VOCs) and ammonia (NH₃). Primary particles are found in both the fine and coarse fractions, whereas secondary particles, such as sulphates and nitrates, are found predominantly in the fine fraction. Both primary and secondary PM can result from either natural or (human) anthropogenic sources.

Particle size is considered to be the most important parameter in characterizing the physical behaviour of particulate matter in the atmosphere. Extremely small ("ultrafine") particles less than 0.1 µm in diameter (the nuclei mode) are formed primarily from the condensation of hot vapours during high temperature combustion processes and from the nucleation of atmospheric species to form new particles. While the greatest concentration of airborne particles is found in the nuclei mode, these particles contribute little to overall particle mass loading due to their tiny size. They are subject to random motion and to coagulation processes in which particles collide to quickly yield larger particles. Consequently, these tiny particles have short atmospheric residence times.

Particles in the size range of 0.1-2.0 µm (the accumulation mode) result from the coagulation of particles in the nuclei mode and from the condensation of vapours onto existing particles which then grow into this size range. These particles account for most of the particle surface area and much of the particle mass in the atmosphere. The accumulation mode is so-named since atmospheric removal processes are least efficient in this size range. These fine particles can remain in the atmosphere for days to weeks. Dry deposition and precipitation scavenging are the primary processes by which these fine particles are eventually removed from the atmosphere. It is calculated that precipitation scavenging accounts for about 80-90% of the mass of particles removed from the atmosphere.

Particles larger than 2.0 µm (the sedimentation or coarse mode) are typically associated with mechanical processes such as wind erosion, breaking ocean waves and grinding operations, which result in the physical breakdown of larger particles into smaller ones to yield particles such as windblown soil, sea salt spray, and dust from quarrying operations. These particles are efficiently removed by gravitational settling, and therefore remain in the atmosphere for shorter periods of a few hours to a few days. They contribute little to particle number concentrations but significantly to total particle mass.

Other physical characteristics which affect particle behaviour include particle shape and density, and bulk properties such as chemical composition, vapour pressure, hygroscopicity (water attracting nature), deliquescence and refractive index. Particles (such as sulphates and nitrates) remain dry with increasing relative humidity until their deliquescent point is reached (which varies with the chemistry of the particle), at which time a sudden uptake of water occurs with a corresponding increase in particle size. The resultant particles are usually within the size range that are most efficient at scattering light. Therefore, particle growth through deliquescence has a large potential impact on atmospheric visibility.

Surface properties such as electrostatic charge, the presence of surface films and surface irregularities may also influence particle behaviour. Small particles are characterized by a large surface area relative to their mass, which, when combined with surface irregularities and internal pores, leads to greater reactivity of fine particles compared to coarse particles.

As a consequence of their different sources and mechanisms of production, fine and coarse particles have markedly different chemical properties also. Coarse particles consist primarily of particles derived from the earth's crust, and are therefore rich in oxides of iron, calcium, silicon and aluminum, and are typically basic in nature. Particles in coastal regions are enriched with sodium chloride. Fine particles are composed mainly of sulphate, nitrate, ammonium, inorganic and organic carbon compounds, and heavy metals such as lead and cadmium, all of which are indicators of anthropogenic production processes. Fine particles tend to be acidic in nature. Sulphate has repeatedly been shown to be the most abundant single component of fine particles. However, only a few of the numerous organic carbon compounds have been identified and together these may comprise approximately 50% of the fine particle mass. Since many of the compounds making up the carbonaceous mass are likely to be toxic, further elucidation of the carbon moiety of particulate matter is clearly required.