Atmospheric Particulates across Scales

Atmospheric particulates, also called particulate matter (PM), are a complex mixture of extremely small solid particles and liquid droplets suspended in the air. Particles are made up of several components, including acids (i.e., nitrates and sulfates), organic chemicals, metals, and soil or dust. PM sources can be anthropogenic or natural. Natural sources include volcanoes, dust storms, vegetation fires, and sea spray. Anthropogenic sources include coil and oil combustion, fossil-fuel power plants, and other industrial activity. The anthropogenic contribution to total atmospheric PM is estimated to be approximately 10%. Some of these particles are emitted directly into the air (primary emissions), while some are emitted as gases and subsequently form particles in the air (secondary emissions).

PM is categorized by size fractions. As particles are often nonspherical, categorization depends on their aerodynamic diameter. A particle with an [Page 150]aerodynamic diameter of 10 µm (micrometers) moves in the air like a sphere with a diameter of 10 µm. PM diameters range from less than 10 nm (nanometers) to more than 10 µm. These dimensions represent the continuum from a few molecules up to the size where particles can no longer be carried by the atmosphere and settle because of gravity or turbulent impact on the surface. A typical notation such as PM10 is used to describe particles of 10 µm or less, which are called coarse particles; and PM2.5 represents fine particles equal to or less than 2.5 µm in aerodynamic diameter. Particles between these two sizes are considered inhalable coarse particles. Everything below 100 nm, down to the size of individual molecules, is classified as ultrafine particles (UFP). In comparison, an average human hair is about 70 µm in diameter.

In general, the smaller and lighter a particle is, the longer it will stay in the air. Larger particles (greater than 10 µm in diameter) tend to settle to the ground in a matter of hours, whereas the smallest particles (less than 1 µm) can stay in the atmosphere for weeks and are mostly removed by precipitation (see Table 1).

The composition of particulates is determined by their source. Wind-blown mineral dust tends to be made of mineral oxides and other material, whereas sea salt consists mainly of sodium chloride originated from sea spray yet can include magnesium, sulfate, calcium, and potassium.

Secondary particles derive from the oxidation of primary gases such as sulfur and nitrogen oxides into sulfuric acid (liquid) and nitric acid (gaseous). The precursors for these acid aerosols may have an anthropogenic origin (e.g., coal combustion) and/or a biogenic origin. The chemical composition of PM affects how it interacts with solar radiation, determining how much light is scattered and absorbed. In this manner, particulates can influence the global climate. Recent evidence suggests that anthropogenic PM can control cloud development by altering the size distribution of cloud condensation nuclei.

The effects of inhaling PM have been widely studied in humans and animals and include asthma, lung cancer, cardiovascular disease, and premature death. The large number of deaths and other health problems associated with particulate pollution was first demonstrated in the early 1970s. As a result, PM is subject to strict regulatory control.

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