Aerosols are a continuous suspension of fine solid particles or liquid droplets in the atmosphere. This mixture is ubiquitous, existing everywhere from remote mountain peaks to dense urban centers. Common examples include fog, mist, haze, and smoke. Aerosols are a natural part of the environment, playing complex roles in atmospheric chemistry and global climate regulation. Understanding their composition is key to determining their origin and effect on the planet.
Defining the Two Core Components
An aerosol is fundamentally a two-phase system requiring both a gas and particulate matter. The first component is the continuous phase, which is the gas medium, almost always atmospheric air. This gas provides the volume in which the second component is dispersed and suspended.
The second component is the dispersed phase, consisting of microscopic liquid droplets or solid particles floating within the gas. These particles, ranging from a few nanometers up to tens of micrometers, define the aerosol’s physical and chemical identity. They remain airborne for periods ranging from hours to years because their suspension prevents them from quickly settling out.
Primary Sources of Aerosol Particles
Aerosol particles originate from diverse processes, broadly categorized as natural and anthropogenic (human-related) sources. Natural processes contribute significantly to the total atmospheric aerosol load. Examples include wind erosion lifting mineral dust and the mechanical action of ocean waves injecting sea salt particles. Biological sources also release biogenic aerosols such as pollen, fungal spores, and organic compounds from vegetation.
Anthropogenic activities generate aerosols largely through combustion and industrial processes. Burning fossil fuels in power plants and vehicles releases particles directly through exhaust. Industrial activities like cement manufacturing and metallurgy also produce primary aerosol particles, as does the incomplete combustion of biomass in wildfires or agricultural burning.
Categorizing Aerosols by Chemical Composition
Aerosol particles are defined by their specific chemistry, often existing as a complex mixture of compounds.
Inorganic Salts
This major group frequently includes sulfates and nitrates. Sulfate aerosols form secondarily when sulfur dioxide gas, released from volcanic eruptions or burning sulfur-containing fuels, is oxidized. Nitrate aerosols are similarly formed from the oxidation of nitrogen oxides, linking them to both natural processes and combustion emissions.
Carbonaceous Aerosols
This substantial category splits into two main types: black carbon (BC) and organic carbon (OC). Black carbon, or elemental carbon, is soot—a product of incomplete combustion highly effective at absorbing sunlight and heating the surrounding air. Organic carbon is a vast mixture of thousands of different compounds. Some are directly emitted as primary organic aerosols, while others form secondarily when volatile organic gases condense after chemical reactions.
Particles may also contain water, dust, and heavy metals, depending on their source. Sea salt is a dominant component in marine environments.
How Particle Size Determines Behavior
The physical size of an aerosol particle, measured in micrometers (µm), dictates its atmospheric behavior and interaction with biological systems. Atmospheric scientists categorize particulate matter based on size, such as PM10 (particles 10 µm or less) and PM2.5 (finer particles 2.5 µm or less). Smaller particles, like those in the PM2.5 range, have a longer atmospheric residence time because their low mass makes them less susceptible to gravitational settling.
Particle size also determines how deeply an aerosol penetrates the human respiratory system. Larger, coarse particles (greater than 10 µm) are typically trapped in the nose and throat. In contrast, fine particles (PM2.5 and smaller) bypass upper respiratory defenses and travel deep into the lung’s tracheobronchial and alveolar regions. Particles less than 1 µm are concerning because they can penetrate the deepest parts of the lung.