Aerosols from Fossil Fuel Burning


The byproducts of fossil fuel burning are injected into the atmosphere from both stationary sources, such as factories and powerplants, and mobile sources, such as motor vehicles. Aerosol particles produced by fossil fuel burning are concentrated over North America, Europe, China, Japan, India, and other industrialized regions. The primary aerosol particles released from fossil fuel burning generally fall into two categories: (1) soot, and (2) fly ash. Soot is carbonaceous; it includes both elemental ("black" carbon) and organic compounds. Soot particles generally belong to the fine particle mode. Fly ash is the non-organic byproduct of coal burning. Coal contains a lot of residual mineral material, including clays, shale, sulfides, carbonates, chlorides and various trace metals. When coal is burned, these parent materials are released unreacted, or thermally transformed. They take the form of spherical glassy particles in the coarse particle mode. The combination of soot and fly ash released from coal furnaces results in a bimodal size distribution of primary aerosol particles. Diesel engines are the dominant source of soot in some urban environments, but worldwide, coal burning releases about ten times as much black carbon into the atmosphere than diesel fuel combustion. Small spherical soot particles coagulate to form chain-like aggregates. These irregularly shaped particles have been found to be fractal, and have special optical properties. The mass median diameter of black carbon in the atmosphere is very small - between 0.1 and 0.5 µm. In urban air, two size modes can often be detected, a smaller mode resulting from freshly formed primary aerosol and a larger mode resulting from coagulation processes. The more remote the location, the more dominant the larger size mode becomes, reflecting the importance of coagulation during the aging of carbonaceous aerosol as it is transported from the site of combustion. The concentration of organic carbon in aerosol (as opposed to black carbon) ranges from about 0.6 to 2.6 µg m-3 in rural and remote continental sites of the United States, with an arithmetic mean of 1.0 µg m-3. This contrasts with the concentrations found in polluted urban conditions, which range between about 5 to 35 µg m-3. In the continental United States, the remote organic carbon aerosol is believed to be about 70% anthropogenic in origin. In forested tropical regions, similar organic carbon concentrations (up to 2.6 µg m-3) have been measured in the troposphere, but these result from the condensation of volatile organic compounds (isoprene, terpene) released from vegetation. Particles containing alkanes (saturated hydrocarbons) result both from fossil fuel combustion and primary biogenic processes (vascular plant waxes, pollen, etc.). Interestingly, clues about the origin of alkane aerosol are obtained from the ratio of the concentration of n-alkanes of odd numbers to even numbers of carbon atoms. Land-based vegetation preferentially releases alkanes with odd carbon numbers in the C19-C35 range, whereas alkanes of anthropogenic or marine origin lack such a preference. This parameter is called the Carbon Preference Index (CPI). Very recently released organic matter can have a CPI as high as 6-8. The mass mean diameter (MMD) of anthropogenic alkane aerosol is smaller (0.7-0.9 µm) than the MMD of biogenic alkane aerosol. The combustion of light organic compounds, such as propane and methane, yields soot (primary particles), and polycyclic aromatic hydrocarbons (PAH). Benzene and its homologues are primarily of interest due to their mutagenic and carcinogenic properties; they are not believed to play an important role in atmospheric processes. PAHs in aerosol particles are believed to be attached to black carbon, and are detected in the fine particle mode.