The impacts of combustion emissions on air quality and climate – From coal to biofuels and beyond Jeffrey S. Gaffney * , Nancy A. Marley University of Arkansas at Little Rock, 2801 South University Avenue SCLB 451, Little Rock, AR 72204-1099, United States Keywords: Fossil fuels Emissions Coal Petroleum Biofuels Biodiesel Oxyfuels Combustion products Natural gas Liquefied petroleum gas Nitrogen oxides VOCs Sulfur dioxide Mercury Natural radioactivity abstract Combustion processes have inherent characteristics that lead to the release in the environment of both gaseous and particulate pollutants that have primary and secondary impacts on air quality, human health, and climate. The emissions from the combustion of fossil fuels and biofuels and their atmospheric impacts are reviewed here with attention given to the emissions of the currently regulated pollutant gasses, primary aerosols, and secondary aerosol precursors as well as the emissions of non-regulated pollutants. Fuels ranging from coal, petroleum, liquefied petroleum gas (LPG), natural gas, as well as the biofuels; ethanol, methanol, methyl tertiary-butyl ether (MTBE), ethyl tertiary-butyl ether (ETBE), and biodiesel, are discussed in terms of the known air quality and climate impacts of the currently regulated pollutants. The potential importance of the non-regulated emissions of both gasses and aerosols in air quality issues and climate is also discussed with principal focus on aldehydes and other oxygenated organics, polycyclic aromatic hydrocarbons (PAHs), and nitrated organics. The connection between air quality and climate change is also addressed with attention given to ozone and aerosols as potentially important greenhouse species. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Since man first discovered that he could control fire and combust fuels for heat and cooking, he has had to deal with the byproducts of the combustion of organic fuels. These byproducts include the major combustion products of carbon dioxide (CO 2 ) and water vapor (H 2 O), along with a variety of trace gasses and aerosol emissions that have many impacts on air quality, human health, and climate. When man’s population levels were at a more or less constant level of a few million for the first two million years of his existence on the planet, and his fuel usage was limited to the combustion of wood in simple campfires, the impacts of the emitted pollutants were limited to the near vicinity of the combustion. Research has found that continual exposure of early man to campfires used as heat sources in enclosed areas contributed to increased incidences of nasal cancer (Zimmer- man, 2004). As the human population grew and the combustion of wood and coal became more widespread, the air pollution impacts of combustion emissions were still generally limited to the major cities where the population and combustion levels were concen- trated into confined areas. Air pollution has been recognized for some time as an urban problem. Moses Maimonides, a Jewish leader, philosopher, and physician living in the middle ages in Cairo, Egypt from 1165 to 1204, noted in his writings (Finlayson-Pitts and Pitts, 1986): ‘‘.Comparing the air of cities to the air of deserts and arid lands is like comparing waters that are befouled and turbid to waters that are fine and pure. In the city, because of the height of its buildings, the narrowness of its streets, and all that pours forth from its inhabitants and their superfluities. the air becomes stagnant, turbid, thick, misty, and foggy. If there is no choice in this matter, for we have grown up in the cities and have become accustomed to them, you should. select from the cities one of open horizons. endeavor at least to dwell at the outskirts of the city.’’ His discussion clearly indicates that, in the twelfth century, air pollution was already associated with urban environments and their higher population densities. During the fourteenth century, coal burning had become such a problem in London, that King Edward I passed a law banning coal burning when the English Parliament was in session (Freese, 2003). This was the first known legislation aimed at reducing human exposures to air pollution. In 1661, John Evelyn Esquire was commissioned by King Charles II to write an assessment of the air * Corresponding author. Tel.: þ1 501 569 8840; fax: þ1 501 569 8838. E-mail address: jsgaffney@ualr.edu (J.S. Gaffney). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.09.016 Atmospheric Environment 43 (2009) 23–36