Nature generates pollutants in sufficient volumes to impact Earth's climate. Particulates and sulfur compounds released in volcanic eruptions can lower mean global temperatures for a few years. However, natural air pollutants are generally less persistent in the atmosphere and have a more transient impact on climate than do human-generated ones.

The term "air pollution" evokes images of belching industrial smokestacks, rush-hour freeways, and smog-shrouded city skylines: human-caused (anthropogenic) air pollution. Natural processes, however, generate a number of gaseous compounds and particulates that would be regarded as pollutants if human activity had produced them. Among these are oxides of carbon, nitrogen, and sulfur; hydrocarbons; ozone; volatile organic compounds (VOCs); and ash, soot, and other particulates. Nature emits many of these in quantities great enough to affect air quality and global climate.

Carbon dioxide (CO2) occurs naturally in Earth's atmosphere. Only in the late twentieth and early twenty-first centuries did it come to be regarded as a pollutant, when anthropogenic (human-generated) CO2 was suspected to have a role in global climate change. Along with water vapor, CO2 is one of the atmosphere's chief absorbers of infrared radiation and is considered a greenhouse gas (GHG)--that is, a gas that keeps solar energy from reradiating into space. The presence of greenhouse gases in Earth's atmosphere allowed the planet to develop a climate conducive to life. However, in the later decades of the twentieth century concerns began to arise that a buildup of greenhouse gases from fossil-fuel use and other human activity could significantly and irreversibly raise temperatures around the globe.

Ozone (O3) is a highly reactive form of oxygen well known for its role in protecting Earth's surface from damaging ultraviolet (UV) radiation. In the troposphere (lower portion of the atmosphere), however, O3 acts as a GHG, contributing to increased surface temperatures. At ground level, O3 is the main component of smog. Sources of O3 in the troposphere include O3 that migrated down from the overlying stratosphere and O3 produced photochemically from nitrogen oxides (NOx). Higher up, within the stratosphere, is where O3 performs its UV-absorbing function. StratosphericO3 also absorbs visible solar radiation that would otherwise warm the Earth's surface. A decrease in stratospheric O3 or an increase in tropospheric O3 results in a rise in surface temperatures.

The hydrocarbon compound methane, CH4, is classed as a hazardous substance, primarily due to its combustibility. This GHG occurs in the atmosphere at lower concentrations than CO2, but according to the Intergovernmental Panel on Climate Change (IPCC) its global warming potential over a one-hundred-year period is twenty-five times higher than that of CO2. Because of CH4's strong global warming potential, combined with its comparatively short lifetime in the atmosphere (roughly twelve years), curbing CH4 emissions has the potential to mitigate global warming over the next few decades.

Nitrous oxide, N2O, by contrast, has an atmospheric lifetime of about 120 years and a 100-year global warming potential 298 times that of CO2. Other nitrogen oxide compounds (NOx), while unlikely to contribute directly to climate change, react with volatile organic compounds (VOCs) in the presence of heat and sunlight to form tropospheric O3. Atmospheric NOx, which can travel long distances from its source, also causes acid precipitation and is a major component of smog. Sulfur dioxide, SO2, another cause of smog and acid precipitation, absorbs infrared radiation. However, its chief climate-altering ability is not as a GHG but as a stratospheric aerosol. Clouds of SO2 aerosol absorb the Sun's energy and cause a resulting drop in tropospheric temperatures.

VOCs, also found in smog, are carbon-containing compounds that readily become gas or vapor. While they do not directly influence climate, they are important O3 precursors, especially at the ground level. By enhancing tropospheric O3 concentrations, they promote global warming.

Carbon monoxide, CO, is a toxic air pollutant. A weak absorber of infrared radiation, it has little direct impact on global climate. However, it contributes to climate change through chemical reactions that boost concentrations of CH4 and O3 in the troposphere. CO ultimately oxidizes to CO2.

Particulate matter includes tiny solid and liquid particles such as dust, salt, smoke, soot, ash, and droplets of sulfates and nitrates. Injected into the atmosphere by anthropogenic or natural processes, these form aerosols, suspensions of particles in air. The length of time these particulates remain in the atmosphere is related to the altitude at which they were introduced and their particle size. Aerosols influence climate directly by reflecting and absorbing atmospheric solar and infrared radiation. While some aerosols cause surface-temperature increases and others cause decreases, the overall effect of aerosols is to lower temperatures. Aerosols influence climate indirectly by serving as condensation nuclei for cloud formation or altering optical properties and lifetimes of clouds.

Volcanic eruptions, the chief source of natural air pollutants, have a demonstrated and complex impact on climate. Gaseous and particulate emissions cause O3 depletion as well as global atmospheric warming and cooling.

Volcanic eruptions damage O3 by injecting SO2 into the stratosphere, where the gas is converted to a sulfate aerosol. The aerosol particles interact with chlorine and bromine in anthropogenic chlorofluorocarbons(once widely used as aerosol can propellants and refrigerants) to produce compounds that break down O3 molecules. While volcanoes also produce the O3-degrading compound hydrochloric acid (HCl), it remains largely confined to the troposphere, where it can be washed out by rains.

Volcanism is a significant source of CO2. According to the United States Geological Survey, subaerial and submarine volcanoes emit an annual  total of 130 to 230 million metric tons of CO2. By comparison, the Carbon Dioxide Information Analysis Center estimates the total global CO2 emissions from fossil fuel burning in 2007 to have been 8.47 billion metric tons.

Despite their GHG emissions, volcanic eruptions produce a net cooling effect on surface temperatures. This is due in part to dust and ash that remain suspended in the atmosphere after an eruption. These particulates lower mean global temperatures by blocking sunlight, thereby reducing the amount of solar radiation that can reach the planet's surface. The dominant cooling effect, however, results from SO2 blasted into the stratosphere. The gas combines with water vapor to form droplets of sulfuric acid. This aerosol can remain suspended in the atmosphere, where the droplets absorb solar radiation and cause a decrease in tropospheric temperatures. Global cooling associated with volcanic activity diminishes after a few years as the particles settle out of the atmosphere.

Major volcanic activity in past centuries--notably the 1783 Laki eruption in Iceland, the 1815  eruption of Mount Tambora in Indonesia, and the 1883 eruption of Tambora's neighbor Krakatoa (Krakatau)--have been followed by several months of abnormally cold weather around the globe. More recently, the eruptions of Mount Pinatubo in the Philippines and Mount Hudson in Chile, both during 1991, caused a decrease in mean world temperatures of approximately 1° Celsius over the next two years.

As wildfires burn, they release carbon stored in vegetation to the atmosphere in the form of CO2, CO, and CH4. Researchers Christine Wiedinmyer and Jason Neff estimate that average annual CO2 emissions from wildfires during the years 2002 through 2006 were 213 (±50 standard deviation) million metric tons for the contiguous United States and 80 (±89 standard deviation) million metric tons for Alaska. This contribution of greenhouse gases--the equivalent of 4 to 6 percent of North America's anthropogenic emissions during that period--has the potential to exacerbate global warming, which can in turn create a hotter, drier environment conducive to larger, more devastating wildfires.

Wildfire combustion products include the GHGs CO2, CH4, and N2O, as well as CO, nitric oxide, and VOCs, all of which promote global warming by enhancing tropospheric O3. Methyl bromide produced during wildfires can also contribute to global warming by destroying stratospheric O3.

Wildfires generate large volumes of particulates in the form of smoke, ash, and soot. Clouds of fire-related particulates both absorb and block sunlight, so that their tropospheric effects are both warming and cooling. Particle color influences whether energy is absorbed (dark particles) to produce warming or reflected (light particles) to cause cooling. If soot settles out of the atmosphere onto snow or ice, its dark particles reduce the reflectivity of the frozen surface while enhancing sunlight absorption. Heating, and accelerated melting of the snow or ice, result.

Oceans and oceanic processes generate a number of natural air pollutants. According to the United Nations Environment Programme (UNEP), oceans produce about 90 billion metric tons of CO2 annually, emissions that are offset by the estimated 92 billion metric tons that oceans absorb. Oceanic phytoplankton releases dimethyl sulfide, which forms SO2 as an oxidation product. Oceans also contribute CO, CH4, N2O, and NOx to the atmosphere. Ocean spray sends sea-salt particles aloft, where they decompose in the presence of sunlight to release chlorine molecules that can interact with anthropogenic air pollutants to produce tropospheric O3.

On land, according to UNEP, vegetation emits 540 billion metric tons of CO2 but takes in 610 billion metric tons. Another GHG, CH4, is emitted from a host of terrestrial sources: digestive processes in wild animals and termites; decomposition of wild animal wastes; lakes and wetlands; tundra; and natural oil and gas seeps. Warmer global temperatures could lead to an increase in CH4 emissions from regions where there is now permafrost.

Other naturally occurring air pollutants include CO from vegetation and the oxidation of naturally occurring hydrocarbons; N2O and NOx produced during bacterial denitrification of soil; NOx created within thunderstorms by lightning; hydrogen sulfide (H2S) generated during decay underground or under water; SO2 produced through the oxidation of H2S; VOCs emitted from coniferous and eucalyptus forests and other vegetation; ammonia released from wild animal wastes; radon gas produced as radium in rock and soil undergoes radioactive decay; and particulate matter in the form of ultrafine soil, dust, pollen, and spores.

Understanding how naturally occurring substances and processes influence climate is a vital part of comprehending the roles that human activity and anthropogenic materials play. The boundaries between the natural and the anthropogenic are often indistinct. Naturally occurring gases and particulates interact with human-made ones to produce O3 in the troposphere or damage it in the stratosphere. Deforested areas are left vulnerable to wind erosion that carries soil particulates into the atmosphere. Lightning causes massive wildfires, but so do arsonists and human carelessness.

What is clear is that natural pollutants are not the major concern. Although  anthropogenicCO2 is dwarfed by the natural carbon cycle and there have been episodes where natural emissions have been huge and have had long-lasting effects (such as the possible massive volcanic sulfur emission at the end of the Permian epoch and the possible methane release in the Eocene), human effects can be more important because (1) humans can change the atmosphere on a very rapid time scale compared to natural effects, and (2) humans introduce substances that have a potent effect on the atmosphere and are not normally found in nature, such as chlorofluorocarbons (CFCs). Hence, anthropogenic substances can surpass naturally occurring ones in their current and long-term impacts on the balance between incoming solar radiation and outgoing infrared radiation within Earth's atmosphere.

Bibliography:

1)         Intergovernmental Panel on Climate Change. Climate Change, 2007--The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.

2)         Knipping, E. M., and D. Dabdub. "Impact of Chlorine Emissions from Sea-Salt Aerosol on Coastal Urban Ozone." Environmental Science and Technology 37, no. 2 (2003): 275-284.

3)         Robock, Alan. "Volcanic Eruptions and Climate." Reviews of Geophysics 38, no. 2 (May, 2000): 191-219.

4)         Wiedinmyer, Christine, and Jason C. Neff. "Estimates of CO2 from Fires in the United States: Implications for Carbon Management." Carbon Balance and Management 2, no. 10 (November 1, 2007).

5)         Wuebbles, Donald J., and Jae Edmonds. Primer on Greenhouse Gases. Chelsea, Mich.: Lewis, 1991.

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