Global Warming

▪ Natural Air Pollution and Pollutants

Essay, Custom Research Paper: Research Paper on Natural Air Pollution and Pollutants

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