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The Habitable Planet: A Systems Approach to Environmental Science 

Atmospheric Pollution

Many forms of atmospheric pollution affect human health and the environment at levels from local to global. These contaminants are emitted from diverse sources, and some of them react together to form new compounds in the air. Industrialized nations have made important progress toward controlling some pollutants in recent decades, but air quality is much worse in many developing countries, and global circulation patterns can transport some types of pollution rapidly around the world. In this unit, discover the basic chemistry of atmospheric pollution and learn which human activities have the greatest impacts on air quality.


acid rain
Rainfall with a greater acidity than normal.

Liquid or solid particles that are suspended in air or a gas. Also referred to as particulate matter.

Surrounding, encircling.

carbon monoxide
Odorless, colorless gas that interferes with the delivery of oxygen in the blood to the rest of the body. It is produced as a result of incomplete burning of carbon-containing fuels including coal, wood, charcoal, natural gas, and fuel oil. Depending on the amount inhaled, this gas can impede coordination, worsen cardiovascular conditions, and produce fatigue, headache, weakness, confusion, disorientation, nausea, and dizziness. Very high levels can cause death.

Any of several organic compounds composed of carbon, fluorine, chlorine, and hydrogen. They were formerly used widely in industry, for example as refrigerants, propellants, and cleaning solvents.

hydroxyl radical
The neutral form of the hydroxide ion, often referred to as the “detergent” of the troposphere because it reacts with many pollutants, often acting as the first step to their removal.

Intergovernmental Panel on Climate Change (IPCC)
Established in 1988 by two United Nations organizations to assess the risk of human-induced climate change.

Montreal Protocol on Substances That Deplete the Ozone Layer
A 1987 international agreement, subsequently amended in 1990, 1992, 1995, and 1997, that establishes in participating countries a schedule for the phaseout of chloroflourocarbons and other substances with an excessive ozone-depleting potential.

National Ambient Air Quality Standards
Standards established by the EPA and required by The Clean Air Act (last amended in 1990) for pollutants considered harmful to public health and the environment.

nitrogen oxides
A group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2), along with particles in the air, can often be seen as a reddish-brown layer over many urban areas.

nonattainment areas
Defined by The Clean Air Act as a locality where air pollution levels persistently exceed National Ambient Air Quality Standards, or that contributes to ambient air quality in a nearby area that fails to meet standards.

A triatomic molecule consisting of three oxygen atoms. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. On the other hand, ozone in the upper atmosphere protects living organisms by preventing damaging ultraviolet light from reaching the Earth’s surface.

particulate matter (PM)
The sum of all solid and liquid particles suspended in air, many of which are hazardous.

A chemical process by which molecules are broken down into smaller units through the absorption of light.

primary air pollutants
Pollutants that are pumped into our atmosphere and directly pollute the air. Examples include carbon monoxide from car exhausts and sulfur dioxide from the combustion of coal as well as nitrogen oxides, hydrocarbons, and particulate matter (both solid and liquid).

Atomic or molecular species with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions.

secondary air pollutants
Pollutant not directly emitted but forms when other pollutants (primary pollutants) react in the atmosphere. Examples include ozone, formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight; NO2, formed as NO combines with oxygen in the air; and acid rain, formed when sulfur dioxide or nitrogen oxides react with water.

A kind of air pollution; the word “smog” is a combination of smoke and fog. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulphur dioxide.

sulfur dioxide
A colorless, extremely irritating gas or liquid (SO2), used in many industrial processes, especially the manufacture of sulfuric acid. In the atmosphere it can combine with water vapor to form sulfuric acid, a major component of acid rain.

volatile organic compounds
Organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere.

Beyond the Habitable Planet: Monitoring Air Pollution from Space

by Kelly Chance

Monitoring Air Pollution from Space

The European Space Agency launched ERS-2 in April 1995 to observe Earth and its environment. It carried the GOME (Global Ozone Monitoring Experiment). It is just one of several Earth monitoring satellites.

Scientists around the world are using remote sensing instruments to monitor changing patterns of the major categories of air pollution.


Measuring Tropospheric Ozone

This is a movie of 5½ years of tropospheric column ozone derived from GOME spectral measurements.
Source: Liu, X., K. Chance CfA

GOME (Global Ozone Monitoring Experiment) is a downward scanning ultraviolet and visible spectrometer on the European Space Agency ER-2 satellite. Light reaching the atmosphere from the Sun is scattered back toward the spectrometer, which measures the strength of the characteristic signature of ozone in the wavelengths of light reaching the detector. The ozone profile/tropospheric ozone algorithm separates the low level ozone concentrations (“bad ozone”) from the stratospheric ozone (“good ozone”). Low level, or tropospheric, ozone damages lung tissue and can pose a serious threat to human health, contributing to “bad air” days when people are advised to stay indoors. Stratospheric ozone, on the other hand, benefits life by shielding us from harmful UV radiation. The amount of ozone varies over time and place, but the main sources of dangerous ground-level ozone are vehicles, industry, and the burning of forests.

Ozone Data from GOME

This orbit (from October 22, 1997) is ozone profile information derived from ozone spectra. Data displayed here is one orbit, with location on the horizontal axis and altitude on the vertical axis. The red line is the tropopause, the boundary between the troposphere and the stratosphere.
Source: K. Chance, Liu, X., CfA

The GOME satellite is in a polar orbit, so over several orbits it passes above the entire planet. This graph shows data collected in one orbit. The Antarctic ozone hole has been forming every October, and is revealed in this plot as a thinning of the green-colored ozone layer at the far left of the scan. The scan also shows high levels of tropospheric ozone pollution from the Indonesian fires that occurred at this time.

Ozone Crossing the Oceans


Global tropospheric maps show streams of tropospheric ozone crossing the oceans. The monthly mean maps of tropospheric ozone show pollution streaming from the U.S., Europe, and China to the west in summer and pollution from biomass burning in the equatorial zone.
Source: NASA

The AURA (Latin for breeze) satellite was launched July 15, 2004. Aura is part of NASA’s Earth Observing System, a program dedicated to using satellites and data systems to monitor the complex interactions that affect the globe. A key tool is the Ozone Monitoring Instrument, or OMI. OMI observations tell how much ozone is over a particular area, and how much the ozone the area is gaining or losing over time.

Formaldehyde in the Atmosphere

A global map of formaldehyde, a proxy for VOCs, the precursors of ground-level ozone.
Source: K. Chance, CfA

Formaldehyde (chemical formula: H2CO or HCHO) is one of the primary measurable proxies for volatile organic compounds, or VOCs, which are the precursors to tropospheric ozone. Tropospheric measurements derived from GOME have been used to improve VOC emission inventories. OMI measurements, which have much higher spatial resolution, show promise for substantially improving the knowledge of VOC emission sources. This image shows tropospheric HCHO measurements from GOME for summertime (July 1996). Over the U.S., isoprene from trees is a major source of HCHO, while for Southeast Asia, sources include vegetation, biomass burning, and human activity (agriculture, fossil-fuel use).

Isoprene from Trees and Vegetation

Seasonal and interannual variability of North American isoprene emissions as determined by formaldehyde column emissions from GOME.
Source: K. Chance, CfA

Isoprene is a common hydrocarbon formed by plants and animals. Plants give off this compound naturally when exposed to light. Isoprene contributes to atmospheric ozone and its concentrations are related to the concentrations of formaldehyde, which can be measured by GOME. This shows how isoprene emissions vary over the year, as derived from GOME spectra. Seasonal and interannual variability of North American isoprene emissions are linked to the exposure of large areas of vegetation in North America to strong sunlight.

Sources of Volatile Organic Compounds


Non-methane volatile organic compounds (NMVOC) abundances can be measured using GOME. This graph compares measurements from space (GOME) and from the Earth (“Bottom-Up”).
Source: K. Chance, CfA

Volatile Organic Compounds (VOCs) can come from many sources: internal combustion engines (“anthropogenic” sources); biomass burning to clear fields and forests for farming; and directly from plants and animals (“biogenic” sources).


Kelly Chance

Is an astrophysicist in the Atomic and Molecular Physics Division at the Harvard-Smithsonian Center for Astrophysics. His research interests include molecular spectroscopy, structure and dynamics and their application to atmospheric studies, including laboratory spectroscopy and balloon-, aircraft-, and satellite-borne measurements of the earth’s atmosphere; atmospheric composition and radiative transfer; and chemical astrophysics.


Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
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  • ISBN: 1-57680-883-1