Ozone in the lower atmosphere (troposphere) is toxic to human beings and to many other living things that breathe it. Before scientists began to track the global travels of ozone in the troposphere with satellite data and measurements made from aircraft, they assumed that much of that part of the atmosphere was relatively free of ozone. But after combining satellite observations with data-rich models that simulate the atmospheres chemistry and dynamics, they are finding tropospheric ozone in some unexpected places. Tropospheric ozone turns out to be an intercontinental traveler, crossing geographic and political boundaries. Where ozone forms and where it travels have become key concerns for international health and economic policy-making. On the stage of global change, ozone plays the role of both hero and villain.
Ozone in the stratosphere (upper atmosphere) protects us from harmful
ultraviolet radiation from the sun, but ozone plays a different role in the
troposphere, where we live, because it is toxic to living things. Ozone in the upper troposphere is also a greenhouse gas, meaning that its presence contributes somewhat to global warming. On the other hand, tropospheric ozone plays a role that is key to enhancing human health and well being, since it is involved in chemical reactions that cleanse the troposphere of some pollutants. Therefore considerable research is now underway to understand the conditions under which ozone forms and how ozone travels from its source.
|To learn more about good vs. bad ozone, read: Ozone in the Stratosphere|
Understanding the chemical and physical dynamics of ozone and other trace
gases is becoming increasingly urgent as world population rises and economic
activity increases among developing nations. Increased combustion of fossil
fuels, which produces chemicals that contribute to ozone formation ("precursors") accompanies that
economic activity. Asian economic development is proceeding particularly
rapidly, and most Asian governments do not strictly regulate emissions from
fossil fuel combustion. The lifetime of ozones precursors in the
troposphere is sufficiently long that they can produce ozone hundreds or even
thousands of miles away before further chemical reactions transform ozone into
oxygen and other chemicals. Of course, Asia is not the only problem area. Air
currents move pollution from all developed and developing nations to other parts
of the world. Governments need to adopt a global perspective when designing a
strategy to meet regional air quality objectives for limiting ozone.
Air currents bring ozone in the lower atmosphere (troposphere) from North America to Europe in this animation of observations made from July 1 through July 31, 1999. Values range from zero to about eighty Dobson Units, with high concentrations of ozone appearing in yellow and brown. Ozones intercontinental pathways cross political as well as geographic boundaries. (Data from EarthProbe/TOMS and IGARRS; Animation by Robert Simmon)
Since 1978 the Total Ozone Mapping Spectrometer (TOMS) instruments have been measuring global ozone levels from a variety of satellites. Beginning in 2003 the instruments aboard the Aura satellite will continue the long term monitoring of ozone.
A global perspective is one of the great gifts of satellites. Analysis of
global-scale data from sensors such as NASAs TOMS (Total Ozone Mapping
Spectrometer) and those aboard the Aura satellite to be launched in 2004 will allow scientists to study ozone chemistry as air masses move across continents and oceans.
|The last week of 2001 saw the worst levels of air pollution in central Israel for the year, as seen at this beach in Tel Aviv. Pollution transported from Europe mixes with local pollution in the populated area of central Israel. Burning fossil fuels in industries and automobiles results in higher aerosol concentrations (often visible, as in this image) and in higher ozone concentrations in the atmosphere (invisible) in many industrialized and populated regions of the world. (Photograph courtesy Yoram Kaufman)|
Scientists combine satellite data with data from observations made at the surface, from ozonesondes (meteorological balloons that carry ozone sensors), from aircraft and from models (simulations of the atmosphere on computers), to study ozone on a range of spatial scales and to ensure that satellite measurements are accurate. Atmospheric scientists began to recognize the extent of ozones travels relatively recently. Anne Thompson, an atmospheric chemist at NASA s Goddard Space Flight Center, explains, What were trying to do is to parse out what ozone comes from natural causes and what ozone comes from human activity. (Natural causes include lightning, production of methane from decomposition of organic materials, and some emissions from plantsisoprene and terpenes.) Its extremely hard to separate natural and manmade sources of gases. The precursors of ozone such as nitrogen oxides are not labeled I came from an aircraft engine, I came from the stratosphere, I came from the ground, or I came from lightning. You have to measure other related chemicals that fingerprint the source.
In the upper photograph, Anne Thompson and Agnes Phahlane prepare for a balloon launch during the SAFARI-2000 campaign in Zambia. The balloon carried both an ozonesonde to measure ozone and a radiosonde to measure temperature, pressure, and relative humidity–conditions that can affect ozone concentration and distribution. In the bottom photograph, a woodland burns in Zambia. Biomass burning of woodlands and croplands produces nitrogen and carbon compounds that are involved in ozone production. (Photograph of balloon launch preparation courtesy of Jacquelyn Witte, Photograph of fire copyright Peter G. H. Frost)
|NASAs Aura spacecraft (to be launched in 2004) will
provide us with the first truly global view of tropospheric ozone quantity,
distribution, and mixing with other gases in the troposphere. Well be
able to track ozone both regionally within continents, and from one continent to
another, explains Reinhold Beer, principal investigator for the
Emission Spectrometer (TES), one of four instruments on Aura.
Understanding air quality on a global scale will continue to be important for the foreseeable future. For the cooperation required to control air pollution, we need an international agreement, explains Brasseur. But before we can move to such an agreement, we need to understand the problem scientifically. Satellites are key to our research.
|In this animated model of the intercontinental travels of air pollution, each mass of color represents a different source of carbon monoxide (CO), at a concentration of 100 parts of CO per billion parts of air (ppb). Both the combustion of fossil fuels and the burning of forests and fields release CO, and contribute to ozone formation over time. In the animation, one concentration of CO represented in purple originates in western and eastern Europe and travels across northern Asia and the Arctic. Another represented in bright pink originates in southern and southeastern Asia and travels eastward across the Pacific Ocean. A third CO concentration in orange, probably the result of burning forests and fields for agriculture, travels west from Africa to South America. CO in gray from North America moves across the Atlantic Ocean, and some of it reaches Europe. The animation represents approximately three months of data (January through March, 2000), with four frames per day. Scientists obtained these results with the chemistry transport model MOZART 2. (Animation provided by B. Khattatov and J-F. Lamarque, NCAR. The MOZART model has been developed by scientists from NCAR and the Max Planck Institute.)|