Highways of A Global Traveler



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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 atmosphere’s 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

Map of Tropospheric Ozone

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 ozone’s 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. Ozone’s 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.


Photograph of Smog Overhanging Tel Aviv Beach

A global perspective is one of the great gifts of satellites. Analysis of global-scale data from sensors such as NASA’s 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)

Photograph of Anne Thompson Holding Weather Balloon

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 ozone’s travels relatively recently. Anne Thompson, an atmospheric chemist at NASA ’s Goddard Space Flight Center, explains, “What we’re 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 plants—isoprene and terpenes.) “It’s 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.”

next The Dynamic and Puzzling Atmosphere

The data used in this study are available in one or more of NASA's Earth Science Data Centers.


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)


The Dynamic and Puzzling Atmosphere

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It is often difficult for scientists to make sense of their observations. “I’ve developed a healthy respect for how variable the atmosphere is,” notes Thompson. “Weather systems in the troposphere are constantly moving and mixing the air, and at the same time, chemical reactions are changing the air’s chemical composition.”

  Photograph of Low-Pressure Weather System from the Space Shuttle

In studies of the atmosphere between Western Europe and Eastern North America in 1997, Thompson and her colleagues wanted to learn how to predict ozone concentration and distribution. Using a variety of satellite and forecast tools, they would make predictions, and then they would carry out observations to check their predictions. “We would combine all the data we could get from various sources. We would get real-time weather data from many sources, including the NASA Data Assimilation Office. We would get real-time satellite data on ozone concentrations from the Total Ozone Mapping Spectrometer (TOMS) and the Optical Transient Detector (OTD) Lightning Sensor. Then we would combine all of them to make our best forecast of where the ozone, dust, biomass burning, and lightning would make their impact, and so where we should direct the NASA research aircraft to make observations for us. We would forecast and fly, analyze our observations, and then forecast and fly again to extend our understanding.” (Lightning produces ozone.)

Major patterns of air circulation have enormous impact on ozone and its precursors. “In our work over the South Atlantic Ocean, we would look at the satellite data and see a big blob of ozone over the ocean,” continues Thompson. We would measure the ozone from an airplane observation, run a model, and explain what we saw in terms of the chemistry. But why was the ozone there and not somewhere else? The reason was that its presence is due to major air circulation patterns.” There’s an anticyclone in the South Atlantic Ocean that brings pollution from both African and South American fires over the Atlantic. Other African air masses travel in the opposite direction—even to Australia and the Pacific Ocean.

Other major air circulation patterns appear to carry ozone from one continent to another. Although a complete, detailed, global picture of how natural and human activities on one continent influence the air quality over other continents and oceans requires more research, some trends are becoming clearer. According to modeling studies at Harvard University, background concentrations (amounts that are usually there) of ozone in surface air over the United States range from 25 to 55 parts of ozone per billion parts of air (ppb) and can be largely attributed to transport from outside the United States. This amount of ozone is significant for a country where the national air quality standard is 80 ppb over 8 hours, not to be exceeded more than three times per year. The same Harvard study had implications particularly for the western part of North America, which receives more pollution from Asia than the eastern part does. In fact, if people in North America succeeded in reducing their emissions of nitrogen oxides and hydrocarbons (ozone precursors) by 25 percent, the expected tripling of Asian emissions by 2010 could more than offset that North American effort.

Because air currents move both east and west over the Atlantic Ocean, Europe and North America influence each others’ air quality. One study based on models in combination with data from ozonesondes (ozone-measuring instruments often flown on balloons) found that ozone produced in the lower troposphere over North America contributes an average of about 5 ppb to surface ozone at the study location in Ireland, but sometimes as much as 10-15 ppb. Other intercontinental and inter-regional air movements carry pollution from one part of the Earth to another. For example, sometimes circulation in the troposphere can funnel northern mid-latitude pollution (including pollution from eastern Asia) over the Middle East. Pollution from Indonesian fires during 1997 reached India. Air pollution from China sometimes drifts directly over Japan. The point is that some countries (such as Japan) cannot have clean air until other countries (such as China) do.

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  The ever-changing and complex nature of the lower atmosphere (troposphere) makes it difficult to trace the chemical reactions that produce ozone. (Shuttle Photograph STS108-723-081 courtesy NASA Johnson Space Center)


The Key Role of Modeling

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An integral part of studying the complex atmosphere is using computer models where the chemistry and dynamics are simulated by mathematical equations. “You can look at a cloud and say, ‘How can I describe what it’s doing quantitatively?’ I can’t! The system is far too complicated. But should I give up? No! I can simplify the system so that it’s tractable mathematically,” says Daniel Jacob, at Harvard University. “Then I can run the model and compare the results with what I see nature doing.” In places where scientists see that the model and nature aren’t the same, they can work to understand the chemistry better.

“When we see differences between our models and data from actual observations in the field or from satellites, we’re not happy,” explains Guy Brasseur, a modeling expert at the Max Planck Institute for Meteorology and the National Center for Atmospheric Research. “But we often learn about a process that’s new to us that way.”

“Models are a means of testing our understanding, so that we can make predictions,” continues Brasseur. “Now our understanding of the atmosphere is developed in a partnership between observations, work in the lab, and modeling.”

  Comaprison of Two Nitrous Oxide Models

Satellite observations are producing large amounts of data and are changing the way atmospheric chemistry is done. “I think this is a fantastic time for young people to enter the field of atmospheric chemistry because our field is undergoing a revolution,” adds Jacob. “My community is having to think about satellite observations. Interpreting satellite observations is a very difficult task. Satellite sensors observe the Earth through most of the atmosphere, and the data we want are in small quantities buried in the “noise” of other data. But we need to understand what satellite observations are saying to us, because we need that global scale [perspective].”

A global systems approach is prominent in Brasseur’s mind. “More and more, we’ve learned that we cannot look at the atmosphere in isolation from the rest of the Earth,” he says. “We must see it in the context of its interactions with the oceans, the biosphere, and human activities—with the whole Earth as a system. We must be developing global, comprehensive, integrated Earth system science models.”

Daniel Jacob explains, “Now we have global three-dimensional models. We’re not yet ready to put together the biosphere and the atmosphere and economics, but there are people who are dreaming about this.”

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Scientists increasingly use satellite observations to evaluate model simulations of tropospheric ozone. The GEOS-CHEM mathematical model of atmospheric chemistry successfully predicts concentrations of nitrogen dioxide, a compound involved in ozone’s formation. In a comparison of actual observations for July 1996 by the European Space Agency’s Global Ozone Monitoring Experiment (GOME, above) with the GOES-CHEM model (below), geographic regions with high concentrations of nitrogen dioxide appear in yellows, oranges, and reds. (Images Courtesy Randall V. Martin, Kelly V. Chance, Daniel J. Jacob)


Next Steps in Tracking Ozone and Its Precursors

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“The troposphere is not yet that well monitored on a global scale,” explains James Drummond of the University of Toronto. Drummond monitors carbon monoxide, an ozone precursor, with the Measurements Of Pollution In The Troposphere (MOPITT) sensor flying aboard NASA’s Terra satellite. There’s still a great deal to do,” he admits, “and lots of collaborating and hard work ahead of us.”




Model of Carbon Monoxide in the Atmosphere

  “NASA’s 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. We’ll be able to track ozone both regionally within continents, and from one continent to another,” explains Reinhold Beer, principal investigator for the Tropospheric 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.”

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  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.)