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)
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