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.
 

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.
 

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.
 

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.”

 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)