Satellite Data Help Assess Ground-level Ozone Effect on Crops

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SAN FRANCISCO — While scientists know that high concentrations of ozone near the ground can cause significant harm to important crops, that harm has been difficult to measure on a regional, national or global scale, due to a lack of ground-based sensors to monitor this type of pollution. In the United States, for example, the U.S. Environmental Protection Agency (EPA) monitoring stations are concentrated in populated areas rather than in farming regions.

To pave the way for more widespread studies of ground-level ozone, officials from NASA and the U.S. Agriculture Research Service conducted a pilot project that relied on five years of data drawn from satellite sensors, ground-based sensors and crop yield reports to evaluate the impact of ground-level ozone on soybean crops in Indiana, Iowa and Illinois. That study provided a clear indication that once ozone levels near the ground reach a certain threshold, of about 50 parts per billion, soybean yields decrease by approximately 10 percent, said Elizabeth Ainsworth, a molecular biologist at the ARS Global Change and Photosynthesis Research Unit in Urbana, Ill.

That decrease can have a significant economic impact. One study estimated that high concentrations of ground-level ozone around the world may cost $14 billion to $26 billion annually in reduced rice, soybeans, corn and wheat yields, Ainsworth said. “It’s an expensive pollutant that probably causes more damage than any pest or pathogen,” she added.

Ozone pollution enters plants through the small pores in the leaves, damaging those cells and interfering with their ability to conduct photosynthesis. Ozone also can accelerate the development of plants, causing them to mature more quickly.

By comparing data drawn from EPA sensors on the ground with satellite data, scientists were able to show that space-based instruments are a valuable tool in measuring ozone concentrations near the ground, Jack Fishman, a professor at St. Louis University, said.

Fishman, who previously worked at NASA’s Langley Research Center in Hampton, Va., has spent decades developing and validating space-based techniques to measure ozone in the lower atmosphere. His work is designed not only to shed light on ozone’s impact on agriculture but also to provide insight into the pollutant’s harmful effects on human health.

Ground-level ozone is produced when sunlight reacts with hydrocarbons in the air and nitrogen oxides, which are emitted by automobile and industrial plants.

Better techniques to monitor and publicize high levels of ozone near the ground could help to avert some of its harmful health effects because ozone is most dangerous when its concentration rises above certain thresholds, Fishman said. When ground-level ozone concentrations are predicted to spike, community leaders could take steps to reduce that danger by limiting automobile use or shutting down local power plants, Fishman said.

While the impact of ground-level ozone on certain crops has been widely reported from laboratory studies and field experiments, it has been difficult to measure on a global basis due to the uneven concentration of ground-based sensors and the difficulty in mapping ozone levels at various portions of the atmosphere from space.

Fishman devised a technique that uses NASA’s Total Ozone Mapping Spectrometer, which was launched on NASA’s Nimbus-7 satellite in 1978, and the Ozone Monitoring Instrument, which has been flying on NASA’s Earth observing Aura satellite since 2005. Those instruments provide data on the total concentration of ozone in a column of air.

He then derived information on ozone in the troposphere, the part of the atmosphere closest to the ground, with the help of data drawn from the Solar Backscatter Ultraviolet (SBUV) instrument onboard a National Oceanic and Atmospheric Administration satellite operating in polar orbit. Fishman used SBUV to determine stratospheric ozone levels and subtracted those levels from the total column ozone to derive information on ground-level ozone.

The NASA-Department of Agriculture study provides the first indication that this technique provides an accurate indication of ground-level ozone because the results were comparable with readings made by ground-based sensors, Fishman said. While this technique offers a useful tool in making global calculations of tropospheric ozone, more detailed and accurate measurements are likely to come from future space-based sensors.

Fishman is particularly eager to gain access to data from geostationary satellites, which can offer information on ozone levels throughout the day in contrast to the polar-orbiting spacecraft that provide only a single daily measurement. Data currently being gathered can help scientists evaluate decreases in crop yields that occur during a season, but a geostationary satellite could measure ozone levels several times a day or even hourly, which would enable scientists to alert the public when pollution reaches dangerous levels, he said.

The Geostationary Coastal and Air Pollution Events mission, which the National Research Council recommended for funding in its 2007 report on Earth science projects, would gather data on ozone and aerosols, the spread of pollution and coastal ecosystems. That proposed mission would measure ozone concentrations in five- to 10-kilometer areas and offer scientists insight into how ozone-concentration levels change from hour to hour, Fishman said.