A team of scientists have used the Ozone Monitoring
Instrument (OMI) on NASA’s Aura satellite to confirm major reductions in the
levels of a key air pollutant generated by coal power plants in the eastern United States.
The pollutant, sulfur dioxide, contributes to the formation of acid rain and
can cause serious health problems.
The scientists, led by an Environment Canada researcher,
have shown that sulfur dioxide levels in the vicinity of major coal power
plants have fallen by nearly half since 2005. The new findings, the first
satellite observations of this type, confirm ground-based measurements of
declining sulfur dioxide levels and demonstrate that scientists can potentially
measure levels of harmful emissions throughout the world, even in places where
ground monitoring is not extensive or does not exist. About two-thirds of
sulfur dioxide pollution in American air comes from coal power plants. Geophysical
Research Letters published details of the new research.
The scientists attribute the decline in sulfur dioxide to
the Clean Air Interstate Rule, a rule passed by the U.S. Environmental
Protection Agency in 2005 that called for deep cuts in sulfur dioxide
emissions. In response to that rule, many power plants in the United States
have installed desulfurization devices and taken other steps that limit the
release of sulfur dioxide. The rule put a cap on emissions, but left it up to
power companies to determine how to reduce emissions and allowed companies to
trade pollution credits.
While scientists have used the Ozone Monitoring Instrument
to observe sulfur dioxide levels within large plumes of volcanic ash and over
heavily polluted parts of China in the past, this is the first time they have
observed such subtle details over the United States, a region of the world that
in comparison to fast-growing parts of Asia now has relatively modest sulfur
dioxide emissions. Just a few decades ago, sulfur dioxide pollution was quite
severe in the United States.
Levels of the pollutant have dropped by about 75% since the 1980s due largely
to the passage of the Clean Air Act.
Vitali
Fioletov, a scientist based in Toronto
at Environment Canada, and his colleagues developed a new mathematical approach
that made the improved measurements a reality. The approach centers on
averaging measurements within a 30 miles radius (50 km) of a sulfur dioxide
source over several years. “Vitali has developed an extremely powerful
technique that makes it possible to detect emissions even when levels of sulfur
dioxide are about four times lower than what we could detect previously,”
says Nickolay Krotkov, a researcher based at NASA’s Goddard Space Flight Center
in Greenbelt, Md., and a coauthor of the new paper.
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The
technique allowed Fioletov and his colleagues to pinpoint the sulfur dioxide
signals from the 40 largest sulfur dioxide sources in the United States—generally
coal power plants that emit more than 70 kilotons of sulfur dioxide per year.
The scientists observed major declines in sulfur dioxide emissions from power
plants in Alabama, Georgia,
Indiana, Kentucky,
North Carolina, Ohio,
Pennsylvania, and West Virginia by comparing levels of the
pollutant for an average of the period 2005 to 2007 with another average from
2008 to 2010.
“What
we’re seeing in these satellite observations represents a major environmental
accomplishment,” says Bryan Bloomer, an Environmental Protection Agency
scientist familiar with the new satellite observations. “This is a huge
success story for the EPA and the Clean Air Interstate Rule,” he says.
The
researchers focused their analysis on the United States to take advantage of
the presence of a robust network of ground-based instruments that monitor
sulfur dioxide emissions inside power plant smokestacks. The ground-based
instruments have logged a 46 percent decline in sulfur dioxide levels since
2005—a finding consistent with the 40% reduction observed by OMI.
“Now
that we’ve confirmed that the technique works, the next step is to use it for
other parts of the world that don’t have ground-based sensors,” says
Krotkov. “The real beauty of using satellites is that we can apply the
same technique to the entire globe in a consistent way.” In addition, the
team plans to use a similar technique to monitor other important pollutants
that coal power plants release, such as nitrogen dioxide, a precursor to ozone.
OMI, a Dutch
and Finnish built instrument, was launched in 2004, as one of four instruments
on the NASA Aura satellite, and can measure sulfur dioxide more accurately than
any satellite instrument flown to date. Though OMI remains in very good condition
and scientists expect it to continue producing high-quality data for many
years, the researchers also hope to use data from an upcoming Dutch-built OMI
follow-on instrument called TROPOMI that is expected to launch on a European
Space Agency satellite in 2014.
On July 6,
2011, the U.S. Environmental Protection Agency (EPA) finalized the Cross-State
Air Pollution Rule (CSAPR), requiring 27 states to significantly reduce power
plant emissions that contribute to ozone and fine particle pollution in other
states. This rule replaces EPA’s 2005 Clean Air Interstate Rule (CAIR). A
December 2008 court decision kept the requirements of CAIR in place temporarily
but directed EPA to issue a new rule to implement Clean Air Act requirements
concerning the transport of air pollution across state boundaries. This action
responds to the court’s concerns.