ESRL Quarterly Newsletter - Summer 2009

Powering Science

Coal-fired power plants help ESRL scientists and colleagues verify satellite pollution measurements

Summer 2005 NO2 column measurements by satellite, with greens, blues, and reds indicating increasing concentration. Boxes highlight plumes from power plants (P) and cities (C).

In a strange twist, polluting power plants in the West are actually helping advance air quality research. A new study by ESRL scientists and colleagues used the large pollution plumes from rural Western power plants to understand satellite measurements of nitrogen dioxide (NO2) pollution over urban areas including Los Angeles, Las Vegas, Denver, and Phoenix. There’s good news in the data: urban NO2 emissions appear to be much lower than estimated using the latest vetted conventional, bottom-up approach.

By fine-tuning space-based measurements with such an approach, scientists believe satellites could offer a new view on emissions to improve air quality policy-making. NO2 and other nitrogen oxide gases are emitted by industry and motor vehicles and are key ingredients in ground-level ozone—smog—a lung-damaging pollutant.

“Because there are very few other large sources of NO2 in areas like the Four Corners, pollution plumes from coal-fired power plants in this region are easily visible from space,” said atmospheric chemist Greg Frost, a researcher in ESRL’s Chemical Sciences Division and the University of Colorado’s Cooperative Institute for Research in Environmental Sciences (CIRES). Frost is a coauthor of the new study, published in the Journal of Geophysical Research—Atmospheres. “These plumes provide a unique reference point against which we can test the accuracy of satellite measurements of NO2, because power plants measure the emissions coming from their smokestacks,” Frost said.

In urban areas, emissions from cars and trucks are often the dominant source of NO2, which complicates pollution monitoring, said Frost. While power plants are required to keep close tabs on smokestack emissions, regulators have no reliable way to track all tailpipe emissions. Instead, most urban emission inventories are modeled off estimates of miles traveled per vehicle type, and checked with spot roadside measurements, said Frost. Such bottom-up estimates are updated only every few years. “Satellites are one of the few tools available to us on a daily basis that can help us evaluate emissions of dangerous air pollutants from the top down,” said study lead author Si-Wan Kim, with ESRL’s Chemical Sciences Division and CIRES, who has also used satellites to study changes in power plant emissions in the eastern United States.

Other studies have investigated large-scale changes in global air pollution patterns with satellites, but the new study is the first to test whether satellites can accurately measure urban emissions by comparison floor with power plant emissions. The authors hope their work will inform air quality policy making at a local scale.

Using the NO2 emissions records from Western power plants, Kim and Frost evaluated three satellite datasets that captured NO2 pollution plumes from space. They also ran a weather and air quality model incorporating emission data from a regional inventory and power plant records.

The satellite data and models agreed on NO2 emissions from power plants, the study reported. However, Kim and Frost found large discrepancies in NO2 measured over cities, with satellites consistently documenting less NO2 than the inventory-based models predicted.

The satellite measurements also show that NO2 over cities has declined in recent years. Ongoing work should help the researchers come up with a more quantitative understanding of the causes of the model-measurement differences and satellite changes.

—Contributed by Adriana Bailey, CIRES

Read the Paper

Kim S. -W. et al, 2009, NO2 columns in the western United States observed from space and simulated by a regional chemistry model and their implications for NOx emissions, J. Geophys. Res., 114, D11301, doi:10.1029/2008JD011343.