Detecting Air Polution from Space

Moderate Resolution Imaging Spectroradiometer

The wildfires that ravaged southern California in 2003 not only scarred the landscape but also dumped pollutants into the air. These fires provide an example of how satellite data can reveal the impact of intense local sources of air pollution on air quality on a regional or even global scale. This true-color image was taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's EOS Aqua satellite and clearly shows the smoke plumes of ten raging fires. MODIS data can assist in monitoring the transport of aerosolized pollutants. (SeaSpace Corp.)

Detecting Air Polution from Space

Leslie Belsma

The use of satellite data for air-quality applications has been hindered by a historical lack of collaboration between air-quality and satellite scientists. Aerospace is well positioned to help bridge the gap between these two communities.

Satellite data have traditionally been underexploited by the air-quality community. The Environmental Protection Agency (EPA), together with state and regional air-quality agencies, relies instead on an extensive ground-based network to monitor and predict urban air quality. Recent and planned technological advancements in remote sensing are demonstrating that space-based measurements can be a valuable tool for forecasting air quality, providing information not available from traditional monitoring stations. Satellite data can aid in the detection, tracking, and understanding of pollutant transport by providing observations over large spatial domains and at varying altitudes. Satellites can be the only data source in rural and remote areas where no ground-based measurements are taken. Satellite data can be used qualitatively to provide a regional view of pollutants and to help assess the impact of events such as biomass burning or dust transport from remote sources. Space-based data can also be used quantitatively to initialize and validate air-quality models.

The Aerospace Corporation has a long history of support to meteorological satellite programs such as the Defense Meteorological Satellite System (DMSP), the Polar-orbiting Operational Environment Satellites (POES), and the Geostationary Operational Environment Satellites (GOES). More recently, this support has extended to environmental satellite systems such as NASA's Earth Observing System (EOS) and the future National Polar-orbiting Operational Environmental Satellite System (NPOESS), which will merge DMSP and POES weather satellites into an integrated environmental observation system. These systems could play a prominent role in forecasting and improving air quality over urban centers.

Federal Regulations

The Clean Air Act gives EPA the authority to regulate emissions that cause air pollution. Accordingly, the agency sets national ambient air-quality standards (NAAQS) for six air pollutants: carbon monoxide, nitrogen dioxide, ozone, lead, sulfur dioxide, and particulate matter under 10 microns. The EPA has also set standards recently for fine particulates under 2.5 microns (see sidebar, The Air We Breathe).

All states must comply with the NAAQS. Those that do not can be denied federal funding for highways and other projects. The EPA requires that each state have a plan to implement federal smog-reduction laws. States therefore need to model the weather as well as the transport, dispersion, and chemical and physical transformation of pollutants to determine the impact of emission sources and set regulatory policy. The EPA provides guidelines for regulatory modeling, but while these models are quite sophisticated, they make little use of ground or space-based measurements to improve forecast accuracy.

The MOPITT sensor (Measurement Of Pollution In The Troposphere) aboard NASA's EOS Terra satellite is designed specifically to measure carbon monoxide profiles and total column methane (a hydrocarbon). Carbon monoxide, produced as a result of incomplete combustion during burning processes, is a good indicator of atmospheric pollution. This false-color MOPITT image shows the atmospheric column of carbon monoxide resulting from the southern California wildfires of 2003. Yellow and red indicate high levels of pollution (gray areas show where no data were taken, probably because of cloud cover). Pollutants can be seen spreading over the western states and into the Pacific Ocean. Carbon Monoxide Column (x 1018 mol/cm2) (NCAR/University of Toronto MOPITT)

Many air-quality agencies issue continuous operational air-quality forecasts, which are based on ground-based measurements and predicted weather conditions. Meteorological satellite data are used in the generation of weather forecasts, but no space-based pollution data are used in predicting air quality. Recently, NOAA and EPA entered an agreement to provide national forecasts of ozone and fine particulates. Under the terms of this agreement, EPA will model emission sources and NOAA will run the national weather-forecast and air-quality models continuously. This represents a new era in air-quality modeling in which satellite data will become essential for establishing the background and boundary conditions necessary to forecast air quality operationally on a national scale (see sidebar, Extreme Living).

Space-Based Data Sources

The combination of measurements from current and planned environmental satellite sensors that monitor the troposphere will play an increasingly important role in explaining pollutant chemistry and transport processes in the lower atmosphere. Satellite-based measurements of ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and aerosols have been compared with EPA ground-based data to demonstrate the potential benefit of satellite data in tracking emissions and their transport.

A European project has demonstrated the use of Landsat and SPOT satellite data to provide a relative quantitative scale of urban air pollution—specifically, fine particulates and sulfur dioxide. The third NASA EOS satellite, Aura, is designed to study Earth's ozone, air quality, and climate; it houses one sensor designed specifically to measure trace gases in the troposphere.

The Geoscience Laser Altimeter System aboard NASA's ICESat satellite measures backscattered light to determine the vertical structure of clouds, pollution, and smoke plumes in the atmosphere. The observation here, taken October 28, 2003, shows the thick smoke plumes emanating from the California wildfires. The image represents a vertical slice of Earth's atmosphere along the satellite path, as shown by the green line superimposed on a MODIS image (insert) taken 7 hours earlier. The zigzag features are the smoke plumes from the fires rising up as high as 5 kilometers. The thin features toward the upper right are high-level cirrus clouds. The large black feature jutting up above sea level is the mountain range separating Santa Barbara from the San Joaquin Valley. Note the low-lying pollution over San Joaquin. (Steve Palm, ICESat/NASA Goddard Space Flight Center)

Numerous satellite sensors can detect at least some type of aerosols—including smoke plumes from fires—and can thus provide a basis for deriving emissions estimates. Soil moisture can be detected from space and is an essential piece of information for estimating how much dust (a type of particulate matter) is contributing to atmospheric haze. Satellite imagery has traditionally been used to characterize land cover to estimate biogenic emissions, and this imagery is now being used more directly to derive biogenic emissions through an inverse analysis retrieval technique.

Several satellite missions designed to detect stratospheric ozone can also provide information on tropospheric ozone levels. There is much potential benefit in combining these initial efforts by scientists to monitor air quality from space with the remote-sensing retrieval and calibration technologies developed at Aerospace to support defense satellite programs.

NPOESS

NPOESS marks a new era in advanced environmental monitoring from space. Though air-quality agencies did not play a role in defining requirements for the baseline system, many of the eleven baseline sensors aboard NPOESS will provide data directly applicable to monitoring air pollutants. For example, the Visible Infrared Imaging Radiometer Suite will provide accurate aerosol detection. In addition, the NPOESS mission will include an instrument dedicated to aerosol detection, the Aerosol Polarimeter Sensor, which is scheduled to fly for the first time aboard a NASA mission in 2007. Thermodynamic soundings of high spatial resolution will be provided by the Crosstrack Infrared Sounder; these soundings can contribute to the detection of trace-gas concentrations. The Ozone Mapping and Profiler Suite consists of a nadir system for both total column ozone and profile ozone observations, as well as a limb system for profile ozone observations at high vertical resolution.

The "aerosol optical depth" is one of the many products from the MODIS sensor aboard the EOS Terra and Aqua satellites. These data can dramatically improve the manual detection of pollution by the air-quality forecaster. This space-based view enables researchers to monitor air pollution events over extended periods and geographical areas. A relationship between MODIS aerosol optical depth and ground-based hourly fine particulate (down to 2.5 microns) allows the MODIS data to be used qualitatively and quantitatively to estimate EPA air-quality categories. (SeaSpace Corp.)

In support of the NPOESS program, Aerospace leads the technical support for the acquisition of many of these sensors and the algorithms to retrieve various environmental parameters. The Crosstrack Infrared Sounder, Visible Infrared Imaging Radiometer Suite, and Ozone Mapping and Profiler Suite sensors will fly for the first time in 2006 on the NPOESS Preparatory Project satellite, an NPOESS risk-reduction mission that will also provide a bridge between NASA's EOS Aqua and Terra and the first NPOESS satellite in 2009. The higher resolution and more timely data from NPOESS will enable more accurate short-term air-quality forecasts and warnings.

Conclusion

Just as new satellite data have helped advance the science of weather prediction, so can they assist the science of air-quality forecasting. The amount of satellite data available is going to increase substantially in the coming years, including information about pollutant concentrations not well measured previously. Aerospace is working to help air-quality agencies fully exploit the wealth of current and planned space-based environmental data to improve air-quality forecasting.