US: Boundary-layer concentrations of carbon monoxide (CO), a vital pollutant, can now be measured from space by exploiting the multispectral capabilities of MOPITT (Measurements of Pollution in the Troposphere), according to an article published in SPIE. The article, Space-based measurements of boundary-layer carbon monoxide, is authored by Merritt Deeter, Helen Worden, David Edwards and John Gille.
Authors claim that a wide array of atmospheric parameters is now routinely measured from polar-orbiting satellites using remote sensing methods. These techniques yield long-term global datasets which are unavailable using ground-based or aircraft-based instruments. For monitoring air quality, satellite remote sensing provides a means for studying both the sources of pollution and its movement in the atmosphere.
Major sources of CO, such as fossil fuel combustion and large-scale wildfires, are mainly located at the earth’s surface. However, existing satellite methods for measuring CO typically exhibit poor sensitivity in the planetary boundary layer (i.e., the lowest 1–2km in the atmosphere). Most satellite products for CO are based solely on thermal-infrared (TIR) measurements, exploiting a series of vibrational-rotational absorption lines near 4.7μm. Such measurements are sensitive not only to the CO vertical distribution, but also the atmospheric temperature profile and surface temperature.
When the surface ‘skin temperature’ and temperatures in the boundary layer are similar, CO molecules in the boundary layer produce a relatively weak effect on the measured radiances. TIR measurements are thus best suited to measuring CO concentrations in the tropospheric layer between about 2 and 12km. An important exception to the prevalence of TIR-based satellite CO products is the European SCIAMACHY instrument, which retrieves CO using near-infrared (NIR) measurements at about 2.3μm. NIR radiances, which sense absorption by CO in reflected solar radiation, yield a measurement of the CO total column, i.e., the total number of CO molecules (per unit area) from the surface to the top of the atmosphere.
Using an experimental version of the MOPITT (Measurements of Pollution in the Troposphere) retrieval algorithm along with actual observations, authors demonstrated the value of merging MOPITT NIR and TIR measurements for determining CO concentrations in the lowermost troposphere. Their studies demonstrate that the combined effects of (1) MOPITT’s moving field of view and (2) sub-pixel variability of the earth-surface reflectance result in a highly variable source of error in the calibrated radiances. New ‘Version 5’ MOPITT data processing algorithms have been adapted to quantify this noise term for each observation and fully account for its effect on the retrieval uncertainties.
Satellite-based measurements of CO improved their understanding of pollution sources and atmospheric transport. Such measurements are also being used to produce ‘chemical weather’ forecasts ). Compared to existing products, authors concluded that new MOPITT retrievals based on simultaneous NIR and TIR measurements will provide atmospheric scientists a much clearer view of CO in the boundary layer, allowing more detailed analyses of CO sources. These new products may also improve estimates of other pollutants which are not readily observed from space but which are related to CO through emission ratios.
The NCAR MOPITT project is supported by the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) Program. The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation.