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Remote Sensing

Space-based measurements of boundary-layer carbon monoxide

Boundary-layer concentrations of carbon monoxide, an important pollutant, can now be measured from space by exploiting the multispectral capabilities of MOPITT (Measurements of Pollution in the Troposphere).
20 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201011.003431

A wide array of atmospheric parameters are 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. Carbon monoxide (CO) is a particularly important component of pollution, with both natural and human-caused sources. CO is not only toxic itself (at high concentrations), but is also a precursor to tropospheric ozone, another major pollutant and greenhouse gas. The atmospheric lifetime of CO is between several weeks and several months which is ideal for studies of pollution transport. (Chemical species with short lifetimes may only be observed close to their respective sources; species with long lifetimes, like CO2, become well mixed 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 (which is typical), 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.1 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.

Figure 1. Mean surface-level concentrations of carbon monoxide (volume mixing ratio) based on MOPITT near-infrared and thermal-infrared measurements during the months September, October, and November and years 2005–2008.

The MOPITT (Measurements of Pollution in the Troposphere, http://www.acd.ucar.edu/mopitt) instrument, which was launched on NASA's Earth Observing System (EOS) Terra platform on Dec. 19, 1999, is uniquely equipped with both NIR and TIR detection systems.2 MOPITT employs gas correlation radiometry, whereby the radiation from earth is spectrally modulated by internal gas-filled cells before being fed to a phase-sensitive detection system. MOPITT's NIR and TIR gas correlation radiometers yield calibrated radiances which are primarily weighted by narrow spectral regions containing the CO absorption lines. These CO-sensitive radiances are fed to a retrieval algorithm which exploits a fast radiative transfer model to determine the most probable CO vertical profile consistent with both the measurements and background ‘a priori’ information.3 The retrieval algorithm also yields uncertainties for each retrieved profile, which largely depend on the estimated uncertainties in the calibrated radiances.

Using an experimental version of the MOPITT retrieval algorithm along with actual observations, we recently demonstrated the value of merging MOPITT NIR and TIR measurements for determining CO concentrations in the lowermost troposphere.4 See Figure 1. Industrialized regions in China surrounding major urban centers are clearly indicated by high boundary-layer CO concentrations. In comparison, TIR-only products do not permit such a detailed view of CO sources. However, complex ‘geophysical noise’ processes affecting the NIR radiances have, so far, prevented their use in operational MOPITT retrieval products. Recent 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 improve our understanding of pollution sources and atmospheric transport. Such measurements are also being used to produce ‘chemical weather’ forecasts5 (http://www.acd.ucar.edu/acresp/forecast). Compared to existing products, 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 new MOPITT products are currently being validated through comparisons with a large set of ‘in-situ’ aircraft measurements. Soon, they will be produced operationally and freely distributed through NASA's data archives (http://wist.echo.nasa.gov). The multispectral CO retrieval concept pioneered by MOPITT may also be exploited in future air-quality satellite missions such as the NASA GEO-Cape (Geostationary Coastal and Air Pollution Events, http://geo-cape.larc.nasa.gov) mission, which is currently scheduled for launch later this decade.

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.

Merritt Deeter, Helen Worden, David Edwards, John Gille
National Center for Atmospheric Research
Boulder, CO

Merritt Deeter obtained the Ph.D. in Optical Sciences from the University of Arizona in 1988. He currently serves as the MOPITT Project Leader within the Atmospheric Chemistry Division at the National Center for Atmospheric Research.

Helen Worden received the Ph.D. degree in particle physics from Cornell University, Ithaca NY, in 1991. She is a science team member on the MOPITT and TES (Tropospheric Emission Spectrometer) experiments and works in the Atmospheric Chemistry Division, NCAR, in Boulder, CO.

David Edwards is an NCAR Senior Scientist, a Co-Investigator on the MOPITT mission, and leads the Program in Atmospheric Composition Remote Sensing and Prediction (ACRESP) at NCAR. He is also a member of the Science Working Group of the GEO-CAPE Decadal Survey mission.

John Gille is the U.S. Principal Investigator for MOPITT. He has been involved with numerous satellite experiments to measure atmospheric composition, most recently MOPITT and HIRDLS. Activities include instrument and algorithm development, and use of data for scientific studies.

1. M. Buchwitz, I. Khlystova, H. Bovensmann, J. P. Burrows, Three years of global carbon monoxide from SCIAMACHY: Comparison with MOPITT and first results related to the detection of enhanced CO over cities, Atmos. Chem. Phys. 7, pp. 2399-2411, 2007. doi:10.5194/acp-7-2399-2007
2. J. R. Drummond, J. Zou, F. Nichitiu, J. Kar, R. Deschambaut, J. Hackett, A review of 9-year performance and operation of the MOPITT instrument, Adv. Space Res. 45, pp. 760-774, 2010. doi:10.1016/j.asr.2009.11.019
3. M. N. Deeter, D. P. Edwards, J. C. Gille, L. K. Emmons, G. Francis, S.-P. Ho, D. Mao, D. Masters, H. M. Worden, James R. Drummond, Paul C. Novelli, The MOPITT version 4 CO product: Algorithm enhancements, validation, and long-term stability, J. Geophys. Res. 115, 2010. doi:10.1029/2009JD013005
4. H. M. Worden, M. N. Deeter, D. P. Edwards, J. C. Gille, J. R. Drummond, P. Nedelec, Observations of near-surface carbon monoxide from space using MOPITT multispectral retrievals, J. Geophys. Res. 115, 2010. doi:10.1029/2010JD014242
5. A. F. Arellano, Jr., K. Raeder, J. L. Anderson, P. G. Hess, L. K. Emmons, D. P. Edwards, G. G. Pfister, T. L. Campos, G. W. Sachse, Evaluating model performance of an ensemble-based chemical data assimilation system during INTEX-B field mission, Atmos. Chem. Phys. 7, pp. 5695-5710, 2007. doi:10.5194/acp-7-5695-2007