Appendix D
Technologies for Measuring Emissions by Large Local Sources

New technologies that have great potential for measuring the local increase in greenhouse gases around large point sources include the same multispecies gas analyzers that are suitable for commercial aircraft applications and spectrometers similar to the Total Carbon Column Observing Network (TCCON). Concurrent measurements of wind speed, wind direction, humidity, solar radiation, and boundary-layer height would allow adjustments for seasonal and interannual variability in atmospheric mixing. When “simple,” low-cost trace gas sensors or remote sensing tools are used, one has to be careful that air density and water vapor variations are not misinterpreted as changes of the trace gas dry air mole fraction, which is the only property containing information about recent sources. Mobile as well as fixed trace gas measurement approaches are likely to be important in characterizing urban to remote gradients and may be particularly useful for identifying point sources of methane (CH4), nitrous oxide (N2O), and halocompounds. For non-carbon dioxide (CO2) greenhouse gases, an urban network may provide new first-order information about different source terms—again allowing for more effective policy that targets specific industries or practices.

Airborne lidar (light detection and ranging) techniques offer the potential for monitoring greenhouse gas emissions downwind of sources. An aircraft-deployed differential absorption lidar (DIAL), flown within or just above the boundary layer across an emission plume downwind of the source, can measure the dimensions and column content of gases within and outside of the plume. Combining the in-plume concentration measurements with estimates of the wind, obtained via models or measured directly from the aircraft, enables a calculation of the emission rate of gases exported from the source. The technique has been demonstrated for estimation of ozone fluxes in plumes downwind of urban areas and oil refineries (e.g., Senff et al., 2006) using observations from a downward-looking lidar flown on a low-flying National Oceanic and Atmospheric Administration (NOAA) Twin Otter and wind extrapolated from wind profiler measurements.

DIAL techniques have been shown to be potentially feasible for measuring both profiles and column content of important greenhouse gases. Ehret et al. (2008) estimated errors in measurements of CO2, CH4, and N2O columns from satellite-borne DIAL instruments for global monitoring. They examined DIAL systems operating in several absorbing infrared spectral regions and concluded that such measurements were indeed feasible, with systematic errors estimated to be less than 0.4 percent for CO2, 0.6 percent for CH4, and 0.3 percent for N2O. Because DIAL observations from low-flying aircraft are significantly easier than satellite measurements due to much shorter distance to surface and the lack of interference from high- and mid-level clouds, the aircraft measurement problem is potentially feasible with much smaller instruments than those assumed for the satellite study. Note that lidar measures concentration, not mole fraction. The measurements rely on the dry air density being the same inside and outside of a plume and on very well characterized surface elevations. These measurements are thus local



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Appendix D Technologies for Measuring Emissions by Large Local Sources N ew technologies that have great potential for the plume. Combining the in-plume concentration measuring the local increase in greenhouse measurements with estimates of the wind, obtained via gases around large point sources include the models or measured directly from the aircraft, enables a same multispecies gas analyzers that are suitable for calculation of the emission rate of gases exported from commercial aircraft applications and spectrometers the source. The technique has been demonstrated for similar to the Total Carbon Column Observing Net- estimation of ozone fluxes in plumes downwind of work (TCCON). Concurrent measurements of wind urban areas and oil refineries (e.g., Senff et al., 2006) speed, wind direction, humidity, solar radiation, and using observations from a downward-looking lidar boundary-layer height would allow adjustments for flown on a low-flying National Oceanic and Atmo- seasonal and interannual variability in atmospheric spheric Administration (NOAA) Twin Otter and wind mixing. When “simple,” low-cost trace gas sensors or extrapolated from wind profiler measurements. remote sensing tools are used, one has to be careful that DIAL techniques have been shown to be poten- air density and water vapor variations are not misinter- tially feasible for measuring both profiles and column preted as changes of the trace gas dry air mole fraction, content of important greenhouse gases. Ehret et al. which is the only property containing information (2008) estimated errors in measurements of CO2, CH4, about recent sources. Mobile as well as fixed trace gas and N2O columns from satellite-borne DIAL instru- measurement approaches are likely to be important ments for global monitoring. They examined DIAL in characterizing urban to remote gradients and may systems operating in several absorbing infrared spectral be particularly useful for identifying point sources of regions and concluded that such measurements were methane (CH4), nitrous oxide (N2O), and halocom- indeed feasible, with systematic errors estimated to be pounds. For non-carbon dioxide (CO2) greenhouse less than 0.4 percent for CO2, 0.6 percent for CH4, and gases, an urban network may provide new first-order 0.3 percent for N2O. Because DIAL observations from i nformation about different source terms—again low-flying aircraft are significantly easier than satellite allowing for more effective policy that targets specific measurements due to much shorter distance to surface industries or practices. and the lack of interference from high- and mid-level Airborne lidar (light detection and ranging) tech- clouds, the aircraft measurement problem is potentially niques offer the potential for monitoring greenhouse gas feasible with much smaller instruments than those emissions downwind of sources. An aircraft-deployed assumed for the satellite study. Note that lidar measures differential absorption lidar (DIAL), flown within or concentration, not mole fraction. The measurements just above the boundary layer across an emission plume rely on the dry air density being the same inside and downwind of the source, can measure the dimensions outside of a plume and on very well characterized and column content of gases within and outside of surface elevations. These measurements are thus local 0

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0 APPENDIX D the lidar wind measurement is about 10 cm s–1. These and cannot readily be compared with mole fraction measurements at other places and times. combined techniques could thus be applied to obtain Research efforts are under way, primarily within measurements of greenhouse gas emission without the the National Aeronautics and Space Administration need for application of models. (NASA), to demonstrate airborne DIAL measure- ments of CO2 as a step toward eventual space-based REFERENCES measurements. Browell et al. (2008) and Abshire et Abshire, J.B., H. Riris, G.R. Allan, C. Weaver, J. Mao, and W. al. (2009) have both reported on airborne measure- Hasselbrack, 2009, Airborne lidar measurements of atmospheric ments of CO2 optical thickness at 1.57 µm. To avoid CO2 column absorption and line shapes from 3-11 km altitudes, the problem of having to convert a column absorption Geophysical Research Abstracts, 11, EGU2009-11507. measurement of a trace gas to a mole fraction, one can Browell, E.V., M.F. Dobbs, J. Dobler, S. Kooi, Y. Choi, F.W. Harrison, B. Moore III, and T.S. Zaccheo, 2008, Airborne deploy a second lidar that simultaneously measures demonstration of 1.57-micron laser absorption spectrometer the column amount of oxygen (O2), which is a very for atmospheric CO2 measurements, in P roceedings of the th accurate indicator of the total amount of dry air in the International Laser Radar Conference, Boulder, Colo., January 12-15, pp. 697. column. The challenges in developing high-precision Ehret, G., C. Kiemle, M. Wirth, A. Amediek, A. Fix, and S. Hou- DIAL systems for column content measurements weling, 2008, Space-borne remote sensing of CO2, CH4, and are associated primarily with instrumentation issues N2O by integrated path differential absorption lidar: A sensitiv- such as maintaining and monitoring long-term laser ity analysis, Applied Physics B, 90, 593-608. Hardesty, R.M., W.A. Brewer, C.J. Senff, B.J. McCarty, G. Ehret, stability. However, given the resources currently being A. Fix, C. Kiemle, and E.I. Tollerud, 2008, Structure of me- directed toward the problem in both the United States ridional moisture transport over the U.S. southern Great Plains and Europe, there is a high likelihood that such chal- observed by co-deployed airborne wind and water vapor lidars, lenges will be met. in Symposium on Recent Developments in Atmospheric Applications of Radar and Lidar, American Meteorological Society, January Hardesty et al. (2008) described co-deployment of 21-24. a Doppler wind lidar and water vapor DIAL instru- Senff, C.J., R.J. Alvarez II, R.M. Hardesty, R.M. Banta, L.S. Darby, ment on a research aircraft to measure horizontal A.M. Weickmann, S.P. Sandberg, D.C. Law, R.D. Marchbanks, W.A. Brewer, D.A. Merritt, and J.L. Machol, 2008, Airborne transport of moisture over the southern Great Plains. lidar measurements of ozone flux and production downwind of In this way, airborne lidar techniques can be used as Houston and Dallas, in Proceedings of the th International Laser improvements over application of models for observing Radar Conference, Boulder, Colo., June 23-27, pp. 659-662. wind characteristics within the plume. The accuracy of