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