AIRBORNE DEMONSTRATION OF 1.57-MICRON LASER ABSORPTION SPECTROMETER FOR ATMOSPHERIC CO2 MEASUREMENTS Edward V. Browell1, Michael E. Dobbs2, Jeremy Dobler2, Susan Kooi1, Yonghoon Choi1, F. Wallace Harrison1, Berrien Moore III3, T. Scott Zaccheo4 1
NASA Langley Research Center, Hampton, Virginia 23681, USA,
[email protected] 2 ITT Corporation, Fort Wayne, Indiana 46801, USA,
[email protected] 3 University of New Hampshire, Durham, New Hampshire 03824, USA,
[email protected] 4 Atmospheric and Environmental Research, Inc., Lexington, Massachusetts 02421, USA,
[email protected] ABSTRACT A unique, multi-frequency, single-beam, laser absorption spectrometer (LAS) that operates at 1.57 µm has been developed for a future space-based mission to determine the global distribution of sources and sinks of atmospheric carbon dioxide (CO2). A prototype of the space-based LAS system was developed by ITT, and it has been successfully flight tested in five airborne campaigns conducted in different geographic regions over the last three years. Flight tests were conducted over Oklahoma, Michigan, New Hampshire, and Virginia under a wide range of atmospheric conditions. Remote LAS measurements were compared to high-quality in situ measurements obtained from instrumentation on the same aircraft on spirals under the ground track of the LAS. LAS flights were conducted over a wide range of land and water reflectances and in the presence of scattered clouds. An extensive data set of CO2 measurements has been obtained for evaluating the LAS performance. LAS CO2 measurements with a signal-to-noise in excess of 250 were obtained for a 1-s average over land. Absolute comparisons of CO2 remote and in situ measurements showed agreement over a range of altitudes to better than 2 percent. Details of flight test campaigns and results are discussed in this paper. 1. INTRODUCTION The U.S. National Research Council's report entitled Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond identified the need for a near-term space mission of Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) [1]. The primary objectives of the ASCENDS mission are to: 1) make CO2 column measurements across the troposphere during the day and night over all latitudes and all seasons and in the presence of scattered clouds; 2) use these active CO2 measurements to greatly reduce uncertainties in global estimates of CO2 sources and sinks; and 3) provide increased understanding of the connection between climate and CO2 exchange. The results from this mission will be used to: 1) improve climate models and
predictions of atmospheric CO2; 2) identify humangenerated CO2 sources and sinks and enable effective carbon trading; and 3) close carbon budget for improved forecasting and policy decisions [1]. The ASCENDS mission requires an active sensor that has the needed CO2 measurement precision from space to determine the variability of CO2 in the troposphere and thereby determine CO2 sources and sinks at the surface and CO2 long-term variations and trends. The critical component in the ASCENDS mission is the laser absorption spectrometer (LAS) which is operated in the 1.57-µm region for CO2 column measurements in the mid to lower troposphere. The following sections describe characteristics of the 1.57-µm LAS system; discuss the various field campaigns that have been conducted; present examples of the LAS CO2 measurements; and summarize results of the flight tests. 2.
1.57-µm LAS SYSTEM
The 1.57-µm LAS Engineering Development Unit (EDU) was developed by ITT [2], and it uses CW fiber laser technology from the communications industry to produce several simultaneous laser wavelengths across a CO2 absorption line to provide total column measurements of CO2 at line center and weighted column measurements in the lower troposphere on the side of the line. The LAS sensor for CO2 is currently configured to transmit up to three laser lines; however, the architecture is expandable to transmit more lines depending on the desired spectroscopic configuration. From a high-altitude platform, the LAS can use three laser lines; one at the center of the R24 CO2 absorption line, one offset by 10 pm and one offset by 50 pm. The rationale for three laser lines is indicated in Figure 1. The off-line is considered to be at the 50-pm offset position, and the curves in Figure 1 represent the altitude-dependent weighting functions for the different on-line laser wavelengths assuming a nominal CO2 mixing ratio of 375 ppm. These weighting functions are normalized to a value of 1 at their maximum values for relative comparison purposes. The on-line operation at line center will have a broad weighting
developed and demonstrated, and the automatic data collection system was successfully operated. The automatic in-flight transmitter-receiver alignment algorithm was also successfully demonstrated. From the flight data, the radiometric system performance was shown to match predictions. Pictures of the LAS installation on the Lear-25 at the Newport News/ Williamsburg airport in Virginia are shown in Figure 2.
Figure 1. Normalized, differential CO2 optical depth weighting functions for on-line wavelength positions from top to bottom: line center at 1571.11 nm and offsets from line center of 5, 10, 15, 20, and 25 pm, respectively. across the lower stratosphere to the surface while operation at the 10 pm offset position will be mostly weighted towards the lower troposphere which will be more sensitive to recent sources and sinks of CO2. From a low altitude platform, there is little advantage of using the side-line measurement, and as a result, the primary CO2 column measurement used in these flight tests is the line-center position. The CO2 LAS detects the scattered laser light from the Earth's surface or cloud tops, and the differential optical depths are calculated from the ratios of the on-line return signals at line-center and at +10-pm offset (sideline) to the off-line return signal at +50-pm offset. Assuming the laser energy reference signals, which monitor the outgoing laser energy, have signal to noise ratios at least ten times larger than the return signals, the CO2 column measurement precision (dn/n) is related to the signal to noise (SNR) in the return signal ratios and the one-way differential optical depth (τ) by: dn/n~(-1/(2τ*SNR)). Using optical depths for the line position for the R24 line yields a required SNR of ~174 at τ ~1.8 for the line-center ratio and ~822 at τ ~0.41 for the side-line ratio. These SNR's need to be achieved from space with an integration time of 10 s which corresponds to a horizontal distance of about 70 km. 3.
FLIGHT DEMONSTRATIONS
3.1 Engineering Flight Demonstration In December 2004, an engineering flight demonstration of the LAS EDU was undertaken in a Lear-25 aircraft. The LAS was successfully integrated with the aircraft avionics, power and mechanical subsystems, and sustained operation using aircraft power and cabin air handling systems was achieved. Flight procedures were
Figure 2. Photos from initial LAS engineering flights at DOE ARM site in May 2005. 3.2 Validation Experiment Requirements In order to validate the remote LAS CO2 column measurements, the following requirements need to be met. High-precision in situ CO2 profile measurements need to be made under the LAS for direct comparisons to the remote measurements. Nearby, contemporaneous radiosonde profile measurements of temperature (T), pressure (P), and relative humidity (RH) are needed to account for atmospheric affects on CO2 spectroscopy. Simultaneous laser altimeter measurements are needed from the LAS aircraft to determine the range to the scattering surface for the LAS. This allows for the binning of LAS CO2 data from cloud tops and surface returns. Accurate aircraft GPS information on geographic position and altitude and accurate aircraft attitude information are also needed. All of these requirements were met during the flight validation campaigns discussed here. The flight validation campaigns were designed to permit validation tests under a number of measurement conditions. LAS flight legs of about 100 km in length were flown repeatedly at the same altitude and at several different altitudes to determine the consistency of the remote CO2 measurements and their correlation with altitude. When possible the in situ CO2 profiles were obtained from as low an altitude as possible to the LAS operational altitude on spirals at the beginning and end of the flight with the entire flight lasting about 2.5 hours. Flights were conducted both during the day and
at night, over a wide range of surface conditions (land and water), and in clear and scattered-cloud conditions. In addition, a horizontal laser range was established at the airport so that horizontal calibration tests of the LAS could be conducted between flights as necessary.
conclusion that the LAS instrument is performing as expected and that the model is accurately representing all the radiometric and noise characteristics of the LAS. Some results from the May and October 2007 flight campaigns will be presented in the next section.
3.3 Flight Campaigns - General Five separate LAS flight campaigns have been conducted over the last three years. Table 1 presents the dates, locations, and some of the flight-test information.
3.4 2007 Flight Campaigns Extensive flights were conducted during May and October 2007 over eastern Virginia and eastern North Carolina and over the Chesapeake Bay and Atlantic Ocean. The flight tracks used during these campaigns are shown in Figure 3. The base of operations was located at the Newport News/Williamsburg Airport, and the 2.5-hr flight duration of the Lear-25 allowed for two complete spirals at the center of a leg and five complete passes at different altitudes along the length of the leg. On each flight the in situ CO2 profile(s) and radiosonde profiles of T, P, and RH were used to generate the modeled CO2 measurements, which were compared with the LAS CO2 measurements.
Table 1. LAS Flight Campaigns May 21-25, 2005 Ponca City, Oklahoma (DOE ARM Site) (5 Science Flights: Land, Day & Night, )
June 20-26, 2006 Alpena, Michigan (6 Science Flights: Land & Water, Day & Night)
October 20-24, 2006 Portsmouth, New Hampshire (4 Science Flights: Land (inc. mountains) & Water, Day & Night)
May 20-24, 2007 Newport News, Virginia (8 Science Flights: Land & Water, Day & Night)
October 17-22, 2007 Newport News, Virginia (9 Science Flights: Land & Water, Day & Night, Clear & Cloudy)
In May 2005, the LaRC-ITT-UNH Team flew the CO2 LAS and a high-precision in-situ CO2 instrument in the first science test flights over the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Central Facility (CF) near Ponca City, Oklahoma. During these initial demonstration flights, more than 10 hours of useable flight data were obtained, and the comparisons between the remote and in situ CO2 measurements were very encouraging. In the summer of 2006, we conducted a weeklong campaign operating out of Alpena, Michigan where we were able to overfly mixed vegetation, urban, and extensive water tracks. In the fall of 2006, we conducted a similar duration campaign based in Portsmouth, New Hampshire, and flights were flown over mountainous and snow-covered terrain as well as over the ocean. The signal levels predicted by a performance model of the LAS instrument compared very favorably with the measured signal levels under a wide range of conditions, including horizontally variable terrain and clouds and during day and night. The signal-to-noise (SNR) predicted by the instrument model also compared very well with the measured SNR under a wide range of conditions. These results support the
Figure 3. Flight tracks used during May and October 2007 field campaigns. An example of the simultaneous laser signals obtained along Track 4 from 3.2 km on a pre-dawn flight on October 23, 2007 is shown in Figure 4. Note that two off-line measurements were made along with the linecenter measurement. The surface albedo varied by a factor of 2.5 along the track with the transitions being extremely rapid upon transitioning between different types of surface types, e.g., from agricultural land to stands of trees. Because the LAS signals are obtained simultaneously, the temporal correlation in the signal variations is excellent. This permits the albedo variations to be eliminated when the ratios are taken between these signals, which is shown in Figure 5.
major instrument errors resulting from spatial mismatch of beams or signal cross talk. There is a slight slope in the on-line/off-line ratio along the track, which indicates that the LAS is detecting some CO2 column variations due to actual atmospheric CO2 variability. Figure 6 shows an expanded view of the CO2 two-way optical depth (OD) variations derived from the data shown in Figure 5. The average OD trend along the track corresponds to a change in the CO2 column of ~3.3 ppm. This was not surprising on this pre-dawn flight with enhanced CO2 levels near the surface.
Figure 6. Measured CO2 optical depth variation along Track 4.
Figure 4. Normalized LAS signal variations for line-center (top), off-line-1 at -50 pm (center), and off-line-2 at +50 pm (bottom) from 3.2 km altitude along Track 4 on October 23, 2007. The horizontal lines represents the average value which is given at the top of each plot. The ratio plots are provided on highly expanded scales (less than 0.1% full scale). The surface albedo variations along the track have been successfully eliminated because of the simultaneous measurements. The off/off ratio in Figure 5 shows that there are no
3.5 Flight Demonstration Results Summary Extensive in situ and LAS data sets were collected over a wide range of land and water surfaces, including in the presence of clouds, and the relative reflectance of these surfaces have been determined. LAS CO2 OD measurements were found to have high signal-to-noise ratios with values in excess of 250 for 1-s averages over land and for 10-s averages over water. These were obtained at a return signal strength of about 100 pW and an OD of about 0.35. LAS OD measurements were found to scale linearly with altitude from 500-7500 m with R2 >0.996. LAS CO2 ODs were found to be within 2% of the expected ODs calculated from the in situ CO2 profiles. On-going data analysis is expected to even further improve the above results. 4. ACKNOWLEDGEMENTS The authors would like to thank all the team members from ITT, NASA Langley, AER, and L-3 Aviation who assisted in these campaigns. This work was supported with funding from ITT, NASA Langley, UNH, and NASA's Earth Science Division. REFERENCES [1] NRC, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, The National Academies Press, Washington, D.C., 2007.
Figure 5. Signal ratios of line-center/off-line-2 (top) and off-line-1/off-line-2 (bottom).
[2] Dobbs, M, Dobler, M. Braun, D. McGregor, J. Overbeck, B. Moore III, E. Browell, and T. Zaccheo, 2008: A Modulated CW Fiber Laser-Lidar Suite for the ASCENDS Mission, Proc. 24ILRC, this issue.
