Airborne active remote sensor for atmospheric carbon dioxide
Worldwide efforts for monitoring atmospheric carbon dioxide (CO2) are currently taking place. These efforts are important for understanding the interactions of this gas and its transport around Earth. In turn, this is essential for studying the carbon cycle and for making climate predictions that address critical issues such as global warming. Although various techniques are used for CO2 monitoring,1 there are still high levels of uncertainty associated with quantifying the sources and sinks of the gas. The ambiguity in CO2 monitoring results from limited spatial and temporal mapping techniques that have insufficient accuracy and resolution. To resolve these issues, more accurate and rapid CO2 monitoring is thus required.
Such accurate and rapid monitoring can be achieved through active remote sensing, which has the potential to enhance CO2 measurement capabilities. Several teams around the world are currently engaged in the development of CO2 sensors that use different transmitters and detection methods.2 All of these approaches are based on the differential absorption lidar (DIAL) technique, yet CO2 DIAL has never been attempted from space.
For over 20 years, several 2μm pulsed DIAL systems and technologies have been developed at NASA's Langley Research Center (LaRC).3 As a result, we have been able to produce high-energy double-pulse lasers with high repetition rates. In our lasers, each of the generated pulses can be tuned and locked independently around the 2μm wavelength (i.e., an optimum spectral region for CO2 sensing). We have implemented this type of laser as a transmitter in an integrated path differential absorption (IPDA) lidar. In this setup (see Figure 1) we tune the first and second pulses to high and low CO2 absorption (i.e., on-line and off-line wavelengths), respectively, and we aim the transmitted pulses at hard targets. The differences between the on-line and off-line return signals (after the transmitted energies have been normalized) correlate to the total amount of CO2 that is within the column space (i.e., between the instrument and the hard target).
We have applied this technique to develop an airborne double-pulse 2μm IPDA lidar for CO2 measurement, which consists of a transmitter and a receiver.2, 4 Our double-pulse 2μm laser transmitter is capable of producing 90 and 40mJ pulses with 10Hz repetition rate that are precisely tuned and locked to the on-line and off-line return signals, respectively. We have also used direct detection to develop and integrate a state-of-the-art receiver. This receiver consists of a telescope that focuses the radiation, through aft-optics, onto quantum detectors. After signal conditioning, we digitize and store the detected signals with the use of a data acquisition unit. In addition, we have developed and verified instrument operating software, data reduction algorithms, IPDA modeling, and spectroscopy. We have also ruggedized the instrument so that it is suitable for both ground and airborne demonstrations and evaluations.4
During ground testing and demonstration of our IPDA lidar system at LaRC, we used a mobile trailer and we aimed the instrument at a set of calibrated hard targets.2, 4 As such, we were able to rigorously test the instrument and evaluate its performance under different operating conditions. Our on-site support facilities included the Chemistry and Physics Atmospheric Boundary Layer Experiment (CAPABLE) for meteorology and a CO2 in situ sensor.4 A nearby incinerator provided a CO2 source that biased the environment. We verified our measurements against models. The IPDA-driven CO2 dry-air mixing ratio is compared with our in situ measurement in Figure 2.
We have restricted the size, weight, and power consumption of our 2μm CO2 IPDA lidar instrument to the payload of a small aircraft. As such, we installed our IPDA on board the NASA B-200 aircraft, along with other supporting instruments (including an in situ CO2 sensor, a global positioning system instrument for global timing and position records, and a video recorder for target identification). Furthermore, the aircraft's built-in sensors were used to provide meteorological measurements. We thus used this suite of instruments to conduct airborne testing of our IPDA over the course of 10 flights that spanned about 27 hours.
In one airborne test, we examined the coarse sensitivity of our IPDA lidar by subjecting the instrument to high CO2 plumes generated from a power plant incinerator (see Figure 3). Our against-wind flight track provides a CO2 profile that distinguishes the plume mitigation, the plume peak, and the normal clear background. In another test we compared the results from our airborne IPDA lidar instrument measurements with those from CO2 air sampling conducted during a National Oceanic and Atmospheric Administration flight. Our comparison indicated an IPDA column-average dry-air CO2 mixing ratio measurement of 405.22ppm, with 1.14ppm (0.28%) offset and 4.15 (1.02%) standard deviation for 6km altitude and 4GHz on-line operation.
The cloud slicing technique (see Figure 4) is another beneficial capability of our IPDA lidar system. Although scattered cumulus clouds that form above the boundary layer prevent the IPDA line-of-sight from reaching the ground, scattering from the clouds' upper surfaces can permit a measurement of tropospheric column CO2.2 The difference between the total column (i.e., to the ground) and free troposphere measurements provides the CO2 content of the boundary layer.
In this work we have successfully demonstrated the CO2 measurement capabilities of our 2μm double-pulse IPDA lidar system. We have shown that our precise, rugged, and compact IPDA instrument can be operated on either ground or airborne platforms to provide accurate and rapid CO2 measurements. We are currently extending the capabilities of our IPDA to include a third transmitted pulse.5 Our new triple-pulse IPDA lidar will enable simultaneous and independent CO2 and water vapor measurements under two different sets of conditions. Achieving accurate measurements of two species with a single compact instrument is thus a major step toward future space applications. As such, the current focus of our development is to meet or exceed the requirements for space-borne CO2 measurements.
Upendra Singh is the associate director for sensors systems, and is an internationally recognized atmospheric scientist and laser remote sensing expert, with more than 30 years of experience. He has chaired more than 40 international conferences and has authored or co-authored over 300 articles in the fields of atmospheric science and remote sensing.