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).

Figure 1. Concept of the 2μm double-pulse integrated path differential absorption (IPDA) lidar for carbon dioxide (CO₂) measurement. By targeting the CO₂ R30 line (labeled in the top panel), the first and second transmitted pulses are tuned and locked to a high and low absorption, respectively. In this instrument, the pulse separation is 150μs and has a 10Hz repetition rate. By aiming the laser at hard targets, the difference between the normalized return signals can be correlated to the average amount of CO₂ between the instrument and the 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.

Figure 2. CO₂ measurement results from the 2μm double-pulse IPDA lidar, compared with the results from an in situ sensor. These results were obtained during an IPDA ground testing demonstration. The IPDA measurements rely on either calibrated energy monitors (PDEM) or residual scattering (RS). Hr: Hour. Min: Minute.

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.

Figure 3. Cartoon illustrating CO₂ plume detection using the 2μm double-pulse IPDA lidar. Flying above a power plant incinerator against the wind flight track results in the CO₂ profile shown, which distinguishes plume mitigation, the plume peak, and normal clear background.

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.

Figure 4. Cartoon illustrating the concept of cloud slicing for CO₂ measurement within the boundary layer using the 2μm IPDA lidar. The difference between the measurements obtained in clear conditions and from thick cumulus cloud cover can be used to estimate the CO₂ content within 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.