Carbon dioxide (CO2) is an important greenhouse gas that significantly contributes to the carbon cycle and global radiation budget on Earth. CO2 role on Earth’s climate is rather complicated due to different interactions with various climate components that include the atmosphere, the biosphere and the hydrosphere [1-2]. These interactions define CO2 sources and sinks that influence the gas transport fluxes worldwide. Understanding the interactions and transport of atmospheric CO2 around Earth is critical for carbon cycle studies and climate predictions through environment models [3-5]. Although extensive worldwide efforts for monitoring atmospheric CO2 through various techniques, including in-situ and passive sensors, are taking place high uncertainties exist in quantifying CO2 sources and sinks mainly due to insufficient spacial and temporal monitoring of the gas. Therefore it is required to have more rapid and accurate CO2 monitoring with higher uniform coverage and higher resolution. This was addressed by many international satellite missions. Satellites offered many advantages including the ability of continuously measuring CO2 in tropical regions and over southern oceans [1-2]. Present satellite instruments monitoring CO2 from space include SCIAMACHY, TES, AIRS, IASI and GOSAT [6-10]. To focus only on CO2 and address the issue of the gas sources and sinks, OCO-2 is fully dedicated for CO2 monitoring . Some of these systems have shown the potential to meet the spatial coverage to improve CO2 flux estimates on continental scales. However, satellite passive remote sensors are unable to meet the accuracy required to aid in better quantifying the terrestrial sources and sinks due to some limitations. For instance, shortwave infrared instruments rely on solar illumination which restricts their orbits and latitudinal coverage. Alternatively, thermal infrared systems relying on Earth’s radiation are not sensitive to the lower atmosphere where the largest CO2 interactions occur. Furthermore, passive remote sensing systems involve retrieval complexities which suffer from aerosol and cloud contamination and radiation path length uncertainties . Active remote sensing of CO2 is an alternative technique that has the potential to overcome the limitation of the passive sensors.
CO2 active remote sensing has been demonstrated using the differential absorption lidar (DIAL) technique [13-21]. Both 1.6 and 2.0 μm are suitable for atmospheric CO2 measurements due to the existence of distinct absorption feathers for the gas at these particular wavelengths. Although CO2 DIAL systems demonstrations were provided for systems validity from ground or airborne platforms, a complete CO2 DIAL mission that contributes to the science community has not been established. A number of worldwide teams have been engaged in developing CO2 DIAL instruments using different transmitters and detection methods. In France, a CO2 DIAL was developed based on 2-μm pulsed crystal-open path cavity transmitter and heterodyne detection . In Germany a 1.6-μm pulsed optical parametric oscillator transmitter with direct detection has been developed . In Japan similar systems were developed for ground based measurement [15-16]. In the USA, the National Research Council Decadal Survey recommended an active laser-based CO2 mission, “Active Sensing of CO2 Emissions over Night, Days, and Seasons (ASCENDS)”, to increase our understanding of CO2 sources, sinks, and fluxes worldwide . Research groups at NASA are currently involved in developing different CO2 DIAL instruments. Two of these instruments operate at 1.6 μm have been developed and deployed as airborne systems for atmospheric CO2 column measurements [17, 19]. One instrument is based on an intensity modulated continue wave (CW) approach , the other on a high pulse repletion frequency, low pulse-energy approach . These airborne CO2 DIAL systems operating at 1.57-μm utilize mature laser and detector technologies by taking advantage of the technology development outcomes in the telecom industry.
CO2 DIAL operating in the 2-μm band offer better near-surface CO2 measurement sensitivity due to the intrinsically stronger absorption lines. Using a 2.05-μm CW laser absorption spectrometer employing coherent detection method, airborne measurements of CO2 column abundance has been demonstrated . For more than 15 years, NASA Langley Research Center (LaRC) contributed in developing several 2-μm CO2 DIAL systems and technologies. This paper focuses on the current development of the airborne double-pulsed 2-μm CO2 integrated path differential absorption (IPDA) lidar system at NASA LaRC. This includes the IPDA system development and integration. Results from ground and airborne CO2 IPDA testing will be presented. The potential of scaling such technology to a space mission will be addressed.
TWO-MICRON CARBON DIOXIDE IPDA LIADR SYSTEM
Double-pulse 2-μm lasers have been developed with energy as high as 600 mJ and up to 10 Hz repetition rate . A single pump pulse produces the two laser pulses at the 2-μm wavelength which are separated by 150 μs. Both pulses can be tuned and locked separately around 2.05μm wavelength. Implementing this laser for a CO2 IPDA system, the first and second pulses are tuned and locked to strong and weak absorption features of the molecule known as the on-line and off-line wavelengths, respectively. Applying the double-pulse laser in IPDA system enhances the CO2 measurement capability by increasing the overlap of the sampled volume between the on-line and off-line [23-24]. IPDA offers measurements with higher signal-to-noise ratio compared to conventional range-resolved DIAL. This is due to the fact that IPDA rely on the much stronger energy monitors and hard target returns that are best suited for airborne platforms, rather than weak atmospheric scattering returns provided in range-resolved DIAL. In absence of range profiling, the IPDA technique measures the total integrated column content from the instrument to the hard target but with weighting that can be tuned by controlling the transmitted wavelengths. Therefore, the transmitter could be tuned to weight the column measurement to the surface for optimum CO2 interaction studies or up to the free troposphere for optimum transport studies. Currently, NASA LaRC is developing and enhancing IPDA data processing algorithms for CO2 column measurement from an airborne platform [23-24]. Fig. 1 shows an isometric view of the key components of the 2-μm CO2 IPDA system.
IPDA Lidar Transmitter
The compact, rugged, highly reliable CO2 IPDA laser transmitter is capable of generating 100 mJ at 10 Hz [22-23]. The transmitter is based on the Ho:Tm:YLF high-energy 2-μm pulsed laser technology. This laser transmitter is side pumped by AlGaAs diode arrays at 792 nm and designed to operate in a unique double pulse format to mitigate the effect of the surface reflection difference between the on-and-off line pulses on the precision of the IPDA measurement. When the Ho upper laser level population reaches its maximum value at the end of the pump cycle, a first Q-switched pulse is generated which extracts the energy stored in the Ho 5I7 upper laser level, resulting in a sharp decrease in the upper laser level population. Then, a new population equilibrium between the Tm 3F4 and Ho 5I7 manifolds is established by energy transfer from the excited Tm ions towards Ho ions even though the pump no longer exists. The population at Ho upper laser level 5I7 comes to its second maximum about 150 μs after the first pulse. The second Q-switch pulse is triggered at this moment resulting in the desired double pulse operation. A unique feature of this laser operation is that it provides two Q-switched pulses with a single pump pulse .
The exact wavelengths of the pulsed laser transmitter are controlled by the wavelength control unit. The first pulse and the second pulse are injection seeded alternately by the on-line and the off-line wavelengths. All the optical mounts are custom designed with space heritage. They are designed to be adjustable and lockable and hardened to withstand vibrations that can occur during operation in an airborne environment. Fig. 2a shows a picture of the engineering packaged laser transmitter. This laser transmitter is 11.5×26.5×6.4 inch (29×67.3×16.5 cm) in size, and weighted less than 70lbs (31.75 kg). Transmitter packaging includes a beam expander at the laser outlet which results in a final 2.54 cm output beam diameter.
IPDA Lidar Receiver
The IPDA receiver consists of a telescope that focuses the radiation onto two detectors through aft-optics. The receiver telescope is a custom designed Newtonian type with 40cm diameter aluminium primary mirror. The shape of the primary mirror is hyperbolic to minimize the aberration, so that the returning signal can be focused to less than 300 μm diameter spot size compatible with the radiation detectors. The telescope is designed to maintain the focus point position in the temperature range between 5 and 35 °C. 300 μm diameter InGaAs pin photodiodes (Hamamatsu; G5853) were selected for this mission. Two detectors accommodate high and low gain channels after beam splitting. Detector characterization resulted in a noise-equivalent-power of 6.8×10-14 W/Hz0.5 at 30°C that is suited for the IPDA lidar application. After amplification the lidar signals are digitized and stored by a data acquisition unit. The data acquisition unit is based on two digitizers. The first is a 10-Bit, 2 GS/s digitizer (Agilent; U1065A) for laser energy monitoring and the second is a 12-bit, 420 MS/s digitizer (Agilent; U1066A) for measuring the hard target return. Detectors are coupled to the digitizers through variable gain, high speed trans-impedance amplifiers (FEMTO; DHPCA-100). Digitizers and data storage are hosted through a personal computer that runs Microsoft XP with a 64-bit/66 MHz PCI bus. The system is capable of transferring data at sustained rates up to 400 MB/s to the host computer. Fig. 2b shows a schematic of the IPDA receiver including ray tracing of the transmitted and collected radiation [23-24].
IPDA GROUND TESTING
The integrated IPDA lidar was installed inside a mobile trailer, shown in fig. 3a, for initial testing and alignment verification. A 24 inch flat mirror was installed underneath the telescope at 45° for turning the transmitted beam and telescope field-of-view from nadir to horizontal direction through a side window, as shown in fig. 3b. This allows pointing the IPDA to a set of calibrated hard targets, with known reflectivity, located at about 857 m away from the trailer. Fig. 4a shows an aerial picture of the test site at NASA LaRC. Collocated in the site is the Chemistry and Physics Atmospheric Boundary Layer Experiment (CAPABLE). CAPABLE is a ground-based observation site for studying atmospheric conditions in the Tidewater region of Virginia, which operates through collaborative effort between the Science Directorate at NASA LaRC, the U.S. Environmental Protection Agency and the Virginia Department of Environmental Quality . CAPABLE instruments continuously measure pollutants, such as ozone, nitrogen dioxide, carbon monoxide and aerosols. Besides, CAPABLE provides continuous ground meteorological monitoring, such as pressure, temperature and relative humidity. This data is valuable for the 2-μm CO2 IPDA lidar modelling for instrument validation on ground. In addition, an in-situ CO2 and H2O gas analyser (LiCor; LI-840A) was installed at fixed location as shown in fig. 4a. This allows better estimates of the CO2 mixing ratio in the test location for better evaluation of the 2-μm CO2 IPDA lidar instrument. Finally, it is worth mentioning the collocation of the Hampton-NASA Steam Plant , which is a solid waste incinerator (fig. 4a) used for steam generation. Depending on the incinerator operating cycles, higher than normal CO2 mixing ratio was observed at the IPDA test location.
IPDA lidar ground testing included different operating conditions, in terms of signal conditioning settings, target reflectivity and on-line offsets from the CO2 R30 line center. Ranging capability is another advantage of the pulsed IPDA lidar. Range measurement between the IPDA and the hard target was achieved by monitoring the time delay between the transmitted and return pulses. Converting the time delay into distance using the speed of light the IPDA column length is determined, as demonstrated in fig. 4b, for both on-line and off-line wavelengths. Results are consistent with the nominal setting of the trailer relative to the hard target. Preliminary analysis of the ground test data indicated IPDA lidar sensitivity to atmospheric CO2 concentration. Fig. 5a compares the CO2 differential optical depth measured by the IPDA lidar to the theoretical value. The theoretical differential optical depth was derived using the US standard atmospheric model . Besides, the IPDA measured differential optical depth was converted to CO2 dry mixing ratio, using metrological data obtained from CAPABLE. Fig. 5b compares the CO2 dry mixing ratio calculated using the IPDA results to the measured using the in-situ sensor. General temporal profile agreement is observed between both curves, after correcting for 50 ppm offset which is caused due to nonlinearities.
IPDA AIRBORNE TESTING
The 2-μm CO2 IPDA lidar is designed for integration into a small research aircraft. The IPDA instrument size, weight and power consumption were restricted to the NASA B-200 payload requirements. This allows the system to be easily adopted in any larger airborne research platform, such as the NASA DC-8 aircraft, for future missions. In addition to the IPDA lidar, other housekeeping instruments were integrated into the B-200 aircraft. These included the in-situ sensor (LiCor) for CO2 dry mixing ratio measurement, GPS for aircraft position, altitude and angles measurements and video recorder for target identification. Besides, aircraft built-in sensors provided altitude, pressure, temperature and relative humidity sampling at the flight position. Time stamps were adjusted to the GPS global timing. Fig. 6 shows the NASA B-200 aircraft and the integrated IPDA instrument inside. Two operators are required to accompany the instrument during operation.
The 2-μm CO2 IPDA lidar airborne testing was conducted during ten daytime flights, spanning more than 20 hours, during March 20, 2014 through April 10, 2014. Flights were conducted from NASA LaRC through Langley Air force Base, Hampton, Virginia. Meteorological balloon radiosonde was independently lunched from CAPABLE site, mostly during the beginning and the end of each flight. This allows for atmospheric pressure, temperature and relative humidity vertical profiling estimates for IPDA modelling verifications. IPDA lidar airborne testing included different operating and environmental conditions. Environmental conditions included different flight altitude up to 6 km, different ground target conditions such as vegetation, soil, ocean, snow and sand and different cloud conditions. Besides, some flights targeted power plant incinerators for investigating the IPDA sensitivity to CO2 plums. Preliminary analysis of the airborne test data indicated the IPDA lidar sensitivity to atmospheric CO2 up to 6 km altitude over ocean target. In spite of the mechanical stresses due to shaking and vibration in the small aircraft environment, the IPDA lidar did not losses alignment resulting in about 190 GB worth of raw data.
On April 5, 2014, the NASA B-200 flight coincided with another NOAA air sampling flight . Due to flight control restrictions, there was a 30 minute time lag between NOAA and NASA flights. Nevertheless, the IPDA lidar on board NASA flight sampled the same geographical location as the NOAA flask samples over the Atlantic Ocean out of the east shore of New Jersey. CO2 flask-sampling results and meteorological data provided by NOAA were valuable for the NASA LaRC IPDA lidar instrument testing. Fig. 7 shows the averaged IPDA return on-line and off-line signals at different altitudes. In each record, the first pulse is attributed to systematic effects from aircraft window and/or telescope secondary mirror radiation leaks. Similar to ground testing, the altitude information can be deducted by comparing the time delay between the first pulse and the ground return pulse. Fig. 8a compares the flight altitude, obtained from the GPS to the range calculated from the IPDA data. The GPS flight altitude was converted to line-of-sight measurement after correcting for the aircraft pitch and roll angles, obtained from the GPS. Fig. 8b compares the CO2 differential optical depth obtained from the IPDA data and modelled from NOAA flask sampling data. IPDA indicated a consistent CO2 differential optical depth offset of about 0.07, similar to what was observed on ground testing.
SUMMARY AND CONCLUSIONS
Understanding atmospheric CO2 interactions and transport dynamics is important for studying carbon cycle and global radiation budget on Earth. In-suit and satellite based passive remote sensors have several limitations that could be recovered with active remote sensors. CO2 active remote sensing has been demonstrated at NASA LaRC using the DIAL technique. NASA LaRC developed a double-pulse, 2-μm integrated path differential absorption (IPDA) lidar instrument for atmospheric CO2 measurement. Advantages of the IPDA remote sensing technique include high signal-to-noise ratio measurement with accurate ranging. The 2-μm CO2 IPDA transmitter is capable of producing 100mJ energy per pulse at 10 Hz repetition rate. High accuracy, stable and repeatable wavelength control and switching unit have been integrated within the transmitter. The IPDA also include a high quality 16 inch telescope and a commercial detector, electronics and data acquisition that has been integrated. The whole IPDA lidar structure is compactly and ruggedly packaged to fit in the NASA B-200 research aircraft. Ground and airborne testing of the 2-μm IPDA lidar was conducted at NASA LaRC through several validation procedures. This included instrument performance modeling through standard atmosphere and meteorological sampling. IPDA CO2 differential optical depth measurement results agree with ground in-situ measurements and with CO2 airborne sampling conducted by NOAA. Consistent differential optical depth offset, of about 0.1, was observed in the measurement. Further detailed data processing is under work. This airborne 2-μm IPDA lidar provides a unique CO2 measurement tool that could be scaled to future space missions.
The authors would like to thank NASA Earth Science Technology Office for funding this program. The authors acknowledge the support from the Engineering and Research Services Directorates at NASA Langley Research Center. Thanks are also due to the dedicated efforts of the Research Systems Integration Branch that made airborne validation possible. Acknowledgement are also due to the CAPABLE team and NOAA for providing public information that is significant for the science validation process.