Flight Demonstration of a 2-Micron, Double Plused CO2 IPDA Lidar Instrument

NASA Langley Research Center (LaRC) developed a double pulsed, high energy 2-micron Integrated Path Differential Absorption (IPDA) lidar instrument to measure atmospheric CO2 column density. The 2-μm double pulsed IPDA lidar was flown ten times in March and April of 2014. It was determined that the IPDA lidar measurement is in good agreement with an in-situ CO2 measurement by a collocated NOAA flight. The average column CO2 density difference between the IPDA lidar measurements and the NOAA air samples is 1.48ppm in the flight altitudes of 3 to 6.1 km.


INTRODUCTION
NASA Langley Research Center (LaRC) has developed a double-pulsed, high energy, 2-micron direct detection IPDA lidar instrument (Yu et.al). Lidars operating in the 2 µm band offer high nearsurface CO2 measurement sensitivity due to the intrinsically stronger absorption lines (Menzies andTratt 2003, Caron andDurand 2009). The objective of the airborne demonstration of the newly developed 2-micron pulsed IPDA lidar is to demonstrate the functionality and capability of the lidar instrument. The airborne IPDA lidar made measurements at different flight altitudes up to 8.3 km limited by aircraft capability and different ground target conditions such as vegetation, soil, ocean surface, snow and sand, and different cloud conditions. Strong lidar return signals were obtained for both on and off-line wavelengths at all flight altitudes. The CO2 column dry mixing ratio is derived from the IPDA lidar measurement and available meteorological data profiles. This paper describes the measurement results of the 2-micron pulsed IPDA lidar instrument during this airborne campaign demonstration.

Data Signal to Noise Ratio
Strong lidar return signals were obtained for both on and off-line wavelengths at different altitudes. Figure 1 shows lidar signal examples at two different lidar operating conditions. There are two pair of peaks in the return signal. The first pair of signal peaks are the on and off-line pulse signals reflected from the airplane window. The second pair of peaks are on and off-line pulse signals from the hard target echo. The off-line signals are intentionally offset from the on-line return signals in order to see the on and off-line return signals clearly. In fact, they are overlapped with each other in terms of range. Fig. 1a corresponds to a lidar signal with a flight at 1372 meters with preamplifier setting at 10^3. Since the airplane is at a low altitude, there is little absorption for the on-line pulse, thus, the amplitudes of the on-line and offline returning signals are close to the amplitudes of the on and off-line laser pulses recorded by an energy monitor. Fig. 1b shows the lidar signal with an airplane flying at 6096 meters with preamplifier gain setting 10^5. Due to the longer absorption path length relative to the conditions in Fig 1a, the return signal amplitude from the on-line pulse is reduced. Therefore, the lidar signals from on and off-line pulses are comparable as shown in Fig. 1b. The inserts in Fig. 1 are the enlarged ground return signals. It clearly shows that the ground return signal is strong with high signal to noise ratio. The SNR is defined as the ratio of the area integrated under the return lidar signal waveform divided by baseline RMS noise, times the same time interval used to integrate the lidar signal.

DAOD Measurement Statistics
The lidar measures the backscattered signals from hard targets normalized to their emitted energy samples recorded by an energy monitor. The key measurement parameter is the differential absorption optical depth (DAOD), which is defined as the optical depth difference between the on and off-line frequency. DAOD can be calculated according to equation 1.
where the Pi is the lidar return signal power, Ei is the transmitted laser energy, and ti is the effective pulse width of the return signal at the on or off-line frequency.
The accuracy of the DAOD measurements depends on the lidar signal and noise characteristics, and lidar system bias errors. Since the objective of the flights is to demonstrate functionality of the newly developed instrument, many lidar instrument settings were adjusted during the flights. Adjustments include the pre-amplifier gain, the online frequency shift from the R30 absorption peak, the receiver bandwidth, and the laser output energy. The instrument measured DAOD is compared with a model simulated DAOD value. The model used here is the US standard atmospheric model with an assumed atmospheric CO2 concentration of 395 ppm.   Fig. 2b shows the result with 100 points moving average, which corresponds to 10 seconds average. The standard deviation is improved to 0.0123 for the lidar data as shown in Fig. 2b Flying the IPDA lidar over the ocean provides a target with near consistent surface reflectivity, which tends to reduce measurement uncertainty compared to elevated continental grounds that varies in both reflectivity and scattering surface elevation. The NOAA flight collected data at seven different altitudes, starting from 6126 meters and gradually descending to 912 meters. (6126, 5243, 3977, 3052, 2127, 1505, 912 m). It provided coarse vertical CO2 and meteorological data profiles. Due to airspace restriction, our flight flew over the same location half an hour after the NOAA flight. The IPDA lidar flew at the same altitudes as the NOAA flight. The on-line frequency was set at 4 GHz from the R30 line absorption peak for the flight altitude above 3052 meters. The on-line frequency was changed to 3 GHz from the R30 line absorption peak below a flight altitude of 3052 meters because of less absorption due to shorter range. At an altitude of 3052 meters, the data with on-line frequency shift at both 3 and 4 GHz was taken.
The profiles of CO2 mixing ratio xcd, temperature, pressure, and water vapor from the ground to 8 km can be obtained by linear extrapolation of the NOAA data. To make the direct comparison to the IPDA lidar column density measurement, the CO2 weighted-average column dry-air volume-mixing ratio, Xcd, c, can be calculated. At a certain altitude, it is a weighted integration of xcd from that altitude to the surface. IPDA lidar measures the DAOD according to equation 2.
Using the NOAA measured meteorological data profile, the CO2 weightedaverage column dry-air volume-mixing ratio, Xcd, m, can be obtained with the lidar measured DAOD value. Then, the lidar measured Xcd, m can be directly compared to the NOAA measurement Xcd, c. The subscripts m and c represent the IPDA lidar measurement and the calculated result from them respectively. As shown in Fig. 2b there appears to be a small drift in the measured DAOD value, or a gradient in the CO2 column value, due to land condition changes over the flight track.  Figure 3 shows the Xcd comparison between the IPDA lidar instrument measurement Xcd, m, and the model from the NOAA in-situ instrument, Xcd, c. The Xcd, m is the result of the 100 pulse average to reduce the error introduced from random noise. The direct comparison between Xcd, c, and Xcd, m, revealed that the column integrated CO2 mixing ratio measured by the IPDA lidar instrument is higher than that derived from NOAA flask air sampling. The average difference is 1.4775 ppm, which corresponds to a 0.36% difference between the two instruments. This direct comparison between the two independent measurements validates the high precision measurement capability of the 2-µm double-pulsed IPDA lidar instrument.

CONCLUSIONS
NASA LaRC developed a double-pulse, 2-μm integrated path differential absorption (IPDA) lidar instrument for atmospheric CO2 measurement. Advantages of the 2-micron high energy pulsed IPDA remote sensing technique include a high signal-to-noise ratio measurement with accurate ranging; favorable weighting function towards the ground surface to measure the source and sinks of the CO2; and the capability to directly eliminate contaminations from aerosols and clouds to yield high accuracy CO2 column measurements. IPDA CO2 differential optical depth measurement results agree well with model prediction. With 10 s average, the standard deviation of the DAOD measurement is 0.0145. Compared to the CO2 mixing ratio measured by NOAA flask sampling data, the 2-micron IPDA lidar provided an accurate measurement with 0.36% difference.

ACKNOWLEDGEMENT
The authors would like to thank the NASA Earth Science Technology Office for funding this project. The authors acknowledge support from the Engineering and Research Services Directorates at NASA Langley Research Center. Thanks also go to the dedicated efforts of the Research Systems Integration Branch personnel who made the airborne flight testing possible.