Abstract
Abstract. The National Institute of Information and Communications Technology (NICT) has made a great deal of effort to develop a coherent 2 μm differential absorption and wind lidar (Co2DiaWiL) for measuring CO2 and wind speed. First, coherent Integrated Path Differential Absorption (IPDA) lidar experiments were conducted using the Co2DiaWiL and a foothill target (tree and ground surface) located about 7.12 km south of NICT on 11, 27, and 28 December 2010. The detection sensitivity of a 2 μm IPDA lidar was examined in detail using the CO2 concentration measured by the foothill reflection. The precisions of CO2 measurements for the foothill target and 900, 4500 and 27 000 shot pairs were 6.5, 2.8, and 1.2%, respectively. The results indicated that a coherent IPDA lidar with a laser operating at a high pulse repetition frequency of a few tens of KHz is necessary for XCO2 (column-averaged dry air mixing ratio of CO2) measurement with a precision of 1–2 ppm in order to observe temporal and spatial variations in the CO2. Statistical comparisons indicated that, although a small amount of in situ data and the fact that they were not co-located with the foothill target made comparison difficult, the CO2 volume mixing ratio obtained by the Co2DiaWiL measurements for the foothill target and atmospheric returns was about −5 ppm lower than the 5 min running averages of the in situ sensor. Not only actual difference of sensing volume or the natural variability of CO2 but also the fluctuations of temperature could cause this difference. The statistical results indicated that there were no biases between the foothill target and atmospheric return measurements. The 2 μm coherent IPDA lidar can detect the CO2 volume mixing ratio change of 3% in the 5 min signal integration. In order to detect the position of the foothill target, to measure a range with a high SNR (signal-to-noise ratio), and to reduce uncertainty due to the presence of aerosols and clouds, it is important to make a precise range measurement with a Q-switched laser and a range-gated receiver.
Highlights
The results indicated that a coherent IPDA lidar with a laser operating at a high pulse repetition frequency of a few tens of KHz is necessary for XCO2 measurement with a precision of 1–2 ppm in order to observe temporal and spatial variations in the CO2
Because of the presence of CO2 sinks such as the oceans or terrestrial ecosystems, atmospheric CO2 increases at only half the rate of anthropogenic CO2 emissions; hnoenwteavl earn, dinthneattuerme,pothrealsvpaartiiaaStli-ooscnlasildeinfErtohame rCrteOhg2iosninalkstoarceonntoi-t well understood due to limited observations (Le Quereet al., 2009)
Spaceborne XCO2 measurement with a bias-free high precision of 1–2 ppm is necessary to improve our knowledge of the carbon cycle (NASA Science Definition and Planning Workshop Report: Active Sensing of CO2 Emissions over Nights, Days, and Seasons, 2008)
Summary
The Co2DiaWiL has a single-frequency Q-switched Tm,Ho:YLF laser with laser frequency offset locking technique, a 10 cmaperture Mersenne off-axis telescope, a two-axis scanning device, two heterodyne detectors, and signal processing devices. Based on the signal-to-noise ratio, we set the wavelength of the on-line laser at 2051.058 nm in order to conduct long-range CO2 measurements. A small portion of the pulsed laser beam is detected using the heterodyne technique to monitor the frequency and lasing time of the outgoing laser pulse on a balanced InGaAs-PIN photodiode. The outputs of these detectors are digitized at 500 MHz by using 8-bit analog-to-digital (AD) converters. The ratio of discarded laser shot pairs was only around 5 % in the emitted laser shot pairs
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