The National Institute of Information and Communications Technology (NICT)has made a great deal of effort to develop a coherent 2 μmdifferential absorption and wind lidar (Co2DiaWiL) for measuring CO andwind 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, and28 December 2010. The detection sensitivity of a 2 μm IPDAlidar was examined in detail using the CO concentration measured by thefoothill reflection. The precisions of CO measurements for the foothilltarget 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 laseroperating at a high pulse repetition frequency of a few tens of KHz isnecessary for XCO (column-averaged dry air mixing ratio of CO) measurement with a precision of 1–2 ppm in order toobserve temporal and spatial variations in the CO. Statisticalcomparisons indicated that, although a small amount of in situ data and thefact that they were not co-located with the foothill target made comparisondifficult, the CO volume mixing ratio obtained by the Co2DiaWiLmeasurements for the foothill target and atmospheric returns was about−5 ppm lower than the 5 min running averages of the in situ sensor. Notonly actual difference of sensing volume or the natural variability ofCO but also the fluctuations of temperature could cause thisdifference. The statistical results indicated that there were no biasesbetween the foothill target and atmospheric return measurements. The2 μm coherent IPDA lidar can detect the CO volume mixingratio change of 3% in the 5 min signal integration. In order to detectthe position of the foothill target, to measure a range with a high SNR (signal-to-noise ratio), and toreduce uncertainty due to the presence of aerosols and clouds, it isimportant to make a precise range measurement with a Q-switched laser and arange-gated receiver.
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