An airborne amplitude-modulated 1.57 μm differential laser absorption spectrometer: simultaneous measurement of partial column-averaged dry air mixing ratio of CO₂ and target range

Abstract. Simultaneous measurements of the partial column-averaged dry air mixing ratio of CO₂ (XCO₂) and target range were demonstrated using airborne amplitude-modulated 1.57 μm differential laser absorption spectrometer (LAS). The LAS system is useful for discriminating between ground and cloud return signals and has a demonstrated ability to suppress the impact of integrated aerosol signals on atmospheric CO₂ measurements. A high correlation coefficient (R) of 0.987 between XCO₂ observed by LAS and XCO₂ calculated from in situ measurements was obtained. The averaged difference in XCO₂ obtained from LAS and validation data was within 1.5 ppm for all spiral measurements. An interesting vertical profile was observed for both XCO_2LAS and XCO_2val, in which lower altitude CO₂ decreases compared to higher altitude CO₂ attributed to the photosynthesis over grassland in the summer. In the case of an urban area where there are boundary-layer enhanced CO₂ and aerosol in the winter, the difference of XCO_2LAS to XCO_2val is a negative bias of 1.5 ppm, and XCO_2LAS is in agreement with XCO_2val within the measurement precision of 2.4 ppm (1 SD).