On-line wavenumber optimization for a ground-based CH4-DIAL

Methane (CH4) is the second most important anthropogenic greenhouse gas that contributes to global warming. The global warming potential of CH4 is 72 times that of CO2 per molecule. However, the uncertainties of the sinks and sources of CH4 remain large. Unfortunately, range-resolved atmospheric CH4 concentrations have been rarely observed, thereby hindering the accurate understanding of some of the key features of carbon cycle. Differential absorption Lidar (DIAL) is a powerful and promising means of obtaining range-resolved CH4 concentrations and is helpful for estimating the emissions of anthropogenic and natural CH4 sources, as well as uncovering previously unknown aspects of the carbon cycle. Prior to developing a CH4-DIAL, a comprehensive preliminary study on the selection of on-line wavenumber must be performed to guarantee a high signal-to-noise ratio (SNR) of the differential signal and minimize the errors due to interference from other gases. This study aims to find the optimal selection of on-line wavenumbers in terms of atmospheric factors and detection scenarios. After roughly estimating the on-line wavenumbers using an evaluation index, the 1K temperature error and 1 hPa pressure error were separately and qualitatively analyzed. Then, the weighting function of the potential wavenumbers based on pressure was analyzed. Results show that 6076.938 cm(-1) is the most suitable wavenumber for vertical range-resolved measurements. The errors caused by uncertainties in temperature and pressure are only 0.33%/K and 0.11%/hPa when 6076.938 cm(-1) is selected as the on-line wavenumber. In addition, our simulation experiments indicated that the influences of H2O and CO2 can be neglected if a reasonable pair ofon-line and off-line wavenumbers was selected. Moreover, we demonstrated the relationship between on-line wavenumber and precision in different background concentrations for horizontal detection, which determines the optimal on-line wavenumbers for horizontal measurements under different circumstances. (C) 2019 Published by Elsevier Ltd.