δ¹³C carbon isotopic composition of CO₂ in the atmosphere by Lidar. A preliminary study with a CDIAL system at 2-µm

Abstract. Our understanding of the global carbon cycle needs for new observations of CO₂ concentration at different space and time scales but also would benefit from observations of additional tracers of intra-atmospheric or surface-atmosphere exchanges to characterize sources and sinks. Lidar is a well-known promising technology for this research as it can provide, at the same time, structure of the atmosphere, dynamics and composition of several trace gas concentration. In this framework, a coherent differential absorption lidar (CDIAL) has been developed at LMD to measure simultaneously and separately ¹²CO₂ and ¹³CO₂ isotopic composition of CO₂ in the atmosphere. It also provides the wind speed along the line of sight of the laser with an additional Doppler ability. This paper investigates the methodology of three wavelengths DIAL in the spectral domain of 2-µm to obtain range-resolved CO₂ isotopic ratio δ¹³C. The set-up of the lidar as well as the signal processing is described in details. First atmospheric measurements along three days are achieved in the surface layer above the suburban area of Ecole Polytechnique campus, Palaiseau, France. Typical performances of the instrument (median values along 70 h of measurement) with 10 min of time averaging show: (1) a precision around 0.6 % for 1.2 km range resolution for ¹²CO₂ mixing ratio (2) a precision around 3.2 % for 1.6 km range resolution for ¹³CO₂ mixing ratio. In situ co-located gas analyser measurements are used to correct for biases that are explained neither by the spectroscopic database accuracy nor the signal processing and will need further investigation. Nevertheless, this preliminary study enables to make a useful state of the art for current lidar ability to provide δ¹³C measurements in the atmosphere with respect to geophysical expected anomalies and to predict the necessary performances of a future optimized instrument.