Lidar sounding of carbon dioxide and water vapor with absorption spectroscopy techniques

. A possibility of monitoring greenhouse gases (CO 2 and H 2 O) along horizontal paths in the troposphere with two-channel near-IR lidar system based on an absorption spectroscopy measurement technique is estimated. The main units and components of the IR lidar under development are described. A backscattered signal is simulated in the informative range of IR lidar for sensing target greenhouse gases.


Introduction
Intensifying anthropogenic impact on the atmosphere increases both the concentrations of harmful gases and the total content of greenhouse gases. Assessment of gaseous emissions from industrial areas is among urgent applied problems. The increase in the total content of greenhouse gases is a determining factor of the climate change. The main greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and ozone (O3). Ozone sounding is commonly limited to the UV region. Strong H2O, CO2, and CH4 absorption lines lie in the infrared (IR) region. At V.E. Zuev Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences (IAO SB RAS) methane is sounded in the IR region by lidar systems designed at the Institute [1,2]. The development of approaches and tools for sounding H2O and CO2 is of interest, which stimulates works on the development of a corresponding two-channel lidar system for recording backscattered radiation at IAO SB RAS. The results of the development and testing of this system make it possible to estimate, correct, and reduce measurement errors of CO2 and H2O vapors along horizontal tropospheric paths using absorption spectroscopy techniques.

Material and methods
Two channels for recording lidar signals are based on absorption spectroscopy techniques: -the differential absorption and scattering method (DAS) [3,4], which ensures spatially resolved lidar signals and allows retrieving the concentration of target gases at a certain section of the sounding path (in an atmospheric strobe of ΔR in length); -the differential absorption optical spectroscopy method (DOAS) [5], which allows spectrally resolved measurements in a wide band and, thus, identification of several target gases even in the case of overlapping absorption bands.
The lidar system under development includes the following units: 1) a transmitting unit of the lidar (a laser with a tunable near-IR radiation of 3824-5571 cm -1 (1795-2615 nm), measuring tools to control the output laser radiation energy, a mirror collimator to reduce the laser beam divergence). The lidar transmitter model is shown in Figure 1.  The DAS channel (Figure 3a) is used to retrieve the spatial distribution of a target gas concentration using a dichroic mirror, focusing mirrors, and two photodiode detectors. The DOAS channel (Figure 3b) is used to obtain integral values of the concentration of an atmospheric gas under study along a sounding path to correct the spatially resolved gas concentration profiles retrieved with the use of the DAS recording channel and spectrally resolving equipment with a line detector.
The lidar is to be mounted at IAO SB RAS, the sounding path is to be directed to the Tomsk thermal power plant (Figure 4). The fragment of a satellite map from an Internet resource is used in Figure 4 [6].

Results and discussion
Using Thorlabs meters [7], the adjustment output pulse energy curves are derived in the lidar transmitter radiation tuning range ( Figure 5). In the IR region of the lidar transmitter, we analyzed the absorption bands of the target gases under study (CO2 and H2O) using the HITRAN spectral database [8] for mid-latitude summer atmospheric conditions. The USA Standard model with corrected concentration values measured in the industrial area of Tomsk with the TOR (tropospheric ozone research) station of IAO SB RAS was used [9,10]. The sounding path was 1 km long; the laser line width was 6.5 and 0.1 cm -1 . The concentrations of foreign gases corresponded to the background state of the atmosphere in the industrial area of Tomsk. The calculation of the transmission spectrum of the atmosphere has shown the spectral range 4800-4900 cm -1 (2040-2083 nm) and the subrange 4878-4894 cm -1 (2043-2050 nm) are preferable for DOAS and DAS sounding, where spectral intervals informative for CO2 and H2O sounding for DOAS measurements are distinguished, as well as νon(λon) and νoff(λoff) f values or DAS measurements.
Then, lidar echoes were numerically simulated in the spectral range 4800-4900 cm -1 (2040-2083 nm) during sounding H2O and CO2 with a two-channel IR lidar. The following input data were used in the simulation: H2O concentration of 18800 ppm, CO2 concentration of 402.1 ppm, energy of pulse ≤4 mJ, horizontal sounding path, sounding range ≤3 km, noise equivalent power (NEP) = 1  10 -12 W/Hz 0.5 . Input data for numerical simulation are systematized in Table 1. The lidar signal simulation results are shown in Figure 6. In Figure 6, the level of the lidar signal exceeds the NEP of the photodetector for the absorption band in the selected spectral sounding range 4878-4894 cm -1 . This points out to a possibility of recording lidar signals with the two-channel IR lidar system. Thus, the lidar signal recording in the two channels and subsequent signal processing are to ensure retrieving the spatial distribution and average concentrations of atmospheric gases under study along a sounding path.

Conclusions
A possibility is shown of monitoring atmospheric gases in the sounding informative range with the use of the IR lidar system developed, with two backscattered signal recording channels. The first, DAS channel makes it possible to retrieve the spatial distribution of target gas concentrations along a sounding path. The second, DOAS channel makes it possible to retrieve the averaged concentrations of gases under study along a sounding path. Thus, the use of two lidar signal recording channels are to make it possible to correct and reduce carbon dioxide and water vapor measurement errors in industrial areas.