A differential absorption lidar (DIAL) system designed on the basis of optical parametric oscillators (OPO) with nonlinear KTiOAsO₄ (KTA) and KTiOPO₄ (KTP) crystals is described. The crystals allow laser radiation tuning in the infrared region (IR) wavelength region. The measurements in the 3.30–3.50 μm spectral range, which includes a strong absorption band of methane, are carried out. Lidar backscattered signals in the spectral band 3.30–3.50 μm has been measured and analyzed along the horizontal path in the atmosphere. Based on the experimental results, CH₄ concentrations ~2.085 ppm along a 800 m surface path are retrieved in the spectral range under study with a spatial resolution of 100 m.

The results of modernization of a mobile lidar for the airborne monitoring of the methane content in the atmosphere are presented. The modernization was carried out on the basis of in situ tests, several engineering solutions, and preliminary numerical simulations. The in situ tests showed a possibility of sounding background tropospheric methane concentrations along a 500 m surface path. During the modernization, the airborne lidar for methane monitoring was supplemented with an off-axis mirror collimator, which made it possible to reduce the divergence of laser radiation by a factor of 4. The overlapping function was simulated for a biaxial scheme of the mobile lidar with radii of the light-sensitive zone of the receiving optics of 0.1, 0.3, 0.5, 0.8 and 1 mm. The dimensions of the light-sensitive zone were found to provide complete coverage of the field of view of the telescope and a laser beam; the length of the “dead” zone was estimated when a laser beam propagated parallel to the optical axis of the telescope. Airborne methane monitoring in the atmosphere in the informative wavelength range (2916.55–2917 cm⁻¹ on-line and 2915.00 cm⁻¹ off-line) was numerically simulated for midlatitude and Arctic summer. Thus, on the basis of the work carried out, the design of the mobile airborne lidar is substantiated, which is to operate as a part of the Tu-134 “Optik” aircraft laboratory of IAO SB RAS and to perform methane monitoring vertically downwards. The airborne lidar was tested during test flights and the Arctic expedition in 2022. The first experimental results of lidar measurements of the averaged methane concentration vertically downwards from sounding altitudes of 2000–3000, 380, and 270 m were obtained for mid-latitude summer and Arctic summer.

Subject of study. Methane concentration in near-the-surface tropospheric sensing paths was investigated under background conditions using a mobile differential absorption lidar. Aim of study. A mobile differential absorption lidar for remote sensing of atmospheric methane in the mid-infrared spectral range was designed, calibrated, and tested in natural field experiments. Method. The designed lidar enables investigation in the atmosphere using the differential absorption method. This method is based on the effect of resonant absorption of laser emission by gases. The emission source of the lidar has two operating wavelengths, one (on-line) in the center of the methane absorption line and the other (off-line) on the wing of the absorption line. The lidar signals obtained at on- and off-line wavelengths enable retrieval of the methane concentration under background conditions. Main results. The designed mobile IR differential absorption lidar for investigation of atmospheric methane is described. The mobile IR emission source of the differential absorption lidar was calibrated in the informative range of methane sensing near 3400 nm. The results of a natural field test of the mobile IR lidar for detection of atmospheric response at the calibrated sensing wavelengths and retrieval of methane background concentrations of approximately 2.0 ppm in horizontal surface atmospheric sensing paths are presented. Practical significance. The technical solutions for the design of the mobile lidar for remote methane sensing proposed in this paper enable formulation of the requirements on its further improvement aiming to increase the measurement range, design a vertical configuration for remote sensing from an aircraft, and use it at arctic latitudes.