High-resolution and accurate profiles of atmospheric temperature have wide applications in weather analysis and forecasting, climate change assessment, atmospheric chemistry research, and remote-sensing measurements. Current primary tools for measuring atmospheric temperature profiles include radiosondes, microwave radiometers, and lidars. While conventional radiosondes provide temperature profiles with satisfactory accuracy and excellent height resolution, their inadequate temporal resolution (approximately 12 hours) impedes the capture of relatively rapid atmospheric processes. Temperature profiles retrieved from microwave radiometer measurements generally suffer from relatively low accuracy, as they rely on statistics-based inversion algorithms that incorporate artificially introduced assumptions. Single-line-extracted pure rotational Raman (PRR) lidar represents an active remote sensing technique capable of obtaining accurate, high-range-resolution atmospheric temperature profiles from the lower troposphere to the lower stratosphere. Originally proposed by Cooney in 1972, this concept remained unrealized for over fifty years due to technical challenges: the atmospheric pure rotational Raman spectrum consists of mixed spectral lines from nitrogen and oxygen molecules, and the bandwidth of commercially-available optical filters was excessive. Although many research teams worldwide have contributed to its development, the technical implementation was not achieved until recently. By analyzing the PRR spectra of air molecules, we identified that two individual Stokes N₂ PRR lines corresponding to rotational quantum numbers J=4 and J=14 can be optically isolated using commercially available interference filters (IF) combined with Fabry-Perot interferometer (FPI). Based on this finding, we developed a single-line-extracted PRR lidar system for accurate atmospheric temperature profiling and simultaneous retrieval of aerosol/cloud backscatter coefficients.

The lidar utilizes the second harmonic (532.237 nm) of a seeded Nd∶YAG laser system with injection as an emitted light. The receiving unit comprises two different-aperture (0.3 m and 1.0 m) receivers, dedicated to near-range and far-range measurements, respectively. Each receiver is equipped with a custom-built three-channel polychromator. This advanced polychromator isolates two individual Stokes N₂ PRR lines corresponding to rotational quantum numbers J=4 and J=14 (in the two Raman channels), along with the Rayleigh/Mie backscatter signal (in the elastic channel). The two Raman lines correspond to the vacuum wavelengths of 533.479 nm and 535.749 nm, respectively. Each Raman channel uses a set of signal extraction components consisting of two identical interference filters and a temperature-controlled sandwich solid Fabry-Perot interferometer. The interference filters are centered at the respective Raman wavelengths (533.479 nm or 535.749 nm), with a bandwidth of 0.16 nm, a peak transmission of approximately 60%, and an elastic-signal rejection ratio exceeding 10⁴. The elastic channel is equipped with a single interference filter with a bandwidth of 0.3 nm and a peak transmission of about 80% at 532.237 nm.

The atmospheric temperature T(z) has been derived from a rigorous analytical expression that relates temperature to the ratio Q(z) of the signal photon counts in the two Raman channels. This analytical expression estalishes a simple functional relation between T(z) and Q(z) involving two constant parameters, A and B. Parameter A is obtained through theoretical calculation, while B is determined via laboratory measurement, eliminating the need for external calibration using data from other instruments. Using the derived temperature profiles, aerosol and cloud backscatter coefficients can be rigorously derived from the signal photon counts in the J=4 Raman channel and the elastic channel, eliminating the need for additional assumptions such as a constant lidar ratio or ?ngstr?m relationship. This provides for the first time a valid and straightforward approach for profiling the optical properties of aerosols and clouds. The performance of the lidar system was evaluated through observational examples and statistical comparisons at a subtropical site in Wuhan, China, using an integration time of 1 hour and a vertical resolution of 150 m. During nighttime operations, the lidar effectively measured temperature profiles from approximately 2 km to 35 km in altitude. The results showed excellent agreement with simultaneous radiosonde data within the 2?16 km altitude range (the altitude range of available sonde data). The temperature measurement uncertainty in nighttime was below 1 K at altitudes up to 21 km. Under daytime conditions, the lidar successfully retrieved temperature profiles from ~2 km to 16 km, showing good consistency with simultaneous sonde data. The daytime lidar temperature profiles exhibited statistical uncertainty of less than 1 K within an altitude range from 2 to 11 km. Such a large altitude range is unprecedented for daytime lidar temperature measurements. Additionally, the pure rotational Raman lidar also demonstrated an excellent performance in revealing small-scale temperature structures, such as temperature inversions, in the troposphere.

Single-line-extracted pure rotational Raman lidar has been develpoed for accurate profiling of atmospheric temperature and aerosol/cloud backscatter coefficients. A seeded frequency-doubled Nd∶YAG laser is utilized as the light source. Light backscattered from atmosphere is collected by two receivers with diameters of 0.3 m and 1.0 m, respectively. Each receiver is equipped with a custom-built three-channel polychromator that isolates respectively two individual Stokes N₂ PRR lines corresponding to rotational quantum numbers J=4 and J=14, along with the Rayleigh/Mie backscatter signal. The logarithm of the ratio between two PRR channel signals is a linear function of the reciprocal of atmospheric temperature. The two parameters in this function are a physical constant and a laboratory-measurable quantity, respectively. Therefore, the atmospheric temperature profiles can be obtained accurately derived from the ratio of the two PRR line signals without requiring external calibration. Based on the derived temperature profiles, the aerosol/cloud backscatter and extinction coefficient profiles can be further rigorously determined from the measured PRR J=4 signal and elastic backscatter signal, without relying on additional assumptions. The derived aerosol/cloud parameters and resulting lidar ratio provide benchmarks for aerosol and cloud lidar measurements. Field experiments have demonstrated that this lidar achieves a wide altitude coverage, high time and altitude resolutions, and unprecedented daytime performance, making it widely applicable in the fields of meteorology and atmospheric environment research.