Optical design of a reflective double-grating spectrometer to detect one branch in the pure rotational Raman spectrum of N2.

This paper presents the optical design of a high-resolution double-grating spectrometer for extracting the multiple lines in the Stokes or anti-Stokes branch of the pure rotational Raman spectra of nitrogen. The spectrometer is composed of collimating and focusing mirrors, two reflective gratings, and a linear detector. The structural parameters were calculated using geometric configuration, dispersion, and aberrational theory, and conditions for first-order correction of keystone distortion with divergent grating illumination were derived. Based on this method, we simulated a spectrometer with a 16-channel linear array photomultiplier tube, resulting in uniformly distributed single-branch lines on each detector channel. The resolution reached 0.225 nm per channel, and the keystone distortion was less than 0.7%. The spectrometer avoids the interference of elastic signals by not detecting them, enabling the extraction of atmospheric temperature profiles via separated single-branch lines with high precision. Our design provides a promising solution to extract atmospheric temperature profiles for pure rotational Raman lidar.

Simulation and misalignment analysis of the gain ratio of a polarization Mie Raman lidar.

The gain ratio is a critical parameter in a polarization Mie lidar. Calibrating the gain ratio is essential in aerosol classification studies. We developed a ray-tracing-based simulation method to investigate the impact of mounting errors on the gain ratio. In this method, a computational model for each element of the lidar was built, and Zemax was used to simulate the lidar receiver to obtain the optical gain ratio by theoretical calculations. This method can analyze the influence of each element's mounting errors and offer a theoretical foundation for the machining and mounting accuracy of the lidar design. The correctness of the model was verified by applying it to a single-wavelength polarization Mie Raman lidar.

Simulation and misalignment analysis of the gain ratio of a polarization Mie Raman lidar: publisher's note.

This publisher's note serves to correct an error in Appl. Opt.61, 2881 (2022)APOPAI0003-693510.1364/AO.453852.

Dual-wavelength-excitation aerosol fluorescence spectra detection using combined spectrometer with Czerny-Turner design.

We developed a dual-wavelength-excitation aerosol fluorescence spectra detection device prototype. In our system, the 263 nm and 355 nm lasers are used to sequentially excite the fluorescence of aerosol stream, which is located spatially and temporally by two crossed infrared lasers; a bifurcated fiber bundle is applied to receive the fluorescence spectra of 274-463 nm and 374-565 nm. Besides, with a 32-channel photomultiplier tube as detector, a self-developed combined spectrometer with Czerny-Turner design is employed to detect the two band spectra in a preset timing sequence. Experiments show that the system can detect the fluorescence spectra, after dual-wavelength-excitation, of three intrinsic fluorophore samples and three bioaerosol samples.

Retrieval of aerosol liquid water content from high spectral resolution lidar.

Aerosol liquid water content (ALWC) has significant effects on aerosol optical properties, radiative forcing, and the development of severe pollution events. In this study, the vertical distribution and temporal evolution of ALWC were determined through linear particle depolarization measured by a high spectral resolution lidar (HSRL) from December 9 to 12, 2020. Near-surface ALWC datasets retrieved by HSRL were validated by measurements from a three-wavelength humidified nephelometer. The ALWC datasets derived by two methods were highly correlated (R = 0.94, N = 192), illustrating the feasibility of retrieving the ALWC by HSRL. A positive correlation between the ALWC and the enhancement of aerosol scattering coefficient F calculated by the scattering coefficient at 525 nm measured in dry and ambient states proves the reliability of the ALWC obtained from HSRL. However, previous research has implied that fine mode particles dominating the total aerosol loading are required to precisely retrieve the ALWC, while the uncertainty of ALWC data will be large when the particle depolarization ratio is larger than 0.07. When it is less than 0.07, the ALWC derived from HSRL has high precision. By analyzing the aerosol property measurements (e.g., PM2.5, PM10, particle depolarization ratio, and scattering coefficient) near the surface, we found that ALWC contributes greatly to the deterioration of visibility. The variability of optical parameters in the vertical direction showed that ALWC significantly promotes the enhancement of aerosol extinction coefficients. Moreover, high ALWC significantly increases the scattering capacity of aerosols, leading to an enhanced cooling effect on the climate system.

Profiling Aerosol Liquid Water Content Using a Polarization Lidar.

Aerosol liquid water content (ALWC) plays fundamental roles in atmospheric radiation and chemical processes. However, there is little information about ALWC vertical distribution due to the lack of sufficient measurement. In this study, a novel method to retrieve ALWC using a polarization lidar is proposed. By analyzing lidar measurement combined with in situ chemical composition measurements at the surface, the particle linear depolarization ratio δp is found to be well correlated with the liquid water mass fraction. The method is built upon a valid relationship between δp and the ratio of ALWC to the particle backscatter coefficient. ALWC can be retrieved with a relative error of 30% with this method. A case study shows that the ALWC in upper levels of the boundary layer may be different from that at the ground, suggesting the importance of measuring ALWC vertical profiles during haze episodes. The study proves that polarization lidars have the potential to retrieve vertical distributions of ALWC which will benefit studies on haze formation.