Impact of aerosols on the polarization patterns of full-sky background radiation.

Regarding aerosol particle-laded turbid atmospheres, full-sky background radiation polarization patterns can be adversely affected, an important factor limiting their effective near-ground observation and acquisition. We established a multiple-scattering polarization computational model and measurement system and conducted the following three tasks. (a) We thoroughly analyzed the impact of aerosol scattering characteristics on polarization distributions, calculating the degree of polarization (DOP) and angle of polarization (AOP) patterns for a more comprehensive set of atmospheric aerosol compositions and aerosol optical depth (AOD) values than calculated in previous studies. (b) We assessed the uniqueness of the DOP and AOP patterns as a function of AOD. (c) By employing a new polarized radiation acquisition system for measurements, we demonstrated that our computational models are more representative of the DOP and AOP patterns under actual atmospheric conditions. We found that under a clear sky without clouds, the impact of the AOD on the DOP was detectable. With increasing AOD, the DOP decreased, and the decreasing trend became increasingly obvious. When the AOD was above 0.3, the maximum DOP did not exceed 0.5. The AOP pattern did not change notably and remained stable, except for the contraction point at the sun position under an AOD of 2.

Does the degree of polarization of vector beams remain unchanged on atmospheric propagation?

All roads lead to Rome. In this article we propose a novel theoretical framework to demonstrate vector beams whose degree of polarization does not change on atmospheric propagation. Inspired by the Fresnel equations, we derive the reflected and refracted field of vector beams propagating through a phase screen by employing the continuity of electromagnetic field. We generalize the conventional split-step beam propagation method by considering the vectorial properties in the vacuum diffraction and the refractive properties of a single phase screen. Based on this vectorial propagation model, we extensively calculate the change of degree of polarization (DOP) of vector beams under different beam parameters and turbulence parameters both in free-space and satellite-mediated links. Our result is that whatever in the free-space or satellite-mediated regime, the change of DOP mainly fluctuates around the order of 10-13 to 10-6, which is almost negligible.

Optomechanical Design and Application of Solar-Skylight Spectroradiometer.

Using a solar radiometer is an effective approach for improving the remote sensing of solar irradiance distribution and atmospheric composition. Long-term development of a solar scanning radiometer enables frequent and reliable measurement of atmospheric parameters such as the water vapor column and aerosol optical properties. However, the discrete wavelength radiometer has encountered a bottleneck with respect to its insufficient spectral resolution and limited observation waveband, and it has been unable to satisfy the needs of refined and intelligent on-site experiments. This study proposes a solar-skylight spectroradiometer for obtaining visible and near-IR fine spectrum with two types of measurement: direct-sun irradiance and diffuse-sky radiance. The instrument adopts distributed control architecture composed of the ARM-Linux embedded platform and sensor networks. The detailed design of the measuring light-path, two-axis turntable, and master control system will be addressed in this study. To determine all coefficients needed to convert instrument outputs to physical quantities, integrating sphere and Langley extrapolation methods are introduced for diffuse-sky and direct-sun calibration, respectively. Finally, the agreement of experimental results between spectroradiometers and measuring benchmarks (DTF sun-photometer, microwave radiometer, and Combined Atmospheric Radiative Transfer simulation) verifies the feasibility of the spectroradiometer system, and the radiation information of feature wavelengths can be used to retrieve the characteristics of atmospheric optics.

Linear depolarization ratios of nitrate-coated mineral dust particles in haze episodes.

This study reports an internal mixed particle model of dust and nitrate aerosols using the actual haze condition. We performed accurate calculations of linear depolarization ratios (LDR) of nitrate-coated mineral dust particles at three wavelengths (0.35, 0.53, and 1.06 µm) using the T-matrix method. The LDRs of the mono-disperse aerosol particles evolve differently as expressions in the Rayleigh and Mie domains. In the Rayleigh domain, the LDRs increase with the core-shell ratio and the aspect ratio and decrease when the wavelength increases. The forward and backward LDRs depend more on aspect ratio than on the core-shell ratio. In the Mie domain, the LDRs overall increase with the core-shell ratio and the aspect ratio, but there is no significant regular change. When the wavelength increases, the gradual change can be explained by the size parameter of the particles in the vicinity of the Rayleigh domain. For poly-disperse particles, the core-shell ratio mainly affects the position of the side-scattering peak, whereas aspect ratio affects the LDRs. The backscattering LDRs depend more on the variation of aspect ratio, and the core-shell ratio only affects LDRs in a small range. Furthermore, our results on the LDRs are highly promising for remote sensing of the non-spherical and inhomogeneous properties of fine aerosols compared with AERONET measurements. Our results provide a comprehensive understanding of the LDR evolution for coated non-spherical particles in a haze atmosphere. The LDRs can be used as an empirical reference for remote sensing to distinguish coated non-spherical particles.

[Retrieval of the Optical Thickness and Cloud Top Height of Cirrus Clouds Based on AIRS IR High Spectral Resolution Data].

A study was carried out to retrieve optical thickness and cloud top height of cirrus clouds from the Atmospheric Infrared Sounder (AIRS) high spectral resolution data in 1070~1135 cm-1 IR band using a Combined Atmospheric Radiative Transfer model (CART) by brightness temperature difference between model simulation and AIRS observation. The research is based on AIRS LIB high spectral infrared observation data combined with Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product data. Brightness temperature spectra based, on the retrieved cirrus optical thickness and cloud top height were simulated and compared with brightness temperature spectra of AIRS observation in the 650~1150 cm-1 band. The cirrus optical thickness and cloud top height retrieved were compared with brightness temperature of AIRS for channel 760 (900.56 cm-1, 11. 1 µm) and cirrus reflectance of MODIS cloud product. And cloud top height retrieved was compared with cloud top height from MODIS. Results show that the brightness temperature spectra simulated were basically consistent with AIRS observation under the condition of retrieval in the 650~1150 cm-1 band. It means that CART can be used to simulate AIRS brightness temperature spectra. The retrieved cirrus parameters are consistent with brightness temperature of AIRS for channel 11. 1 µm with low brightness temperature corresponding to large cirrus optical thickness and high cloud top height. And the retrieved cirrus parameters are consistent with cirrus reflectance of MODIS cloud product with high cirrus reflectance corresponding to large cirrus optical thickness and high cloud top height. Correlation coefficient of brightness temperature between retrieved cloud top height and MODIS cloud top height was relatively high. They are mostly located in the range of 8. 5~11.5 km, and their probability distribution trend is approximately identical. CART model is feasible to retrieve cirrus properties, and the retrieval is reliable.