Optimized Classification of Suspended Particles in Seawater by Dense Sampling of Polarized Light Pulses.

Suspended particles affect the state and vitality of the marine ecosystem. In situ probing and accurately classifying the suspended particles in seawater have an important impact on ecological research and environmental monitoring. Individual measurement of the optical polarization parameters scattered by the suspended particles has been proven to be a powerful tool to classify the particulate compositions in seawater. In previous works, the temporal polarized light pulses are sampled and averaged to evaluate the polarization parameters. In this paper, a method based on dense sampling of polarized light pulses is proposed and the experimental setup is built. The experimental results show that the dense sampling method optimizes the classification and increases the average accuracy by at least 16% than the average method. We demonstrate the feasibility of dense sampling method by classifying the multiple types of particles in mixed suspensions and show its excellent generalization ability by multi-classification of the particles. Additional analysis indicates that the dense sampling method basically takes advantage of the high-quality polarization parameters to optimize the classification performance. The above results suggest that the proposed dense sampling method has the potential to probe the suspended particles in seawater in red-tide early warning, as well as sediment and microplastics monitoring.

Atmospheric microplastics at a southern China metropolis: Occurrence, deposition flux, exposure risk and washout effect of rainfall.

Atmospheric microplastics (AMPs) have raised much concern for public health due to their potential for exposure. In this study, temporal distribution, characteristics and exposure risk of AMPs were studied in the urban area of Guangzhou, a metropolis in Southern China, and the washout effect of rainfall on AMPs was investigated. It was found that AMP abundances in Guangzhou were in a range of 0.01-0.44 items/m3, with higher abundance in the wet season (0.19 ± 0.01 items/m3) than in the dry season (0.15 ± 0.02 items/m3). The distribution of AMPs did not correspond to that of common air pollutants (e.g., PM2.5 and PM10), implying that their pollution sources might be distinct. In Guangzhou, a total of 1.26 × 1011 items AMPs are in the air every year, and annual inhalation exposure of adults was estimated to be in the range of 79.65-3.50 × 103 items. The annual deposition flux of AMPs is 65.94 ± 7.53 items/m2/d, and the deposition flux in the wet season (84.00 ± 6.95 items/m2/d) was much greater than that in the dry season (47.88 ± 8.35 items/m2/d). Furthermore, rainfall has an effective mechanism for removing AMPs from the atmosphere, with an average washout ratio of (19.39 ± 6.48) × 104 for rainfall washing AMPs out. Compared to moderate rain (2.5-10 mm/h) and heavy rain (10-50 mm/h), light rain (rainfall intensity <2.5 mm/h) had a better washout effect. This study contributes to the evaluation of AMP exposure risk and understanding of AMP environmental behavior and fate by providing long-term monitoring data on AMPs and quantifying the washout effect of rainfall on AMPs for the first time.

Recognition of microplastics suspended in seawater via refractive index by Mueller matrix polarimetry.

Microplastics have become the marine pollution posing a human health risk, but they are difficult to be detected and recognized for different materials, irregular shapes, and broad size distributions. Microplastics' refractive index (RI) is related to the materials and can be characterized by the Mueller matrix. In this work, the particles are suspended in water and their Mueller matrices are measured by a particulate Mueller matrix polarimetry setup. Four kinds of spherical particles including microplastics are effectively classified by their Mueller matrices. Moreover, two kinds of common microplastics with broad size distributions, irregular shapes, and random orientations are also well recognized by the Mueller matrix. These results imply that RI plays a vital role in the recognition of microplastics suspended in water. By using the Mie theory and discrete dipole approximation simulation, the discussions explain in physics origin how RI affects Mueller matrix coupling with size and structure, and give some decoupling methods. Results in this work help advance future tools to in situ recognize the microplastics in seawater.

Probing Individual Particles in Aquatic Suspensions by Simultaneously Measuring Polarized Light Scattering and Fluorescence.

Suspended particles play a significant role in aquatic systems. However, existing methods to probe suspended particles have several limitations. In this paper, we present a portable prototype to in situ probe individual particles in aquatic suspensions by simultaneously measuring polarized light scattering and fluorescence, aiming to obtain an effective classification of microplastics and microalgae. Results show that the obtained classification accuracy is significantly higher than that for either of these two methods. The setup also successfully measures submicron particles and discriminates two species of Synechococcus. Our study demonstrates the feasibility of simultaneously measuring polarized light scattering and fluorescence, and the promising capability of our method for further aquatic environmental monitoring.

Two-dimensional backscattering Mueller matrix of sphere-cylinder scattering medium.

We present the experimental results for the two-dimensional backscattering Mueller matrix of a scattering medium containing polystyrene microspheres and silk fibers and simulate the same Mueller matrix using a polarization-sensitive Monte Carlo program with both layered and homogeneous sphere-cylinder scattering models. We discuss the characteristic features in each Mueller matrix element and their relations with the parameters of the spherical and cylindrical scatterers in the medium. Both experiments and simulations suggest that the Mueller matrix elements can be used to characterize the structural and optical properties of anisotropic scattering media.