Optical Properties of Aerosols in Bohai Bay Based on Shipboard Platform Sun-Photometer

Hai Zhu; Jianyu Li; Honghua Huang; Gang Xu; Congming Dai; Heli Wei

doi:10.3788/AOS221169

Abstract

Results and Discussions Shipboard sun-photometer has been successfully developed to meet the performance indexes, and long-term observation has been carried out in Bohai bay. The experimental results show that the daily average aerosol optical thickness in Bohai bay is mostly concentrated in the range of 0.1-0.3. Although human-made pollution is increasing in recent years, the frequency distribution of the daily average ?ngstr?m index in Bohai bay is basically Gaussian, which indicates that the sea area has not caused large pollution with the relatively clean atmosphere (Fig. 7). The aerosol in Bohai bay shows a bimodal spectrum structure, and there are significant differences between August, September, and November. In August and September, the atmosphere is dominated by small particles, while in late autumn, November, is dominated by large particles. The average ?ngstr?m index of the two periods is about 1.1 and 0.5 (Fig. 6).Objective

Aerosols play an important role in the balance of the earth's atmospheric radiation budget due to their complex composition and increasing particle concentration. Sun-photometer is an effective measurement device for remote sensing atmospheric parameters by measuring solar spectral radiation and is widely employed in ground-based remote sensing of aerosols. However, the sea covers a large area. In the research on the optical characteristics of aerosols on earth and climate change, there is an urgent need for a device that can measure the optical characteristics of aerosols with high accuracy under the shipboard platform to compensate for the lack of atmospheric data at sea.

Methods

The traditional sun-photometer is difficult to track the sun on a moving shipboard platform, which cannot meet the needs of sea aerosol observation. The research group develops a novel shipboard sun-photometer with a new tracking method. The sun can be tracked on a shipboard platform with an accuracy of better than 1'. This new sun-photometer could obtain radiation information in nine spectral bands of 400, 440, 532, 550, 780, 870, 940, 1050, and 1064 nm at once. The aerosol information of the sea area can be obtained by direct solar radiation remote sensing on the mobile platform.

The instrument adopts a two-segment image tracking method instead of the traditional four-quadrant tracking method. Firstly, the fish-eye imaging system is employed to obtain the whole sky image, and the coarse tracking of the sun is completed. Then, the precision tracking imaging system is leveraged to improve the resolution of the solar image to improve the tracking accuracy. The development process of the shipboard sun-photometer is introduced in detail, including the instrument's two-dimensional turntable, image tracking system, and measurement optical path. The function and working flow of each main structure of the instrument are described in detail with the two-dimensional turntable of the shipboard sun-photometer as the starting point. After that, the theoretical tracking accuracy of the image tracking system is calculated in detail by image processing technology and can reach 0.744' to meet the tracking accuracy requirements of offshore measurement. Finally, a spectrometric measurement system is introduced based on the integrated design of spectrometric measurement systems. The structure and operation flow of the spectrometric measurement system are analyzed specifically, and the influence of stray light and detector saturation on the measurement results is thoroughly considered with corresponding improvement approaches.

Conclusions

The Langley calibration method and the improved Langley method are adopted to calibrate the bands without atmospheric molecular absorption and water vapor absorption, respectively. The 550 nm aerosol optical thickness and ?ngstr?m index measured by the shipboard platform sun-photometer are compared with the measurement results of the POM-01 MK III marine sun-photometer. The diurnal variation trend of the 550 nm aerosol optical thickness is basically similar, the determination coefficient is 0.968, and the average relative measurement error is 4.83%. The ?ngstr?m index has an average relative measurement error of 2.55%. The reliability and stability of the shipboard sun-photometer are verified, and the optical properties of other atmospheric parameters can be further retrieved via the radiation information of visible and near-infrared bands. This instrument enriches the technical means for measuring the parameters of aerosol optical characteristics at sea and lays a solid experimental technical foundation for the research on space remote sensing, climate change, and atmospheric environment.