| During the past decades, the climate and sea ice environment in the polar region has been undergoing tremendous changes. Meanwhile, the polar ocean ecosystem has also been changing significantly. One of the representative indicators of these changes is the dynamics of the ice-algae bio-mass. However, the short-term and destructive limitation of the traditional ice-coreing makes it impossible to monitor the dynamic change of ice-algae seasonally; therefore new methods are needed. An effective way to identify the presence of ice-algae and evaluate its bio-mass is to measure the spectral variation of solar radiation field at different depths of sea ice and proper modeling of these data. Therefore, in this project, we aim at developing such a long-term and dynamic solar irradiance profiling system.Based on the knowledge on the reflection, absorption and scattering performance of solar radiation when it transmits through the media, and the current methods for irradiance measurement in the sea ice environment, a profiling system has been proposed to monitor the solar irradiance at different depths autonomously. A photodiode-array spectrometer module is the core of this profiling system, with multiple-fiber probes located at different locations within and under sea ice. A prototype of this system is developed in laboratory which has twelve fiber probes in total, with ten of them for irradiance profiling measurement and the other two for downwelling and upwelling solar irradiance measurement above the ice. The collected light signal by these probes are guided by corresponding optical fibers, and measured by the spectrometer module. Switch among these fibers is controlled by a fiber alignment control system, which mainly consists of a servo motor and an optical encoder. To enhance signal coupling between the spectrometer and each fiber probe, a collimating lens is attached to the optical entrance of the spectrometer. Additionally, auxiliary sensors such as GPS and Iridium transceiver, and proper software design, are also accommodated in this system to ensure its autonomous monitoring function.The extremely low temperature in the sea ice environment has severe influence on the irradiance measurement by the system. To solve this problem, tremendous efforts have been devoted to temperature dependence evaluation of the spectrometer and its driver circuitry, optical fibers and the mechanical repeatability, from -40 to 25.6℃ in laboratory. After temperature biases of the driver circuitry are corrected, temperature dependence of the spectrometer dark output is fully depicted and thus a prediction model for dark output is developed on the basis of linear interpolation and the least squares method. The uncertainty of this model is within ±0.25% for a spectral range of 320～800 nm. Temperature dependence of spectrometer signal output is also fully analyzed, based on which a multi-integration-time multi-temperature model for signal output is developed for correction. The uncertainty of this model is smaller than ±1.0% at 400～700 nm, which is less than one third of the single-integration-time multi-temperature model used before. Combining these two models with the spectral sensitivity and immersion factor correction model, a correction solution is proposed for the entire system, having an uncertainty of smaller than ±1.0%. Following this solution, the actual solar irradiance spectral intensity at different depths in the sea ice environment could be inferred from the spectrometer measurements with different integration time and temperature. A test has been conducted with an ice column in the laboratory and the results indicate that the whole system works well and its measurement sensitivity is around 1.0×10-3 μw cm-2 nm-1, which is comparable with that of the commercially available oceanographic spectra-radiometers. Additionally, the experimental verification in a -20℃ walk-in cooler indicates that the system is promising for working in cold polar environment in a continuous and stable manner. |
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