Jinjia Guo

Direct-detection Doppler wind measurements with a Cabannes-Mie lidar: A. Comparison between iodine vapor filter and Fabry-Perot interferometer methods

Atmospheric line-of-sight (LOS) wind measurement by means of incoherent Cabannes-Mie lidar with three frequency analyzers with nearly the same maximum transmission of ~80% that could be fielded at different wavelengths is analytically considered. These frequency analyzers are (a) a double-edge Fabry-Perot interferometer (FPI) at 1064 nm (IR-FPI), (b) a double-edge Fabry-Perot interferometer at 355 nm (UV-FPI), and (c) an iodine vapor filter (IVF) at 532 nm with two different methods, using either one absorption edge, single edge (se-IVF), or both absorption edges, double edge (de-IVF). The effect of the backscattered aerosol mixing ratio, R(b), defined as the ratio of the aerosol volume backscatter coefficient to molecular volume backscatter coefficient, on LOS wind uncertainty is discussed. Assuming a known aerosol mixing ratio, R(b), and 100,000 photons owing to Cabannes scattering to the receiver, in shot-noise-limited detection without sky background, the LOS wind uncertainty of the UV-FPI in the aerosol-free air (R(b)=0), is lower by ~16% than that of de-IVF, which has the lowest uncertainty for R(b) between 0.02 and 0.08; for R(b)>0.08, the IR-FPI yielded the lowest wind uncertainty. The wind uncertainty for se-IVF is always higher than that of de-IVF, but by less than a factor of 2 under all aerosol conditions, if the split between the reference and measurement channels is optimized. The design flexibility, which allows the desensitization of either aerosol or molecular scattering, exists only with the FPI system, leading to the common practice of using IR-FPI for the planetary boundary layer and using UV-FPI for higher altitudes. Without this design flexibility, there is little choice but to use a single wavelength IVF system at 532 nm for all atmospheric altitudes.

Direct-detection Doppler wind measurements with a Cabannes–Mie lidar: B. Impact of aerosol variation on iodine vapor filter methods

Atmospheric line-of-sight (LOS) wind measurement by means of incoherent Cabannes- Mie lidar with three frequency analyzers, two double-edge Fabry-Perot interferometers, one at 1064 nm (IR-FPI) and another at 355 nm (UV-FPI), as well as an iodine vapor filter (IVF) at 532 nm, utilizing either a single absorption edge, single edge (se-IVF), or both absorption edges, double edge (de-IVF), was considered in a companion paper [Appl. Opt. 46, 4434 (2007)], assuming known atmospheric temperature and aerosol mixing ratio, Rb. The effects of temperature and aerosol variations on the uncertainty of LOS wind measurements are investigated and it is found that while the effect of temperature variation is small, the variation in R(b) can cause significant errors in wind measurements with IVF systems. Thus the means to incorporate a credible determination of R(b) into the wind measurement are presented as well as an assessment of the impact on wind measurement uncertainty. Unlike with IVF methods, researchers can take advantage of design flexibility with FPI methods to desensitize either molecular scattering for IR-FPI or aerosol scattering for UV-FPI. The additional wind measurement uncertainty caused by R(b) variation with FPI methods is thus negligible for these configurations. Assuming 100,000 photons from Cabannes scattering, and accounting for the Rb measurement incorporated into the IVF method in this paper, it is found that the lowest wind uncertainty at low wind speeds in aerosol-free air is still with UV-FPI, ~32% lower than with de-IVF. For 0.050.07, the IR-FPI outperforms all other methods. In addition to LOS wind uncertainty comparison under high wind speed conditions, the need of an appropriate and readily available narrowband filter for operating the wind lidar at visible wavelengths under sunlit condition is discussed; with such a filter the degradation of LOS wind measurement attributable to clear sky background is estimated to be 5% or less for practical lidar systems.

Investigation of two novel approaches for detection of sulfate ion and methane dissolved in sediment pore water using Raman spectroscopy.

The levels of dissolved sulfate and methane are crucial indicators in the geochemical analysis of pore water. Compositional analysis of pore water samples obtained from sea trials was conducted using Raman spectroscopy. It was found that the concentration of SO42− in pore water samples decreases as the depth increases, while the expected Raman signal of methane has not been observed. A possible reason for this is that the methane escaped after sampling and the remaining concentration of methane is too low to be detected. To find more effective ways to analyze the composition of pore water, two novel approaches are proposed. One is based on Liquid Core Optical Fiber (LCOF) for detection of SO42−. The other one is an enrichment process for the detection of CH4. With the aid of LCOF, the Raman signal of SO42− is found to be enhanced over 10 times compared to that obtained by a conventional Raman setup. The enrichment process is also found to be effective in the investigation to the prepared sample of methane dissolved in water. By CCl4 extraction, methane at a concentration below 1.14 mmol/L has been detected by conventional Raman spectroscopy. All the obtained results suggest that the approach proposed in this paper has great potential to be developed as a sensor for SO42− and CH4 detection in pore water.

Diurnal Variability in Chlorophyll-a, Carotenoids, CDOM and SO42− Intensity of Offshore Seawater Detected by an Underwater Fluorescence-Raman Spectral System

A newly developed integrated fluorescence-Raman spectral system (λex = 532 nm) for detecting Chlorophyll-a (chl-a), Chromophoric Dissolved Organic Matter (CDOM), carotenoids and SO42− in situ was used to successfully investigate the diurnal variability of all above. Simultaneously using the integration of fluorescence spectroscopy and Raman spectroscopy techniques provided comprehensive marine information due to the complementarity between the different excitation mechanisms and different selection rules. The investigation took place in offshore seawater of the Yellow Sea (36°05′40′′ N, 120°31′32′′ E) in October 2014. To detect chl-a, CDOM, carotenoids and SO42−, the fluorescence-Raman spectral system was deployed. It was found that troughs of chl-a and CDOM fluorescence signal intensity were observed during high tides, while the signal intensity showed high values with larger fluctuations during ebb-tide. Chl-a and carotenoids were influenced by solar radiation within a day cycle by different detection techniques, as well as displaying similar and synchronous tendency. CDOM fluorescence cause interference to the measurement of SO42−. To avoid such interference, the backup Raman spectroscopy system with λex = 785 nm was employed to detect SO42− concentration on the following day. The results demonstrated that the fluorescence-Raman spectral system has great potential in detection of chl-a, carotenoids, CDOM and SO42− in the ocean.

Highly sensitive Raman system for dissolved gas analysis in water

The detection of dissolved gases in seawater plays an important role in ocean observation and exploration. As a potential technique for oceanic applications, Raman spectroscopy has already proved its advantages in the simultaneous detection of multiple species during previous deep-sea explorations. Due to the low sensitivity of conventional Raman measurements, there have been many reports of Raman applications on direct seawater detection in high-concentration areas, but few on undersea dissolved gas detection. In this work, we have presented a highly sensitive Raman spectroscopy (HSRS) system with a special designed gas chamber for small amounts of underwater gas extraction. Systematic experiments have been carried out for system evaluation, and the results have shown that the Raman signals obtained by the innovation of a near-concentric cavity was about 21 times stronger than those of conventional side-scattering Raman measurements. Based on this system, we have achieved a low limit of detection of 2.32 and 0.44  μmol/L for CO2 and CH4, respectively, in the lab. A test-out experiment has also been accomplished with a gas-liquid separator coupled to the Raman system, and signals of O2 and CO2 were detected after 1 h of degasification. This system may show potential for gas detection in water, and further work would be done for the improvement of in situ detection.

Feasibility investigation on deep ocean compact autonomous Raman spectrometer developed for in-situ detection of acid radical ions

A newly developed Deep Ocean Compact Autonomous Raman Spectrometer (DOCARS) system is introduced and used for in-situ detection of acid radical ions in this paper. To evaluate the feasibility and capability of DOCARS for quantitative analysis of the acid radical ions in the deep ocean, extensive investigations have been carried out both in laboratory and sea trials during the development phase. In the laboratory investigations, Raman spectra of the prepared samples (acid radical ions solutions) were obtained, and analyzed using the method of internal standard normalization in data processing. The Raman signal of acid radical ions was normalized by that of water molecules. The calibration curve showed that the normalized Raman signal intensity of SO42−, NO3−, and HCO2− increases linearly as the concentration rises with correlation coefficient R2 of 0.99, 0.99, and 0.98 respectively. The linear function obtained from the calibration curve was then used for the analysis of the spectra data acquired in the sea trial under a simulating chemical field in the deep-sea environment. It was found that the detected concentration of NO3− according to the linear function can reflect the concentration changes of NO3− after the sample was released, and the detection accuracy of the DOCARS system for SO42− is 8%. All the results showed that the DOCARS system has great potential in quantitative detection of acid radical ions under the deep-sea environment, while the sensitivity of the DOCARS system is expected to be improved.

Research on characters of the marine atmospheric boundary layer structure and aerosol profiles by high spectral resolution lidar

We briefly discuss the characters of aerosol profiles in the marine atmospheric boundary layer (MABL) detected by a high spectral resolution lidar (HSRL) system developed by Ocean University of China. In this 532-nm lidar system, an iodine filter is used to reject the Mie scattering signal and extract the Rayleigh signal. We deduce the aerosol scattering ratio (Rb) by the HSRL and analyze the correlation index of Rb and relative humidity (RH) under different weather conditions. The height of MABL is derived from the Rb profiles using a wavelet method.

A Direct Bicarbonate Detection Method Based on a Near-Concentric Cavity-Enhanced Raman Spectroscopy System

Raman spectroscopy has great potential as a tool in a variety of hydrothermal science applications. However, its low sensitivity has limited its use in common sea areas. In this paper, we develop a near-concentric cavity-enhanced Raman spectroscopy system to directly detect bicarbonate in seawater for the first time. With the aid of this near-concentric cavity-enhanced Raman spectroscopy system, a significant enhancement in HCO3− detection has been achieved. The obtained limit of detection (LOD) is determined to be 0.37 mmol/L—much lower than the typical concentration of HCO3− in seawater. By introducing a specially developed data processing scheme, the weak HCO3− signal is extracted from the strong sulfate signal background, hence a quantitative analysis with R2 of 0.951 is made possible. Based on the spectra taken from deep sea seawater sampling, the concentration of HCO3− has been determined to be 1.91 mmol/L, with a relative error of 2.1% from the reported value (1.95 mmol/L) of seawater in the ocean. It is expected that the near-concentric cavity-enhanced Raman spectroscopy system could be developed and used for in-situ ocean observation in the near future.

A Study to Measure Optical Properties of Waters by Oceanographic Lidar with variable Field-of-View

Return signal of oceanographic Lidar is a decaying exponential function of the attenuation coefficient which is related to the optical properties of water. The Lidar attenuation coefficient ( Klidar ) obtained from traditional oceanographic Lidar with single field of view (FOV) cannot be used to effectively estimate parameters of the water optical properties due to the deficiency of Lidar equation. However, this exponential decay of elastic backscattering from water is strongly dependent on both FOV of Lidar and optical properties of water. Thus, an approach using a variable FOV receiver has become a potential way to compensate the deficiency in the measurement by traditional Lidar. Three major parts are presented in the paper. Firstly, on the basis of historical models of oceanographic Lidar, a practical model between Klidar and FOV angles is derived to estimate the optical property parameters, namely a (absorption coefficient), b (scattering coefficient) and Kd (diffuse attenuation coefficient). Secondly, for the purpose of getting the appropriate FOVs of a shipboard oceanographic Lidar, return signals are simulated under different FOVs to analyze the relationship between Klidar and the water optical parameters with the impact of background noise taken into account. Finally, the experiments are conducted to measure optical properties of water by using oceanographic Lidar with different FOVs. The optical properties of water bodies are estimated in the cases of different FOVs, and the accuracy of measurements is analyzed. Experimental analysis verifies the feasibility of measuring multiple optical properties of seawater by oceanographic Lidar with variable FOV.

Analysis and Modeling Methodologies for Heat Exchanges of Deep-Sea In Situ Spectroscopy Detection System Based on ROV

In recent years, cabled ocean observation technology has been increasingly used for deep sea in situ research. As sophisticated sensor or measurement system starts to be applied on a remotely operated vehicle (ROV), it presents the requirement to maintain a stable condition of measurement system cabin. In this paper, we introduce one kind of ROV-based Raman spectroscopy measurement system (DOCARS) and discuss the development characteristics of its cabin condition during profile measurement process. An available and straightforward modeling methodology is proposed to realize predictive control for this trend. This methodology is based on the Autoregressive Exogenous (ARX) model and is optimized through a series of sea-going test data. The fitting result demonstrates that during profile measurement processes this model can availably predict the development trends of DORCAS’s cabin condition during the profile measurement process.