Calibration of Automatic Sun Photometer with Temperature Correction in Field Environment

Aerosol optical depth (AOD) is an important atmospheric correction parameter in remote sensing. In order to obtain AOD accurately, the surface-based automatic sun photometer needs to carry out calibration regularly. The normally used Langley method can be effective only when the AOD and the calibration coefficients of the instrument remain unchanged throughout the day. However, when observing the AOD with CE318 sun photometer in field environment, it was found that the AOD of silicon (Si) detector at 1020 nm and indium gallium arsenide (InGaAs) detector at 1639 nm was strongly influenced by temperature due to the large temperature difference at the Dunhuang site. Based on the corresponding relationship between AOD and wavelength, the model of the calibration coefficients varying with temperature was established by nonlinear regression method in field environment. By comparing the AOD before and after temperature correction with the theoretical one, the ratio of data with relative error (RE) less than 5% increased from 0.195 and 0.14 to 0.894 and 0.355, respectively. By this method, calibration can be carried out without the limit of constant AOD. In addition, it is simpler, more convenient, and less costly to perform temperature correction in a field environment than in a laboratory.

Spectral-Coding-Based Compressive Single-Pixel NIR Spectroscopy in the Sub-Millisecond Regime

In this contribution, we present a high-speed, multiplex, grating spectrometer based on a spectral coding approach that is founded on principles of compressive sensing. The spectrometer employs a single-pixel InGaAs detector to measure the signals encoded by an amplitude spatial light modulator (digital micromirror device, DMD). This approach leads to a speed advantage and multiplex sensitivity advantage atypical for standard dispersive systems. Exploiting the 18.2 kHz pattern rate of the DMD, we demonstrated 4.2 ms acquisition times for full spectra with a bandwidth of 450 nm (5250–4300 cm−1; 1.9–2.33 µm). Due to the programmability of the DMD, spectral regions of interest can be chosen freely, thus reducing acquisition times further, down to the sub-millisecond regime. The adjustable resolving power of the system accessed by means of computer simulations is discussed, quantified for different measurement modes, and verified by comparison with a state-of-the-art Fourier-transform infrared spectrometer. We show measurements of characteristic polymer absorption bands in different operation regimes of the spectrometer. The theoretical multiplex advantage of 8 was experimentally verified by comparison of the noise behavior of the spectral coding approach and a standard line-scan approach.

A new site: ground-based FTIR XCO₂, XCH₄ and XCO measurements at Xianghe, China

The column-averaged dry-air mole fractions of CO2 (XCO2), CH4 (XCH4) and CO (XCO) have been measured with a Bruker IFS 125HR Fourier transform infrared spectrometer (FTIR) at Xianghe (39.75° N, 116.96° E, North China) since June 2018. The site and the FTIR system are described in this study. The instrumental setup follows the guidelines of the Total Carbon Column Observing Network (TCCON), and the near-infrared spectra are recorded by an InGaAs detector together with a CaF2 beam splitter. The HCl cell measurements that are recorded regularly to derive the instrument line shape (ILS) show that the instrument is correctly aligned. The Xianghe site lies in a polluted area in North China where there are currently no TCCON sites. It can fill the TCCON gap in this region and expand the global coverage of the TCCON measurements. The TCCON standard retrieval code (GGG2014) is applied to retrieve XCO2, XCH4 and XCO. The time series, seasonal cycles and day-to-day variations of XCO2, XCH4 and XCO measurements at Xianghe between June 2018 and July 2019 are shown and discussed. In addition, the FTIR measurements have been used to validate Orbiting Carbon Observatory-2 (OCO-2) and Tropospheric Monitoring Instrument (TROPOMI) satellite observations, as also shown in this paper. The Xianghe FTIR CO2, CH4 and CO data can be accessed at https://doi.org/10.18758/71021049 (Yang et al., 2019).

High-Frequency Raman Analysis in Biological Tissues Using Dual-Wavelength Excitation Raman Spectroscopy

A dual-wavelength excitation Raman probe with laser inputs at 866 nm or 1064 nm is customized and integrated into a compact Raman spectrometer that is based on an InGaAs detector. Under 1064 nm illumination, the spectrometer detects fingerprint Raman signals below 2000 cm–1. While under 866 nm illumination, the spectral range is extended to cover high-frequency region (2400–4000 cm–1) that includes major C–H and O–H Raman vibrations. We demonstrate that the dual excitation InGaAs Raman is beneficial in detecting high-frequency Raman signals, especially water contents in high-fluorescent biological samples such as human dental tissues, grape skin, and plum skin due to the suppressed fluorescence interference.