An improved method of calibration has been devised for instruments that utilize tunable lasers to measure the absorption spectra of atmospheric gases in order to determine the relative abundances of the gases. In this method, CO2 in the atmosphere is used as a natural calibration standard. Unlike in one prior calibration method, it is not necessary to perform calibration measurements in advance of use of the instrument and to risk deterioration of accuracy with time during use. Unlike in another prior calibration method, it is not necessary to include a calibration gas standard (and the attendant additional hardware) in the instrument and to interrupt the acquisition of atmospheric data to perform calibration measurements.
In the operation of an instrument of this type, the beam from a tunable diode laser or a tunable quantum-cascade laser is directed along a path through the atmosphere, the laser is made to scan in wavelength over an infrared spectral region that contains one or two absorption spectral lines of a gas of interest, and the transmission (and, thereby, the absorption) of the beam is measured. The concentration of the gas of interest can then be calculated from the observed depth of the absorption line(s), given the temperature, pressure, and path length.
CO2is nearly ideal as a natural calibration gas for the following reasons: CO2 has numerous rotation/vibration infrared spectral lines, many of which are near absorption lines of other gases. The concentration of CO2 relative to the concentrations of the major constituents of the atmosphere is well known and varies slowly and by a small enough amount to be considered constant for calibration in the present context. Hence, absorption-spectral measurements of the concentrations of gases of interest can be normalized to the concentrations of CO2. Because at least one CO2 calibration line is present in every spectral scan of the laser during absorption measurements, the atmospheric CO2 serves continuously as a calibration standard for every measurement point.
Figure 1 depicts simulated spectral transmission measurements in a wave-number range that contains two absorption lines of N2O and one of CO2. The simulations were performed for two different upper-atmospheric pressures for an airborne instrument that has a path length of 80 m. The relative abundance of CO2 in air was assumed to be 360 parts per million by volume (approximately its natural level in terrestrial air). In applying the present method to measurements like these, one could average the signals from the two N2O absorption lines and normalize their magnitudes to that of the CO2 absorption line. Other gases wit which this calibration method can be used include H2O, CH4, CO, NO, NO2, HOCl, C2H2, NH3, O3, and HCN.
One can also take advantage of this method to eliminate an atmospheric-pressure gauge and thereby reduce the mass of the instrument: The atmospheric pressure can be calculated from the temperature, the known relative abundance of CO2, and the concentration of CO2 as measured by spectral absorption.
Natural CO2 levels on Mars provide an ideal calibration standard. Figure 2 shows a second example of the application of this method to Mars atmospheric gas measurements. For sticky gases like H2O, the method is particularly powerful, since water is notoriously difficult to handle at low concentrations in pre-flight calibration procedures.
This work was done by Chris Webster of Caltech for NASA's Jet Propulsion Laboratory.
NPO-30401
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Calibrating Laser Gas Measurements by Use of natural CO2
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Overview
The document discusses a novel calibration method for measuring atmospheric gases using tunable diode lasers (TDL) and quantum-cascade (QC) lasers, particularly in the context of planetary missions such as those to Mars. The primary focus is on utilizing carbon dioxide (CO2) as a natural in-flight calibration standard, which addresses the challenges associated with traditional calibration methods that require additional hardware and can compromise measurement accuracy over long durations.
The need for reliable in-flight calibration arises from the lengthy time between pre-launch calibration and the actual measurement period, which can span years. During this time, changes in instrument electronics or the calibration and response of supporting pressure and temperature instrumentation can affect the absolute accuracy of gas measurements. The conventional approach often involves carrying an onboard calibration gas standard, which is difficult to maintain at the same pressure and temperature as the atmospheric gas being measured, and requires complex hardware that increases the instrument's mass.
The proposed method involves scanning the tunable laser over the absorption lines of the gas of interest while simultaneously scanning an adjacent CO2 line. This allows for automatic calibration of the gas measurements without the need for additional calibration cycles, thus enabling continuous data collection. The advantages of this method include reduced instrument mass, elimination of the need for extra pressure gauges, and the ability to take uninterrupted measurements, which is particularly beneficial for long-duration planetary missions.
The document highlights the potential applications of this calibration method for various atmospheric gases, including H2O, N2O, CH4, CO, NO, NO2, HOCl, C2H2, NH3, O3, and HCN, making it versatile for both Earth and Mars applications. The innovative approach not only simplifies the calibration process but also enhances the accuracy and reliability of gas concentration measurements in challenging environments.
In summary, this document presents a significant advancement in atmospheric gas measurement technology, offering a streamlined and effective solution for in-flight calibration that could improve the quality of data collected during planetary exploration missions.
