Abstract
Abstract. We present a novel spectral method to measure atmospheric carbon dioxide (CO2) with high precision and stability without resorting to calibration tanks during long-term operation. This spectral null method improves precision by reducing spectral proportional noise associated with laser emission instabilities. We employ sealed quartz cells with known CO2 column densities to serve as the permanent internal references in the null method, which improve the instrument's stability and accuracy. A prototype instrument – ABsolute Carbon dioxide (ABC) is developed using this new approach. The instrument has a one-second precision of 0.02 ppm, which averages down to 0.007 ppm within one minute. Long-term stability of within 0.1 ppm is achieved without any calibrations for over a one-month period. These results have the potential for eliminating the need for calibration cylinders for high accuracy field measurements of carbon dioxide.
Highlights
Introduction der extreme environmental conditionsone common disadvantage with most previous instruments is that cal-Knowledge on the global budget and the transport of atmospheric carbon dioxide (CO2) is essential to understanding the climate change effects introduced by this important greenhouse gas (GHG)
The null method has been employed by many spectroscopic instruments, such as NDIR analyzers and other quantum cascade laser spectrometers (QCLS) that we have developed
In order to compare the measurements made by Licor and ABsolute Carbon dioxide (ABC), we use the relative molar response (RMR) provided by Tohjima et al (2009) to correct the Licor measurements for isotopic variation and to express the results as measurements of the primary (626) isotopologue of CO2
Summary
The fundamental technique we use for measuring ambient CO2 mixing ratio is Tunable Infrared Laser Differential Absorption Spectrometry (TILDAS). The sample absorption is measured via nonlinear least square Voigt fitting (Olivero and Longbothum, 1977) using simulated transition lines with known spectral parameters (line shape and position from HITRAN, and pressure and temperature from measurements) to obtain a quantitative mixing ratio. The experimental setup of ABC is distinct in the way that we aim to measure the nearly nulled ratio spectra between sample and reference paths. In this way the signal to noise ratio (SNR) and long-term stability are greatly improved. We discuss in detail the special features of this instrument as compared to the other TILDAS spectrometers previously developed in our laboratory
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
