Atmospheric CO 2 monitoring with single-cell NDIR-based analyzers

Abstract. We describe CO2 concentration measurement systems based on relatively inexpensive single-cell non-dispersive infrared CO2 sensors. The systems utilize signal averaging to obtain precision (1-σ in 100 s) of 0.1 parts per million dry air mole fraction (ppm), frequent calibrations and sample drying in order to achieve state-of-the-art compatibility, and can run autonomously for months at a time. Laboratory tests indicate compatibility among four to six systems to be ±0.1 ppm (1-σ), and field measurements of known reference-gases yield median errors of 0.01 to 0.17 ppm with 1-σ variance of ±0.1 to 0.2 ppm. From May to August 2007, a system co-located with a NOAA-ESRL dual-cell NDIR system at the WLEF tall tower in Wisconsin measured daytime-only daily averages of CO2 that differ by 0.26 ± 0.15 ppm (median ± 1 σ), and from August 2005 to April 2011 a system co-located with weekly NOAA-ESRL network flask collection at Niwot Ridge, Colorado measured coincident CO2 concentrations that differed by −0.06 ± 0.30 ppm (n = 585). Data from these systems are now supporting a wide range of analyses and this approach may be applicable in future studies where accuracy and initial cost of the sensors are priorities.

� Current attempts to infer surface CO 2 fluxes from concentration measurements are limited by both errors in modeling atmospheric transport (Lin et al., 2004) and sparseness in the available data (Gurney et al., 2002).data from these systems are sufficient; a number of these systems are still operating in the field and will continue to do so for years to come; low cost than laser-based instruments; many of the gas-handling, sample drying, calibration, and intercomparison issues are relevant to any atmospheric CO 2 measurement system regardless of sensor type.� The CO 2 levels are measured using a single-cell sensor employing NDIR absorption spectroscopy (LI-COR, model LI-820).The LI-820 actively and precisely controls the optical bench to 50℃ and has good stability with respect to ambient temperature of around 0.1 ppm℃ -1 .

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� The LI-820 also provides a measurement of cell pressure and applies its own internal concentration-dependent pressure correction.
� The enclosure is temperature controlled to 30℃, with a fan to minimize temperature gradients.
� Automated leak tests are performed following every calibration cycle.
� Differences in AIRCOA system � The AIRCOA data-processing software uses measured temperature variations and changes in pressure during leak checks to define monthly empirical temperature and concentration-dependent pressure effects on the LI-820 data.
� Calibration using field standards � Even with internal or post-processing temperature and pressure compensation, the LI-820 CO 2 calibration typically drifts by 0.3 ppmday -1 with fluctuations of similar magnitude possible over several hours.� Known cylinder measurements    � Nanjing, as a developed city, we want to know its increase in CO 2 concentration caused by urbanization.

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Use of atmospheric CO 2 concentration measurements to estimate CO 2 fluxes has become finer scale in recent years.

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The history of atmospheric CO 2 measurements: flask-collection program dual-cell NDIR-based analyzer single-cell NDIR-based analyzer laser-based instrument � Reasons for select single-cell NDIR-based analyzer:

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Fig. 1.PSU system schematics of the single-cell NDIR-based CO 2 concentration measurement systems.

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Fig. 5. Results from field measurements of cylinders of known concentration for PSU systems.

Fig. 6 .Fig. 7 .
Fig. 6. Results from field measurements of cylinders of known concentration for AIRCOA systems.

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Fig. 9. Map of Nanjing City showing locations of the urban-forest gradient and measurement sites from S1 to S9 (ZHAO De-Hua et al,.2009 ).

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Fig. 10.Select for Nanjing road and this is likely because many of the potential causes of error in CO 2 systems relate to things other than sensor stability.Suggestions: recommend introducing target or surveillance gases through the entire inlet system; the post-processing pressure, temperature, and flushing corrections can improve the quality of the final data; a program of multiple intercomparison activities.China has been considered responsible for two thirds of the global increase in anthropogenic carbon dioxide emissions of 3.1% in 2007 (Yan andYang,2010).
� Once calibrations are performed, the performance of lessexpensive NDIR-based measurements is not dramatically disparate from that shown in field tests of laser-based instruments (Richardson et al., 2011), � � Among the largest carbon dioxide emitters in the world (IEA,2009), � CO 2 concentration measured with mobile observation method is very few in China (ZHAO De-Hua et al,.2009).