scholarly journals Comparison of two closed-path cavity based spectrometers for measuring air-water CO<sub>2</sub> and CH<sub>4</sub> fluxes by eddy covariance

2016 ◽  
Author(s):  
Mingxi Yang ◽  
John Prytherch ◽  
Elena Kozlova ◽  
Margaret J. Yelland ◽  
Deepulal Parenkat Mony ◽  
...  

Abstract. In recent years several commercialized closed-path cavity based spectroscopic instruments designed for eddy covariance flux measurements of carbon dioxide (CO2), methane (CH4), and water vapor (H2O) have become available. Here we compare the performance of two state-of-the-art models – the Picarro G2311-f and the Los Gatos Research (LGR) FGGA at a coastal site. Both instruments can compute dry mixing ratios of CO2 and CH4 based on the concurrently measured H2O. Additionally, we used a high throughput Nafion dryer to physically remove H2O from the Picarro air stream. Observed air-sea CO2 and CH4 fluxes from these two analyzers, averaging about 12 mmol m−2 d−1 and 0.12 mmol m−2 d−1 respectively, agree within the measurement uncertainties. For the purpose of quantifying dry CO2 and CH4 fluxes, the numerical H2O corrections appear to be effective and lead to results that are comparable to physical removal of H2O with a Nafion dryer. We estimate the high frequency attenuation of fluxes in our closed-path setup, which was relatively small (≤ 10 %) for CO2 and CH4 but very large for the much stickier H2O. The Picarro showed significantly lower noise and flux detection limits than the LGR. The hourly flux detection limit for the Picarro was about 2 mmol m−2 d−1 for CO2 and 0.02 mmol m−2 d−1 for CH4. For the LGR these detection limits were about 8 mmol m−2 d−1 and 0.05 mmol m−2 d−1. Using global maps of monthly-mean air-sea CO2 flux as reference, we estimate that the Picarro and LGR can resolve hourly CO2 fluxes from roughly 40 % and 4 % of the world’s oceans, respectively. Averaging over longer timescales would be required in regions with smaller fluxes. Hourly flux detection limits of CH4 from both instruments are generally higher than the expected emissions from the open ocean, though the signal to noise of this measurement may improve closer to the coast.

2016 ◽  
Vol 9 (11) ◽  
pp. 5509-5522 ◽  
Author(s):  
Mingxi Yang ◽  
John Prytherch ◽  
Elena Kozlova ◽  
Margaret J. Yelland ◽  
Deepulal Parenkat Mony ◽  
...  

Abstract. In recent years several commercialised closed-path cavity-based spectroscopic instruments designed for eddy covariance flux measurements of carbon dioxide (CO2), methane (CH4), and water vapour (H2O) have become available. Here we compare the performance of two leading models – the Picarro G2311-f and the Los Gatos Research (LGR) Fast Greenhouse Gas Analyzer (FGGA) at a coastal site. Both instruments can compute dry mixing ratios of CO2 and CH4 based on concurrently measured H2O, temperature, and pressure. Additionally, we used a high throughput Nafion dryer to physically remove H2O from the Picarro airstream. Observed air–sea CO2 and CH4 fluxes from these two analysers, averaging about 12 and 0.12 mmol m−2 day−1 respectively, agree within the measurement uncertainties. For the purpose of quantifying dry CO2 and CH4 fluxes downstream of a long inlet, the numerical H2O corrections appear to be reasonably effective and lead to results that are comparable to physical removal of H2O with a Nafion dryer in the mean. We estimate the high-frequency attenuation of fluxes in our closed-path set-up, which was relatively small ( ≤  10 %) for CO2 and CH4 but very large for the more polar H2O. The Picarro showed significantly lower noise and flux detection limits than the LGR. The hourly flux detection limit for the Picarro was about 2 mmol m−2 day−1 for CO2 and 0.02 mmol m−2 day−1 for CH4. For the LGR these detection limits were about 8 and 0.05 mmol m−2 day−1. Using global maps of monthly mean air–sea CO2 flux as reference, we estimate that the Picarro and LGR can resolve hourly CO2 fluxes from roughly 40 and 4 % of the world's oceans respectively. Averaging over longer timescales would be required in regions with smaller fluxes. Hourly flux detection limits of CH4 from both instruments are generally higher than the expected emissions from the open ocean, though the signal to noise of this measurement may improve closer to the coast.


2018 ◽  
Vol 11 (11) ◽  
pp. 6075-6090 ◽  
Author(s):  
Brian J. Butterworth ◽  
Brent G. T. Else

Abstract. The Arctic marine environment plays an important role in the global carbon cycle. However, there remain large uncertainties in how sea ice affects air–sea fluxes of carbon dioxide (CO2), partially due to disagreement between the two main methods (enclosure and eddy covariance) for measuring CO2 flux (FCO2). The enclosure method has appeared to produce more credible FCO2 than eddy covariance (EC), but is not suited for collecting long-term, ecosystem-scale flux datasets in such remote regions. Here we describe the design and performance of an EC system to measure FCO2 over landfast sea ice that addresses the shortcomings of previous EC systems. The system was installed on a 10 m tower on Qikirtaarjuk Island – a small rock outcrop in Dease Strait located roughly 35 km west of Cambridge Bay, Nunavut, in the Canadian Arctic Archipelago. The system incorporates recent developments in the field of air–sea gas exchange by measuring atmospheric CO2 using a closed-path infrared gas analyzer (IRGA) with a dried sample airstream, thus avoiding the known water vapor issues associated with using open-path IRGAs in low-flux environments. A description of the methods and the results from 4 months of continuous flux measurements from May through August 2017 are presented, highlighting the winter to summer transition from ice cover to open water. We show that the dried, closed-path EC system greatly reduces the magnitude of measured FCO2 compared to simultaneous open-path EC measurements, and for the first time reconciles EC and enclosure flux measurements over sea ice. This novel EC installation is capable of operating year-round on solar and wind power, and therefore promises to deliver new insights into the magnitude of CO2 fluxes and their driving processes through the annual sea ice cycle.


2013 ◽  
Vol 10 (11) ◽  
pp. 18309-18335 ◽  
Author(s):  
E. Podgrajsek ◽  
E. Sahlée ◽  
D. Bastviken ◽  
J. Holst ◽  
A. Lindroth ◽  
...  

Abstract. Fluxes of carbon dioxide (CO2) and methane (CH4) from lakes may have a large impact on the magnitude of the terrestrial carbon sink. Traditionally lake fluxes have been measured using the floating chambers (FC) technique, however, several recent studies use the eddy covariance (EC) method. We present simultaneous flux measurements using both methods at the lake Tämnaren in Sweden during field campaigns in 2011 and 2012. Only very few similar studies exist. For CO2 flux, the two methods agree relatively well during some periods, but deviate substantially at other times. The large discrepancies might be caused by heterogeneity of partial pressure of CO2 (pCO2w) in the EC flux footprint. The methods agree better for CH4 fluxes, it is, however, clear that short-term discontinuous FC measurements are likely to miss important high flux events.


2018 ◽  
Author(s):  
Brian J. Butterworth ◽  
Brent G. T. Else

Abstract. The Arctic marine environment plays an important role in the global carbon cycle. However, there remain large uncertainties in how sea ice affects air–sea fluxes of carbon dioxide (CO2), partially due to disagreement between the two main methods (enclosure and eddy covariance) for measuring CO2 flux (FCO2). The enclosure method has appeared to produce more credible FCO2 than eddy covariance (EC), but is not suited for collecting long-term, ecosystem-scale flux datasets in such remote regions. Here we describe the design and performance of an EC system to measure FCO2 over landfast sea ice that addresses the shortcomings of previous EC systems. The system was installed on a 10-m tower on Qikirtaarjuk Island – a small rock outcrop in Dease Strait located roughly 35 km west of Cambridge Bay, Nunavut in the Canadian Arctic Archipelago. The system incorporates recent developments in the field of air–sea gas exchange by measuring atmospheric CO2 using a closed-path infrared gas analyzer (IRGA) with a dried sample airstream, thus avoiding the known water vapor issues associated with using open-path IRGAs in low-flux environments. A description of the methods and the results from four months of continuous flux measurements from May through August 2017 are presented, highlighting the winter to summer transition from ice cover to open water. We show that the dried, closed-path EC system greatly reduces the magnitude of measured FCO2 compared to simultaneous open-path EC measurements, and for the first time reconciles EC and enclosure flux measurements over sea ice. This novel EC installation is capable of operating year-round on solar/wind power, and therefore promises to deliver new insights into the magnitude of CO2 fluxes and their driving processes through the annual sea ice cycle.


2015 ◽  
Vol 8 (10) ◽  
pp. 4123-4131 ◽  
Author(s):  
J. B. Wu ◽  
X. Y. Zhou ◽  
A. Z. Wang ◽  
F. H. Yuan

Abstract. Eddy covariance using infrared gas analyzes has been a useful tool for gas exchange measurements between soil, vegetation and the atmosphere. So far, comparisons between the open- and closed-path eddy covariance (CP) system have been extensively made on CO2 flux estimations, while lacking in the comparison of water vapor flux estimations. In this study, the specific performance of water vapor flux measurements of an open-path eddy covariance (OP) system was compared against a CP system over a tall temperate forest in northeastern China. The results show that the fluxes from the OP system (LEop) were generally greater than the LEcp though the two systems shared one sonic anemometer. The tube delay of closed-path analyzer depended on relative humidity, and the fixed median time lag contributed to a significant underestimation of LEcp between the forest and atmosphere, while slight systematic overestimation was also found for covariance maximization method with single broad time lag search window. After the optimized time lag compensation was made, the average difference between the 30 min LEop and LEcp was generally within 6.0 %. Integrated over the annual cycle, the CP system yielded a 5.1 % underestimation of forest evapotranspiration as compared to the OP system measurements (493 vs. 469 mm yr−1). This study indicates the importance to estimate the sampling tube delay accurately for water vapor flux calculations with closed-path analyzers, and it also suggests that some of the imbalance of the surface energy budget in flux sites is possibly caused by the systematic underestimation of water vapor fluxes measured with closed-path eddy covariance systems.


2015 ◽  
Vol 8 (5) ◽  
pp. 4711-4736
Author(s):  
J. B. Wu ◽  
X. Y. Zhou ◽  
A. Z. Wang ◽  
F. H. Yuan

Abstract. Eddy covariance using infrared gas analyses has been a useful tool for gas exchange measurements between soil, vegetation and atmosphere. So far, comparisons between the open- and closed-path eddy covariance (CP) system have been extensively made on CO2 flux estimations, while lacking in the comparison of water vapor flux estimations. In this study, the specific performance of water vapor flux measurements of an open-path eddy covariance (OP) system was compared against a CP system over a tall temperate forest in Northeast China. The results show that the fluxes from the OP system (LEop) were generally greater than the (LEcp though the two systems shared one sonic anemometer. The tube delay of closed-path analyser depended on relative humidity, and the fixed median time lag contributed to a significant underestimation of (LEcp between the forest and atmosphere, while slight systematic overestimation was also found for covariance maximization method with single broad time lag search window. After the optimized time lag compensation was made, the average difference between the 30 min (LEop and (LEcp was generally within 6%. Integrated over the annual cycle, the CP system yielded a 5.1% underestimation of forest evapotranspiration as compared to the OP system measurements (493 vs. 469 mm yr−1). This study indicates the importance to estimate the sampling tube delay accurately for water vapor flux calculations with closed-path analysers, and it also suggests that when discuss the energy balance closure problem in flux sites with closed-path eddy covariance systems, it has to be aware that some of the imbalance is possibly caused by the systematic underestimation of water vapor fluxes.


2014 ◽  
Vol 11 (15) ◽  
pp. 4225-4233 ◽  
Author(s):  
E. Podgrajsek ◽  
E. Sahlée ◽  
D. Bastviken ◽  
J. Holst ◽  
A. Lindroth ◽  
...  

Abstract. Fluxes of carbon dioxide (CO2) and methane (CH4) from lakes may have a large impact on the magnitude of the terrestrial carbon sink. Traditionally lake fluxes have been measured using the floating chamber (FC) technique; however, several recent studies use the eddy covariance (EC) method. We present simultaneous flux measurements using both methods at lake Tämnaren in Sweden during field campaigns in 2011 and 2012. Only very few similar studies exist. For CO2 flux, the two methods agree relatively well during some periods, but deviate substantially at other times. The large discrepancies might be caused by heterogeneity of partial pressure of CO2 (pCO2w) in the EC flux footprint. The methods agree better for CH4 fluxes. It is, however, clear that short-term discontinuous FC measurements are likely to miss important high flux events.


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