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An integrated flask sample collection system for greenhouse gas measurements

Atmospheric Measurement Techniques (2012) 5(9) 2321-2327
DOI: 10.5194/amt-5-2321-2012
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Abstract

A one hour integrated flask sampling system to collect air in automated NOAA/ESRL 12-flask packages is described. The integrating compressor system uses a mass flow controller to regulate the flow of air through a 15 l volume, thus providing a mixture of air collected over an hour-long period. By beginning with a high flow rate of 3.8 standard liters per minute and gradually decreasing the flow rate over time to 0.29 standard liters per minute it is possible to obtain a nearly uniformly time averaged sample of air and collect it into a pressurized 0.7 l flask. The weighting function determining the air mixture obtained is described in detail. Laboratory and field tests demonstrate that the integrated sample approximates a simple mean of air collected during the one-hour sampling time. © 2012 Author(s).

Figures

  • Fig. 1. Integrating compressor flask sample collection system schematic.
  • Fig. 2. Measured CO2 mole fraction through time, starting with “ambient” 378.95 ppm CO2 air inside integrating volume. Air entering the volume was switched to “zero” 0 ppm CO2 air at time zero. Flow rate was set at 3.8 SLPM.
  • Fig. 3. Weighting function for air inside the 15 l integrating volume after 60 min, under different flow scenarios. Black line: ideal linear weighting function where each timestep contributes an equal amount to the final sample (1.67 % from each minute). Solid and dotted grey lines: constant flow at 1 and 4 SLPM, respectively. Red line: the weighting function we selected changing the flow rate every 12 min from 3.8 SLPM to 1 SLPM to 0.55 SLPM to 0.38 SLPM to 0.29 SLPM. The value at minute zero for each curve is the sum total of all air remaining in the flask from times prior to the last 60 min.
  • Fig. 4. Test of weighting function. Cylinder air at room pressure was flowed through the integrating system at the indicated flow rates (blue). Note that the flow rates in this experiment were slightly different than our standard protocol used in Fig. 3. Natural 363.5 ppm CO2 air was flowed through the system for 20 min prior to the start of the experiment and during the experiment, except during the periods shaded in grey, when incoming air was switched to zero air. The measured CO2 mole fraction exiting the system is indicated in black, and was 335.3 ppm at the end of the integrating time (60 min). Predicted values at each time step are shown in red, and the final predicted value was 335.1 ppm.
  • Fig. 5. Mean flask pair differences, typically averaged over 12 flask pairs. Measured mean pair differences for each species are shown in red, and the acceptable maximum pair difference is shown in black. Vertical axis uses a log scale for clarity, units vary by species. CO2 is reported in ppm, CO, CH4, N2O and H2 are in ppb, δ 13CO2 and δ13CH4 are in ‰, all other species are in reported in ppt.
  • Fig. 6. Difference between predicted and measured flask CO2 mole fraction, plotted against the CO2 mole fraction. Predicted mole fractions were calculated from in situ (∼ 2 s) CO2 measurements. Red points use the full weighting function to calculate the predicted CO2 mole fraction; blue points use a simple average of the in situ values during the flask sampling time. Error bars on the blue points indicate the standard deviation of the in situ measurements during the sampling time.
  • Fig. 7. Difference between in situ mean and flask CO2 mole fraction for 50 flasks sampled at two sites near Indianapolis. Error bars are the standard deviation of the in situ values during the sampling time. Mean difference is 0.04 ppm.

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APA

Turnbull, J., Guenther, D., Karion, A., Sweeney, C., Anderson, E., Andrews, A., … Tans, P. (2012). An integrated flask sample collection system for greenhouse gas measurements. Atmospheric Measurement Techniques, 5(9), 2321–2327. https://doi.org/10.5194/amt-5-2321-2012

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