Simultaneous measurement of N2O and CH4 emissions from agriculture using photoacoustic

detection and QCL laser

Arto Branders, Senior Client Partner, Gasera Ltd. Pittcon 2016, 7.3.2016, 3:05 PM

Gasera Ltd. Tykistökatu 4, 20520 Turku, Finland

Background

Agriculture contributes significantly to the anthropogenic emissions of non-CO2 greenhouse gases (GHG) methane (CH4) and nitrous oxide (N2O). GHG emissions comes from livestock, agricultural soils, and rice production. Methane released to the atmosphere by domestic ruminant livestock represents one of the largest global sources of methane [1].

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Characteristics

Real time continuous standalone monitoring. Low ambient background level for analytes. Ambient water vapor interferes with analytes. Single digit ppb level sensitivity is required for accurate detection of changes in the ambient background level. Presence of dust and corrosive gases. Changing seasonal conditions for ambient temperature and humidity.

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Concept

Cantilever-enhanced photoacoustic spectroscopy with tunable laser source Photoacoustic detection is based on a silicon MEMS cantilever sensor coupled with an optical interferometric readout system [2] Laser exciting fundamental vibrational bands at Mid-IR wavelengths (quantum cascade or gas laser) provides good performance One tunable distributed feedback quantum cascade laser (DFB-QCL) source covers the spectral lines of both target gases

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Concept

Laser wavelength range is selected so that single absorption lines characteristic to both target molecules can be used With tunable laser a wavelength modulation across the absorption line can be performed in order to minimize background signal The photoacoustic signal is generated in the double frequency, while possible signal from interfering gases is in the fundamental frequency

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Cantilever sensor coupled with an optical readout system provides more than 100 times higher sensitivity compared to condenser microphones [3], [4] Contactless optical measurement based on laser interferometer is used in converting the cantilever sensor displacement in to electric signal The optical cantilever microphone has highly linear response over a wide dynamic range The microphone output signal is directly in digital form, and therefore, the dynamic range is not limited by the analog circuitry The response of the cantilever remains stabile, even if the ambient temperature varies

Optical microphone

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Laser selection

Simulation of CH4 and N2O spectra from 1277 to 1279 cm-1 at 300 mbar sample pressure. Water is included. Single QCL covers both gases

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Realization

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Realization

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Key elements and parameters

Multi-purpose analyzer bench equipped with a commercial continuous wave DFB-QCL mounted on a high heat load package, Peltier elements with water cooling, gas exchange system, CPU and user interface QCL tuning range is approx. 1275 – 1283 cm-1, power ~ 50 mW and initial operating temperature 30 ºC Laser scan period is 5 seconds for each gas Measurement response time is 30 seconds Measurement pressure is 300 mbar Calibration with both analyte gases in Nitrogen base and also with dry Nitrogen (6.0) for BG verification Photoacoustic relaxation was not noticed, therefore sample gas doesn’t need humidification Ambient moisture and carbon dioxide doesn’t interfere with analyte gases

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Laser scan spectra for analytes

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Laser scan for CH4 101 ppm in N2. Laser scan for N2O 5.16 ppm in synthetic air.

X-axis is relative to the laser drive parameter, Y-axis is relative to the photoacoustic signal

Laser scan spectra for background

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Laser scan for mixture of CO2 505 ppm, H2O 1.0 %, O2 10 % , and balance N2.

Performance

The detection limits for CH4 and N2O were estimated by calculating 2xRMS noise of analysis with N2 6.0. Channel integration time (CIT) for both lasers was 5 s. Detection limit is 27 ppb and 9.0 ppb for CH4 and N2O, respectively. The measurement repeatability for CH4 is 0.68 % (1xRMS) in N2, and for N2O 0.77 % (1xRMS) in synthetic air.

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CH4 10.1 ppm analysis results with 5 s CIT. N2O 10 ppm analysis results with 5 s CIT.

Con

cent

ratio

n (p

pm)

Performance

Accuracy was estimated by measuring different concentrations of both analyte gases with the instrument.

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N2O analysis end errors for different concentrations

CH4 analysis end errors for different concentrations

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cent

ratio

n (p

pm)

Conclusions

Livestock emissions as well as biological and abiological processes in soil represent a major source of Greenhouse Gases. Therefore, the measurement of agriculture and animal husbandry based GHGs represents an important part of climate change research. Cantilever-enhanced photoacoustic detection combined with widely tunable mid-IR laser source provides an attractive platform for selective and sensitive simultaneous monitoring of methane and nitrous oxide without the interference of other atmospheric gases. One tunable distributed feedback quantum cascade laser source can cover the spectral lines of both CH4 and N2O, and therefore, both target molecules can be measured with one instrument. The platform presented is versatile in terms of sensitivity, repeatability, wide dynamic measurement range and speed and therefore can be used in various air quality applications.

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References

[1] E. Negussie, MTT Agrifood Research Finland, Biotechnology & Food Research [2] T. Kuusela, J. Kauppinen. Appl. Spectrosc. Rev., 42, (2007) [3] V. Koskinen, J. Fonsen, J. Kauppinen, I. Kauppinen, Vibr. Spectrosc. 42, (2006) [4] J. Fonsen, V. Koskinen, K. Roth, J. Kauppinen, Vibr. Spectrosc. 50, (2009)

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MADE TO MEASURE

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Further presentations

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Analysis of Damaged Floor Coverings Emissions in Indoor Air Quality with Cantilever-Enhanced Photoacoustic Spectroscopy Session: Environmental Air Quality and Analysis Tuesday, March 8th, 10:25 AM, Room B403 Ship Emissions Monitoring with Laser-Based Cantilever-Enhanced Photoacoustic Detection Session: Environmental Air Quality and Analysis Tuesday, March 8th, 10:45 AM, Room B403

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Thank you! Visit us at booth #1657