Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm
Abstract
:1. Introduction
2. Experiment
3. Results and Discussion
3.1. Single-Pass Absorption Spectrum of NH3
3.2. Off-Axis Coupling of the GSFC to the Cavity Without Target Species
3.3. GSFC Application to off-axis Cavity-Enhanced Detection of NH3 at 6604–6607 cm−1
3.4. Uncertainties and Stability Considerations
- One of the smallest systematic uncertainties is the measured cavity length which was estimated to be Δd = ±1 mm (≈0.2%).
- Another general systematic error is caused by the dependence of the mirrors’ reflectivity on the shift of the laser beam away from the center of the mirror. The reflectivity at the center of the cavity mirror was measured with an absorption spectrometer to be R = 0.9960 for normal incidence. At an offset from the mirror center of ≈4 mm, the reflectivity was found to be R = 0.9940. Due to the finite diameter of the beam in the reflectivity measurement and also in the CEAS measurements with off-axis alignment (GSFC beam diameter ∼1 mm), the uncertainty ΔR was estimated to be ±5 × 10−4. This uncertainty results in an error of Δ(1−R) ≈ 8.3%, which is relevant for the evaluation of the absorption coefficient. Based on this systematic error alone, absolute gas concentrations are affected, causing an error in the detection limit of approximately ±0.3 ppmv.
- A systematic uncertainty is also caused by the small inherent intensity noise of the GSFC coupling to the cavity in on- and off-axis configuration, which was estimated by recording 50 consecutive transmission spectra (30 s integration time) of the cavity at intervals of 40 s. At each wavenumber in the spectrum, the standard deviation of the 50 measurements was evaluated for on- and off-axis (shift of 4 mm) measurements. The mean 1σ standard deviations,, averaged over all wavenumbers for on- and off-axis configurations were found to be 0.042 (=4.2%) and 0.0014 (=0.14%), respectively. The maximum values of standard deviation, , in on- and off-axis configuration were 0.3331 (=33.3%) at 6604.74 cm −1 and 0.005 (=0.5%) at 6604.78 cm−1, respectively. Assuming the latter maximum deviation as the sole systematic error concerning the intensity (ΔI = 0.5%), an optimal (theoretically achievable) lower limit of the minimum detectable absorption coefficient of 5.4 ×10−7 cm−1 for an integration time of 20 s can be estimated. This corresponds to a detection limit of 830 ppbv, i.e., approximately a factor of 4.5 below the detection limit evaluated from the absorption measurement of NH3 used here to determine the experimental detection limit.
- The absorption cross-sections as well as the air- and self-broadening parameters in the HITRAN database have uncertainties that add to the overall absolute systematic error of the calculated absorption coefficients (and number densities). The five strongest absorption features of NH3 in the region between 6604 and 6607 cm−1 are given in Table S1 (supplementary material), together with the relative uncertainties of the corresponding absorption strengths (ΔS), all of which are below 10%. For all other (weaker) absorption features in the relevant region, the uncertainties are generally larger than 2% and smaller than 20% [26]. The maximum relative uncertainty of the self- and air-broadening coefficients for the absorption features at 6604.728, 6605.104 and 6605.609 cm−1 is 5%, for features at 6605.190 and 6605.652 cm−1, it is 20% [26]. As a conservative estimate for the overall systematic error, ΔSoverall, arising from these uncertainties, we simply used the sum of the maximum uncertainty of the strongest feature at 6605.609 cm−1 (10%) and the average of its line broadening uncertainties (5%); i.e., ΔSNH3 = 15%. Finally, the calculated HITRAN reference spectrum was also calculated for approximate conditions (in terms of partial pressures and temperature for Doppler broadening) and was only linearly scaled (with background correction) to the measured data in the least-square fit to the measured spectrum, which leads to a small systematic discrepancy, which is treated as negligible.
3.5. Selectivity in the Near IR range and Potentially Interfering Species
3.6. Benchmarking the NH3 Detection in the Near IR Range
4. Conclusions
Supplementary Materials
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Chandran, S.; Ruth, A.A.; Martin, E.P.; Alexander, J.K.; Peters, F.H.; Anandarajah, P.M. Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm. Sensors 2019, 19, 5217. https://doi.org/10.3390/s19235217
Chandran S, Ruth AA, Martin EP, Alexander JK, Peters FH, Anandarajah PM. Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm. Sensors. 2019; 19(23):5217. https://doi.org/10.3390/s19235217
Chicago/Turabian StyleChandran, Satheesh, Albert A. Ruth, Eamonn P. Martin, Justin K. Alexander, Frank H. Peters, and Prince M. Anandarajah. 2019. "Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm" Sensors 19, no. 23: 5217. https://doi.org/10.3390/s19235217
APA StyleChandran, S., Ruth, A. A., Martin, E. P., Alexander, J. K., Peters, F. H., & Anandarajah, P. M. (2019). Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm. Sensors, 19(23), 5217. https://doi.org/10.3390/s19235217