Inline multi-harmonic calibration method for open-path atmospheric ammonia measurements
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- Sun, K., Tao, L., Miller, D.J. et al. Appl. Phys. B (2013) 110: 213. doi:10.1007/s00340-012-5231-2
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We demonstrate a real-time inline calibration method for an open-path ammonia sensor using a quantum cascade laser (QCL) at 9.06 μm. Ethylene (C2H4) has an absorption feature partially offset from the ammonia absorption feature, and the ethylene signal serves as a reference signal for ammonia concentration in real time. Spectroscopic parameters of ammonia and ethylene are measured and compared with the HITRAN database to ensure the accuracy of the calibration. Multiple harmonic wavelength modulation spectroscopy (WMS) signals are used to separate the ambient ammonia and reference ethylene absorption signals. The ammonia signal is detected with the second harmonic (2f), while it is calibrated simultaneously with a high-harmonic (6–12f) signal of ethylene. The interference of ambient ammonia absorption on the ethylene reference signal is shown to be negligible when using ultra high-harmonics (≥6f). This in situ calibration method yields a field precision of 3 % and accuracy of 20 % for open-path atmospheric ammonia measurements.
Atmospheric ammonia (NH3) is a key component in the global nitrogen cycle. As the dominant alkaline atmospheric species, ammonia reacts readily with atmospheric acidic species such as sulfuric acid (H2SO4) and nitric acid (HNO3) to form ammoniated aerosols, with strong implications for regional air quality and global radiative forcing [1–3]. Ammonia also plays an important role in the deposition of reactive nitrogen in sensitive ecosystems . Despite the importance of atmospheric ammonia, its spatial and temporal variability is poorly characterized due to its low atmospheric concentration and high reactivity .
Traditional ammonia measurements utilize passive filters and denuders with long integration times, and they are usually labor-intensive in operation and maintenance . State-of-the-art techniques include chemical ionization mass spectrometry (CIMS) [7–9], tunable laser absorption spectroscopy [10–12], photoacoustic spectroscopy , and cavity ring down spectroscopy . All of these techniques need to sample ammonia into a closed-path system and thus involve direct contact with sampling surface to which ammonia readily adsorbs. Closed-path measurements of ammonia are complicated by significant backgrounds, unknown buffering of large changes in concentration, and ambiguity between ammonia and ammonium due to phase transitions in sampling lines . For field deployments where conditions can change rapidly, the simplicity and automation of calibration needs improvement at typical ambient mole fractions [parts per billion by volume (ppbv)].
To address the sampling issue of closed-path techniques, we have developed an open-path ammonia sensor using a quantum cascade laser (QCL) operating at 9.06 μm for atmospheric measurements. Wavelength modulation spectroscopy (WMS) is used to enhance the signal to noise ratio (SNR) and resolve air-broadened absorption lines. Given the complexity of WMS systems, calibrations with reference samples are widely used to make accurate measurements. However, the same problem with the calibration of a closed-path ammonia sensor remains for an open-path sensor: one needs to introduce a known concentration of ammonia for calibration. Ethylene (C2H4) has an abundance of absorption lines in the ν7 band near the ammonia ν2 band in mid-IR. Previous research has already shown that ethylene can be used in ammonia sensors as a reference of laser wavelength at 10.34 μm  and as a reference for ammonia concentration calibration at 9.06 μm . In this study, we present a new in situ calibration method with an inline ethylene reference cell using multi-harmonic WMS. Ethylene is a stable, relatively inert gas and has line strengths two orders of magnitude smaller than ammonia near 9.06 μm. Thus ethylene does not cause interference at typical atmospheric mixing ratios (sub-ppbv) , which are comparable to ammonia mixing ratios. At a low pressure (<100 hPa), high gas concentration (1 %), and short path length (~10 cm), ethylene shows a stable absorption signal partially offset from the ammonia absorption feature, and the ethylene signal can serve as a reference for ammonia concentration in real time. This calibration method can also compensate for the effect of laser drifting by line locking to the sharp ethylene peak instead of the air-broadened ammonia peak and is particularly useful near the detection limit.
Comparing with conventional WMS, this technique has advantages in accuracy, frequency, simplicity, and automation. The ammonia concentrations are retrieved by fitting the second harmonic (2f) spectra, so theoretically the precision should be the same as traditional 2f detection. The accuracy is ensured by experimental calibrations of the spectroscopic parameters of both ammonia and ethylene, which are independent of ammonia concentrations. However, the accuracies of conventional calibration methods are limited by the uncertainties of ammonia standards, which can be quite large at ambient levels (ppbv) due to the adsorption effects of the gas delivery system . In long-term field measurements, frequent calibrations are usually needed to account for system drift. The traditional solution is by periodically calibrating the system with some standards, which can be expensive, labor-intensive, or subject to loss of measurement points. By checking the absorption signals of a fixed concentration reference cell, this in situ calibration method enables continuous and unattended measurements, which are very important in rapidly changing conditions in the field.
2 Experimental and model details
2.2 Simulation of WMS signals
In order to interpret and predict the multi-harmonic signal from the reference cell, we have developed a numerical model based on the general WMS theories [20–23]. The equations are rewritten to involve more variables for open-path atmospheric measurements. An infinite impulse response (IIR) filtering algorithm enables a direct comparison between the model and the signal output from a lock-in amplifier.
For a single absorption line, the line center value of the Nth harmonic WMS signal is then denoted by X(line center, N).
3 Spectroscopic calibration
In order to use ethylene as a reference absorption signal to calibrate ammonia, precise knowledge of the absorption cross-sections of both ammonia and ethylene is critically important. For instance, a variation in the relative line strengths of ammonia or ethylene of 10 % leads to a direct variation of 10 % on the ammonia concentration retrieval. The spectroscopic parameters that determine the absorption cross-section are given by the HITRAN database . However, HITRAN data can have large uncertainties and sometimes differ significantly from experimental validation . For example, the uncertainties of HITRAN ammonia line strengths are estimated to be 10–20 % [28, 29], and there are no reported uncertainties for the parameters of ethylene. We remeasure the spectroscopic parameters for both ethylene and ammonia precisely using direct absorption, 2f, and 4f signals.
The ethylene and ammonia spectroscopic calibrations generally agree with the HITRAN 2008 database. The only exception is the ethylene collision line width, which we measure to be 15 % lower than HITRAN for both lines. The uncertainties for line strength measurements mainly come from the uncertainties of the concentration of the gas mixture we use (2 % for ethylene and 10 % for ammonia). The accuracy of this calibration method is 20 %, according to propagation of errors of the gas concentration and spectral fitting.
4 Ammonia calibration using an inline ethylene reference cell
An inline calibration cell with a reference gas has been used in laser spectroscopy with isolated lines , but there are significant challenges when the reference absorption line overlaps with the target absorption line. As shown in Fig. 2, one ethylene absorption line (even under reduced pressure) sits on the shoulder of the ammonia absorption feature. Due to the constraint of QCL tuning rate, we also need to use this ethylene line for calibration.
In c and d of Fig. 7, we use the numerical WMS model to simulate the same conditions as the experiment. The concentration and pressure/temperature of ammonia and ethylene are fixed at the measured or stated values. The spectroscopic parameters are those from HITRAN 2008 except where we use our ethylene collision line width determined previously. The simulated 2f spectra are scaled to the same magnitude of the experimental 2f spectra, and the simulated 6f spectra are scaled using the same factor. The excellent agreement between experiment and simulation shows that the model can capture the relative values of different harmonic signals. It also indicates that we can fit the multi-harmonic spectra with the simulation results and retrieve ammonia concentrations.
The maximum of r(6) occurs at a relatively small modulation depth, but Fig. 8a shows that r(2) keeps decreasing if the modulation depth increases. This implies that we can use even higher harmonics for the ethylene reference signal, so that the r(Nref) is larger and reaches its maximum at larger modulation depth. Figure 8c investigates the ethylene/ammonia ratio at 12f. r(12) is about 400 under optimal conditions, indicating that the ammonia signal is vanishingly small compared to the ethylene reference signal at ultra high harmonics. At the same time, r(2) is much smaller when r(12) is maximized than it is when r(6) is maximized. This indicates a general trend that higher harmonic gives better separation between the ambient ammonia and reference ethylene signals. However, it will be ultimately limited by signal-to-noise ratio since the intensity decreases as N increases. Generally, we use 6–12f depending on sensor configurations.
In polluted urban areas, ambient ethylene concentration may reach up to 30 ppbv [33, 34], which gives a signal about 0.01 % of the low-pressure ethylene reference signal at high harmonics. Hence the interferences from ambient ethylene are also negligible to the ethylene reference signal. Ambient ethylene may cause interferences to ambient ammonia signals at 2f when the ethylene concentration is >100 times higher than ammonia. However, these conditions are unlikely to happen and the signals can still be separated by spectral fitting.
5 Field demonstration
This inline calibration technique has been used in a prototype ammonia sensor and tested in Baltimore Ecosystem Study (BES) in October 2011. An ethylene reference cell (L = 5 cm; P = 50 hPa; filled with 2 % ethylene in nitrogen) was fixed in series with an open-path cylindrical multi-pass cell with a path length of 40 m. Instead of the digital lock-in amplifier, a software-based virtual lock-in amplifier was used to extract the WMS signals. Due to the constraint of the AD sampling rate (1 MHz) and optical fringing caused by the multi-pass cell, only 8f was used as the reference signal. The ammonia absorption used in Fig. 9 is equivalent to 750 ppbv ammonia in a path length of 40 m. However, ambient ammonia concentration is rarely higher than 100 ppbv even under polluted conditions . Therefore the ambient level ammonia signal does not cause any significant interference on the reference signal with the sensor configuration.
The base length of the open-path multi-pass cell is 50 cm, leading to optical fringing with free spectral range (FSR) comparable to the Voigt line width of ethylene absorption line at 50 hPa. When the FSR is comparable to the line width, it has the largest interference with the absorption signal. Since this FSR is much smaller than ammonia Voigt line width, its influence on ammonia signal is very small.
Significant thermal drifting of laser wavelength was observed under the field conditions. The wavelength drifting was prevented by actively changing the laser current offset to lock the peak position of the ethylene 8f signal. However, the changes of intensity and tuning rate, which are potentially affected by wavelength drifting, cannot be fully quantified.
We demonstrate a real-time inline calibration of ambient ammonia measurements with an ethylene reference cell. Simultaneous multi-harmonic detection is used to resolve and optimize the overlapping ammonia and ethylene reference signals. We detect ammonia at 2f and ethylene at higher harmonics, up to 12f. By controlling the pressure in the reference cell and the modulation depth, we optimize ammonia and ethylene signals, respectively, and simultaneously. The interferences from ambient ammonia absorption to ethylene reference signals are less than 2 % even for extremely large ammonia signals. The laboratory precision of high-harmonic ethylene reference signal is within 1 %. Initial tests of this calibration method have been performed on the prototype open-path ammonia sensor in the Baltimore Ecosystem Study. Future improvements include using the inline approach with a smaller concentration reference cell to calibrate at cleaner ammonia conditions in the atmosphere (sub-ppbv).
We acknowledge support from NSF-ERC-0540832 and a private gift from Thomas and Lynn Ou. David J. Miller is supported by a Graduate Research Fellowship from the National Science Foundation. We thank W. Wang for helpful discussion on spectral fitting.