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• W2106379005 abstract "Because of their accuracy and precision for measuring gas concentrations, gas chromatographs (GC) are standard analytical instruments used in investigations of nitrous oxide (N2O) and carbon dioxide (CO2) exchange between the soil and the atmosphere. Iqbal et al. (2012) indicate that photoacoustic spectroscopy (PAS) performs similar to GCs to this end. We welcome this addition to the literature, given the increasing number of studies using PAS (e.g. Predotova et al., 2009; Leytem et al., 2011) and the few comparative analyses available (Ambus & Robertson, 1998; Yamulki & Jarvis, 1999). However, poor performance of PAS in some assessments (Flechard et al., 2005; Akdeniz et al., 2009) and data from our own tests (reported below) raise questions about whether Iqbal et al.'s (2012) results are generally applicable to PAS instruments or unique to the experimental conditions and calibration of their instruments. We tested three PAS instruments (PAS 1, 2, and 3: INNOVA Lumasense Technologies models 1412, 1302, and 1312, respectively) for their accuracy and precision by measuring gas samples of known concentration in a laboratory environment. Experiments with PAS 1 and 2 took place under the same experimental conditions and tested differences in performance between two instruments. Gas samples of known concentration were passed directly into the instruments commensurate with manufacturer's specifications (Lumasence, 2009). Separate experiments were conducted on PAS 3 to examine how ambient air temperature and sample moisture content affect the instrument's performance. We generated gases of known concentration by mixing calibration gas with synthetic air. Two meters of common airline allowed for proper mixing. A dew point generator was used to vary sample moisture content. Each gas cylinder was equipped with a flow regulator and the gas flow was maintained constant throughout the experiments. An Aerodyne Quantum Cascade Laser was used to verify the concentration of the gas mixture and showed that the targeted and obtained concentrations differed less than 0.5%. The manufacturer calibrated PAS 1, 2, and 3, 1, 6, and 4 months prior to the experiments, respectively, within the recommended once-yearly factory calibration cycle. These experiments build on those by Iqbal et al. (2012) by testing instrument performance (i) at lower greenhouse gas concentrations; (ii) across a range of gas concentrations; and (iii) for instruments calibrated at different times. Data from our experiments suggest PAS is less accurate and precise than that suggested by Iqbal et al. (2012). Measurement of N2O by PAS 1 and 2 deviated from known concentration by nearly twice the amount reported by Iqbal et al. (2012) and the deviation of average PAS 1 and 2 measurements were outside the upper bound of Iqbal et al.'s (2012) range (Table 1). The magnitude of deviation we found (16%) was greater than that in previous evaluations: 12% (Predotova et al., 2009) and 5% (van Groenigen et al., 2004). The most parsimonious explanations for variation among instrument performances are differences in experimental conditions or calibration algorithms that account for interference among gas and water vapor absorption spectra and for cross interferences among the targeted molecules (Fechard et al.,2005). However, differences may also be attributable to the gas concentrations tested. The most significant deviation we found was at concentrations 331 ppb N2O, close to atmospheric concentration, less than half the concentration of 719 ppb tested by Iqbal et al. (2012). Measurements of 649 and 1020 ppb N2O concentration were in closer agreement with previous results. Iqbal et al. (2012) report changes in headspace concentrations of 50–600 ppb N2O min−1, roughly equivalent to 600–7000 g N2O-N m−2 h−1 (or 50–600 kg N ha−1 yr−1) when assuming a 15 cm high chamber. Soil fluxes of that magnitude are rarely found and only occur under high emission conditions. It is thus particularly germane that analytical instruments measure N2O concentrations accurately below those tested by Iqbal et al. (2012). Photoacoustic spectroscopy (PAS) is typically used to measure other greenhouse gas molecules in addition to N2O. Measurements of CO2 with our PAS were reasonable, with estimates deviating 4% or less from known concentration; a degree of accuracy that agrees with that found by Iqbal et al. (2012). PAS's ability to measure methane (CH4) is also worth consideration given its use to measure these emissions too. Measurements of CH4 by our machines deviated by 95–364% from actual gas concentrations (Table 1), so that under our experimental conditions it is not recommendable to use PAS for CH4 flux measurements. In contrast to Iqbal et al. (2012), we found that the operating conditions affect gas measurements with PAS. Measurements of the same gas concentration (e.g. 409 ppb N2O) changed by more than 100% depending on the ambient air temperature and the moisture content of the gas sample (Fig. 1). Measured concentration estimates were sensitive to both the water vapor of the air sample and the ambient air temperature surrounding the instrument. These findings indicate that the measurements are affected heterogeneously as environmental conditions change over various time intervals, within an hour when the PAS is exposed to sunshine, over the course of a day, and among days over the course of measurement campaign. Therefore, our results show that ‘manufacturer calibration’ for moisture and temperature can be insufficient to overcome errors due to experimental conditions (Fig. 1). It may be feasible to correct PAS estimates after measurement as we found N2O and CO2 to be linearly misestimated across gas concentrations for all three instruments (see N2O in Fig. 1). Recall though that PAS's estimates appear to be temperature and moisture dependent (Fig. 1). It is, therefore, unreasonable to use standard curves generated in the lab to calibrate field measurements, where ambient temperature and humidity may differ and change continually. Furthermore, such corrections rely on the assumption that the performance of the instrument is stable over time. Data from PAS 3 indicate it may not be. Measurements taken 5 days apart, using the same instrument and N2O gas concentration, and under the same moisture and temperature conditions, varied by 15%. The findings do not invalidate the potential of post measurement correction; simply it would require monitoring calibration at equal time scale to instrument drift. The potential need for near-daily in-field calibration monitoring contrasts sharply with the once yearly calibration typically used with PAS monitors as recommended by the manufacturer. Positive results of Iqbal et al. (2012) and others (Ambus & Robertson, 1998) suggest PAS may be an efficient and effective analytical tool under certain conditions. However, reliable use of PAS for measurements of greenhouse gases requires a better understanding of the mechanisms driving variation in PAS performance. Plausible causes are interference in absorption spectra between gases and/or with water vapor and the temperature/moisture sensitivity of the electrical board. Perhaps PAS is most useful when targeting a single gas molecule under specific humidity and temperature ranges, conditions that can only be replicated in the laboratory. Or adopting protocols that include running standard curves for N2O in the field, possibly multiple times per day and adjusting for drift between standards, might make PAS a more reliable technology for measurement of N2O and CO2. Unfortunately, at this point of time, too little information exists to set boundary conditions of this instrument's functionality. On the basis of our results and the variable and somewhat contradictory results found in the literature, one should be careful when considering using PAS for measurements of greenhouse gases." @default.
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• W2106379005 date "2013-10-20" @default.
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• W2106379005 title "Accuracy and precision of photoacoustic spectroscopy not guaranteed" @default.
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