Evaluation of Aeris MIRA, Picarro CRDS G2307, and DNPH-based sampling for long-term formaldehyde monitoring efforts
Abstract. Current formaldehyde measurement networks rely on the TO-11A offline chemical derivatization technique, which can be resource intensive and limited in temporal resolution. In this work, we evaluate the field performance of three new commercial instruments for continuous in-situ formaldehyde monitoring: the Picarro cavity ringdown spectroscopy (CRDS) G2307 gas concentration analyzer and Aeris Technologies’ mid-infrared absorption (MIRA) Pico and Ultra gas analyzers. All instruments require regular drift correction, with baseline drifts over a 1-week period of ambient sampling of 1 ppb, 4 ppb, and 20 ppb for the G2307, Ultra, and Pico, respectively. Baseline drifts are easily corrected with frequent instrument zeroing using DNPH scrubbers, while Drierite, molecular sieves, and heated hopcalite fail to remove all incoming HCHO. Drift-corrected 3σ limits of detection (LOD) determined from regular instrument zeroing were relatively comparable at 0.055 ppb (Picarro G2307), 0.065 ppb (Aeris Ultra), and 0.08 ppb (Aeris Pico) for a 20 min integration time. We find that after correcting for a 30–40 % bias in the Pico measurements, all instruments agree within 5 % and are well correlated with each other (all R2≥0.70). Picarro G2307 HCHO observations are more than 50 % higher than co-located TO-11A HCHO measurements (R2 = 0.92, slope = 1.47, int = 1 ppb HCHO), which is in contrast to previous comparisons where measurements were biased low by 1–2 ppb. We attribute this discrepancy to the previous versions of the spectral fitting algorithm as well as the zeroing method. The temperature stabilization upgrade of the Ultra offers improved baseline stability over the previously described Pico version, reducing the maximum drift rate by a factor of 13 and improves precision of a 10 min average by 13 ppt. Using a 6-month deployment period, we demonstrate that all instruments provide a reliable measurement of ambient HCHO concentrations in an urban environment and, when compared with previous observations, find that midday summertime HCHO concentrations have reduced by approximately 50 % in the last two decades.
Asher P. Mouat et al.
Status: final response (author comments only)
- RC1: 'Review of “Evaluation of Aeris MIRA, Picarro CRDS G2307, and DNPH-based sampling for long-term formaldehyde monitoring efforts” by Mouat et al.', Anonymous Referee #1, 27 Apr 2023
- RC2: 'Comment on egusphere-2023-703', Anonymous Referee #2, 15 May 2023
Asher P. Mouat et al.
Asher P. Mouat et al.
Viewed (geographical distribution)
This manuscript provides describes a series of collocations between three different ambient formaldehyde instruments, including the Picarro G2307 cavity ringdown spectrometer and two different models (the Pico and the Ultra) of the Aeris Technologies MIRA formaldehyde analyzer. Comparisons between different formaldehyde measurement methods are common and typically have shown mixed results, suggesting that issues still need to be resolved in making accurate and comparable measurements of formaldehyde at ambient air concentrations. However, this manuscript compares three different commercially available, easy-to-operate platforms and compares it to the standard EPA Method TO-11A. Therefore, there is likely some additional value in this work beyond previous studies, which often used instrumentation that is more resource intensive or was research-grade and less easy to run in a routine situation. I had some general and specific concerns about this manuscript that I suggest the authors consider (and possibly address) prior to publication. I recommend moderate revisions (between major and minor) before acceptance.
I had several general concerns with this paper.
The first has to do with how the calibrations were performed. The Picarro was calibrated at 100x to 1000x higher concentrations than the range measured in the field, and was also calibrated in a different matrix gas (N2 vs Air). The authors did point out that they assume linearity, but they do not provide strong evidence for linearity from single-point calibrations, and certainly not linearity over 3 orders of magnitude. The Aeris instruments either weren’t calibrated (Ultra) or were post-corrected based on comparison with the Picarro (Pico). Therefore, the slope and intercept in Figure 7 are essentially a test of how good the Aeris calibration is. Given the 15+ months from manufacturing to use, the strong comparison is impressive but perhaps not very useful. Given the importance of appropriate calibrations for making these measurements, I recommend the authors spend additional time discussing recommendations for how to calibrate the instruments. Even suggestions based on experience (if presented as such) would be better than completely ignoring the issue.
The second major concern is with the suggestion that the Picarro G2307 be run with a DNPH scrubber for regular zeros. I would make sure to mention that doing this would likely (and probably should) void the manufacturer’s warranty and/or any maintenance or service agreements and could also reduce the lifetime of the instrument. The LpDNPH S10L cartridges are made by coating silica gel with 2,4-dinitrophenylhydrazine in an acidified solvent solution. As a result, the LpDNPH S10L cartridges will emit small amounts of solvents (possibly acetonitrile or methanol), acid gases (e.g., HCl or H2SO4), and possibly volatilized byproducts of the DNPH reactions. It is not immediately clear what impact those might have on the cell or mirrors in the G2307, or whether any of those compound might contribute to spectral interferences in the formaldehyde retrievals in the 5626 cm-1 region. This should also be considered when discussing the ~0.5 ppbv offset difference between the DR / DR+MS baseline and the DNPH baseline for the Picarro instrument. Although the DNPH cartridge makes sense for doing humidity-matched baseline corrections, it appears (Figure 3) that the Picarro is plumed such that the zeros are performed from indoor air passing through the DNPH scrubber. In this configuration the instrument loses the ability to perform humidity-matched zeros, which seems to negate the major reason for using DNPH over a different scrubber (e.g., humidified zero air or just dry zero air). A zero air cylinder would last longer than a DNPH cartridge, not have the potential concerns about acid / solvent off-gassing, and could last indefinitely with an appropriate zero air generator.
Nowhere in the manuscript is the issue of the actual accuracy of the Aeris or Picarro instruments addressed. Given the large discrepancy between the Picarro G2307 and the TO-11A, there are clearly accuracy issues with one or both methods. In addition, the discrepancies observed here are larger than those observed in many other studies comparing DNPH to spectroscopic measurements. I also have concerns about the 0.5 ppbv difference in baseline using the DNPH versus DR and whether that could contribute to the 1 ug/m3 intercept in the Picarro vs DNPH comparison. Nowhere do the authors make a convincing argument that the DNPH zero is a “true zero” versus the DR or other zero / scrubber mechanism.
PLEASE use consistent units when talking about formaldehyde concentrations. Choose either ppbv or µg·m-3 and don’t keep switching back and forth in different parts of the text and different figures. Conversion from one unit set to the other is straightforward, but it’s very difficult as written to compare concentration ranges in different sections because the units are not consistent.
Specific Comments and Suggestions:
Line 36: Recommend “Because HCHO photolysis / oxidation is a source of …” instead of just “HCHO is a source of”
Line 44: Recommend “the standard EPA approach” rather than “EPA-standard”
Lines 45-46: Please cite the 1999 version of EPA Method TO-11A. You cite Riggin, 1984, which is before the TO-11A “Method” existed. (https://www.epa.gov/sites/default/files/2019-11/documents/to-11ar.pdf)
Line 46: I believe it should be “Sample collection and analysis are” instead of “is”
Line 47: “…long sampling times” is very ambiguous. TO-11A is generally used for 1 hour to 24 hour sampling in ambient air – it is not effective if the time is too short (not enough HCHO collected) or too long (you get breakthrough).
Line 47-48: “EPA Method TO-11A measurements in the PAMS and NATTS networks are 8 or 24 h …”
Technically TO-11A measurements can be any length – you are specifically talking about current PAMS and NATTS required sampling frequency / duration.
Line 51: “the method…” here seems to refer to the “previous approaches” in the sentence prior rather than “Method TO-11A” (which I believe is the intended target for “the method”).
Line 51: Perhaps mention “the DNPH method…” because Method TO-11A specifically addresses the O3 interference (which was actually a large impetus for publication of TO-11A versus staying with TO-11).
Lines 57 – 61: There are a number of datasets with about 1 month or longer of continuous spectroscopic formaldehyde measurements at ground level, generally using TDLAS. See, for example, Coggon et al. (2021) (https://doi.org/10.1073/pnas.2026653118, Figure S20) or Spinei et al. (2018) (https://doi.org/10.5194/amt-11-4943-2018, Figure 3).
Line 63: “A more suitable long-term HCHO monitoring instrument…” – more suitable than what? And suitable for what purpose? I recommend rewriting this entire sentence – it’s a bit confusing as written.
Line 77: “relies on the HDO line” – which HDO line? Either specify a line or say “a HDO line”
Line 88-89: “This updated algorithm…” – are you referring to the algorithm used in Glowania et al. (2021) or the post-Glowania algorithm update to resolve the issues reported in Glowania et al. (2021).
Line 91: Technically, the Picarro G2307 does not “rely” on periodic instrument baseline zeroing. Once calibrated, it should be stable for months without needing to zero. Regular zeroing is recommended for the highest (sub 1 ppbv) precision (e.g., minimize baseline drift).
Line 92 – 94: A commonly used scrubber for HCHO-free air is a heated catalytic hydrocarbon scrubber. This is often used in cases where humidity-matched zeros are necessary. See, e.g., Herndon et al. (2007) (https://doi.org/10.1029/2006JD007600). I believe it is also used by Fried et al. on various aircraft studies. This is also used in commercial zero air generator systems to produce HCHO-free air.
Lines 150 – 160: It’s odd for the authors to calibrate the HCHO at > 1 ppmv but make most of their measurements in the 1 – 10 ppbv range. This is a 3 order of magnitude difference between the calibrated range and the measured range. Linearity across 3+ orders of magnitude is a major assumption, especially given the potential influence of peak shape on formaldehyde retrievals at different concentrations over a 3 order of magnitude range. Because these are single-point calibrations, the authors do not even test the linearity of the instrument across any range.
In addition, the calibrations were done in an N2 bath gas, whereas the zero and measurements were done with a N2/O2 mix (air). In an ideal situation, calibration matrix would match the measurement matrix as closely as possible, especially considering the high potential for matrix effects in a high-reflectivity cell with > 1 km effective pathlength. Given the 50% discrepancies observed versus DNPH, I recommend some of the assumptions made during the calibration be reconsidered (or at least discussed more thoroughly).
Lines 150 – 160: The concentration of HCHO in reference gas cylinders typically decrease over time at a pseudo-linear rate. It would be helpful to know when the gas cylinders were certified by Apel-Riemer / Airgas versus when they were used to perform the calibration checks.
Line 166: The manufacturer’s literature / spec sheets describe a 13 m pathlength for the instruments. I recommend you check whether 1.3 m or 13 m is the correct pathlength.
Line 178 / 179: Figure 2 is a map. I believe the authors intend to refer to Fig 3a / Fig 3b in these lines. Figures should also, in general, be added to the manuscript in the order they are referred to in the text, which would make Fig 3 the first figure (Fig 1).
Lines 177 – 179: The “Scrubbing ambient air rather than indoor air…” part is confusing, since indoor air also has sufficient water vapor to maintain a laser line lock (and, in fact, Aeris markets their instruments for indoor air measurements of HCHO as well). Scrubbing ambient air will provide humidity-matched (or very close to humidity matched) background versus sample gases, whereas scrubbing indoor air would produce a near constant humidity for the zeros but a varying humidity for the sample gases.
Line 179: It is not clear why authors choose to sample ambient air for 180 s and scrubbed air for 30 s, versus the scheduling used by Shutter et al. or recommended by the manufacturer.
Line 216-217: Please provide the correct (1999) reference for EPA Method TO-11A
Lines 219 – 221: What was the temperature of the heated inlet? What type of ozone denuder? I’m assuming based on the ATEC sampler that the ozone denuder was a KI-coated copper tube heated to 50 °C, but this is important to mention. Particularly as there are concerns with some types of ozone scrubbers in sampling formaldehyde (see, e.g., Ho et al. 2013, https://doi.org/10.4209/aaqr.2012.11.0313).
Lines 220 – 221: Please provide additional details about the cartridge (e.g., sorbent, DNPH loading, bed size, etc.). Supelco DNPH-C-18 is not a standard item – their standard DNPH cartridges use silica gel, not C-18. This seems to be an unusual / custom cartridge type.
Lines 230 – 234: The 12% uncertainty seems very low. Estimated collection efficiencies are generally about 70 – 100% (see, e.g., https://projects.erg.com/conferences/ambientair/conf18/MacGregor_Ian_AirToxics_8-15_800_SalonE_POST_508.pdf) for HCHO. In addition, there is likely to be a negative bias in the DNPH because of breakthrough of HCHO, reverse derivatization reactions, degradation of hyrdozone, etc. In contrast, for a good chromatographic program (that resolves the NO2 artifacts) there should not be a positive bias in the measurements.
Figure 9: It is very challenging to see the difference between the three symbol types in this figure. I recommend choosing a different presentation if you want to clearly distinguish between the Picarro and two Aeris by the symbol style.
If I understand this correctly, the Aeris Ultra essentially used the manufacturer’s calibration, the Picarro was calibrated at 1 ppmv in N2, and the Aeris Pico was corrected based on the Picarro. Therefore, the comparability of the Aeris Pico, Aeris Ultra, and Picarro G2307 (especially the slope and intercept) seem to be a test of how comparable the Ultra calibration is to the Picarro calibration. Essentially, this section seems to be a test of the Aeris Technologies calibration more than actual instrument-versus-instrument comparability.
It might be more interesting to look more closely at: