From the journal Environmental Science: Atmospheres Peer review history

05-Jan-2024

Dear Dr Bittner:

Manuscript ID: EA-ART-12-2023-000170
TITLE: Spatial Comparison of Wildland Fire PM_2.5 Concentrations from Vehicle Add-On Mobile Monitoring System (VAMMS) in Remote Locations

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************

Reviewer 1

see attached PDF

Reviewer 2

Comments on Manuscript ID EA-ART-12-2023-000170

Spatial Comparison of Wildland Fire PM2.5 Concentrations from Vehicle Add-On Mobile Monitoring System (VAMMS) in Remote Locations

1) Ln 107-109: “The VAMMS samples through ¼” conductive tubing attached to a ¼” stainless steel probe with a 10 deg cone and 1/16” inlet that provides isokinetic sampling at 35 mph at 3.5 LPM sample flow for the PM2.5 cyclone.”

Please describe how it was determined that the inlet provided isokinetic sampling for PM2.5 at 35 MPH at 3.5 LPM.

Is the vehicle profile expected to have an impact on the inlet’s performance?


2) Ln 203-205: “Only data from PurpleAir labeled as ‘outdoor’ were considered. We required that the difference of the A and B cf=1 channels be < 70% or < 5 μg m-3 at the highest time resolution available, a data quality assurance step described in detail elsewhere33.”

The “outdoor” designation being used to select identify sensors that are believed to deployed outside and monitoring ambient? But the ‘CF1” data was used in analysis?


3) Ln 293-298. Please note the filter concentration ranges for the chamber tests?


4) Ln 314-316: “During the four sampling run periods shown in Fig. 1, the Oakridge FEM monitor was the closest monitor to the VAMMS in all cases except for Fig. 1d when the VAMMS was closer to the regulatory stations in Eugene and Springfield (10:00 to 11:30 PM UTC).”

Please clarify this statement. I assume that for the two sets of markers shown in Fig 1d, the VAMMS was closest to the Oakridge station, i.e. the three regulatory stations are not within 400 m of each other.

5) Ln 324-325: “Including all instances where the VAMMS passed by the Oakridge AQMS, in 11 out of 14 passages (~80%), all four instruments agreed in “approximate AQI” category (displayed as the color in Figure 1)....”

Figure 1 shows 9 sets of markers indicating the VAMMS was within 400 m of the Oakridge monitoring site. The text states 14 passages, presumably these are from sampling periods not shown in Fig 1. The authors should consider including the sampling period discussed in Ln 330-345, since this event takes up a large portion of section’s text.


6)Figure 3b. Please clarify – is the PurpleAir data displayed in Figure post-Holder correction or uncorrected?


7) Ln 462-446: “For small, prescribed fires where meteorological conditions are well characterized, the monitoring duration is short, and near-field plume dynamics are highly variable, stationary monitors may better represent concentrations for personnel on the ground, while the mobile monitor can provide data with higher temporal and spatial extent that would be useful for validating smoke plume models.”

Good point!

We would like to thank both reviewers for their helpful feedback. The revisions involve corrections to the citations, clarifications to the text, a title change, and new analysis and discussion to address both reviewers’ comments regarding anisokinetic sampling conditions.
Below we respond to the reviewer’s comments in detail.
-------------------------------------------------------------------------------------------------------------------------------
Referee: 1

General Comments
The study shows some performance attributes of the VAMMS for wildfire mobile PM2.5
sampling. I think the study is more about the VAMMS general performance in this application
rather than a “spatial comparison”, so I would suggest the title be changed to reflect this. The
study does indeed demonstrate that there is utility for wildfire mobile monitoring with the
VAMMS to obtain higher time and spatial resolution PM2.5 data compared to stationary
monitors, and thus aids air quality messaging to the public in these situations. The study is
within the scope of this journal.
Authors’ response: We changed the title of the manuscript to “Performance of Vehicle Add-on Mobile Monitoring System PM2.5 Measurements During Wildland Fire Episodes”

Specific Comments
Line 23-24: I think this statement needs some refinement. PM10 is not mentioned as one of the major pollutants for which an AQI is calculated. In addition, I don’t think AQIs for all the mentioned pollutants are based on the 24-hour mean concentration.
Authors’ response: We rephrased this statement for accuracy and completeness, mentioning “coarse particulate matter” to refer to the PM10 AQI and indicating that the 24-hour mean concentration is applicable to PM only.

Line 30: Both the 2009 and 2019 Integrated Science Assessments for PM are cited (citations 10 and 11). Is this really necessary to cite both versions to support the statement that PM2.5 exposure causes these various health effects? I think only the most recent one should be cited (2019).
Authors’ response: We removed the 2009 citation.

Line 52-71: In the rest of the paper all citations have a superscript, but in just this one paragraph the inline citations follow a different format.
Authors’ response: We corrected this citation error.

Line 109-111: Are you sure this is correct that this probe is isokinetic at 35 MPH? Because by my own calculation, this described probe is isokinetic at ~65 MPH, not 35 MPH:
Claimed vehicle velocity for isokinetic sampling is 35 MPH
Probe sample flow rate: Q = 3.5 l/min = 5.833e-5 m3/s
Probe opening: D = 1/16 inch = 0.0015875 m; R = D/2 = 7.9375e-4 m
Flow is isokinetic if probe inlet velocity also equals 35 MPH…but
= / = /(π2) = (5.833 × 10−5 3/)/ (π(7.9375 × 10−4 )2)
= 29. 47 / = 66 ≠ 35
Note that EPA Method 5 (PM sampling from a stack) specifies a difference of +/10% between the free stream and probe inlet velocities as acceptable, while EPA Method 201A (PM2.5 sampling from a stack) allows for +/-20% difference between the free stream and probe inlet velocities. If we go with the looser +/-20% tolerance, this means that the described probe only acceptably performs PM2.5 sampling when the vehicle is driven at speeds of 52 to 78 MPH.

This suggests that much of the sampling in this study was done at speeds that disfavor the aspiration of larger particles, which would bias the VAMMS PM2.5 concentrations low. If this is true, that suggests that the regression slopes in Figure 3 and Figure S4 should be even higher than what is shown, if particle sampling was actually performed isokinetically.

I take the point that for many instances of PM2.5 sampling, there may be little impact with non-isokinetic sampling, especially if much of the particle population is in the ultrafine regime. However, the characterization of the probe in lines 109-111 needs to be corrected – either you should state that this probe is isokinetic at ~65 MPH, or the probe inlet diameter is different than what was stated, or the sampling flow rate is different than what was stated. In addition, the authors need to go beyond just telling the reader that this non-isokinetic sampling has “minimal impact” on the sampling of PM2.5; there needs to be a statement that allows the reader to understand quantitatively what the potential magnitude of the bias would be in this study, e.g. calculate the bias in sampling particles of diameter = 2.5 um using this probe and flowrate at the median vehicle velocity in this study.



Authors’ response: We thank the reviewer for pointing out an important error. We modified the probe inlet between the design stage and deployment stage and failed to update our description. The inlet used in this study is 0.084”, which is how we determined that the described probe provides isokinetic sampling at 35 mph:


Probe opening: D = 0.084 inch = 0.00213 m; R = D/2 = 1.065e-3 m

= / = /(π^2) = (5.833 × 10e−5 3/)/ (π(1.065 × 10e−3 )^2)
= 16.37 / = 36.6 ≈ 35

We updated the text to reflect this correction. We also added the above equation and analysis to calculate the bias in sampling particles of diameter 2.5 microns and less using this probe and flowrate at the median vehicle velocities in this study to Section 1 of the supplementary information (see manuscript revisions below). Finally, we made mention of the potential impacts in the relevant results sections and again in the “Limitations and future work” section.

5. Line 166: Can you please recheck the statement that the fire grew from 114,000 to 122,700 acres over 18 days? This doesn’t sound like a huge growth over that time period, and it would be incredible that a fire already started at 114,000 acres on day 1. A look at the Wikipedia article on the Cedar Creek wildfire shows a table where on August 2 it was approximately 5 acres, so perhaps it would be more reasonable to describe the fire as growing from 5 to 122,700 acre over 18 days. https://en.wikipedia.org/wiki/Cedar_Creek_Fire
Or did you mean to write that the fire grew “to 114,000-122,700 acres in size” rather than “from 114,000 to 122,700 acres”?
Authors’ response: We apologize for the confusing statement – this has been rephrased for clarity.
Manuscript text revisions: “The 18-day VAMMS monitoring period started about two months later, during which fire growth (from approximately 114,000 to 122,700 acres in size) and the continued burning and smoldering of interior fuels (typical of any large fire) contributed to heavy smoke. During this time, the border of the fire was within 15 km of the Oakridge, OR regulatory air quality monitoring station (AQMS).


6. Line 206: Is this citation #33 supposed to be Holder et al. or Barkjohn et al.? Because there are two citations numbered 33 in the bibliography.
Authors’ response: This citation is intended to reference Barkjohn at al. 2021. We corrected this duplicative citation error.

7. Line 294: Here is another instance where the inline citation format doesn’t have superscripts like the rest of the manuscript.
Authors’ response: Apologies for the confusion – The two in-text citations in this paragraph were intended to include the authors’ names as part of the narrative. We replaced the “&” symbol with “and” to make this more apparent.

8. Line 389-396 – I had trouble following the logic in this paragraph that led to the conclusion that “rather than the VAMMS overestimating the true concentration in this case [the Monument Fire], the corrected PurpleAir data was likely underestimating the ambient concentration.” Perhaps it needs to be rewritten to be more clear to help the reader also arrive at this conclusion. When I look at Figure 3a and Figure S4, I see a repeated behavior of the VAMMS overestimating at high PM2.5 concentrations (plus the non-isokinetic sampling in this study that could have biased the reported VAMMS PM2.5 concentrations low), so I am more inclined to think that the VAMMS really was overestimating the concentration for the Monument Fire in Figure 3b.
Authors’ response: We rephrased this statement and added additional text for context to explain our logic. We believe the revised text makes the limitations of this analysis more apparent.

9. Line 541: Is the variable sampling velocity and difficulty in getting isokinetic samples not another limitation of this study?
Authors’ response: We agree with the reviewer that these limitations should be explicitly stated. We have added additional text in the “Limitations and future work” section to make this more apparent.

10. Line 648-654: There are two citations numbered 15. I suspect the second citation #15 wasn’t used?
Authors’ response: We corrected this duplicative citation error.

11. Line 702-706: There are two citations numbered 33.
Authors’ response: We corrected this duplicative citation error.

12. Supplemental File 102-135: You mention one of the PurpleAir smoke corrections was Barkjohn 2022, yet in Figure S5 it appears you labeled it as “Johnson et al. 2022” and in the caption you also mention “Johnson et al. 2022”.
Authors’ response: Thank you for noting this error - this author recently changed her surname from “Johnson” to “Barkjohn”; this paper was published under “Barkjohn” so all references to it have been corrected. For clarity, we also added a bibliography to the supplemental file.














Referee: 2

Comments to the Author
Comments on Manuscript ID EA-ART-12-2023-000170
Spatial Comparison of Wildland Fire PM2.5 Concentrations from Vehicle Add-On Mobile Monitoring System (VAMMS) in Remote Locations

1) Ln 107-109: “The VAMMS samples through ¼” conductive tubing attached to a ¼” stainless steel probe with a 10 deg cone and 1/16” inlet that provides isokinetic sampling at 35 mph at 3.5 LPM sample flow for the PM2.5 cyclone.”

Please describe how it was determined that the inlet provided isokinetic sampling for PM2.5 at 35 MPH at 3.5 LPM.
Authors’ response: We added the isokinetic calculation and related discussion to Section 1 of the supplementary information. Please also see response to Reviewer #1 above.


Is the vehicle profile expected to have an impact on the inlet’s performance?

Authors’ response: Most likely, however we did not characterize the potential impact in this study. We added some text in the “Limitations and future work” section to address this.


2) Ln 203-205: “Only data from PurpleAir labeled as ‘outdoor’ were considered. We required that the difference of the A and B cf=1 channels be < 70% or < 5 μg m-3 at the highest time resolution available, a data quality assurance step described in detail elsewhere33.”

The “outdoor” designation being used to select identify sensors that are believed to deployed outside and monitoring ambient? But the ‘CF1” data was used in analysis?

Authors’ response: Correct – we first used the “outdoor” designation to identify sensors believed to be deployed outside in ambient conditions.
The cf=1 and cf=atm are products of undisclosed propriety algorithms developed by sensor manufacturer, Plantower. The PurpleAir webpage indicates that the default on their PurpleAir Map is for ‘indoor’ sensors to display cf=1 data and ‘outdoor’ sensors to display cf=atm data (https://community.purpleair.com/t/what-is-the-difference-between-cf-1-atm-and-alt/6442, last accessed 1/22/24). They discuss why this is the best choice for their purposes, however we note that all PurpleAir sensors report both cf=1 and cf=atm data channels.
The correction equations we applied use the [cf=1] data, as the authors stated that it is more strongly correlated to reference monitors over the full range of concentration they considered (Barkjohn et al. 2021, 2022).
We moved the existing text from “Additional Instrumentation” to the “Corrections” section and added some of the above explanation to better explain our rationale and method.
Manuscript text revisions: “…We then compared the performance of two wildfire-smoke specific correction equations35,36, both of which use the [cf=1] data, as the authors stated it is more strongly correlated to reference monitors over the full range of concentrations considered…”


3) Ln 293-298. Please note the filter concentration ranges for the chamber tests?
Authors’ response: We added the gravimetric filter concentration and some additional text for context.
Manuscript text revisions: “The gravimetric filter concentration (cumulative from two smoke pulse and decay experiments) was 8.19 µg m-3 and the mean pDR-1500 concentration was 14.6 µg m-3, suggesting a linear adjustment factor of 0.56. This is comparable to the Delp and Singer (2020)35 wildfire smoke correction for the pDR-1500 (0.53). Given this, we opted to use Delp and Singer (2020) value to correct all VAMMS pDR-1500 data given that their correction factor was developed using real wildfire smoke over a wider concentration range and for a longer period.”


4) Ln 314-316: “During the four sampling run periods shown in Fig. 1, the Oakridge FEM monitor was the closest monitor to the VAMMS in all cases except for Fig. 1d when the VAMMS was closer to the regulatory stations in Eugene and Springfield (10:00 to 11:30 PM UTC).”

Please clarify this statement. I assume that for the two sets of markers shown in Fig 1d, the VAMMS was closest to the Oakridge station, i.e. the three regulatory stations are not within 400 m of each other.
Authors’ response: Apologies for the confusing language. For the two sets of markers shown in Fig 1d the VAMMS was closest to the Oakridge station (i.e., the three monitors were within 400 m of each other). However, between those two sets of markers, the VAMMS travelled about 70 km away (and back again) toward Eugene and Springfield (which have their own FEM stations). We clarified this statement as:
Manuscript text revisions: “During the four sampling run periods shown in Fig. 1, the closest FEM monitor was at the Oakridge site, except between 10:00 and 11:30 PM UTC on 10/10/22 (Fig. 1d) when the VAMMS travelled roundtrip northwest toward Eugene and Springfield which have their own regulatory stations (data from those monitors is not shown in Fig. 1).”





5) Ln 324-325: “Including all instances where the VAMMS passed by the Oakridge AQMS, in 11 out of 14 passages (~80%), all four instruments agreed in “approximate AQI” category (displayed as the color in Figure 1)....”

Figure 1 shows 9 sets of markers indicating the VAMMS was within 400 m of the Oakridge monitoring site. The text states 14 passages, presumably these are from sampling periods not shown in Fig 1. The authors should consider including the sampling period discussed in Ln 330-345, since this event takes up a large portion of section’s text.
Authors’ response: Correct – there are additional passages not shown in Figure 1. The passages we selected to show in Figure 1 were chosen as they had the most passages within a given time frame (i.e., 2 or 3 passages per sampling run instead of just 1). Though, for completeness, we added the remaining passages as a supplemental figure.
We appreciate and agree with the reviewer’s suggestion to include the sampling period we discussed at length in the text – we replaced panel c) to show that sampling period instead of the original.


6)Figure 3b. Please clarify – is the PurpleAir data displayed in Figure post-Holder correction or uncorrected?
Authors’ response: We updated the caption to clarify that the PurpleAir data shown are corrected.


7) Ln 462-446: “For small, prescribed fires where meteorological conditions are well characterized, the monitoring duration is short, and near-field plume dynamics are highly variable, stationary monitors may better represent concentrations for personnel on the ground, while the mobile monitor can provide data with higher temporal and spatial extent that would be useful for validating smoke plume models.”

Good point!
Authors’ response: We appreciate the comment.

20-Feb-2024

Dear Dr Bittner:

Manuscript ID: EA-ART-12-2023-000170.R1
TITLE: Performance of Vehicle Add-On Mobile Monitoring System PM_2.5 Measurements During Wildland Fire Episodes

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 1

I appreciate the efforts the authors have taken to address both reviewers' comments and am satisfied with the revisions. I only note that in lines 316-317 of the tracked changes PDF, the Percent Difference equation was not displaying properly, so please make sure that is correct before resubmission.