the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2025
Fine-scale fluctuations of PM1, PM2.5, PM10 and SO2 concentrations caused by a prolonged volcanic eruption (Fagradalsfjall 2021, Iceland)
Abstract. The 2021 Fagradalsfjall fissure eruption was the first of ongoing eruptions in the most densely populated part of Iceland (70 % of population within 50 km). It was monitored by an exceptionally dense reference-grade air quality network (14 stations within 40 km), and the first time that a reference-grade timeseries of PM1 was collected during an eruption. We used these measurements to identify fine-scale dispersion patterns of volcanic air pollutants (SO2, PM1, PM2.5, PM10) in populated areas.
Despite its small size the eruption caused a statistically-significant increase in average and peak PM and SO2 concentrations in at least 300 km distance. Peak daily-means of PM1 peak rose to 18–20 µg/m3 from 5–6 µg/m3; and proportion of PM1 increased relative to coarser PM fractions (21–24 % of PM10 compared to 14 % background). Eruption increased PM10 and PM2.5 by ~50 % in populated areas with low background concentrations, but its impact was not measurable in areas with high background sources. This suggests that ash-poor eruptions are one of, or the most, important source of PM1 in Iceland, and potentially in other areas exposed to volcanic emissions.
There were significant fine-scale temporal (≤1 hour) and spatial (<1 km) fluctuations in volcanic pollutant concentrations. In Reykjavík, two stations located <1 km of each other recorded peak hourly-mean concentrations of 480 and 250 µg/m3 SO2, and 5 and 0 exceedance events, respectively, within a ~12-hour plume advection event. This has implications for population exposures estimates.
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Status: open (until 08 May 2025)
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RC1: 'Comment on egusphere-2025-937', Anonymous Referee #1, 30 Mar 2025
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This paper appears to have been written by the corresponding author, and not have been edited by the native English-speaking co-authors. There are several English grammar errors, which are annoying to this reviewers. Reviewers should not be expected to edit the paper. And every author is required to agree to submission before a paper is submitted. Why did they not read it and correct the grammar? Languages like Russian and Chinese do not have articles, and so it is obvious when the paper is missing articles that the author did not learn English well enough to use them correctly.
This paper is about health effects of volcanic emissions, looking at SO2 and PM. But adverse health effects depend on exposure, which includes not just concentration of the pollutants but on how long a person is exposed. There was no discussion of this in the paper. How dangerous are these pollutants? Is SO2 or PM more dangerous? And I have never heard of PM1 before. Is it more or less dangerous than PM2.5? The paper needs a discussion of these issues, and the conclusions need to frame the results in terms of short or long-term health impacts. Were the lifespans of visitors or Icelandic residents really affected by this eruption?
The paper analyzes concentrations of particles, but does not provide any modeling of the particles from the volcanic vents to the sensors. This could be done with various air pollution models, forced by actual meteorology, say from reanalysis, or downscaled with a local high-resolution model. This would explain the variations, and also provide a capability to predict the air pollution from future eruptions. Emissions from a fissure only get to populated places if the wind is blowing that way.
Even without sophisticated models, the paper needs to explain the synoptic situation in the most polluted cases to show how the pollution gets to the people. The wind rose in Fig. B11 is not sufficient. And just to be clear, is it correct that the wind direction is shown as the direction the wind is COMING FROM, as is the standard convention? If so, why are there so few times when the wind comes from the west and northwest?
I recommend major revisions to address these issues.
And the authors need to respond to the 39 comments in the attached annotated manuscript.
Why is there an Appendix B and not an Appendix A? Anyway, the Appendix should be supplemental material. Otherwise the paper is too long, and supplemental material should not be needed to read the gist of the paper.
There are multiple problems with the figures and tables:
The supplemental table needs a caption that can be read. Right now it is buried in Excel spreadsheet cells. The column headings in row 9 are also hard to read. Make them wide enough so the text is easy to read.
Table 1 should be all on one page. Why are you making it so hard for the reviewers to read it, with the column headers split over two pages? This is especially true, since there is empty space after the table on that page?
For Fig. 3, the colors of the eruption and background plots are very similar. Make one red so they can be distinguished. And the diagrams show empty boxes, but I don’t see any on the plots. However, the same type plots in Fig. 4 are easy to see. Why don’t you make the ones in Fig. 3 the same?
Fig. 4: Why are some stars red and some orange?
Table 2 is very poorly done. Because of the large cell margins, Erupt. goes over two lines, and does 0.2 on one line and 5 on the next line mean 0.25? Make the left and right cell margins smaller and use no indentation, and the table will be possible to understand.
Shouldn’t Table 2 be in the supplemental, and have it replaced with a figure? That would make it much easier to compare the numbers in each cell.
Table 3 is also faulty. The caption mentions orange and green rows, but the text is all black. And the ratios written in the headers and the years on the side span two rows and are very hard to understand. This needs to be fixed.
Hint: If a table will not fit in a portrait orientation page, use landscape mode!
Fig. 9 should plot population density (by area), not by municipality.
Fig. 10a is so tiny with miniscule fonts that it cannot be read. Enlarge it and put it on its own separate page.
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RC2: 'Comment on egusphere-2025-937', Anonymous Referee #3, 01 Apr 2025
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The article investigates the impact of the 2021 Fagradalsfjall volcanic eruption on air quality in the most densely populated region of Iceland. Using one of the densest reference-grade air quality monitoring networks in the world, the study finds statistically significant increases in SO₂ and particulate matter (PM₁, PM₂.₅, PM₁₀) concentrations, particularly PM₁. These findings have important implications for assessing population exposure and public health risks associated with volcanic air pollution in inhabited areas.
In the reviewers opinion, a major limitation of the study is the lack of calibration for the low-cost sensors.
Below you will find my comments regarding the text.Major Comments:
Sensor models: Please provide the specific models of the PM and SO₂ sensors used. Were these commercial low-cost sensors (e.g., Alphasense), or custom-built?
Air intake design: How was the air intake for the sensors constructed? Was it a prototype, experimental design, or a standardized market-available solution? This should be clearly described, ideally with a schematic or reference.
Sensor sampling period: The manuscript lacks information on the temporal resolution or sampling interval of the sensors. This is crucial for interpreting short-term variability in pollutant concentrations and for reproducibility.
It may be helpful to add a dedicated supplementary table with detailed specifications for each sensor (manufacturer, model, detection range, resolution, operational dates).
Technical and Formatting Corrections:
Line 138 & 360: “PM2.5” → please use correct subscript formatting (e.g., PM₂.₅) and apply this consistently throughout the manuscript.
Line 192: Correct the citation format – change “(Icelandic Directive, 2016)” to Icelandic Directive (2016).
Line 193–194: Add space before parentheses: “15 µg/m3(World” → 15 µg/m³ (World).
Line 119: please clarify which size fraction is being referenced here (PM₁, PM₂.₅, or PM₁₀?).
Please revise the manuscript to correct improper or missing articles. Numerous instances throughout the manuscript require grammatical revision.Figures and Tables:
Figure 4: Please explain in the legend what is the meaning of full vs empty stars.
Table 1: The table should be kept on a single page to aid readability.
Table 3: The manuscript refers to "Green-coloured" and "Orange-coloured" rows, but these distinctions are not visible in black-and-white print.
Please make the tables more readable.
Consider using a logarithmic scale on selected figures. This would allow better visualization of both low and high values (e.g., Figure B3).Citation: https://doi.org/10.5194/egusphere-2025-937-RC2 -
AC1: 'Reply on RC2', Evgenia Ilyinskaya, 01 Apr 2025
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We thank the Referee for the review and the comments, which we will reply to once the editor's decision has been made.
For the time being we would like to point out that a supplementary table with the details the Referee is asking for* was already included in our submission (Table S1, submitted as a zip file as per the journal guidelines).
*Referee's comment: "It may be helpful to add a dedicated supplementary table with detailed specifications for each sensor (manufacturer, model, detection range, resolution, operational dates)."
Citation: https://doi.org/10.5194/egusphere-2025-937-AC1
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AC1: 'Reply on RC2', Evgenia Ilyinskaya, 01 Apr 2025
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RC3: 'Comment on egusphere-2025-937', Anonymous Referee #2, 17 Apr 2025
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General comments:
This study reports measurements of SO2 and particulate matter concentrations at several Icelandic in-situ measurement stations before and during the 2021 Fagradalsfjall fissure eruption. The study contains some novel results and is of interest to the scientific community, in my opinion. It is generally well written and I don’t have any major objections to the publication of the paper. I ask the authors to consider the following general and specific comments.
One general comment addressed the term „reference-grade”. It is used many times throughout the manuscript and I assume most readers will understand more or less what it means. It would be good, however, to state briefly at the beginning of the paper, what the exact meaning of the term is in this study.
Specific comments:
Line 90: “…, locally known as Reykjanes Fires.”
This is only a minor point, but from the way this is phrased, it is not entirely clear what the last part of the sentence refers to. I assume it is supposed to refer to the eruption, but linguistically it may also refer to “Reykjanes peninsula”.
Line 98: “and over 70% of Iceland’s total population (263,000 out of 369,000 people) lived within 50 km distance, including the capital area of Reykjavík.”
This has been mentioned already two times before and I’m not sure, whether it has to be repeated again.
Line 132: “The sensor accuracy limits during field deployment of (Whitty et al., 2022) were significantly poorer than the detection limits reported by the manufacturer.”
Something is wrong here.
Line 170: “We considered whether the year 2020 had lower PM and PM concentrations”
PM10 and PM2.5?
Line 192: “by the (Icelandic Directive, 2016).”
Opening parenthesis in the wrong place.
Lines 193 and 194: missing spaces before citations.
Figure 2, G3-A: an increase von 14% to 21% is not that large. It would be good to know what the standard deviation of the PM1 fraction before the eruption is.
Line 230: “This is a novel result showing that volcanic plumes contribute a significantly higher proportion of 230 PM1 relative to both PM10 and PM2.5”
Without the standard deviation etc. one cannot really tell, whether the PM1 proportion is significantly enhanced, right?
Table 1: It would be very useful for the reader to add the standard deviation of the SO2 hourly means for the background conditions, to judge how large the variability is.
Figure 3: I don't fully understand what the red asterisks show? The legend states: “The figure also shows whether the number of threshold exceedances at each station exceeded the recommended annual total (n=24, orange horizontal line” and I understand what the horizontal orange line means. But the red asterisks also appear below the line? Sorry, I think, I didn’t get the point here.
Also: because of the large dynamic range of the SO2 values a logarithmic y-axis would be useful here. At the moment it is impossible to tell, whether the values in Table 1 are consistent with the figures. And the boxes and whiskers are not really visible.
Another question about the box and whisker plots: What percentiles do you assume for the whiskers? It cannot be the 0th and 100th percentiles, because you show outliers.
Figure 4: What is the difference between solid and open asterisks? This doesn't seem to be explicitly mentioned?
Figure 5: In some of the boxes the median seems to be missing? Why?
Table 2: The standard deviation would also be useful here.
Line 411: “Figures 7a-7d show the spatio-temporal resolution and ratios of SO2 and PM”
Why “resolution”? Perhaps “variation”? And the plots don’t really show ratios of SO2 and PM. Do you mean the scatter plots here?
Figure 10a: The text in figure and legend is barely legible.
Section 3.5.2: This section only contains quite general and somewhat vague considerations with little quantitative results? What are the main conclusions of section 3.5.2?
There is probably (?) no lower threshold for the harmful consequences of air pollution and it may also be of interest to consider periods with SO2 levels below the assumed threshold.
Citation: https://doi.org/10.5194/egusphere-2025-937-RC3
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