Review of resubmission of Anthropogenic VOC in Abidjan, southern West Africa: from source quantification to atmospheric impacts, by Dominutti et al.
General Comments:
This is my second review of this paper, and the authors have addressed many of my concerns from the first round of reviews, although I would have preferred that the “response to reviewers” included significantly more detail to make it easier to see what changes were made between the original submission and this revised version. Nevertheless, many of the grammatical, technical, and word choice issues have been addressed, and have made the paper somewhat more readable, although there remain a long list of technical issues that I have listed at the end of this review since the authors failed to extrapolate my suggestions to the remainder of the paper.
This paper is a continuation of the Keita et al. (2018) paper, a paper that is referenced frequently in this work, which reports emission factors for a subset of measured VOCs Abidjan. There is scientific value in the observations and analyses presented in this paper, and in the connection of the VOC measurements and source emissions in an urban West Africa city to contribute to and improve global emission inventories, and as such, it would be good to see this research published. Nevertheless, this paper still has several issues that need to be addressed prior to publication, and I outline them below.
Finally, I wish to voice my concern again that the data being discussed and analyzed in this paper is still not available. The authors have provided a website, but as the data are not public, it is not possible for me to assess the calculations that they have described in this paper. My recommendation is that the paper not be published until the data are made available to the reviewers, either at the website provided, or at a secondary temporary data repository.
Specific Comments:
Line 66: Please reference where the AMMA campaign took place (city and country?)
Line 97: It is unclear what is meant by “sensitive places with high anthropogenic pressures”. Please use a more technical description.
Line 102: The authors state that global emission inventories commonly estimate the total mass of VOCS. This needs to be explained or corrected. Many inventories include speciation or at the very least, parameterization of various VOCs or VOC groups, and do not just provide a total VOC mass. The authors go on to suggest that the fate and contribution of each species can change depending on the emission source, and again – this needs to be explained somewhat. The fate of the VOC should not depend on the emission source, unless the reference implies that *other* co-emitted species could determine the lifetime and ultimate fate of the VOC. The source, however, doesn’t not necessarily define the atmospheric fate of all VOCs.
Line 106: Give a better description of the “different compounds” for which Keita et al. reported EFs. Also, perhaps start line 107 “In that study, a comparison of the emissions calculated from the EFs…” to clarify that this isn’t being presented here. Similarly, on line 109, “That study emphasized the importance…” to differentiate work that was presented in another paper from work being presented in this paper.
Line 116: A second Knippertz et al., 2015 seems to be missing from the paper (titled: The DACCIWA Project: Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa, https://doi.org/10.1175/BAMS-D-14-00108.1). Clarify this, and where necessary, edit all references to Knippertz et al., 2015a or Knippertz et al., 2015b.
Lines 139-150 – This paragraph is not a material or a method. It should be moved to the intro.
Section 2.1 Sampling: This section still lacks the clarity and specifics needed to describe how the sampling was performed. The authors need to consider that the reader does not know how the sampling took place and be very explicit. At the very least, a table indicating the number of samples, (including resampling of cartridges), and times for the sampling for each cartridge, or pair of cartridges would be helpful. As well, the campaign needs to be described in terms of duration, times, intensives, etc. Also, describe the preparation of the cartridges *before* a discussion of the sampling, and the results.
Also, for example, on Line 190: 3 samples on Tenax and Carbopack tubes each? Or 3 samples total? One Tenax and two Carbopack? This is why the sampling should be more explicit. These should be described fully. Which were sampled where? Simultaneously, or back to back? How many samples from each location were used in the summary table S1? What kind of uncertainty is introduced from having samples taken at different times of day? Were there some samples that were higher than others? Were there times of time that were more concentrated than others? Were the samples taken sequentially over a few days, and then new samples taken, or were samples taken only at specific times of day and then other cartridges sampled at other times of day?
With no access to the data, I have no idea how many samples were taken at each location, when they were taken, and whether there is any bias introduced into the sampling strategy as a result. This is one of my primary issues with the paper.
Related, the authors need to include a much more robust description of how the VOC measurements were calibrated, an assessment of cartridge blanks, detection limits, uncertainties, etc. Please include a section on the instrumental methodologies that includes all of these details.
Lines 228-229: Carbonyls are ketones, so this sentence doesn’t really make sense. Also, perhaps just list the “6 VOCs of intermediate volatility” to explain what is meant.
Lines 257-269 – there are three references to Keita et al. (2018) in this paragraph alone. The strong dependence on, and inadvertent co-opting of the work that was done in the previous work to this work makes it questionable where the line between the two papers really lies. The authors should consider what is really necessary to include in this paper, and what is just additional hyperbole that was already covered in the Keita paper.
Lines 347-349 – this was moved from elsewhere, but the reason for including this sentence needs to be given. I.e., there needs to be a point given for this reference info.
Lines 373-375 – were the observations reported in Djossou et al. (2018) measured at the exact same locations, or just similar types of locations?
Lines 375-377 – this sentence is very hand-wavy. It should either be more quantitative, or removed.
Line 378: The authors report that the m+p-xylene and toluene contributions to ambient BTEX range from 9-27% and 8-31%, respectively, but the m+p-xylene and toluene components of the pie charts shown in Figure 3 appear to never be less than 25%. This needs to be clarified.
Lines 379-381: These sentences are inconsistent. The authors explain that the heterogeneity in the VOC spatial distribution could be related to the main activities (which is not specific enough – please describe) that are involved in the emission of these compounds. This needs to be backed up and/or clarified. The authors go on to say that “except for higher benzene concentrations observed in some sampling locations, the BTEX profile is rather consistent.”, which seems to be contradictory to the previous sentence. Are the profiles consistent, or not? Do the distributions relate to the emissions, or not? And what about chemical loss processes? Do those have any bearing on the chemical profile in some of the sampling locations?
Lines 392-393: The authors note that similarities in VOC distributions in Abidjan are similar to other mid-latitude megacities, suggesting emissions from fossil fuel combustions for alkanes and aromatics could dominate other regional sources. There is a disconnect in the logic here – alkanes and aromatics in similar profile fossil fuel combustion. While this may be legit, the authors need to tie the observations to the emissions, either using literature values or the co-measured emissions.
Lines 441-442 – the implication that there are terpenes in road transportation emissions is a pretty big stretch, when they are not usually found in road transportation fuels. While they may be present in waste burning, their strongest sources tend to be from biomass emissions (burning or not), and the presence of them in the vicinity road transportation emissions does not mean that they are being emitted from road transportation activities. I recommend the authors be very cautions with this kind of suggestion that isn’t backed up by direct emissions studies. Also, VOC22 is not the only group that contains OVOCs, so this should be more specific.
Lines 447-455 – There are several assumptions being made to connect and compare the OVOC measurements made in this paper to the work in previous studies. I caution the authors to not include comparisons of OVOCs that don’t even include methanol, formaldehyde, acetaldehyde, acetone, etc., which are massive components of many emissions inventories. While the authors include a caveat at the end of this paragraph, I don’t think that there is any need to include these details, as the limited number of OVOCs measured in their technique cannot be justifiably compared to previous studies that include C1-C4 OVOCs.
Technical notes:
Line 30: change to “despite high VOC concentrations near-source…”
Line 33: change “achieved at” to “for”
Line 37: “the VOC atmospheric impacts”
Line 42: Gg yr^-1, Gg year-1, Gg per year… be consistent throughout.
Line 44: “the residential sector is largely underestimated in global emission…”
Line 47: perhaps “for the entire West Africa region…”
Line 49: sometimes “fuelwood” is used, sometimes “fuel wood”. Be consistent. Ideally, “wood fuel” would be better, if the reference is to the burning of wood for fuel.
Line 49: add “burning” or “burning and fabrication” after charcoal to be clear.
Line 58: “will continue to increase…” (they’re already increasing.)
Line 60: “urban centers in the region is a significant issue” – unless you’re going to list others, “one of the main issues” doesn’t have any context.
Line 62: Eliminate “consequently”.
Lines 64-65: “Saraha Desert, biomass burning, and local urban…”
Line 69: “Regional biomass burning is a significant source…”?
Line 72: Perhaps change “important” to “major”. Important implies that the emissions are useful or good.
Line 77: Add a comma after biofuels.
Line 79: “However, emission estimates are uncertain…”
Line 84: change “their emissions…” to “VOC emissions…”
Lines 85-86: eliminate “As VOCs are important pollutants”. This is redundant, and again, implies that they’re “good”.
Line 87: “secondary product formation.”
Line 89: “characterizing VOC to better understand their emission…”
Line 91: “VOC observations have been intensely used…”
Line 95: “For northern mid-latitude cities, discrepancies up to a factor of 10 for VOC emissions have been observed (Borgon…”
Line 99: “involve numerous…”
Line 110: “suitable data, the uncertainties…”
Line 119: “off-line” isn’t necessary here.
Line 123: “…in real-condition operation” is awkward.
Line 125: add a comma after fabrication
Line 126: “additional VOC emissions”? (i.e., the ones not reported in Keita et al.?)
Lines 136-137: be consistent with the Knippertz paper with “Workpackage” vs. work package, and capitalize “Air Pollution and Health”.
Line 140: “20% of the population of the…”
Lines 145, 147, 149, 343: northeasterly, southwesterly, southwesterlies – no hyphen needed.
Line 156 and elsewhere – for C6-C9 and C5-C16 - the numbers should be subscripted.
Line 160: “for 5 hours at 320 °C”
Line 162-163: “They include regional (?) background sites as well as areas impacted…” (Background can mean different things depending on the context.)
Line 167: “100 mL sccm flow rate” is redundant. Delete mL and define sccm.
Line 171: remove all hyphens from this line. Also, consider “wood fuel burning”.
Line 122: “for 15 min”.
Line 213: remove the period from 40° s-1
Line 240: “the mathematical basis” may be more appropriate.
Line 249: there is a + missing from the denominator of Delta-VOC/(Delta-CO + Delta CO2).
Line 251: should emission and background be subscripted?
Line 255: change the semicolon to a comma.
Line 276: There is a Delta symbol missing from the numerator.
Line 282: “et al.”
Lines 284 and 295: The ° symbol should be consistent throughout the paper.
Line 292 and throughout (including the Supplement) Usually, kOH has a subscripted OH.
Line 294 and elsewhere: Spell out molecule. “molec” is not an acceptable shortform.
Line 304: Can VOC – ozone formation potential be written a way that doesn’t look like “VOC minus ozone formation potential”? Similar question for line 317.
Line 306: “Derwent and co-authors (Derwent et al., 2007; 2010a; Jenkin…” – not all the papers listed are by “Derwent et al.”
Line 325: it is not clear what “Table S1, in the 1” means.
Line 337: “… analysed for 2016. Meteological…” (The fact that the field campaigns were in 2016 should be included in the methods, and doesn’t need to be repeated here.)
Line 344: delete the period in m s-1.
Line 352: “Keita et al. (2018)”
Line 353 “8 aromatic hydrocarbons”
Line 354: delete the space after fossil.
Lines 359-360: Delete the sentence “Details on sampling locations can be found in Table 2.” This information is not needed.
Line 367: Delete “(Figure 3.)” The details stated in this sentence are not shown in Figure 3, which shows the relative contributions of the different BTEX components, and the total VOC concentrations.
Line 369: “at each site, and the BTEX…”
Line 373: “maximum aerosol concentrations”.
Line 374: delete “Figure 1” from the two sites. This information isn’t needed.
Line 375: “Besides the dilution process…”
Line 384: “with”; delete “blue points in ”
Lines 391-392: “Our results indicate that ambient VOC distributions in Abidjan are noticeably similar to other northern…”
Line 397: “Figure 5 shows…”
Line 404: “x 10-15 cm… shouldn’t be in parentheses.
Line 424: again – ketones are carbonyls. Rephrase.
Line 426: “according to the chemical function”.
Line 430: “as already described”.
Line 438: Section should be capitalized. And “reported” should be changed to “comprised”.
Line 439: Important is the wrong word. Most significant, perhaps? Also, with regards to what?
Line 457: “charcoal burning” (to differentiate from charcoal fabrication).
Line 470: “described in Section 2.3.”
Lines 479-481: This is still not a complete sentence.
Line 493: “calculated for each source”
Line 493 and Figure 7 – for numbers > 9999, either add a space between the 1000s and the 100s place, or a comma. (80343 80,343 or 80 343).
Line 499: “charcoal burning has…”
Line 500: “SOAP values…”
Line 545: “accounting for 62%” of what?
Line 548: “for atmospheric chemistry”.
Lines 550-552 – this is not a complete sentence.
Line 571: “Figure 10a shows…”
Line 575: “terpene emissions”
Line 577: controlled is not the right word.
Line 579: another example of confusing the results presented in Keita et al. with this work. “In our study” vs. “in that study”…
Lines 587-589 – this is the third time the authors have stated this. Once is enough.
Line 589 – ruled is not the right word.
Line 602 – “World Health Organization” is its official English name.
Lines 597-609 – I believe all references here to Figure 11 (a-c) should be Figure 10.
Line 620 – probably best to use “local burning” rather than “biomass burning”, which usually implies widespread fires. Or just be specific about the kinds of burning that were quantified.
Line 643 – delete “to the annual emissions.” (redundant with Gg year-1).
Figure 2 – the caption states that weekly precipitation is in mm/month, but the x-axis just states mm.
Figure 4 – South Africa is not a city. In the caption, please include the city and country where the measurements were made for all referenced studies.
Figure 6 – Capitalize Table S1 and Table S2.
Figure 7 – it is unclear why there are asterisks after SOAP in the (m) – (p) panels.
Figure 8 – the bottom x-axis should be fixed – “% weighted by kOH” with OH subscripted if possible.
Figure 10 – it is a little sketchy to include aldehydes here, when the number of volatile aldehydes measured is actually pretty small, and didn’t include any C1-C5 aldehydes. If the authors insist on including 10c, they should at least clearly specify which aldehydes are included, i.e., C6+ aldehydes.
Figure 11 – this figure should have error bars showing the uncertainties and/or the range of molar masses observed from the different sources.
Table 2 – AT – What kind of “transport” station? That is too vague. Also, “ancient” is not the right word – it implies centuries-old relics, not decades-old. AD – “Landfill waste burning”. CRE - “much wind” could be amended to something better. “windy” for instance. ZI YOP – “all types of industries…” KSI – delete space after “traffic/”
Table S1 and S2 – the authors are encouraged to check the style guide, but generally, lower case letters are used as superscripts for footnote references, not numbers. Also, define “n.d.”. |
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