| 23 Jun 2026
Measurement report: Size-resolved seasonal study of inorganic ions and isotopic carbon signatures of aerosol particles at the Wadden Sea
Abstract. Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. 13C/12C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The 13C/12C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.
The paper describes size resolved measurements of major ions and total carbon at a site in the Netherlands as well as size resolved carbon isotope analyses. Taken together, the paper addresses relevant scientific questions within the scope of ACP.
The results of the chemical analyses are not really unexpected or novel – they confirm what is known about the composition of the aerosol in the NL. Size resolved composition data, however, are not as commonly available as data for the total aerosol, so the paper adds to existing knowledge. The more interesting part, however, is the part on the carbon isotope analyses. These size-resolved analyses definitely give new data and yield insights into the sources of primary as well as aged OC (fossil, contemporary).
The title clearly reflect the contents of the paper and the abstract provides a concise and complete summary. The overall presentation is well structured and clear and the language is fluent and precise. Proper credit is given to related work. References, however, are missing in several instances (see comments below). This occurs most often for things that are “generally known” (e.g. levoglucosan as tracer for biomass burning) or in the conclusions section when other studies are mentioned for comparison purposes without adding the reference to these other studies right where they are mentioned. This, however, is not an issue in terms of “proper credit to related work”, but an issue of final copy editing.
In summary, this measurement report definitely merits publication after important revisions (see my comments below)
Major point
One of the major results is the influence of shipping emissions on the aerosol at this coastal location. This is definitely interesting, but the main arguments for this are derived only from the ion (and TC) analysis. Sources (biogenic as well as anthropogenic) of nss-sulphate other than shipping emissions should be mentioned. No info is given why ships are sources of sulphate (of course “everyone knows” that shipping fuels have a high S content….), so this issue must be discussed (including proper references on fuel sulphur content as well as dedicated measurements in ship plumes). The relatively small modern carbon fraction (Fig. 4) in the smallest size range is attributed to traffic (cars, ships) but the link between the two sections of the MS (ions, isotopes) is not made. The issue of shipping as a major source of aerosol sulphate is discussed only regarding total sulphate and non-sea-salt sulphate with some disregard of the back trajectory analysis. There is only one “sea” sample, and even there the back trajectories show that the air mass had passed over source areas in Great Britain. Info on elemental carbon would have helped to substantiate the conclusion about the contribution of shipping emissions to the coastal aerosol at the site, but as these data obviously are not available, nothing can be done in this respect. In the absence of more samples, data on source strengths and a clearer distinction between “sea” and “land” trajectories, I would suggest weakening the categorical statements that shipping emissions _are_ _the_ main source for sulphate. They may well be, but this conclusion would need actually a lot more info and data, which would be out of scope of this study. The abstract should also be changed accordingly (Lines 13 – 15: not “the”, but “a” main source)
General comments
Throughout the MS (and most often in section 3.1) mention of concentrations is qualitative instead of quantitative. Statements about a concentration as being “high” or “low” or “maximum” etc. must be quantified.
As there are only 7 samples for “land” and 1 sample for “sea” influence, some of the statements seem to be too general, and there might be some overinterpretation of the scarce data (most importantly, the “sea” sample). I suggest writing e.g. “in the spring samples” instead of “in spring”, as there are only 2 spring samples. I fully appreciate the difficulty of sampling at this site, but generalizing from a couple of samples for a whole season is risky.
CCN are first mentioned in the conclusions section? Of course the data are relevant for CCN issues, but if the issue of CCN should stay in the MS, more discussion is needed (including references), and CCN should be mentioned in the introduction section
Other points (roughly in order of occurrence)
Line 75: “long term sampling campaign” is not appropriate, as it suggests long term _continuous_ sampling. There are only 8 samples in total with sampling times ranging from 1.7 to 10.6 days (Table 1).
Line 105: does the customization of distance rings for the impactor stages have an influence on cut size?
Line 106: info on storage conditions of pre-heated filters is missing, as is any info on field blanks (possible adsorption artefact of OC?)
End of section 2.1: describe attribution of the samples (aliquots?) for the different analyses; add where the different analyses were performed; shipping conditions; all analysis methods: LOD?
Line 128: quantify instrument blank / field blank
P 8, Figure 2: these plots are impossible to interpret. Adding connecting lines might help. A table containing all the values should be given at least as supplementary material, especially as the discussion in the next three pages of text is much too qualitative.
P 9 – 12: quantify all statements referring to “high” “low” etc. concentrations.
Lines 215 ff, discussion of non-sea-salt sulphate: rather vague, no concentrations quantified. The back trajectories for this sample also indicate that the air mass had spent some time over Great Britain, so there could also have been some anthropogenic influence (both in nss-sulfate and carbonaceous material) other than shipping.
Line 230 – 232 the sentence refers to reduced concentrations of ammonium compounds related to successful reductions of sources by legal restrictions. Either add info and relevant references or delete this sentence.
Lines 233 ff: discussions on seasonal differences: the influence of mixing height is mentioned correctly, but otherwise the discussion is focussed on absolute concentrations. The effect of mixing heights (correctly mentioned by the authors) can be removed by using ratios of individual analytes to total analytes. This might change the interpretation of seasonal differences and differences in sources
Lines 250 ff: the statement “In general, in summer more stable weather conditions cause the transport of likely cleaner air masses from the main southwest wind direction. Whereas in autumn the atmosphere over Europe experiences the passage of more low pressure systems, so that the transport of air to the station is less predictable” is unsubstantiated – either add a lot more info or delete.
Lines 260 ff the discussion on chloride, sodium and calcium concentrations in general reads a bit speculative – either substantiate with literature references or modify the language (e.g. use “might be” instead of “is”…..)
line 268: Substantiate statement on reaction rate
Lines 262 – 283 (discussion on chloride, sodium and calcium to deduct the influence of seas salt): the text refers to a figure (B1) giving size distributions of these ions in mol/m³, but this figure is nowhere to be found?
Lines 280 – 283: add reference for particle bounce issue
Lines 283 ff: unclear reason for fine calcium: might be emitted from wood/biomass combustion (e.g. Khalil and Rasmussen, 2003, Atmospheric Environment) or from coal fired power plants (e.g Meij, te Winkel, 2009, J. Aerosol Sci. – for the Netherlands…)
Lines 284ff section on isotope analysis: Adding subsections for the two types of isotope ratios would be helpful to differentiate between the influence of fossil fuels and atmospheric processing. Another subsection heading could go after line 357
Lines 310 ff: the statement: “the impactor samples taken in spring show elevated F (14C, OC) compared to the other seasons” does not seem supported by Figure 4 – the green markings are “drowned” by the grey ones. As this seems to be a problem especially for the large particle range: could the high fraction of “modern” OC in spring be also influenced by pollen? Of course pollen are too large, but the aerodynamic diameter of sub-pollen particles is in this size range
Figure 4: please connect the data points of the individual size distributions to guide the eye
Lines 328 ff: Please put the relevant references right where the different sources are mentioned
Figure 5 and description in the text: add a (few) sentence(s) on why so many of the data points lie outside the regions marked in green (biomass) and grey (traffic) and on possible reasons for the large overlap between “green” and “grey”
Conclusion section: again, please quantify statements such as major, minor, high, low, etc
Line 405: “a one year long sampling campaign“ would imply continuous sampling and not just a total of eight samples – please modify
Line 420: change “inorganic ions” to “other inorganic ions”…..
Lines 424 ff, CCN: see general comment above
A sample of missing references: Lines 410, 431, 437, 440, 442, 443, 444, 449 - please check the whole MS for other instances
Technical points:
Figure 1: label bypass line
Figure 5: explain the bars also in the caption
Figure 6: explain green / grey lines in caption
Typos:
Line 268 change rage to range
Caption Figure 4: change filter to filters