the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jul 2026
Contributions of Fine and Coarse Particles to Light Absorption at an Urban Traffic Site during Street Dust Season
Abstract. Light absorption by urban aerosols is dominated by fine particles (particulate matter ≤2.5 µm; PM2.5), particularly from traffic exhaust and residential combustion. However, coarse particles (2.5–10 µm; PM2.5–10) can also contribute during spring street dust events in northern countries. This study investigated size-resolved optical properties and chemical composition of PM in an urban street canyon during spring. Hourly PM₁₀ concentrations averaged 27.1 µg m⁻³, while PM2.5 and PM1 (≤1 µm) concentrations averaged 7.1 and 4.3 µg m⁻³, respectively. Light absorption across PM₁, PM₂.₅, and PM₇.₂ fractions was dominated by submicron particles, predominantly at short wavelengths, while coarse particles enhanced absorption during dust-resuspension events. Equivalent black carbon (eBC) showed strong size dependence, with PM1 capturing most combustion-derived BC despite low campaign-mean concentrations (0.49 µg m⁻³). Relative to PM₁, hourly eBC increased by 14 % in PM2.5 and 41 % in PM7.2. Absorption Ångström exponent analysis revealed size- and source-dependent spectra differences, with stronger wavelength dependence during high PM10 dust events and values closer to unity under traffic-exhaust-dominated conditions. The inclusion of coarse particles complicated the interpretation of the AAE470/950 and related biomass burning contribution estimates. Elemental analysis revealed elevated concentrations of Si, Fe, and Al in coarse PM, suggesting contributions from crustal material and/or non-exhaust emissions that may also influence optical properties. These findings demonstrate that quantification and source identification of urban aerosol light absorption requires consideration of the selected particle size cut-off.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on ar-2026-28', Anonymous Referee #1, 10 Aug 2026
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RC2: 'Comment on ar-2026-28', Anonymous Referee #2, 07 Sep 2026
The manuscript is a valuable and well-designed study addressing an important and increasingly relevant question concerning the contribution of coarse particles to urban aerosol light absorption. The simultaneous size-resolved absorption measurements, together with elemental composition and supporting air-quality observations, provide a useful dataset, particularly for springtime street-dust conditions. The manuscript is generally well written, the results are interesting, and I believe the study is suitable for publication after revision. My main concerns relate primarily to the interpretation of some results rather than to the experimental design itself.
My first major concern is the interpretation of elemental composition in relation to the observed coarse-particle absorption. The elevated concentrations of Si, Fe and Al clearly support an important contribution from crustal or resuspended road-dust material. However, these measurements do not allow a clear distinction between mineral/resuspended dust and specific direct non-exhaust sources such as brake, tyre or rail wear. In particular, Fe can originate from several different sources, while XRF does not provide information on its chemical form. I therefore suggest that the authors moderate statements attributing the enhanced absorption specifically to non-exhaust wear emissions and distinguish more clearly between evidence for resuspended road/mineral dust and more tentative attribution to individual traffic-wear sources.
A related issue is the difference between the size fractions used for optical and elemental measurements. The coarse absorption contribution is evaluated for PM2.5–7.2, whereas the elemental composition used to interpret it represents PM2.5–10. Particles between 7.2 and 10 µm therefore contribute to the chemical measurements but not to the corresponding absorption measurements. This mismatch should be explicitly discussed and should be considered when interpreting relationships between composition and absorption or when extending the conclusions to the full PM10 fraction.
My second main concern concerns the relatively low eBC/absorption levels and the derivation of the incremental size-fraction absorption by subtraction between separate AE33 instruments. Since the PM1–2.5 and PM2.5–7.2 absorption signals are relatively small, even modest instrument-to-instrument offsets may become important relative to the calculated coarse contribution. The occurrence of slightly negative derived absorption values under some conditions further suggests that measurement uncertainty can be comparable to the incremental signal. Although the intercomparison demonstrates good correlations between the instruments, it would be useful to quantify the absolute differences between instruments and propagate this uncertainty into the derived PM1–2.5 and PM2.5–7.2 absorption values. This would strengthen confidence in the reported coarse-mode contribution.
I also suggest greater caution in the interpretation of eBC in the coarse fraction. In the presence of mineral dust and other non-BC absorbers, the AE33 converts measured attenuation into eBC using a BC-specific mass absorption coefficient. Therefore, the additional eBC measured with the larger inlet should not necessarily be interpreted as an increase in actual black-carbon mass. This distinction is particularly important here because one of the main findings of the study is that coarse non-BC particles can influence the optical measurement. The terminology throughout the manuscript should make this clear.
Finally, the conclusions drawn from the high-dust and high-traffic subsets would benefit from some additional indication of statistical robustness, particularly because these conditions represent a relatively limited fraction of the campaign. Reporting the number of observations or independent episodes contributing to each category, together with some measure of variability or uncertainty, would help demonstrate how representative these results are.
Overall, I consider the manuscript to be a strong and useful contribution. The main findings are relevant and the experimental approach is convincing. Addressing the points above would mainly improve the precision of the source interpretation and the treatment of uncertainty, and I would support publication after revision.
Citation: https://doi.org/10.5194/ar-2026-28-RC2 - AC1: 'Comment on ar-2026-28', Luis Miguel Feijo Barreira, 01 Oct 2026
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- 1
The manuscript addresses an important and timely question: the contribution of coarse particles to aerosol light absorption in an urban street canyon during the spring dust‑resuspension season. The topic is relevant for both air‑quality monitoring and optical‑property interpretation, especially given the widespread use of PM1/PM2.5 inlets in monitoring networks. The experimental design is solid, combining multi‑fraction AE33 measurements, XRF elemental analysis, and auxiliary particle and gas observations.
The study provides valuable insights, but several aspects require clarification or deeper analysis before publication. In particular, the interpretation of absorption and AAE in the presence of coarse particles needs to be strengthened, and the role of chemical composition versus physical effects should be more explicitly disentangled.
In conclusion, the manuscript is promising, but requires substantial strengthening of interpretation, especially regarding coarse‑mode absorption and AAE applicability.
I recommend major revisions.
Major Comments
1) The manuscript reports substantial increases in absorption for PM7.2 relative to PM1 (20–39% depending on wavelength). This is a key result. However, the interpretation remains incomplete that is it is unclear how much of this enhancement is due to:
increased mass in the coarse mode,
metal‑rich particles (Si, Fe, Al, Cu, Zn),
black carbon from tyre wear or graphite from brake pads,
filter‑related artefacts (scattering, loading, multiple scattering).
For example, the abstract states:
“coarse particles enhanced absorption during dust‑resuspension events” but does not quantify the relative contribution of mineral dust vs non‑exhaust traffic emissions.
It could be useful to include a multivariate analysis linking absorption coefficients to XRF elemental concentrations, and normalize absorption by mass to evaluate size‑dependent absorption efficiency.
2) The manuscript shows that AAE370/470 decreases from 1.12 (PM1) to 0.77 (PM2.5) and 0.86 (PM7.2). This is an interesting finding, but the interpretation is too closely dependent on the classical BC/BrC paradigm, which does not hold when coarse particles contain iron oxides, tyre-derived soot, graphite from brakes, mineral dust with wavelength‑dependent absorption.
The authors correctly note:
“The inclusion of coarse particles complicated the interpretation of AAE470/950 and related biomass‑burning contribution estimates.” This point should be expanded substantially. AAE‑based BB% is not valid when coarse particles dominate absorption.
Therefore, it is recommenden to dedicate a subsection discussing the limitations of AAE and BB% when sampling PM2.5–PM10, supported by literature on Fe‑driven absorption and non‑exhaust carbonaceous materials.
3) The intercomparison is well executed, showing a 14% difference between AE33 and MAAP for PM1. However, the manuscript does not discuss whether this discrepancy increases for PM7.2, where scattering artefacts are more severe.
Please, discuss whether AE33 corrections remain valid for coarse‑mode aerosols and whether MAAP comparisons could be extended to PM2.5/PM7.2.
In addition, given that AE33 uses a fixed multiple‑scattering correction (C_ref = 1.39), the applicability of this factor to coarse particles is questionable. This parameter is calibrated for submicron, BC‑dominated aerosols and assumes optical behaviour typical of fine combustion particles. When applied to PM7.2, however, it may introduce systematic biases due to the much stronger coarse‑mode scattering and the presence of non‑BC absorbers (e.g., Fe‑oxides, mineral dust, tyre‑derived carbon black). As a result, absorption in the coarse fraction may be partially overestimated. This limitation should be highlighted both in the data interpretation and in the conclusions, and future datasets including additional high‑PM dust‑resuspension episodes would allow a more robust evaluation of these artefacts and further reinforce the study’s findings.
4) The manuscript provides a detailed description of meteorology and PM dynamics, but does not analyze how coarse‑mode absorption varies with road wetness, wind speed, dust‑resuspension episodes. Given the strong PM10 peaks during dry periods, this is essential.
Please, include an analysis of PM7.2 absorption vs road wetness and wind speed, and discuss changes in AAE during dust events.
5) The introduction mentions ACTRIS and national networks, but the discussion does not elaborate on the implications of size‑cut‑off variability for harmonization of eBC datasets, comparability of PM1 vs PM2.5 vs PM10 absorption, emission factor estimation, model parameterization.
This is a missed opportunity.
It could be useful tobriefly discuss how coarse‑mode absorption affects eBC comparability and model inputs.
6) The analysis in Section 3.4 relies on a limited number of high‑PM episodes; future campaigns capturing additional dust‑resuspension events would allow a more robust statistical evaluation and further strengthen the study’s conclusions.
7) The use of the FIDAS optical spectrometer for coarse-mode characterization presents methodological limitations: its optical response is strongly affected by large-particle scattering and by assumptions on refractive index and particle shape, which may distort or overrepresent dust‑related PM peaks. This constraint should be acknowledged in the data interpretation and conclusions.
Minor Comments
Introduction: add a sentence noting that coarse‑mode absorption is poorly constrained in climate and air‑quality models.
Figure 2: Absorption normalized by mass would improve interpretation.
XRF: Provide uncertainties for elemental concentrations.
References: add literature on Fe‑driven absorption and non‑exhaust carbonaceous particulates. Moreover, in line 94 please insert reference for traffic statistics.
Line 224: correct “PM2.5” with “PM7.2”
Line 520: delete “in the”.