the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 May 2026
Disentangling Mechanistic Controls on Ultrafine Particle Number and Growth Across Seasons in an Urban Street Canyon
Abstract. Ultrafine particles (UFP; <100 nm diameter) play a disproportionate role in human health and atmospheric processes, yet their sources and growth mechanisms in urban environments in different seasons remain poorly constrained. This study presents a comprehensive characterization of UFP number concentrations, size distributions, chemical drivers, and meteorological influences in downtown Munich across summer, winter, and spring. We combine high resolution particle number size measurements with source characterization of organic aerosol (OA), semi-volatile organic aerosol (SVOA) and volatile organic compounds (VOC) to disentangle primary emissions from secondary processes. Results show that UFP number concentrations are driven by traffic- and cooking-related emissions, consistently peaking during nighttime boundary layer collapse due to accumulation. In contrast, ultrafine particle growth (from 40 to 80 nm) arises predominantly from condensation of semi-volatile or low-volatility organic vapours, with distinct seasonal pathways: biogenic and biomass burning OA at night, and photochemically oxidized low-volatility vapours during daytime in summer; biomass-burning and combustion-related SVOA in winter; and a mixed regime involving both primary emissions and moderate photochemistry in spring. No evidence of classical new particle formation appears in these non-nucleation UFP growth events. These findings demonstrate that UFP evolution in Munich is governed by the interplay between boundary-layer dynamics and seasonally varying organic vapour sources, highlighting the need for season-specific mitigation strategies.
Competing interests: At least two of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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: open (until 02 Jul 2026)
- RC1: 'Comment on egusphere-2026-2195', Anonymous Referee #1, 16 Jun 2026 reply
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- 1
Li et al. presents the findings from a seasonal measurement campaign that took place in an urban street canyon in Munich, Germany during April of 2023 and March of 2024. Overall, the dataset is robust, comprising chemical measurements of both particle- and gas-phase composition. Additionally, I believe the topic of the manuscript will be of great interest to the scientific community, and as the authors have mentioned, will be particularly useful as regulatory agencies begin to consider regulation of UFP. However, there are several major revisions that I believe need to be addressed before the work is publishable.
Major comments:
1) The authors present seasonal measurements named to represent summer, winter, and spring. However, the winter measurement campaign only represents a handful of days. While the authors are forthright about this limitation, I believe that the manner in which the data is presented is still misleading to a reader. For example, Figures 2 and 3 present the winter measurements with equal weight as the summer/spring datasets. My recommendation would be to either a) move the figures presenting the winter campaign to the SI or b) combine the winter/spring dataset to conduct a summer vs. winter/spring comparison. In this current form, I do not think comparing diurnal variations with such different sampling duration is scientifically sound.
2) The authors present data collected using a HR-ToF-AMS with a PM2.5 inlet. Subsequent positive matrix factorization is used to determine different source factors for organic aerosol during the campaign. While this in and of itself is interesting, I do not believe the direct inference of UFP drivers of growth using solely HR-ToF-AMS data is scientifically sound. The authors mention several times throughout the manuscript that accumulation mode particles can dominate AMS measurements, and yet the AMS data represents the only particle phase data supporting their conclusions regarding chemical drivers of UFP growth. Additionally, I do not see any data presenting size-resolved number fraction (or mass fraction) for particles greater than ~800 nm. Can the authors provide data to demonstrate that these larger particles are not dominating the AMS composition measurements? Given that UFP will contribute a very small mass fraction of particle composition, the authors must present more convincing evidence that the AMS data is representative of UFP composition. Could they provide a calculation of mass fraction of UFP vs. accumulation mode particles? Could they show size distributions representing the entire PM2.5 range? At this stage, their PMF results are not convincing evidence that these factors can be directly associated with chemical drivers of UFP growth.
Minor comments:
1. I found the introduction to be very well written! It was interesting to consider season-specific mitigation strategies for UFP.
2. The authors may want to consider non-exhaust emissions as a source of UFP (Thomas et al. ESPI 2025, Park et al. Journal of Aerosol Science 2018). Given that many cities are transitioning their vehicular fleet from predominantly combustion-powered to electric, this will likely become an increasingly important component of UFP traffic emissions.
3. The authors may want to consider adding discussion of socioeconomic drivers of UFP exposure (e.g., Batisse Environment International 2025), which further supports the importance of their regulation.
4. Please define FIDAS in line 147.
5. Consider changing the header for 3.1.1 Seasonal Variations in Particle Number Size Distributions. The following paragraph discusses much more than just the number size distributions, so the header is a bit misleading.
6. Consider changing Figure 1 caption to better align with the label axes. Specifically, axis is labeled as "UFP ratio (%)" on plot but "UFP number concentration" in caption.