| 23 Oct 2025
Six-Year Trend of Concentrations of Ultrafine Particles Six Kilometres Away from a Major German Airport
Abstract. Ultrafine particles play a crucial role in the atmosphere, both as a source of larger particles and as a factor influencing human health. We analysed hourly particle number size distributions collected during 2015–2021 from an urban background station in the Rhine-Main area in Germany, with a focus on multiannual trends and potential particle sources. The site is influenced by diffuse regional sources such as motor traffic and domestic heating, as well as Frankfurt Airport, located at a distance of 6 km. The average total particle number concentration (TNC, size range 10–500 nm) was 9.4×103 cm-3. TNC maxima were observed in diurnal cycles at 07:00, 13:00, and 21:00. The midday peak was more distinct during the warm season and dominated by nucleation mode particles (NUC, 10–30 nm), suggesting photochemical particle formation as a source. When the wind was blowing from Frankfurt airport, a 2.5-fold concentration average in NUC was observed compared to other directions (11.2 ×103 cm-3 and 4.3×103 cm-3, 2015–2021). In 2020, during traffic restrictions related to the COVID-19 lockdown, TNC downwind of the airport was 40–60 % lower compared to the average of the four years before. Overall trend analysis for 2015–2021 yielded consistent downward trends for TNC (-2 %/year), atmospheric particulate matter PM10 mass (4 %/year) and nitrogen dioxide NO2 (-5 %/year). While our observations of particle number size distributions show general similarities to other Central European observations, the effect of winds from Frankfurt Airport as a particle source is most prominently seen in the range 10–30 nm. The airport's role as a source of NUC and the rise in flights from 2015 to 2019 may be the cause of lower decline rates when compared to other locations.
The manuscript of Gerwig et al. presents a valuable six-year analysis (2015–2021) of ultrafine particle (UFP) concentrations measured at an urban background site located 6 km downwind of Frankfurt Airport (FRA). The long-term dataset, combined with detailed size-resolved measurements and comprehensive meteorological and auxiliary pollutant data, offers important insights into the role of airports as sources of nucleation-mode particles in urban areas. The work is relevant for both scientific and regulatory communities.
However, the manuscript has some areas that require clarification or strengthening, particularly regarding source attribution, and the interpretation of airport-related influences. Addressing these issues could improve the robustness of the conclusions and the clarity of the narrative. Overall, I find the paper suitable for publication after minor revision.
General comments/suggestions
Specific Comments
Line 37: “The temporal variability of particle number size distributions (PNSD) and UFP concentrations is complex…”, I suggest “The temporal variability of particle number size distributions (PNSD) and UFP concentrations is influenced by multiple interacting factors such as meteorology…”
Line 59-60: Aircraft emit a large fraction of the UFP in the size of NUC particles. These NUC were
primarily caused by VOC emissions”…The sentence is not clear and should be clarified and expanded to better explain the formation mechanisms.”
Line 61 (and throughout the text): The notation of pollutants is inconsistent (e.g., PM₁, PM₂.₅, PM₁₀, SO₂); please use subscripts consistently throughout the text, captions, and tables.”
Line 78: “In this study, we analyse hourly PNSD collected during 2015-2021…”, I suggest “In this study, we analyse hourly PNSD measurements collected from 2015 to 2021…”
Line 152: “Prior to size classification, the aerosol was dried below 50% relative humidity…”; better “Prior to size classification, aerosol samples were dried to below 50% relative humidity…”
Line 180: The sentence “To better compare the contributions to exposure to an air pollutant between the modes…” is unclear. Please rephrase it to clearly state what contributions are being compared and how.”
Line 275: “The highest concentration ACC of 1.6×10³ cm⁻³ was measured from all directions at <2 m/s”. Perhaps the authors mean to say “The highest concentration of ACC (1.6×10³ cm⁻³) occurred under low wind-speed conditions (<2 m/s), largely independent of wind direction”?
Line 268-269: “We conclude that the airport 6 kilometres away has a strong influence on NUC concentrations.” I suggest improving the sentence “These results indicate that Frankfurt Airport significantly influences NUC concentrations at the monitoring site, despite the 6 km distance.”
Line 335, 337: “7 and 9:00 a.m.” Please write the time “7:00 and 9:00 a.m.” (correct standard format).
Line 341: “The high midday TNC peak is attributed to regional or urban photo-nucleation…”I suggest “The pronounced midday TNC peak is consistent with regional or urban photochemical nucleation processes…”.
Did the authors actually identify the days on which the NPF events occurred? I would suggest isolating the secondary aerosol contribution due to NPF events to better quantify the primary emission contribution from the various sources. The distinction between NPF-related NUC and airport-related NUC could be made sharper.
Line 369-371: “Higher concentrations of AIT and, to a lesser extent, ACC, particularly at night…may indicate the influence of increased barbecue and residential wood heating activity”. Please rewrite the sentence more clearly.
Line 468: “We draw the conclusion that aircraft emissions from a major airport within a 6-kilometer radius seem to have an impact on NUC.” Perhaps the sentence is clearer “We conclude that aircraft emissions from a major airport can substantially elevate nucleation-mode particle concentrations at distances up to 6 km downwind.”
Line 474: “The WHO limit for high UFP in 24 hours (>10,000 cm⁻³) per year was exceeded on 30% of all days.” I suggest “According to the WHO threshold for high UFP concentrations (>10,000 cm⁻³ as a 24 h mean), exceedances occurred on approximately 30% of all days during the study period.”