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.