A virtual limit value for ultrafine particles: Frequency of threshold exceedances across Germany based on data from the German Ultrafine Aerosol Network

The recent update of the European Air Quality Directive calls for systematic measurements of ultrafine particles (UFPs) to advance the scientific understanding of their health impacts. However, legally binding limit values for UFPs have yet to be established. In this study, we conduct a threshold exceedance analysis by defining virtual limit values for both short-term (hourly) and longer-term (daily) ambient UFP concentrations, motivated by good-practice statements issued by the World Health Organization. These thresholds were applied to assess spatial and temporal variability over the three-year period 2017–2019, leveraging observations from the 13 sites in the German Ultrafine Aerosol Network (GUAN). Exceedances of the daily virtual limit value occur more frequently than those of the hourly limit and peak at traffic-influenced roadside spots. For example, at roadside spot (Leipzig-Mitte), the daily limit was exceeded up to 65% of available days, compared to 13% of available hours for the hourly limit. In contrast, urban background, rural, and mountainous sites across Germany exhibited few or no exceedances of the hourly threshold, while daily exceedances reached up to 33% at urban background locations. These findings highlight road traffic as an important source for elevated UFP concentrations and demonstrate the added value of a dual-timescale assessment. Overall, the study provides a quantitative framework for characterizing ambient UFP concentrations using binary virtual limit values, offering new insight into temporally resolved concentration patterns that are not yet captured by existing regulatory metrics.