Analysing High Resolution Ultrafine Particle Count Data to Differentiate Local Road Traffic from Background Contributions to Personal Exposure

During personal exposure monitoring, short-term peaks, or “spikes”, in pollutant concentration, were observed frequently in proximity to the local pollution sources, especially road traffic. This phenomenon may influence the overall exposure. The current study examined spikes in particle number concentration (PNC) to differentiate local from background contributions by eliminating spikes from the personal exposure dataset.

Personal exposures were measured on 33 walking trips alongside major and minor roads between 20^th May and 26^th June 2024, in a heavily populated residential area close to the University of Birmingham. A portable miniature particle counter (Testo DiSCmini) was carried in a backpack, measuring particle number concentration with a high temporal resolution of 1 second. Particle Number Count data (10-100nm) were also collected with a reference-grade instrument (Scanning Mobility Particle Sizer (SMPS)) at a local urban background air quality monitoring station.

The time series of PNC contained short-term excursions (spikes) to higher concentrations.  There were several steps involved in removing the spikes from the dataset including baseline calculation, spike identification and removal, and background data interpolation. The first step was done using a moving median with 10 minutes average before and after all data points. Secondly, a threshold i.e., 10% of the baseline was chosen which can capture spikes optimally based on visual observation. Data above the threshold was subsequently identified as spikes and excluded from the data set. Finally, the edited background data was interpolated using a linear method.

The results show that roughly 25% (by time) of the walking data was categorized as short-term peaks. Removal resulted in a reduction of the overall average PNC by nearly 19%. Temporal variation according to weekday/weekend and period of the day revealed a decline in average PNC ranges of 12-34%, with the most significant fall of 34% occurring during weekday mornings (MWD), due to a substantial number of PNC spikes observed during this period.  PNC measured during walking was scaled to SMPS-equivalent values using data from an instrument intercomparison and was compared to urban background SMPS data during measurement. It was shown that the average of corrected de-peaked PNC from walking (11801±8357 #/cm³) was 9% higher than that recorded in the urban background (10816±6711 #/cm³). Local diffuse sources are probably responsible for this higher concentration, while the spikes appear to be due to road traffic emissions and locally operating sources such as off-road mobile machinery.

By separating short-term peak concentrations from personal exposure monitoring data, the data indicate that limiting pollutant hotspots, especially in areas with high population density, may reduce exposure to pollutants, particularly those with significant geographical and temporal variability such as ultrafine particles (UFP).