Spatial Variation of Ultrafine Particle Concentration across an Urban Environment

Ultrafine particles (UFPs) are a significant component of urban air pollution, with serious implications for public health due to their ability to penetrate deep into the respiratory system. Despite their importance, the spatial variation of particle number concentration (PNC) in urban areas remains poorly understood. Since most mobile monitoring has been conducted on-road, reliable mapping of the background concentrations is largely missing. This study was conducted over 4 months at 16 sites across the city of Brisbane, Australia, all situated well away from busy roads, to investigate the background spatial and temporal distribution of PNC, particle mass (PM2.5), and carbon monoxide (CO) concentrations. The results show that concentrations of all three pollutants exhibited limited spatial variation, suggesting a relatively homogeneous urban background thus offering the possibility of using a single fixed monitoring station without the need to employ mobile or multistation measurements. However, pronounced increases in PNC were observed during specific pollution episodes, including bushfire smoke events (74% increase) and new particle formation (NPF) events (117% increase). The temporal analysis highlighted diurnal trends, with elevated PNC levels during high-traffic hours. Exceedance probabilities using Bayesian statistical analysis revealed high-risk locations and times for elevated PNC levels. This study provides novel insights into the drivers of spatial and temporal variability of PNC in an urban environment. These findings emphasize the significant influence of episodic events on urban air quality and are critical for informing air quality management policies, including monitoring network design and strategies to mitigate public exposure to ultrafine particles.