Exposure to ultrafine particles while bicycling in a residential area near Copenhagen International Airport, Denmark: A repeated measures study.

Airports are major sources of ultrafine particles (UFP), raising health concerns among people living in immediate proximity. However, little is known about UFP concentrations in residential areas around airports. In this study, we mapped UFP exposure concentrations in a residential area nearby Copenhagen International Airport (CPH). Particle number concentrations (PNC) were measured using a portable device during 44 bicycling trips on a fixed route of 8.2 km, on weekdays in July and August 2024. The route was located in an area 4 km north of CPH and tracked using GPS. We investigated PNC spatial variation linking measured data to OpenStreetMap. To compare PNC across different times of the day and wind directions, we used Generalized Additive Models (GAM), adjusted for time trends, hourly flights and meteorological variables. We found an overall mean PNC of 7620 pt/cm across 44 repeats, with no significant differences between morning and noon trips. Highest means PNC were observed during south wind (11,594 pt/cm) compared to other wind directions (4189-7069 pt/cm), showing an increasing gradient of PNC from north to south (∼10,000 to ∼13,000 pt/cm, respectively) under south wind conditions. We also observed mean PNC of 8151 pt/cm across all traffic intersections along the route, with peaks at traffic lights on main roads under south wind, up to 16,442 pt/cm. Our findings suggest that airports, together with road traffic, are a significant source of UFPs near residential neighbourhoods. The diffusion of UFP is influenced primarily by wind direction with graduation by proximity to the airport.