Dispersion of traffic derived air pollutants into urban parks

Highlights: • Traffic derived air pollutants are readily detected in urban parks. • Measurements suggest a rapid decay of pollutants from roadside into parks. • Pollutant gradients in parks are explored using an analytical dispersion equation. • Traffic derived pollutants halve after ~ 17 m from the road edge. • Future park design should emphasise a separation between park interiors and roads. Scarce land resources in dense cities means that small urban parks are important as a leisure and amenity resource for the urban population. However, streets with heavily traffic often surround these fragmented parks and increase the potential user exposure to air pollutants from vehicles. The dispersion profiles of PM_2.5 and black carbon from roadside into urban parks at pedestrian level, in Hong Kong, were measured using mobile high time resolution instruments. In the downwind direction, pollutant concentrations decreased rapidly from roadside and by some tens of metres reached relatively constant values. An even sharper gradient is found in the upwind direction, with a rapid increase detected within 2 m of the road edge. The distinct decay profiles were explained with an analytical dispersion model formulated based on the gradient transport theory using an Eulerian approach. The simulations using the dispersion model suggest 17 m as a typical halving distance under normal urban conditions, which is introduced to simplify the description of dispersion profiles. Using Hong Kong as an example, ~ 90% of urban parks, to different extent, overlap with the 17 m halving distance from roads, which means few urban parks in Hong Kong avoid the impact from nearby traffic emissions. Thus, from the perspective of human exposure to air pollutants in urban parks, this study provides observations of relevance for future park design in dense cities.