Urban park layout and exposure to traffic-derived air pollutants

Air quality in small urban parks improves rapidly with distance from surrounding streets. This arises because air pollutants disperse within parks where there are few sources. This was traditionally thought to arise from pollutant uptake by trees, but they can also reduce wind speed and potentially trap pollutants. It is increasingly evident that relatively little pollution is absorbed by trees over short distances typical of small urban parks. Nevertheless, trees and park infrastructure, also constrain the distribution of park users and thus lower their exposure. In this paper, we explore the exposure to traffic-derived air pollutants in the Hong Kong parks (Sha Tin and Sham Shui Po) and examine the way in which PM_2.5 concentration and user distribution is affected by the spatial layout of park features. The modelled pollutant distribution was validated against measurements made in Sha Tin Park. Population-weighted exposure to traffic-derived pollution was quantified by overlapping pollutant and park user distribution. The results reveal how different design features (i.e. vegetation, earth berms and recreation areas) affect exposure during individual time period; most positively in Sha Tin Park where the design leads to a > 50% reduction in exposure, early morning. In Sham Shui Po Park, exposure increases as park users are often attracted to facilities near a busy trunk road. This study reveals the heterogeneity of park user exposure to traffic-derived air pollution, which leads to uncertainty in health benefits provided by urban parks and suggests fine-scale exposure considered in thoughtful park design.