Vehicle-induced air pollutant dispersion and personal exposure in an urban street canyon: Association with air-source heat pump outdoor units combined with building envelope components.

Air-source heat pumps are popular in buildings to provide cooling and heating. However, how the air discharged by air-source heat pump outdoor units affects the dispersion of air pollutants in urban street canyons remains poorly understood. This study used coupled simulations to examine the effects that air-source heat pump outdoor units had on vehicle-induced indoor and outdoor air pollution in an urban street canyon and how these effects varied based on the arrangement of outdoor units or the presence of building envelope components (e.g. overhangs and balconies). Data from wind-tunnel experiments and field measurements were used to validate simulation models. The air pollution exposure in indoor environments and air pollutant dispersion in pedestrian spaces were quantified using the personal intake fraction (P_IF) and net escape velocity (NEV), respectively. Results indicate that when being installed on both north and south sides of each building, outdoor units could reduce the average P_IF by up to 62 %. In the presence of overhangs or balconies, occupants could have greater exposure due to outdoor units, with an increase in the average P_IF of up to 12 %. Outdoor units also facilitated the removal of vehicle-emitted air pollutants from pedestrian spaces, increasing the average NEV by up to 57 %. This positive effect, however, could almost be eliminated by the presence of balconies. The results lead to a better understanding of the role of outdoor units on the dispersion of air pollutants in urban street canyons and provide insights into the potential health benefits gained from outdoor units.