A flow conditioned infiltration metric for traffic-related air pollution ingress and indoor air quality in naturally ventilated buildings

Source abstract

The indoor-outdoor pollutant concentration relationship in naturally ventilated buildings is commonly studied by means of the indoor/outdoor concentration ratio. This metric provides information on instantaneous exposure levels relative to the outdoor environment but does not account for the local air flow condition that govern pollutant transport through the building opening. This work aims at developing a flow conditioned ingress parameter that complements the indoor/outdoor concentration ratio to physically describe the indoor-outdoor relationship. Simultaneous field measurements of indoor and outdoor NO 2 concentrations, traffic flow and wind conditions were performed in a single-sided ventilated office building located in a high aspect-ratio street canyon in Gżira, Malta. The proposed metric is compared with the conventional indoor/outdoor ratio. A low-frequency domain analysis of both parameters showed the conventional indoor/outdoor concentration ratio closely followed the dominant traffic frequencies irrespective of varying wind conditions. On the other hand, the newly proposed metric also showed dependence on wind conditions, which altered its frequency response. Furthermore, the conventional indoor/outdoor ratio showed high scatter during low outdoor concentration events and showed no clear dependence on local flow conditions. The proposed metric demonstrated a clear sensitivity to local flow conditions with its polarity indicating whether favourable or unfavourable ventilation states prevail. The proposed parameter was therefore shown to provide a physically interpretable metric that links gaseous pollutant transport processes with indoor pollutant concentrations that addresses the limitations of conventional metrics used in the design of naturally ventilated buildings exposed to polluted outdoor environments.