An Empirical Study of Particulate Matter Exposure for Passengers Waiting at Bus Stop Shelters in Portland, Oregon, USA

Current guidelines for the location and design of bus stops do not take into account air quality or exposure considerations for waiting passengers. This paper compares the exposure of transit riders waiting at three-sided bus stop shelters that either: 1) face roadway traffic, or 2) face away from roadway traffic. Shelters were instrumented with devices to monitor particulate matter concentration inside and outside the shelter, wind speed and direction, and vehicle counts. Data were collected at three shelters during both the morning and afternoon peak periods. Bus shelter orientation is found to have a significant effect on the concentration of four sizes of particulate matter: ultrafine particles, PM1, PM2.5, and PM10. It was observed that shelters with an opening oriented towards the roadway had consistently higher concentrations inside the shelter than outside the shelter. In contrast, shelters oriented away from the roadway were observed to have lower concentrations inside the shelter than outside the shelter. Particulate concentrations are shown to vary based on both wind speed and direction. Qualitative analysis conducted on select variables suggests temperature and humidity are the dominant influencers of concentrations across all four particulate sizes. Vehicle flows can have significant correlations with ultrafine particulate counts, though not consistently.