Sensor network for traffic-related pollutant monitoring in urban public transport interchanges

Abstract

<p>Public transport interchanges (PTI) are special transportation-impacted micro-environments in Hong Kong where public transport such as buses, taxis and mini-buses pass through or terminate, and where passengers queue for the transport. Hong Kong has 65 PTIs in total and most of these are located under residential or commercial buildings. Mechanical ventilation is included in all PTIs due to very limited natural ventilation and it is intended to limit the accumulation of air pollution from the various vehicles. However, numerous complaints were reported concerning PTIs’ air quality, and data are lacking to characterize pollution in these places. The purpose of this study was to determine the overall nature of pollutants in a sample of PTIs, to identify whether hot spots were present and how these might be related to both ventilation practices and bus activities in PTIs.</p><p> </p><p>8 PTIs were selected for simultaneous measurement of NO, NO<sub>2</sub> and PM<sub>2.5</sub> in 4 days of sampling. A monitoring network was formed by a group of sensor-based air monitoring facilities which were deployed at multiple points in the passenger waiting areas of each selected PTI and also at the ventilation intakes. Specific data calibration and validation protocols were well designed for sensor data control and assurance in such near-source monitoring application. </p><p> </p><p>NO, NO<sub>2</sub> and PM<sub>2.5­ </sub>measured inside the PTIs were compared with the ambient air quality data reported by nearby routine air quality monitoring stations. The average concentration levels of NO<sub>x</sub> were about 4-16 times higher than the ambient levels. NO, NO<sub>2 </sub>and NO<sub>x </sub>in the PTIs themselves showed similar daily repeatable variation patterns and NO concentration levels were always higher than NO<sub>2</sub>’s during the daytime, while the ambient showed opposite patterns. This indicates NO<sub>x</sub> pollutants inside the PTIs were mainly produced by the local bus activities. NO and NO<sub>2</sub> measured at some ventilation intakes had even higher concentration levels than those of PTIs, which means the existing ventilation systems were generally not adequate to control the pollution concentration and sometimes could even make the problem worse. Exceedances of NO<sub>2</sub>’s 1-hour concentration limit (0.30 mg/m<sup>3</sup>) were observed at several monitoring sites and were found mainly located in the middle of the PTIs where ventilation is poorer or close to the bus stops occupied by older buses. PM<sub>2.5</sub> measured inside the PTIs followed the patterns of ambient PM<sub>2.5</sub> and showed comparable concentration levels, which implies traffic emission, especially the exhaust from buses in the PTIs may not be the main source for particle pollution.</p><p> </p><p>Concern was raised on the implementation of pollution mitigation plans inside PTIs to satisfy the urgent health protection need for the commuters and also for the staff of bus companies who work there. To effectively control the PTI pollution and limit exposures, it is necessary to consider the bus volume, bus emission type and ventilation design.  </p>