Field measurement on transient carbon monoxide levels in vehicular tunnels

Carbon monoxide (CO) and oxides of nitrogen (NOx) produced from internal combustion engines would be accumulated to harmful levels in vehicular tunnels if the design and operation of ventilation system are unsatisfactory. There are 11 vehicular tunnels in Hong Kong with an average of over 650,000 vehicles crossing the tunnels daily. To have a preliminary understanding on the background pollutant levels and identifying the further areas to improve the ventilation provisions, a field study on air quality in those tunnels was carried out. CO levels measured in each tunnel are presented in this paper. Results revealed that the demand control ventilation rates derived from CO and NOx are different. Which one is more important is to be decided by the tunnel management. Air quality was assessed by the temperature difference between the ambient and inside the vehicular tunnels. This is because hot gases emitted would be trapped inside the tunnel due to poor ventilation, and so measurement on the temperature difference would give an indication of air quality. A simple model based on pressure balancing is used to calculate the CO levels for justifying the measured values. The levels of nitrogen dioxide (NO2) were derived from the equation suggested by the Environmental Protection Department (EPD) of the local government. The levels are compared with the internationally recommended standards.