Personal exposure to air pollutants in a Bus Rapid Transit System: Impact of fleet age and emission standard

This work documents the extremely high concentrations of fine aerosol particles (PM2.5), equivalent black carbon (eBC) and carbon monoxide (CO) experienced by commuters in one of the world largest Bus Rapid Transit (BRT) systems, located in Bogota, Colombia. A strong relationship between vehicle emissions standard and in-vehicle concentrations was established. Extensive measurements of PM2.5, eBC, and CO were carried out inside the system buses and stations. Measurements were performed during eleven months covering all the system lanes and a significant fraction of the bus fleet and stations. Based on the observed in-bus and in-station concentrations, travel times, and urban ambient levels, the contribution of a typical round trip in the BRT system to the daily inhaled dose was estimated to be 60% for PM2.5, and between 79% and 90% for eBC. The mean PM2.5 dose inhaled by commuters in a typical round trip in the system is 1.2 times the dose a subject would inhale over 24 h exposed to WHO guideline of 25 mu g m(-3). In 52 out of 180 buses sampled the mean in-cabin concentration of eBC was in excess of 100 mu g m(-3). Measurements show that mean ratio of in-bus to urban ambient concentration is 8:1 for PM2.5 and 4:1 for CO. These ratios were observed to be much larger when traveling inside the older fraction of the BRT fleet, reaching 11:1 and 7:1 for PM2.5 and CO respectively. These older buses consist exclusively of Euro II and Euro III nominal emission standard vehicles. Average exposure levels inside Euro II and III buses was twice as large as those measured inside vehicles with stricter emission standards, Euro IV or superior. Overall, the measurements suggest that the entrainment of polluted pockets of air to the semi enclosed micro-environments in the system, namely, buses and stations, explains the extreme observed exposure concentrations. Measurements inside an underground bus-station where traffic is restricted to only BRT buses, and on-board a zero emission fully electric BRT bus suggested most of the pollution in the system comes from entrainment of the exhaust of the diesel-powered BRT buses. The significantly lower exposure and inhaled dose of PM2.5, eBC, and CO observed for commuters in newer vehicles suggests that a fleet renewal could have a disproportionately large impact on reducing population exposure to air pollutants. The extent of the reduction in exposure and inhaled dose through fleet renewal is proportional to the gradient of in bus to urban ambient air pollution levels.