Particulate matter (PM) concentrations in underground and ground-level rail systems of the Los Angeles Metro

Abstract

Abstract Elevated concentrations of particulate matter (PM) have been found in a number of worldwide underground transit systems, with major implications regarding exposure of commuters to PM and its associated health effects. An extensive sampling campaign was conducted in May–August 2010 to measure PM concentrations in two lines of the Los Angeles Metro system – an underground subway line (Metro red line) and a ground-level light-rail line (Metro gold line). The campaign goals were to: 1) determine personal PM exposure of commuters of both lines, and 2) measure and compare PM concentrations at station platforms and inside the train. Considering that a commuter typically spent 75% of time inside the train and 25% of time waiting at a station, subway commuters were exposed on average to PM10 and PM2.5 concentrations that were 1.9 and 1.8 times greater than the light-rail commuters. The average PM10 concentrations for the subway line at station platforms and inside the train were 78.0 μg m−3 and 31.5 μg m−3, respectively; for the light-rail line, corresponding PM10 concentrations were 38.2 μg m−3 and 16.2 μg m−3. Regression analysis demonstrated that personal exposure concentrations for the light-rail line are strongly associated with ambient PM levels (R2 = 0.61), while PM concentrations for the subway line are less influenced by ambient conditions (R2 = 0.38) and have a relatively stable background level of about 21 μg m−3. Our findings suggest that local emissions (i.e., vehicular traffic, road dust) are the main source of airborne PM for the light-rail line. The subway line, on the other hand, has an additional source of PM, most likely generated from the daily operation of trains. Strong inter-correlation of PM10 between the train and station microenvironments shows that airborne PM at stations are the main source of PM inside the trains for both lines (R2 = 0.91 and 0.81 for subway and light-rail line, respectively). In addition, PM2.5 and coarse PM (PM10-2.5) are also strongly correlated for the subway line (R2 = 0.89) and the light-rail line (R2 = 0.52–0.92), suggesting that PM2.5 and coarse PM originate from a common source. Finally, in comparison to worldwide subway systems, the L.A. Metro system is relatively ’clean’. Since the system is comparatively new (in operation since 1993), its ventilation system and braking technology are probably more efficient and more advanced than older subway systems.