10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Airborne Fine Particulate Matter in the Subway System of the Megacity of São Paulo
ADMIR CRÉSO TARGINO, Patricia Krecl, Julián Felipe Segura, Federal University of Technology
Abstract Number: 586 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Subway systems are efficient mass transportation modes, especially in megacities with neighborhoods separated by long distances. Nevertheless, below-grade stations are enclosed spaces that facilitate the build-up of concentrations of airborne particulate matter (PM) entering from both the outside sources and generated by the abrasion of rail tracks, wheels and brake pads. Despite spending only 7-10% of the day within transport microenvironments, urban dwellers can be highly exposed to airborne PM while commuting. The subway system of the megacity of São Paulo comprises five lines, covers 71 km, serves 64 below- and above-grade stations and transports on average 3.7 million passengers daily on weekdays. The system operates electric trains with steel wheels rolling on steel tracks. For the first time, concentrations of black carbon (BC) and particle number (PN) were monitored in the city’s subway system with a high temporal resolution using hand-held condensation particle counters (a P-Trak and a CPC model 3007, TSI, USA) and aethalometers (AE51, Aethlabs, USA). The monitoring was conducted on the green and yellow lines simultaneously during one week in August 2017. The AE51 and the particle counters were operated with temporal resolutions of 10 s and 1 s, respectively. The sampling protocol followed a pattern of boarding and disembarking at each station up to the end of the lines when the researchers returned to complete the loop. Because the smallest detectable particle size of the P-Trak was 20 nm and of the CPC was 10 nm, we collocated the instruments for about one hour before each sampling session to extract linear regression equations using the CPC measurements as reference (R2 > 0.90). Thus, the P-Trak measurements were harmonized by using the slopes and offsets found in the intercomparison. Overall, the concentrations increased rapidly when the researchers disembarked and spiked as trains approached, with values consistently larger at the platforms than in the trains. The average BC and PN concentrations on the yellow line were 11.3 µg/m3 and 37.000 pt/cm3 at the platforms and 10.1 µg/m3 and 30.400 pt/cm3 in the trains, with maximum values of 241 µg/m3 and 287.000 pt/cm3 at República (a below-grade station in São Paulo’s city center). The average BC and PN concentrations on the green line were 9.1 µg/m3 and 22.200 pt/cm3 at the platforms and 7.3 µg/m3 and 18.000 pt/cm3 in the trains. Despite the larger particulate concentrations at the platforms, on some occasions BC and PN did not follow each other and were overall poorly correlated, with maximum R2 of 0.3 on the green line and 0.4 on the yellow line. This is likely to be associated with the generation of metallic particles by the wheel−rail friction, which may not have absorption properties at the wavelength of 880 nm used by the AE51, but can be detected by the particle counters. Comparison between concurrent BC measurements on Paulista Avenue (a thoroughfare with 6.400 vehicles/h at peak hours) and the nearby below-ground Trianon-MASP station showed that the average concentrations were 7.2 and 11.1 µg/m3 at street level and at the platform, respectively. Our results revealed that the concentrations of fine PM in the subway system of São Paulo were large and highly variable with substantial differences in the train and at platforms. We will explore details of the spatial distribution of PM on both lines and identify features in the design and location of stations which may drive the differences in concentrations.