Link between aerosol optical, microphysical and chemical measurements in an underground railway station in Paris

Measurements carried out in Paris Magenta railway station in April–May 2006 underlined a repeatable diurnal cycle of aerosol concentrations and optical properties. The average daytime PM10 and PM2.5 concentrations in such a confined space were approximately 5–30 times higher than those measured in Paris streets. Particles are mainly constituted of dust, with high concentrations of iron and other metals, but are also composed of black and organic carbon. Aerosol levels are linked to the rate at which rain and people pass through the station. Concentrations are also influenced by ambient air from the nearby streets through tunnel ventilation. During daytime approximately 70% of aerosol mass concentrations are governed by coarse absorbing particles with a low Angström exponent (∼0.8) and a low single-scattering albedo (∼0.7). The corresponding aerosol density is about 2 g cm−3 and their complex refractive index at 355 nm is close to 1.56–0.035 i. The high absorption properties are linked to the significant proportion of iron oxides together with black carbon in braking systems. During the night, particles are mostly submicronic, thus presenting a greater Angström exponent (∼2). The aerosol density is lower (1.8 g cm−3) and their complex refractive index presents a lower imaginary part (1.58–0.013 i), associated to a stronger single-scattering albedo (∼0.85–0.90), mostly influenced by the ambient air. For the first time we have assessed the emission (deposition) rates in an underground station for PM10, PM2.5 and black carbon concentrations to be 3314 ± 781(−1164 ± 160), 1186 ± 358(−401 ± 66) and 167 ± 46(−25 ± 9) μg m−2 h−1, respectively.