Physico chemical characterization of PM2.5 in the microenvironment of Shanghai subway
Abstract The Shanghai subway metro system has brought great convenience to the city's travelling public, although passengers are exposed to airborne particles in this built micro environment. However, investigations on the physicochemical characterization of PM 2.5 air pollution in the Shanghai subway system are to date very limited. Three subway stations along the No. 7 line were selected as subway PM 2.5 monitoring sites: Pan'guang, Shanghai University (SHU) and Jing'an, which are located in an outer suburban area, a suburban area and the urban area, respectively, airborne PM 2.5 on the subway station platforms and in the ambient atmosphere above ground was synchronously collected from 19th March to 4th, May, 2012. Cutting edge techniques, including scanning electronic microscopy coupled with energy dispersive X ray (SEM/EDX) inductively coupled plasma mass spectrometry (ICP MS) and X ray absorption near edge structure (XANES) were employed to investigate microscopic characterization, chemical elements and speciation of the main heavy metals in subway PM 2.5 Our results demonstrated that mass levels of PM 2.5 in the subway stations were higher than that in ambient air. Mass levels of PM 2.5 in the subway stations and in ambient air ranged from 49.17 19.7 mg/m 3 to 66.15 25.20 mg/m 3 and 24.52 3.3 mg/m 3 to 65.60 5.6 mg/m 3 respectively. The microscopic characterization of PM 2.5 in ambient air and in subway stations showed marked differences. The PM 2.5 in the subway stations was mainly composed of iron containing particles and mineral particles, while the PM 2.5 in ambient air largely consisted of mineral particles and soot aggregates. Fe was the most abundant element in subway PM 2.5 followed by: major elements (mass level 100 ng/m 3 including Na, Mg, Al, K, Ca, Zn, Mn, Ba; sub major elements (10 ng/m 3 3 including Li, Cr, Ni,Cu, Ga, Sr, Pb; and minor elements (mass level 3 Be, V, As, Se, Rb, Ag, Cd, Tl, Bi. The mass levels of Ca, Al and Zn in ambient PM 2.5 were higher than those in subway PM 2.5 however those of the remaining 26 measured elements in subway PM 2.5 were higher than in ambient PM 2.5 The speciation of Fe in PM 2.5 was in the form of Fe 2 while for Cu, that in the finer fractions 2 but in the PM 2.5 fraction itself, was as Cu 1