Figure 2.

Average percent contribution of elemental constituents to total mass of ${\mathrm{PM}}_{2.5}$ collected on filters in underground stations in three urban transit systems. Each system is represented by the two stations with the highest measured ${\mathrm{PM}}_{2.5}$ concentrations: Government Center–Blue Line ($n=1$) and Broadway in Boston; Christopher Street ($n=4$) and Newport ($n=2$) in the PATH-NYC/NJ system; and Van Ness ($n=2$) and Capitol South ($n=2$) in Washington, DC. Trace elements analysis was conducted with XRF analysis of Teflon filters and carbon analysis was performed on the quartz filters using NIOSH’s 5,040 method (NIOSH 2003). Note: Ba, Barium; Ca, Calcium; Fe, Iron; Mg, Magnesium; Na, Sodium; S, Sulfur; Si, Silicon; TC, total carbon.