Table 2.

Airborne Fe and Si aerosol concentrations in underground subway stations. The contribution of Fe and SI to the total ${\mathrm{PM}}_{2.5}$ mass was calculated using ${\mathrm{PM}}_{2.5}$ gravimetric data at 1 to 4 underground subway stations in three different urban systems.

System, line, and station	Number of samples	$\mathrm{Si}\text{\hspace{0.17em}mean}\pm \text{SD}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	% contribution	$\mathrm{Fe}\text{\hspace{0.17em}mean}\pm \text{SD}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	% contribution
Boston
Government Center–Blue Line	1a	4.5	1.9%	190	78.3%
Broadway	2	$6.0\pm 0.8$	1.5%	$162\pm 14.6$	39.1%
PATH-NYC/NJ
Newport	2	$42.2\pm 18.6$	7.5%	$176\pm 35.8$	32.3%
Christopher Street	4	$92.1\pm 76.7$	6.2%	$329\pm 116$	29.7%
Washington, DC
Van Ness	2	$4.3\pm 1.8$	1.9%	$180\pm 44.7$	76.5%
Capitol South	2	$3.7\pm 2.3$	1.5%	$141\pm 81.1$	56.7%

Note: Fe, iron; SD, standard deviation; Si, silicon: .

^aOne sample was excluded due to a sampling error.