Table 3.

Airborne total, EC, OC carbon concentrations in underground subway stations. The contribution of carbon to the total ${\mathrm{PM}}_{2.5}$ mass was calculated using ${\mathrm{PM}}_{2.5}$ gravimetric data at one to two underground subway stations in three different urban systems.

System, Line, and Station	N	Total carbon		ECa		OCa
		$\text{Mean}\pm \text{SD}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	% contribution	$\text{Mean}\pm \text{SD}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	% contribution	$\text{Mean}\pm \text{SD}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	% contribution
Boston
Government Center–Blue Line	1b	177	72.6%	$<\text{LOD}$		177	72.6%
Broadway	2	$70.6\pm 24.7$	17.1%	$12.2\pm 10.4$	3.0%	$58.4\pm 14.3$	14.1%
PATH-NYC/NJ
Newport	2	$58.9\pm 29.8$	10.9%	$0.2\pm 0.01$	0.03%	$58.7\pm 29.8$	10.6%
Christopher Street	2	$57.2\pm 21.6$	3.7%	$<\text{LOD}$		$57.2\pm 21.6$	3.7%
Washington, DC
Van Ness	2	$65.6\pm 15.7$	27.0%	$9.8\pm 2.0$	4.0%	$55.8\pm 13.7$	23.0%
Capitol South	2	$55.1\pm 6.2$	20.7%	$11.8\pm 5.2$	4.4%	$43.3\pm 1.0$	16.3%

Note: EC, elemental carbon; LOD, limit of detection; OC, organic carbon; SD, standard deviation; TC, total carbon: .

^aThe high iron content in subway ${\mathrm{PM}}_{2.5}$ samples interferes with the thermal ramp used in the analysis of EC and OC.

^bOne sample was excluded due to sampling error.