Table 1.

Real-time measured ${\mathrm{PM}}_{2.5}$ and ${\text{BC}}_{2.5}$ concentrations by location (i.e., underground and aboveground stations, on-train, and aboveground ambient) for each urban transit system. All data, except in Philadelphia (February and August 2015), were collected from June to September 2019. Underground data were collected for 5 to 10 min at underground stations with single or multiple platforms (the latter were counted as multiple locations), whereas aboveground data correspond to aboveground station platforms. Ambient location samples were collected aboveground outside station entrances. On-train samples represent the concentration measured inside a train car during the ride between stations (for interstation rides greater than 3 minutes) and averaged for each run. RH, ${\mathrm{PM}}_{2.5}$ and ${\text{BC}}_{2.5}$ was collected simultaneously. Real-time ${\mathrm{PM}}_{2.5}$ concentrations were adjusted for RH and corrected with gravimetric data.

System and line	Number of locations	${\mathrm{PM}}_{2.5}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)					${\text{BC}}_{2.5}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)					RH (%)
		Number of samples	$\text{Mean}\pm \text{SD}$	Median	Min	Max	Number of samples	$\text{Mean}\pm \text{SD}$	Median	Min	Max	$\text{Mean}\pm \text{SD}$
Boston
Underground	24	96	$327\pm 136$	306	48	638	78	$12.2\pm 12.3$	10.4	1.04	103	$49.9\pm 10.4$
Aboveground	3	12	$14.2\pm 5.9$	13.8	7.2	28.7	10	$2.2\pm 1.3$	1.9	0.2	5.1	$42.8\pm 10.6$
On-train	3	12	$182\pm 59.8$	191	768	267	12	$3.5\pm 2.2$	3.8	0	6.3	$45.8\pm 9.3$
Ambient	3	5	$10.0\pm 2.7$	10.6	7.2	13.8	5	$0.7\pm 0.9$	0.3	0	2.1	$53.0\pm 12.6$
PATH-NYC/NJ
Underground	8	26	$779\pm 242$	714	392	1499	20	$28.5\pm 10.2$	25.3	17.1	50.8	$58.8\pm 8.5$
Aboveground	ND						ND
On-train	2	6	$449\pm 261$	378	219	958	4	$13.9\pm 3.3$	14.7	9.2	16.9	$54.8\pm 8.3$
Ambient	3	3	$20.8\pm 9.3$	20.3	11.8	30.3	2	$2.2\pm 1.9$	2.2	0.9	3.5	$43.2\pm 1.0$
MTA-NYC
Underground	3	12	$547\pm 207$	509	263	959	12	$22.4\pm 7.5$	23.7	10.9	33.7	$59.8\pm 3.5$
Aboveground	ND	ND					ND
On-train	4	4	$343\pm 147$	279	250	563	4	$19.2\pm 22.5$	8.8	6.2	52.8	$50.9\pm 3.1$
Ambient	2	4	$24.1\pm 5.9$	22.1	19.5	32.6	4	$1.8\pm 0.7$	1.8	0.9	2.6	$56.2\pm 6.8$
LIRR
Underground	1	4	$91.2\pm 38.9$	103	36.7	123	4	$6.3\pm 3.8$	7.8	0.7	9.2	$53.8\pm 13.1$
Aboveground	1	3	$13.8\pm 9.6$	10.2	6.5	24.6	3	$2.3\pm 1.5$	2.1	0.9	3.9	$53.6\pm 7.2$
On-train	1	6	$11.6\pm 5.10$	10.7	7.33	18.6	6	$1.1\pm 1.0$	0.7	0.3	2.9	$47.9\pm 5.7$
Ambient	ND	ND					ND
Philadelphia
Underground	14	28	$112\pm 46.7$	100	49.7	226.6	21	$7.1\pm 5.1$	6.2	0.9	19.0	$43.9\pm 5.9$
Aboveground	ND						ND
On-train	1	3	$55.7\pm 2.2$	56.0	53.4	57.8	3	$2.0\pm 1.1$	1.4	1.3	3.3	$37.0\pm 3.7$
Ambient	7	7	$12.6\pm 4.8$	15.5	5.3	17.1	7	$2.3\pm 0.8$	2.4	1.1	3.3	$52.8\pm 24.0$
Washington, DC
Underground	21	84	$341\pm 147$	315	91.7	720	84	$5.5\pm 2.7$	5.1	1.3	16.0	$52.3\pm 8.1$
Aboveground	3	12	$14.8\pm 7.5$	15.6	5.0	24.4	12	$1.9\pm 1.0$	1.8	0.1	3.4	$58.6\pm 12.1$
On-train	2	7	$186\pm 33.5$	190	144	239	7	$2.2\pm 0.9$	2.2	0.9	3.4	$45.7\pm 7.6$
Ambient	1	3	$12.0\pm 7.8$	9.1	6.13	20.8	3	$1.2\pm 0.8$	1.6	0.3	1.6	$58.4\pm 9.6$
All Systems
Underground	71	250	$362\pm 226$	315	36.7	1499	219	$11.1\pm 10.9$	7.6	0.7	103	$51.8\pm 9.7$
Aboveground	7	27	$14.4\pm 6.7$	14.1	5.0	28.7	25	$2.1\pm 1.1$	1.9	0.1	5.1	$51.0\pm 13.1$
On-train	10	38	$205\pm 179$	194	5.9	958	36	$5.6\pm 9.1$	2.9	0	52.8	$47.4\pm 8.4$
Ambient	16	22	$15.1\pm 7.5$	14.7	5.3	32.6	21	$1.6\pm 1.0$	1.6	0	3.5	$53.0\pm 13.2$

Note: BC, black carbon; Max, maximum; Min, minimum; ND, not done; RH, relative humidity.