Measuring In-Cabin School Bus Tailpipe and Crankcase PM_2.5: A New Dual Tracer Method

Figures & data

Table 1a. Tailpipe PM emission rates and PM:tracer mass ratios

Bus	Speed, km/hr (CV %)	Fuel Use, km/L (CV %)	DPMtp, g/km (CV %)	DPMtp:Ir, g:g (CV %)	DPMtp:d-alkane, g:g (CV %)
1	14.4 (8)	2.22 (7)	0.18 (13)	1045 (8)	6939 (57)
2	14.1 (3)	2.04 (7)	0.14 (14)	975 (8)	4082 (24)
Notes: Data shown are averages of three runs each.

Table 1b. Crankcase PM emission rates and PM:tracer mass ratios

Bus	Speed, km/hr (CV %)	PMck, g/km (CV %)	PMck:Ir, g:g (CV %)	PMck:d-alkane, g/g (CV %)
1	12.0 (21)	0.028 (7)	5.69 × 10^5(56)	283 (5)
2	13.5 (23)	0.099 (33)	1.21 × 10^6 (4)	360 (7)
Notes: Data shown are averages of three runs each.

Figure 1. In-bus concentrations of PM_2.5, OC/EC, DPM_tp, and PM_ck.

Table 2. Bus 1 in-cabin PM_2.5, carbonaceous PM, DPM_tp, and PM_ck concentrations (μg/m³)

Run and Direction	Windows	PM2.5		EC + 1.2 ×OC		DPMtp		PMck
		2-hr^a	1-hr^a	2-hr	1-hr	2-hr	1-hr	2-hr	1-hr
1 F^a	C	14.8	19.1	15.8	24.3	0.24	0.54	9.8	6.8
1 R^a	C		23.0		22.6		0.29		10.9
2 F	C	10.4^b	60.2	22.9	26.0	0.36^b	0.67	12.5	9.3
2 R	C		34.9		38.0		0.29		10.1
Average closed	–	14.8	34.1	19.4	27.8	0.24	0.43	11.1	9.4
3 F	O	13.8	17.5	7.5	45.6^c	0.17	0.29	0.5	0.6
3R	O		14.0		12.3		0.18		0.4
4 F & R^d	O	13.8	13.7^d	8.2	9.9^d	0.50	0.43^d	0.2	0.4^d
Average opened	–	13.8	14.8	7.9	20.0	0.34	0.33	0.4	0.5
Notes: C = closed, O = open. ^aEach run consisted of a forward (F) route (from residences to schools) and a reverse (R) route (from schools to residences). A nominal 2-hr duration sample set was collected over the course of a run, as well as a nominal 1-hr duration sample set during the forward and reverse portions. ^bSuspected leak; data not included in calculated averages. ^cOne OC fraction for this sample is suspect, but it has been included in the average shown. ^dSampling during run 4 included only two 2-hr sample sets.

Table 3. Bus 2 in-cabin PM_2.5, carbonaceous PM, DPM_tp, and PM_ck concentrations (μg/m³)

Run and Direction	Windows	PM2.5		EC + 1.2 ×OC		DPMtp		PMck
		2-hr^a	1-hr^a	2-hr	1-hr	2-hr	1-hr	2-hr	1-hr
1 F^a	C	28.4	28.1	31.2	36.1	2.17	2.22	20.8	22.5
1 R^a	C		26.8		33.0		2.75		15.9
2 F	C	9.4^b	33.8	19.9	24.9	1.03^b	3.66	8.9	7.9
2 R	C		22.9		30.4		2.16		6.9
Average closed	–	28.4	28.0	25.5	31.1	2.17	2.71	14.8	13.3
3 F	O	10.5	8.7	8.1	10.2	0.43	0.54	2.5	2.1
3 R	O		2.5		11.6		0.30		1.8
4 F	O	12.5	11.0	12.7	12.2	0.67	0.81	2.1	2.4
4 R	O		16.0		21.0		0.30		2.9
Average opened	–	11.5	9.1	10.4	13.0	0.55	0.53	2.3	2.2
Notes: ^aEach run consisted of a forward (F) route (from residences to schools) and a reverse (R) route (from schools to residences). A nominal 2-hr duration sample set was collected over the course of a run, as well as a nominal 1-hr duration sample set during the forward and reverse portions. ^bSuspected leak; data not included in calculated averages.

Table 4. Average in-cabin PM_2.5, carbonaceous PM, DPM_tp, and PM_ck concentrations (μg/m³)

Bus 1 and Bus 2	PM2.5	EC + 1.2 × OC	DPMtp	PMck
Average closed	26	26	1.4	12
Average opened	12	13	0.44	1.3
Average all	19	19	0.91	6.8