Near-road multipollutant profiles: Associations between volatile organic compounds and a tracer gas surrogate near a busy highway

Figures & data

Figure 1. Aerial view of site. Includes locations of tracer release, summa canisters for whole air sample collection, Autotrac gas chromatograph monitors (for continuous SF₆ measurements), and the open path Fourier transform infrared spectrometer (OP-FTIR). Inset (a) shows a ground-level view of the site. Incoming wind directions are shown for the time periods of (b) the first tracer release (on day 2) and (c) the second tracer release (on day 3).

Table 1. Date and sample number, canister-based sample collection times, analyte, measurement method, time interval of measurement, measurement methods, distances from source, sampling time intervals, and sampling dates and times for SF₆ and VOCs

Date[Day]in 2006	Collection Times	Avg. WindSpeed (km/hr)	Avg. Wind Direction	Approx. Vehicles/5 min	Analyte	Methods	CollectionTime	Distance from Highway (m)^a
August 3 [1.1]	3:12 p.m.–3:47 p.m.	8–16	SW	700	Whole Air: SF6 ^b and 59 VOCs	summa canister; Electron capture gas chromatograph	35 min	13, 19, 25, 31, 43, 58, 74
August 3 [1.2]	5:05 p.m.–5:40 p.m.	8–16	WSW–SW	800
August 7 [2]	10:18 a.m.–10:53 a.m.	8–16	SW–SSW	550
August 8 [3]	8:32 a.m.–9:07 a.m.	˜8	SW–WSW	700
August 10 [4]	10:16 a.m.–10:51 a.m.	8–16		550
August 7 [2]	4:47 a.m.–11:14 a.m.	0–16	SSW–WSWW	Variable 600–800	SF6	Electron capture gas chromatograph	4 min	13, 43, 93, 268
August 8 [3]	8:14 a.m.–9:41 a.m.	˜8
August 7 [2]	4:47 a.m. –11:14 a.m.	0–16	SSW–WSWW	Variable 600–800	SF6	Optical path Fourier transform infrared spectrometer	30 sec	10
August 8 [3]	8:14 a.m. – 9:41 a.m.	˜8
Notes: ^aSF6 source was released 7 m from the highway, so distances shown in this column should be subtracted by 7 to obtain the distance from the SF6 release. ^bSF6 collection occurred on August 7 and August 8.

Figure 2. Summa canister BTEX and sulfur hexafluoride tracer gas concentrations as a function of distance from their respective sources for the canister sample collection periods (distance to I-440 for BTEX, and distance to hose for SF₆).

Table 2. Pearson correlation coefficients for benzene, toluene, ethylbenzene, and m,p- and o-xylene (BTEX) for each sample period

	Benzene	Toluene	Ethylbenzene	m,p-Xylene	o-Xylene
Day 1.1
Benzene	1
Toluene	0.98	1
Ethylbenzene	0.59	0.51	1
m,p-Xylene	0.81	0.84	0.19	1
o-Xylene	0.88	0.91	0.31	0.98	1
Day 1.2
Benzene	1
Toluene	0.88	1
Ethylbenzene	0.92	0.90	1
m,p-Xylene	0.81	0.74	0.66	1
o-Xylene	0.86	0.70	0.74	0.94	1
Day 2
Benzene	1
Toluene	0.91	1
Ethylbenzene	0.81	0.89	1
m,p-Xylene	0.56	0.85	0.75	1
o-Xylene	0.60	0.87	0.72	0.99	1
Day 3
Benzene	1
Toluene	0.95	1
Ethylbenzene	0.83	0.91	1
m,p-Xylene	0.91	0.98	0.92	1
o-Xylene	0.92	0.97	0.89	0.99	1
Day 4
Benzene	1
Toluene	0.98	1
Ethylbenzene	0.83	0.87	1
m,p-Xylene	−0.37	−0.30	−0.36	1
	(0.96)	(0.95)	(0.72)
o-Xylene	0.64	0.66	0.47	0.43	1
Notes: The day 4 m,p-xylene correlations in parentheses indicate the resulting values when anomalous concentration measurements nearest to the roadway are removed.

Table 3. VOC-VOC correlations averaged across all sampling periods

VOC	1-Butene	2,3,4-Trimethylpentane	2,3-Dimethylbutane	2,3-Dimethylpentane	2,4-Dimethylpentane	2-Methylhexane	2-Methylpentane	3-Methylpentane	Benzene	Cyclopentane	Ethylbenzene	Isobutane	Isopentane	m,p-Xylene	m-Ethyltoluene	n-Butane	n-Heptane	n-Hexane	n-Pentane	o-Ethyltoluene	o-Xylene	Propylene
2,3-Dimethylpentane			0.85
2-Methylhexane			0.96	0.87
2-Methylpentane			0.96	0.84		0.95
3-Methylpentane			0.98	0.84		0.96	0.98
α-Pinene	0.83
Benzene			0.91			0.85	0.85	0.87
Cyclopentane			0.88	0.80		0.85	0.86	0.86
Isobutane			0.92		0.80	0.89	0.91	0.90	0.90		0.82
Isopentane			0.98	0.81		0.97	0.96	0.98	0.93	0.85	0.80	0.93
n-Butane			0.83				0.81	0.80	0.82			0.91	0.82
n-Heptane		0.80	0.86		0.80	0.83	0.84	0.84	0.89			0.92	0.87			0.81
n-Hexane			0.97	0.83	0.80	0.95	0.93	0.96	0.90	0.83		0.92	0.97				0.88
n-Pentane			0.97			0.93	0.94	0.96	0.91	0.82		0.94	0.97			0.84	0.88	0.94
o-Xylene														0.86			0.80	0.84		0.83
Propylene		0.83													0.82		0.82	0.84			0.85
Toluene		0.81	0.97	0.81		0.95	0.94	0.96	0.94	0.83	0.82	0.92	0.98			0.81	0.89	0.98	0.95		0.82	0.81
Notes: Only those correlations with R ^2 ≥ 0.8 are shown. A table with average correlation coefficients for all VOCs measured in this study is included in the supplemental materials.

Table 4. Correlations with an average 0.5 or greater between SF₆ and each of the VOCs that had complete data for day 2, day 3, or both

Chemical Classification ^a	Tracer/VOC	Day 2	Day 3	Average
—	SF6	1.00	1.00	1.00
M	Isopropylbenzene		0.98	0.98
A	Isopentane	0.95	0.81	0.88
A	n-Pentane	0.94	0.81	0.88
A	2,3-Dimethylbutane	0.91	0.84	0.87
A	Isobutane	0.91	0.79	0.85
A	3-Methylpentane	0.89	0.81	0.85
M	Benzene	0.91	0.79	0.85
C	Cyclohexane	0.82	0.87	0.85
A	2-Methylpentane	0.84	0.84	0.84
M	Toluene	0.89	0.76	0.83
A	n-Butane	0.85	0.80	0.82
M	p-Ethyltoluene		0.81	0.81
A	n-Hexane	0.81	0.82	0.81
C	Methylcyclopentane	0.77	0.84	0.81
A	2,3,4-trimethylpentane	0.77	0.83	0.80
C	Cyclopentane	0.73	0.86	0.80
E	1,3-Butadiene		0.76	0.76
A	3-Ethylpentane		0.76	0.76
A	2-Methylhexane	0.79	0.74	0.76
M	o-Xylene	0.64	0.87	0.76
M	1,2,4-Trimethylbenzene	0.79	0.72	0.76
M	Ethylbenzene	0.92	0.59	0.75
M	o-Ethyltoluene	0.65	0.84	0.74
A	2,3-Dimethylpentane	0.76	0.72	0.74
C	1-c-3-Dimethylcyclopentane		0.72	0.72
M	m,p-Xylene	0.61	0.80	0.70
A	n-Heptane	0.67	0.69	0.68
M	m-Ethyltoluene	0.41	0.86	0.64
E	Olefin		0.64	0.64
A	2,2-Dimethylbutane	0.62	0.66	0.64
E	Propylene	0.52	0.73	0.62
A	2,4-Dimethylpentane	0.72	0.50	0.61
A	3-Methylheptane	0.47	0.74	0.60
A	2-Methylheptane	0.28	0.90	0.59
L	α-Terpineol		0.51	0.51
Notes: Correlations between SF6 and all 102 VOCs are available in the supplemental materials. ^aA = alkane; C = cycloalkane; E = alkene; L = alcohol; M = monocyclic arene.

Table 5. Average correlations between SF₆ and the 102 distinct VOCs measured on the tracer release days

	Day 2	Day 3	Days 2 and 3 Combined
All VOCs	0.16	0.33	0.23
BTEX	0.79	0.76	0.78
Mobile source	0.26	0.43	0.25
	(68 VOCs)	(73 VOCs)	(80 VOCs)
Not listed as mobile source	−0.22 (3 VOCs)	−0.23 (6 VOCs)	−0.02 (7 VOCs)
Chemical class
Cycloalkane	0.58	0.59	0.59
Alcohol		0.51	0.51
Monocyclic arene	0.27	0.59	0.44
Alkane	0.31	0.37	0.34
Terpene	−0.01	0.40	0.22
Ketone		0.16	0.16
Alkene	−0.08	0.13	0.03
Halogen-containing compounds	−0.19	0.04	−0.06
Alkyne	−0.44	0.22	−0.11
Bicyclic arene	−0.49	0.04	−0.23
C10–C13  aromatics	−0.33	−0.34	−0.34
Ester	−0.44		−0.44
Notes: Day 2 includes 68 distinct VOCs listed as mobile sources, and day 3 includes 73 mobile source VOCs according to the EPA Master List of Compounds Emitted by Mobile Sources (2006).

Figure 3. Number of VOCs having their maximum concentrations at the indicated distances during each sampling period (out of the 59 total VOCs that had complete results for all distances and all sampling periods).