Concentrations of Air Toxics in Motor Vehicle-Dominated Environments

EPA Grant Number: R834677C156
Subproject: this is subproject number 156 , established and managed by the Center Director under grant R834677
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Health Effects Institute (2010 — 2015)
Center Director: Greenbaum, Daniel S.
Title: Concentrations of Air Toxics in Motor Vehicle-Dominated Environments
Investigators: Fujita, Eric M.
Institution: Desert Research Institute , Health Effects Institute (HEI)
EPA Project Officer: Hunt, Sherri
Project Period: April 1, 2015 through March 31, 2015
RFA: Health Effects Institute (2010) RFA Text |  Recipients Lists
Research Category: Health Effects , Air Quality and Air Toxics , Air


Motor vehicles and other combustion sources emit many air toxics whose levels are not regulated by the U.S. Environmental Protection Agency (EPA), but that are known or suspected, with sufficient exposure, to cause adverse health effects. Among these are mobile source air toxics (MSATs), compounds the EPA has identified as derived, at least in part, from motor vehicles and whose emissions need to be reduced.  Although monitoring has been performed by some state and local agencies in some locations, characterization of ambient levels of and personal exposure to air toxics has been limited. Though their ambient levels are generally low, there may be so-called hot spots where concentrations of one or more air toxics, and exposure of the population, are expected to be elevated. Such elevation may be due to the proximity of the hot spot to one or more pollution sources or to transient or sustained localized conditions that lead to elevated levels of particular pollutants. The objective of this research is to identify and characterize potential hot spots.


Investigators measured the concentrations of several MSATs and other pollutants on urban highways in Los Angeles County with a varying mix of gasoline- and diesel-powered traffic and at various fixed sites in the vicinity of the roads. The main goals were to compare on-road concentrations with those at fixed sites and those measured at monitoring sites managed by the South Coast Air Quality Management District (SCAQMD) and to estimate the contributions of gasoline- and diesel-powered vehicles to MSATs, particulate matter ≥ 2.5  μm in aerodynamic diameter (PM2.5), and elemental carbon (EC).

The pollutants measured were carbon monoxide (CO), oxides of nitrogen (NOx), MSATs (benzene, toluene, ethylbenzene, xylene, styrene, naphthalene, n-hexane, 1,3-butadiene [BD], methyl tert -butyl ether, formaldehyde, acetaldehyde, and acrolein) and other volatile organic compounds (VOCs), PM2.5, EC, and black carbon (BC).

The study was conducted in the southern portion of Los Angeles County for several weeks during the summer and fall of 2004. A combination of time integrated and continuous measurements were made in the following location classes: (1) on roads; (2) at sites at various distances from the roads (referred to as spatial surveys); and (3) at three near-road sites with varying proportions of gasoline- and diesel-powered vehicles. For the on-road sampling, a van equipped with monitoring instruments and operating with windows and vents fully opened and a circulating fan turned on was driven for 1 hour on three commuting routes at peak commuting times and on one freeway loop with a higher fraction of diesel-truck traffic (referred to as the truck route). Spatial surveys were conducted immediately after the morning on-road sampling and immediately before the afternoon on-road sampling by stopping the van for a few minutes at locations at various distances from these routes. The three near-road sites, Long Beach, Lynwood, and Diamond Bar, were located in the same general geographic area as the routes for the on-road measurements and were sampled for 24 hours.

Source apportionment was conducted using the chemical mass balance model to estimate the contributions of gasoline- and diesel-powered vehicles to VOCs, the sum of benzene, toluene, ethylbenzene, and toluene (BTEX), total carbon (the sum of EC and organic carbon), and EC. The model uses measured pollutant concentrations, along with source composition information, to determine the contributions of primary sources to the measured concentrations.

Expected Results:

Measured Concentrations

The spatial patterns of on-road concentrations of BC, NOx, CO, and total VOCs (all measured with continuous monitors) differed. Concentrations of CO and VOCs were higher on the commuting routes, whereas concentrations of BC and NOx were higher on the truck route. The spatial and temporal variations of on-road concentrations of BTEX and BD were similar to those of corresponding time-averaged continuous CO and total VOC concentrations — higher on the commuting routes in the morning, and in the fall. Formaldehyde and acetaldehyde showed less diurnal variation. The concentrations of MSATs were consistently higher (by about a factor of 2) on the commuting routes than on the truck routes.

The on-road concentrations of nearly all pollutants were higher than those at the survey sites, the near-road sites, and the SCAQMD sites. Unexpectedly, on-road concentrations of acrolein were lower than those at the SCAQMD air toxics sites (but higher than those at the near-road sites).

The findings of higher on-road concentrations are consistent with results of studies that have found that the levels of pollutants decrease with distance from roads. Meteorologic factors (such as wind direction and speed) play a major role in the shape of the decay curves. However, the Review Committee, in its independent review of the study, thought that in the absence of information about the relationship between the roads and the fixed sites, such as the distance between the sites and the sampled roads and other roads, only general qualitative relationships can be inferred from this study. An additional factor that makes comparisons between on-road and near-road measurements difficult to interpret is the different durations of sampling for MSATs (1 hour on road during peak pollution and 24 hours at the near-road sites).


Source Apportionment

Apportionment of total VOCs to sources showed that gasoline exhaust was the predominant source for both on-road and near-road concentrations, ranging from 70% for some of the samples at the near-road sites to about 100% for on-road samples. The contribution of diesel exhaust was small (22%) and more significant on the truck route. Apportionment of BTEX showed that gasoline was the dominant source (94% to 100%) for all on-road samples (including those from the diesel-dominated road) as well as those from the near-road sites (83% to 100%). Apportionment of ambient total particulate carbon (TC) associated with particulate matter to the near-road samples showed a greater contribution from diesel exhaust (averaging 46% to 52%) than from gasoline exhaust and evaporative emissions, which ranged from 10% to 17% in the summer, but did not show any significant contribution in the fall (0% to 4%).  About 40% to 50% of TC was not apportioned to mobile sources. These results are consistent with our knowledge of the emission compositions of gasoline and diesel vehicles when the study was conducted.

Apportionment of EC showed that diesel exhaust contributed 88% to 94% at the three near-road sites. Although the authors interpret these results to be an indication that EC may be a good surrogate for diesel exhaust, the Review Committee noted that the correlation between EC and truck counts was not as good as that with total traffic counts. Overall, the Committee noted that it was difficult to assess the accuracy of the quantitative split between the contributions from diesel- and gasoline-powered vehicles to EC.

The study’s main conclusions are that (1) on-road concentrations of all pollutants measured, including several MSATs, were higher than those measured at fi xed sites away from the roads, (2) gasoline-powered vehicles are the main sources of VOCs (including BTEX) at the near-road sites, and (3) diesel- and gasoline-powered vehicles contribute about 50% to 60% of TC associated with PM.

Supplemental Keywords:

Health Effects, Air Toxics, VOCs,  mobile source air toxics, ambient air sampling, BTEX, benzene, particulate matter,

Relevant Websites: Exit

Main Center Abstract and Reports:

R834677    Health Effects Institute (2010 — 2015)

Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R834677C149 Development and Application of a Sensitive Method to Determine Concentrations of Acrolein and Other Carbonyls in Ambient Air
R834677C150 Mutagenicity of Stereochemical Configurations of 1,3-Butadiene Epoxy Metabolites in Human Cells
R834677C151 Biologic Effects of Inhaled Diesel Exhaust in Young and Old Mice: A Pilot Project
R834677C152 Evaluating Heterogeneity in Indoor and Outdoor Air Pollution Using Land-Use Regression and Constrained Factor Analysis
R834677C153 Improved Source Apportionment and Speciation of Low-Volume Particulate Matter Samples
R834677C155 The Impact of the Congestion Charging Scheme on Air Quality in London
R834677C156 Concentrations of Air Toxics in Motor Vehicle-Dominated Environments
R834677C158 Air Toxics Exposure from Vehicle Emissions at a U.S. Border Crossing: Buffalo Peace Bridge Study
R834677C159 Role of Neprilysin in Airway Inflammation Induced by Diesel Exhaust Emissions
R834677C160 Personal and Ambient Exposures to Air Toxics in Camden, New Jersey
R834677C162 Assessing the Impact of a Wood Stove Replacement Program on Air Quality and Children’s Health
R834677C163 The London Low Emission Zone Baseline Study
R834677C165 Effects of Controlled Exposure to Diesel Exhaust in Allergic Asthmatic Individuals
R834677C168 Evaluating the Effects of Title IV of the 1990 Clean Air Act Amendments on Air Quality
R834677C172 Potential Air Toxics Hot Spots in Truck Terminals and Cabs
R834677C173 Detection and Characterization of Nanoparticles from Motor Vehicles
R834677C174 Cardiorespiratory Biomarker Responses in Healthy Young Adults to Drastic Air Quality Changes Surrounding the 2008 Beijing Olympics