The researchers found that diesel engines are the primary contributors of fine particles to Houston's air, followed by gasoline-powered vehicles and road dust. Smoke particles from wood burning and fatty acids from meat grilling contributed considerably smaller but nonetheless significant amounts of the particulates in Houston's air.
"There's been a good deal of speculation about the relative contribution of various sources to the particulate matter in air pollution," said lead researcher Matthew P. Fraser, assistant professor of Civil and Environmental Engineering at Rice University. "With this study, we're helping build a scientific body of evidence that policymakers and regulators can use to focus attention on the most significant contributors."
The findings from the Houston study will be published in the May 15 issue of the journal Atmospheric Environment.
The data in the study, a composite of seasonal samples representing a total of four months of records, were collected in 1997-1998 at four locations in the greater Houston-Galveston area. One site, located on Galveston Island, served as a background site because it is commonly upwind of the Houston metropolitan area, with typical winds off the Gulf of Mexico. Of the other sites, one was in a northwestern suburb and two were located near the petrochemical complexes along the Houston Ship Channel.
Collaborators at the Desert Research Institute in Reno, Nev., analyzed each air sample by quantifying the major chemical components making up the fine particles. At Rice, researchers measured individual organic compounds that have been linked to specific sources of air pollution like smokestack emissions or diesel exhaust. By analyzing the amount of these markers in each sample and comparing them to source emissions records, Fraser and graduate students Birnur Buzcu and David Yue were able to determine the precise proportion of particulate matter produced by each source.
The study is part of ongoing national effort to better characterize and understand the sources of fine particles in the atmosphere. Compared to gas pollutants like carbon dioxide and ozone, relatively little research has been conducted in the area of airborne particulates. Air quality researchers want to know more because a growing body of medical evidence suggests that exposure to fine particulate matter can cause serious health problems.
The source apportionment of fine particles did not focus on ammonium sulfate -- a byproduct of large scale, industrial fossil fuel combustion. As the largest single component of particulate matter -- accounting for some 40 percent -- a number of previous studies have investigated the sources of ammonium sulfate. Instead, Fraser's team focused on less understood sources of fine particles like vehicle emissions.
Diesel exhausts, which accounted for only 4 percent of particulates at the Galveston site, accounted for up to 17 percent at urbanized locations. Particulates from gasoline-powered vehicles, which also accounted for only 4 percent of the ambient particles in Galveston, accounted for up to 13 percent of particulates at urban locations.
At all but one location near the ship channel, road dust contributed fewer particulates than vehicle exhaust. At the suburban location, road dust was measured at just over one microgram per cubic meter. While this was 10 times the background level of road dust measured at Galveston, it was still less than less than half the mass of particles collected from diesel exhaust.
The study found that fuel oil combustion, a common practice at the industrial plants located along the ship channel, produced up to 1.5 micrograms of particulates per cubic meter at one location. These types of particles didn't register at all in the background sample.
"This is significant because it's the first time particulates from fuel oil combustion have been found in an apportionment study," said Fraser. "The findings weren't unexpected, given the nature of industrial activity in the city, but they are indicative of an air quality pattern that may be unique to the city."
Fraser said air quality researchers working on apportionment studies for particulate matter in other U.S. metropolitan areas expect to find similar, "signature" patterns. Each pattern will reflect a mix of factors such as meteorology, population density and the city's industrial profile.
The apportionment study was sponsored by the American Chemistry Council's Atmospheric Chemistry Technical Implementation Panel. Sample collection and analysis were sponsored by the Texas Natural Resources Conservation Commission, the Houston Regional Monitoring Corporation and the City of Houston.
Journal
Atmospheric Environment