TechnologyMSolutions Characterizing fine, airborne particulate matter
© 2001 American Chemical Society
different atmospheric chemistry and air pollution sources. Seven additional Supersites studies throughout the United States have since begun. Meanwhile, data from the Atlanta Supersites project are now emerging, some of which were presented last December at the American Geophysical Union meeting in San Francisco, CA. Those who participated in the study say overall that measurement methods for characterizing PM2.5 compared quite well, particularly the real-time methods. Some of the technologies that were deployed in Atlanta have been around for several years, whereas some were being field-tested for the first time. But what participants say is most valuable from the Atlanta study is the information gained through the intercomparison of such a wide array of sophisticated measuring devices. “Atlanta was a chance to compare measurement methods and to determine whether there were systematic differences among these new technologies,” says Susanne Hering of Aerosol Dynamics in Berkeley, CA, who demonstrated a pair of real-time
SANDY DASGUPTA, TEXAS TECH UNIVERSITY
It’s August 1999, and teams of researchers equipped with various instruments have just arrived in Atlanta, GA, for a four-week intensive air-monitoring study. What brings them all together is a chance to evaluate and compare new and emerging technologies for characterizing airborne particulate matter less than 2.5 µm in diameter (PM2.5). Ultimately, they hope the results will assist the U.S. EPA in its efforts to identify and quantify major particulate matter pollutants and determine where these pollutants are coming from. That was the scene nearly two years ago in Atlanta, at the first of EPA’s so-called Supersites projects, an ambient air-monitoring program established to characterize spatial and temporal PM2.5 trends and to support human exposure and health effects studies that are being conducted to address uncertainties in EPA’s PM2.5 standards (Fed. Regist. 1997, 62, 38,651–38,752). EPA’s Supersites program closely follows recommendations outlined in the National Research Council (NRC) report, Research Priorities for Airborne Particulate Matter, first released in March 1998 and updated in August 1999. (Both reports are available at www. nap.edu.) For example, the NRC report recommends that a relationship between particulate matter sources and human exposures be established. Data from the Supersites projects are expected to help pinpoint those sources. To get the program started, EPA chose Atlanta, GA, and Fresno, CA, as the first two Supersites projects because they already had ongoing activities that closely aligned with the program (e.g., the Atlanta Supersites study was linked with the Southern Oxidants Study, a 10-year-old program that investigates the chemistry of tropospheric ozone accumulation in the southeastern United States) and because the cities have distinctly
flash vaporization systems, one for determining nitrate on fine aerosol particles and one for sulfate and carbon. “For the first time ever, single-particle mass spectrometers were run side-by-side in Atlanta,” says Paul Solomon, EPA’s technical lead for the Atlanta Supersites study. Among these were an aerosol time-of-flight mass spectrometer from Kim Prather’s group at the University of California– Riverside, a system called PALMS (particle analysis by laser mass spectrometry) demonstrated by Ann Middlebrook of the National Oceanographic and Atmospheric Administration, and a second-generation rapid single-particle mass spectrometer operated by Tony Wexler of the University of Delaware. In addition, Massachusetts-based Aerodyne demonstrated its commercially available aerosol mass spectrometer (AMS). The Aerodyne AMS system is not a single particle mass spectrometer. “It’s more of a hybrid between a mass spectrometer and a flash vaporization system,” says Hering. One advantage of having the mass spectrometry (MS) data is that they provide size resolution, which could turn out to be critical for assessing health effects. Size often determines
Like an armyoftin soldiers,these high-volume samplersw orked around the clock to monitor the airin Atlanta. MARCH 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SANDY DASGUPTA, TEXAS TECH UNIVERSITY
New devicescollectairborne particleson the rooftopsofmobile laboratoriesin Atlanta.
whether a particle will penetrate the lungs. Unfortunately, MS is not a quantitative technique. “The MS data were actually scaled using the semicontinuous measurements,” points out Rodney Weber of the Georgia Institute of Technology, who is leading the effort to compare the results from four real-time PM2.5 inorganic ion analyzers tested in Atlanta. Other systems under development included continuous or semicontinuous chemical species-specific analyzers, such as nitrate and sulfate particle analyzers, which are capable of sampling on the order of every 5 min to 1 hr. Traditionally, such measurements are performed using integrated filter-based techniques, which sample only once every 12–24 hrs. “The filter-based methods are fine for things like compliance with EPA regulations,” says Weber, “because EPA has a 12–24 hr and a yearly [PM2.5] standard. But they are not as good for understanding atmospheric processes and particulate matter sources because you don’t have that temporal resolution,” he adds. On the other hand, the inorganic ion analyzers operate in real time and therefore provide fine temporal resolution on particles. “It allows us to see plumes going by and diurnal variation,” says Weber. Among those demonstrating realtime nitrate and sulfate particle analyzers in Atlanta were Purnendu “Sandy” Dasgupta of Texas Tech University, who was the first to publish a real-time approach for such measurements (Environ. Sci. Technol. 1995, 29 (6), 1534–1541), Weber from
Georgia Tech, J. Slanina and colleagues from the Netherlands Energy Research Foundation ECN, and Hering with her flash vaporization systems. Each of the four systems is designed with a 2.5-µm cyclone at the entrance so that only particles less than 2.5 µm in diameter can get through. Samples are first sent through a diffusion denuder to remove gases such as SO2, HNO3, and NH3, which can interfere in the particle collection part of the system. Three of the four systems collect the particles in purified water and use ion chromatography (IC) to determine nitrate and sulfate levels, whereas Hering’s method uses an electrically generated thermal pulse to vaporize all of the particles and then measures what is generated in the gas phase. The IC systems all differ from each other in how the particles are collected. In addition, some are capable of measuring the gas-phase species collected by the denuders, whereas others only measure what is in the particle phase. Despite their inability to provide temporal resolution, several traditional 24-hr and 12-hr integrated particle samplers for monitoring PM2.5 mass, ions, trace elements, organic and elemental carbon, and gases such as NH3, HNO3, and SO2 were evaluated alongside the other technologies in Atlanta. Most of the chemical speciation monitors used denuders and reactive filters for collecting semivolatile species and either Teflon or quartz filters for collecting the nonvolatile species. In addition, techniques for continuous and semicontinuous particle mass and particle physical characterization were tested, and gaseous
and meteorological measurements were made to enhance the overall data set. “The idea is to use all of the data to better understand the atmospheric chemistry and determine how best to combat pollution,” says Dasgupta. “If you measure only PM2.5, you will learn nothing. Most particles in the atmosphere are secondary aerosols, meaning that they are generated as a result of reactions of various gaseous species. Sulfate, for example, is formed from the oxidation of gaseous SO2. Unless you understand and characterize the chemistry of the gasphase species, you are not going to understand the chemistry of the particles,” he says. Dasgupta’s team also measured ambient hydrogen peroxide and formaldehyde levels in Atlanta. “Both of these are key indicators of photochemical oxidation processes,” says Dasgupta. Currently, his group is analyzing archived filters from Atlanta by IC/MS and liquid chromatography/ MS, in order to identify organic acids such as oxalate, pyruvate, gamma hydroxy butyrate, and a myriad of others. “Just about every organic acid you can think of between one and six carbons is there,” he adds. They have even seen evidence of low levels of phthalates (commonly used as plasticizers) and methanesulfonate. Realtime methods for monitoring organic compounds in the field, however, currently do not exist. “We can collect gas-phase species on the denuders and analyze them, and we can collect aerosol species on reactive filters with minimal bias. But we are a long way from having a good method for organics,” says EPA’s Solomon. “One of the big advantages of the Atlanta study is that many of these instruments are now being used at other Supersites,” says Weber. “The first study let everyone figure out how to run their instruments,” he says. The Supersites program is expected to continue throughout 2005 and ultimately should assist EPA in its ongoing and future reviews of the PM2.5 standards. But even then, particulate matter research is not expected to grind to a halt. “As long as there is a concern about the haze in the sky, there is going to be an interest in knowing where it comes from. The first handle you have on that is the chemical composition of particulate matter,” says Hering. —BRITT E. ERICKSON
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