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Dust in the wind EPA regulation spurs development of methods to analyze airborne particulate matter
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n July 18,1997, the U.S. Environ- tion is not with levels of particles in the air mental Protection Agency (EPA) but rather is with the change in particle published a controversial new level from the preceding time period, usustandard for airborne particulate matter ally the previous five days. Robert Phalen having a diameter less than 2.5 um (known of the Department of Community and Envias PM2.5), setting an annual standard of ronmental Medicine at the University of 15 ug/m3 and a 24-h standard of 65 ug/m3 California-Irvine calls the promulgation of (1). According to an agency fact sheet, a regulation based solely on such epidemiEPA estimates that these standards will ological evidence "unprecedented". prevent annually approximately 15,000 preDebate continues over just what the mature deaths, 350,000 cases of aggravated best indicator of the observed health efasthma and one million cases of signififects is—the mass of the particles, the total cantly decreased lung function in children. number of individual particles, or some The PM2 5 standard is based solely on particular characteristic such as particle total of particles of a certain size the acidity. "We take mass right now as the current indicator of the health problem not chemical comoosition is not mentioned in the regulation Such a stratesrv is anathema because we think it's fundamentally mass to chemists: After all the chemicals have to [that causes the problem] but because it's the best indicator that we can use right now," says Russell Wiener of the AtmosYet, whether the problem is a particular pheric Methods and Monitoring Branch of chemical species, a number of agents actthe Human Exposure and Atmospheric ing in combination, or simply the physical Sciences Division of EPA's National Expopresence of particles, better analytical sure Research Laboratory methods will be necessary to understand the epidemiological association between There are concerns that standards based solely on mass do not address the problems exposure and the observed respiratory efof the number of particles, because for the fects. A number of research groups are smaller particles the correlation between developing methods to help in that quest. mass and number of particles is very poor. The particles from diesel exhaust and indusBackground trial sources dominate the number of partiPart of the controversy surrounding the cles in the PM2.5 fraction, but, because they standard stems from the fact that the supporting research has all been epidemiologi- are "vanishingly small", they add very little to the overall mass measurement. cal and that the underlying mechanism of Research by Kimberly Prather and her the respiratory effects is unknown. The group at the University of Californiaepidemiological models have all used the total mass of particles smaller than 2.5 um. Riverside indicates that a fortuitous connecEven more controversial is that the associa- tion may exist between particle size, compo-
Analytical Chemistry News & Features, July 1, 1998
sition, and, thus, source. In 1996, Prather's group published results that seem to indicate that the break between mechanically generated particles, such as soil and sea salt particles, and combustion particles is at a point smaller than 2.5 um and that the dividing line between thefineand coarse fractions should be drawn even lower (2). "If you look where 2.5 microns occurs in a size-composition histogram we obtain with aerosol MS, there's not really a clearcut break in chemical composition. Ideally, if you are going to use size as the parameter for regulation, it would be nice to have the break divide fine (i.e., combustion) and coarse (i.e., sea salt, soil) particles. In our data, this actually occurs at smaller sizes or
about 1 micron. This break also represents the break in particles produced from different sources." However, Prather sees 1 micron as being too small because on foggy or humid days, the breaking point between fine and coarse particles can shift to larger sizes. However, even with the shift, coarse particles such as soil are still being regulated at 2.5 microns. Prather adds, "The jury's out [for where the size cut should be] until the 'health effects' people have the data to come back and which particles which composition from which are really the of the health problems " Some researchers question the necessity of identifying the individual organic
components. "I don't believe the organics are important in [the health effects], but I could be wrong. I think it's a matter of irritation," says Morton Lippmann, an environmental toxicologist at the New York University Medical Center. "Cells get irritated and secrete. An organic particle may [cause irritation], but I don't believe it does so with any particular toxicity." Lippmann hypothesizes that the acidic particles may cause the most irritation. He says that recent animal studies of concentrated outdoor aerosols have indicated cardiovascular effects. Particularly interesting was that the aerosols were fresh, that is, neither aged nor surface modified. However, Lippmann has also found recently
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Focus that ultrafine sulfuric acid droplets in the presence of fatty acids can develop a surface monolayer that retards the hygroscopic growth of the particles and impedes ammonia neutralization. He says, "The toxicity of an ultrafine droplet may depend not only on its presence but on whether in this complex mixture it's coated with a fatty acid monolayer." Checking priorities
One thing that the experts agree on is that more research is needed to understand the underlying cause of the observed associations. The new air standard came with a large pot of money for EPA to fund research that helps resolve the scientific uncertainties associated with the health effects of airborne particulate matter. In its fiscal 1998 appropriations, Congress gave EPA $49.6 million for particulate matter research and directed EPA to commission an independent study by the National Research Council (NRC the operating arm of the National Academy of Sciences) to identify the most important research priorities for Darticulate matter The first of four expppted reports from the committee was released March The 20-member committee, which was chaired by Jonathan Samet, an epidemiologist at Johns Hopkins University, identified 10 research priorities, one of which was to "develop advanced mathematical, modeling, and monitoring tools to represent the relationships between specific sources of particulate matter and human exposures." Over the 13-year research timeline that the report proposes, $1 million is the recommended amount for each of the first three years to develop advanced-exposure analytical methods for use in epidemiology studies; and $15 million will be needed in each of years four through six to develop advanced analytical methods for monitoring biological responses of particulate matter (3) As part of the PM2.5 implementation, EPA is establishing a monitoring network. The total network will have approximately 1500 sites, of which —1200 wiil locus on mass concentration measurements and only 300 will focus on chemical characterization. According to the NRC report, the committee is "concerned that the monitor464 A
ing program is moving forward rapidly with too narrow a focus on PM2.5." The committee is "concerned about the scientific value of the data to be collected in these efforts if such monitoring is fully planned and implemented before some of the immediate research priorities are addressed and data gaps arefilled."The committee recommends independent peer-review of EPA's plans for the network at an early enough stage that changes could still be included in the design and implementation. The challenges
"When you think of a single particle, a particle of about a micron is just a picogram of material," says Murray Johnston of the University of Delaware. "You're looking for components that are much smaller than that. If you go to submicron particles, you're looking for very small amounts.
Analytical methods are needed to sharpen the epidemiology and identify sources. There's been increasing interest in the effect of aerosols on atmospheric chemistry. Every atmospheric problem that we face has an aerosol component to it." The total aerosol composition in the air is extremely complex. "You have bioaerosols, secondary organic aerosols, combustion aerosols, and so forth. There may be thousands of different species. Let's say you go to heroic measures and measure a hundred of those. Are you measuring the right hundred?" says Johnston. "How can you get the right parameter that correlates with health? In particles, there are all sorts of components, and what's important from a health perspective is not clear." The most difficult fraction of the particles for determining the composition is the polar organic compounds. "If we just look at the bulk methods, there are pretty reasonable methods available for [measuring] inorganic components and nonpolar organics—that is,
Analytical Chemistry News & Features, July 1, 1998
organics that might be [analyzed] by GC/ MS," says Johnston. "There are problems, though, because less than half of the organic mass in aerosols has been characterized. The uncharacterized fraction is thought to consist mainly of polar organic compounds. We're looking at using MALDI and electrospray ionization to [identify] the polar fraction of organics in aerosols." The question immediately comes to mind: Aren't electrospray and MALDI better suited to large molecules? Johnston answers, "MALDI has problems with matrix peaks that can hide low-mass analytes, but electrospray has been used for small molecule analysis. Both work well for large molecules, but both also work well for small polar molecules." Toward better methods
Ultimately, Phalen says that the perfect instruments would be cheap real-time personal monitors (sophisticated dosimeters) that could record what individuals are breathing in real time. Such instruments are many years down the road, but efforts are underway to develop improved methods and instrumentation for particle characterization. Regardless of whether improved methods play into the PM2.5 regulation, they are needed for the basic research that will sharpen the epidemiological studies and will allow sources to be identified John Ondov of the University of Maryland says, "We need near-real-time or realtime continuous monitors. One might argue that we don't need them for regulatory purposes across the board, but if one is interested in source attribution and atmospheric chemistry, one must be able to take data at a frequency comparable with the fluctuations in the meteorological parameters wm(j direction and mixing height." Most samples are taken for 10-24 h, which averages the temporal information and severely degrades the ability of statistical methods to resolve the 'You want size-resolved composition because different size particles penetrate the lungs differently and have different health effects," says Johnston. "If you do that by bulk methods, you have the advantage that you have the entire arsenal of analytical methods that one would have for organic and inorganic analyses to bring to
bear on that problem. You can be very quantitative; you can look for lots of different types of components; and you can look for trace species." Of course, bulk methods are not without their problems. "Usually they're offline, so it's a slow turnaround time for analysis or a long sampling time," says Johnston. "The bulk sampling and analysis will probably not give you as good an indication of acute effects because of the time resolution. The other problem is that each particle has its own composition signature that's related to the source of the particle and the transformations that it undergoes in the atmosphere." Prather's group is one of the few groups acquiring field measurements of single particles. The method that they use is aerosol time of flight mass spectrometry, which simultaneously provides size and composition information (4). The aerodynamic sizer uses two lasers set at right angles to one another and to the particle flight path. The first laser triggers the timing circuit, and the second laser serves as the stopwatch. The flight time between the lasers can be used to calculate the particle's aerodynamic size The mass spectrometer has two flight tubes so both positive and negative ion mass spectra can be obtained for every particle The ability to simultaneously acriuire positive and negative ion snprtra has made sourrp assignment easier Ahmit a Hn7Pn crrnnnQ worldwidp in have developed related instruments and the OndoVs group is currently testing a semicontinuous monitor for the simultaneous quantification of several trace metals. The system involves a dynamic aerosol concentrator coupled with a multielement atomic absorption spectrometer. Ondov suggests that the future of airborne particle analysis will be the application of conventional methods used in unconventional ways. Kenneth Marcus of Clemson University is combining a particle-beam inlet with glowdischarge atomic emission spectroscopy and MS. The instrument can be used for either near-real-time or filter measurements. Rather than analyzing single particles, he is looking more at averages over short periods, such as five seconds. The interface works by drawing in particles from the air and deliver-
ing them to the plasma. "It's like a vacuum cleaner with a 50-um diameter hose attachment—you can sniff things from mid-air or you can get the stuff that's collected on the floor." Because the plasma operates at reduced pressure, it can be used to analyze for nonmetals such as nitrogen, carbon, and oxygen. The emission spectrometer could be very small. "The interface and the plasma could be literally shoe-box size. Then you'd have a couple of vacuum pumps. Your spectrometer could be one of the diode array spectrometers that are on an interface card," says Marcus. Marcus sees the instrument more for continuous monitoring than for individual particle analysis. "You could see putting one of these with the same type of interface on top of a smoke stack. It will be in the continuous monitoring mode where people are monitoring what they're putting out to the environment. I think in the long run someone's going to want to know composition." Purnendu Dasgupta of Texas Tech University is using continuous wet analysis methods to characterize the soluble components of particulate matter (5). Soluble gases, such as sulfur dioxide and ammonia, are removed with a diffusion denuder. Superheated steam or another condensable vapor condenses on the particles in a "cloud" chamber. The cloud is moved to a rain chamber, where the droplets coalesce and impact on the walls. The air is sampled and analyzed for soluble inorganic ions such as nitrate and sulfate In addition Dasgupta has recently developed a continuous monitoring system to measure strong acidity in aerosol particles (6) The quadrupole ion-trap mass spectrometer is another instrument that can be used for characterizing particulate matter. A group at Oak Ridge National Laboratory headed by Michael Ramsey and William Whitten has developed a method for analyzing single particles in an ion-trap mass spectrometer (7). Particles are injected directly into the trap, and a timing circuit determines when to fire the laser used for ionization. The ion trap allows tandem mass spectra to be obtained for particle constituents improving the identification This ability may be particularly important for identification of the organic components
Better indices The PM2.5 standard may be based on the best information available, but there's still room for improvement. As Lippmann points out, the history of air monitoring has been one of continually improving indices. "We used to measure something called total suspended particulates. That was changed in 1987 to thoracic particles, those that can enter the chest. Those were best approximated by something called PM10. The epidemiology became sharper and was much more closely related to the effects than samples including 'boulders' larger than 10 urn As people looked at still finer fractions that contained more active chemicals they went to PM2 5 to supplement PM10 1 presume that in 2002 [at the fivevpar review of the PM? 5 standard] WP may wellCTAIn another indicator or multi l
indicators as abnnt what is really activp " One vision of the future is to use chemical composition to regulate emission sources. Prather calls the true power of single-particle analysis its ability to identify the composition signature and to accurately pinpoint the sources. With single-particle data, the contribution from different sources could be ascertained and regulated directly rather than having a blanket rule that lumps all particles together from sea salt to diesel exhaust. In addition, knowledge of the composition of single particles will allow researchers who are trying to elucidate the cause of the observed health effects to better simulate atmospheric particles for laboratorybased toxicological studies Celia Henry References (1) Fed. Regist. 1997, 62, 38651-712. (2) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1199630, ,267-80. (3) National Academy of Sciences, Board on Environmental Studies and Toxicology. Research Priorities for Airborne Particulate Matter: I. Immediate Priorities and a Long-Range Research Portfolio. National Academy Press: Washington, D.C., 1998. (4) Gard, E.; Mayer, J. E.. Morrical, B. D.; Dienes, T.; Fergenson, D. P.; Prather, K. A. Anal. Chem. 1197, 69,4083-91. (5) Simon, P. K.; Dasgupta, P. K.Anal. Chem. 1995, 67, 71-78. (6) Ito, K.; Chasteen, C. C; Chung, H.-K.; Poruthoor, S. K.; Genfa, Z.; Dasgupta, P. K. Anal. Chem., 1198, 70, in press. (7) Reilly, P.T.A.; Gieray, R. A; Yang, M.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1998, 69, 36-39.
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