Issue Date: April 12, 2010
Bringing Outdoor Chemistry Indoors
When we think about atmospheric chemistry or air pollution, our thoughts immediately move outdoors. But most people spend the majority of their time indoors, and the indoor environment deserves as much attention as the outdoor environment. A symposium at the American Chemical Society meeting in San Francisco, sponsored by the ACS Division of Environmental Chemistry and the National Science Foundation’s Division of Chemical, Bioengineering, Environmental & Transport Systems, brought together atmospheric and indoor chemists with the goal of shifting some attention inside.
The field of indoor chemistry “is in the early stages of development, especially compared with its older sister, outdoor atmospheric chemistry,” said symposium co-organizer Hugo Destaillats, an indoor chemist at Lawrence Berkeley National Laboratory (LBNL). “Techniques that are developed specifically for outdoor air may also be applicable indoors.”
“We have an enormous amount of information from the outdoor world, primarily driven by regulation and global concerns,” said co-organizer Glenn C. Morrison, an environmental engineering professor at Missouri University of Science & Technology. “Very little of that has been applied to the indoor world. The opportunity is huge.”
Outdoor atmospheric chemistry involves such species as the hydroxyl radical, the nitrate radical, and ozone reacting with other chemicals in the air. Species such as hydroxyl and hydroperoxy radicals are formed photochemically and have such a short lifetime that they rarely make it indoors. In contrast, ozone survives long enough to be a major player in indoor chemistry. Ozone-initiated chemistry can produce other oxidants such as hydroxyl, hydroperoxy, and nitrate radicals indoors at meaningful concentrations.
The chemistry and techniques outdoor and indoor chemists use “are not radically different,” said Barbara J. Finlayson-Pitts, an atmospheric chemist at the University of California, Irvine. “There’s probably more emphasis on surfaces in the indoor community, because the surface-to-volume ratio is so much bigger.”
Cleaning products are a big contributor to indoor air chemistry. That fresh citrus or pine scent that indicates “clean” comes from terpenes, such as limonene or pinene, found in many cleaning products. That fresh scent comes with baggage, though. The terpenes react with ozone to form products such as formaldehyde, ultrafine particles, and dicarbonyls. Dicarbonyls are of interest in part because of their similarity to diacetyl, which has been shown to cause lung damage in workers in microwave popcorn factories (C&EN, Nov. 16, 2009, page 24).
Ray Wells, a scientist at the National Institute for Occupational Safety & Health (NIOSH), is interested not only in dicarbonyls but also in dicarboxylic acids. “When we looked at the yields of dicarbonyls from consumer products, a lot of the carbon was still missing,” he said. That missing carbon led him to dicarboxylic acids.
Wells is developing a method that can be taken into the workplace to measure dicarboxylic acids. He has borrowed from the atmospheric chemistry community a sampling technology known as a denuder. This device consists of a series of interconnected, layered glass tubes with a coating. As air flows through the tubes, gas-phase molecules stick to the coating, but particles move straight through to filters. He initiates a reaction of ozone with terpenes and, using the denuder, captures particles that are formed. Wells then derivatizes carboxylic acids in the particles so they can be analyzed by gas chromatography.
Terpene chemistry has been studied more extensively in the gas phase than on surfaces. “Some terpenes are very volatile, and the chemistry in the gas phase is all that matters,” Morrison said. “But there are a lot of terpenes in these consumer products of low volatility. That means they’re adsorbing to surfaces. Is their chemistry at these surfaces important, and are the rates relevant?”
Morrison studies the chemistry of these compounds in reactors filled with beads that mimic indoor surfaces, including painted walls, windows, and vinyl flooring. Some of the low-volatility compounds have reaction rates on surfaces that are up to 1,000-fold faster than they are in the gas phase, he said.
In addition to monoterpenes, indoor chemists should consider the even more reactive sesquiterpenes, said Allen H. Goldstein, an environmental science professor at UC Berkeley.
“When we look at essential oils in plants, we see similar amounts of sesquiterpenes and monoterpenes,” he said (Environ. Sci. Technol., DOI: 10.1021/es903674m). “My guess would be that if there are monoterpenes in cleaning products, there are sesquiterpenes as well. They react very quickly, so they wouldn’t stay in the air long, but their reaction products might be more persistent in the indoor environment.”
Surfaces play a much larger role indoors than they do outdoors because the average air molecule is much closer to a surface indoors than it is outside. The role of the surface depends on the nature of the surface and on the reaction, said Vicki H. Grassian, a chemist at the University of Iowa. The surfaces found in the real world differ greatly from the highly idealized surfaces studied by surface scientists in vacuum chambers. Real surfaces have water, organic matter, and particles on them, none of which have been well studied from a molecular perspective, Grassian said. “A lot of good characterization methods for surfaces tend to be in ultrahigh vacuum. The question is: What else is present when that same surface is placed in a real environment?”
Many of these surfaces involve thin films of water. There is a “vacuum of knowledge” with respect to understanding how thin water films affect the chemistry, Grassian said.
Finlayson-Pitts also studies reactions involving water at interfaces. “One of the big issues with respect to the indoor situation is if you put water on the surfaces of a room or a building, what does it look like and how does it behave chemically,” she said. The water’s structure doesn’t begin to look like bulk water until the relative humidity reaches 80 or 90%, she added. Other people have described the unusual spectral signature of surface water as “icelike,” but Finlayson-Pitts has banned her group from using that term. “There’s no way I believe that water on most surfaces is going to have an icelike structure at room temperature,” she explained.
Finlayson-Pitts suggested that this unusual structure—whatever it is—could explain the reactivity of water observed at indoor surfaces. “If it isn’t bulk liquid water, that suggests that its chemistry and reactivity are not going to be the same as those of bulk liquid water. I think this is why so much of the chemistry that occurs on these surfaces is much faster than you would expect,” she added.
Some indoor surfaces could be localizing and holding reactants in suitable conformations to react with components from the air. For example, nicotine from tobacco smoke sorbs strongly to indoor surfaces, where it reacts with ozone and nitrous acid, LBNL’s Destaillats said.
The chirality of terpenes on a surface affects reactions with ozone in the air, said Franz M. Geiger, a chemistry professor at Northwestern University. Surface-bound terpenes point their carbon double bonds toward or away from the gas phase depending on their chirality. The reaction rates are higher for those chiral terpenes that orient their carbon double bonds toward the gas phase.
A poorly understood type of pollutant both indoors and outdoors is ultrafine particles, roughly defined as particles smaller than 100 nm. These ultrafine particles are found in every type of environment, from the most polluted urban areas to pristine boreal forests, said William W. Nazaroff, an environmental engineering professor at UC Berkeley. They can form from “nucleation events” in which supersaturated gaseous species suddenly form bursts of particles. Another major outdoor source is motor vehicles, but the mechanism of formation is not well understood. Rather than coming solely from the burning of fuel, the particles might also come from the oil used as a lubricant or from wearing of metal components in the engine.
“It’s very hard to do chemistry on these particles because their mass is really tiny,” Nazaroff said. What scientists can do very well is count and measure the size of such particles, he said. Nazaroff described a study looking at the levels of ultrafine particles in homes his group performed for the California Air Resources Board, the state’s clean air agency.
Without an indoor source of ultrafine particles, the levels indoors would typically be only 30 to 40% of the outdoor levels because some outdoor particles are left outside as air enters buildings and because the particles deposit on indoor surfaces. But Nazaroff’s study found similar levels indoors and outdoors. Cooking, a major source of indoor particles, might be the source of these higher than expected indoor levels. “Even boiling water in a stainless steel pot on an electric range produces copious ultrafine particles,” he said. The particles might be formed from an organic film on the burner, he speculates. Some toasters and even steam irons were found to produce ultrafine particles.
Such sources of ultrafine particles may be impossible to regulate, but it is important to understand them, Nazaroff said. “People spend most of their time indoors,” he said. “If indoor activities are an important source of their exposure, we’d better be able to sort that out.”
Humans affect their indoor environment not only with the products they use and the activities they undertake but by their mere presence.
A leader in this area of studying people’s impact on indoor chemistry is symposium co-organizer Charles J. Weschler, who splits his time between the University of Medicine & Dentistry of New Jersey and the Technical University of Denmark, in Copenhagen. In collaboration with Armin Wisthaler of the University of Innsbruck, in Austria, Weschler studied the chemistry inside a partial mockup of the cabin of a commercial passenger aircraft.
Weschler, Wisthaler, and coworkers were particularly interested in ozone and ozone oxidation products in the aircraft. Ozone can be naturally elevated outside airplanes flying at high altitude and high latitude, and it enters through the planes’ air intakes. But many airplanes aren’t equipped to remove ozone from ventilation air.
In the group’s experiments, an empty plane and a certain rate of ozone generation resulted in ozone levels of approximately 120 ppb in the cabin. With people in the cabin, the same rate of ozone generation yielded air concentrations of only 70 ppb (Environ. Sci. Technol. 2007, 41, 6177). “Right away, we know that these occupants are really big sinks for ozone,” Weschler said. “We’re seeing products of ozone reacting with squalene and unsaturated fatty acids present in our skin oils.”
They have since done similar studies in a simulated office environment (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0904498106). They see different levels of oxidation products depending on the type of indoor environment and the occupant density. Many of the species are secondary products that start to accumulate only after significant amounts of primary products have formed.
Although ozone is the primary oxidant indoors, other species could also play a role in indoor oxidation processes. “We’ve got measurements of ozone and decreases in indoor ozone when a room is occupied, but we have nothing on the nitrate radical or the hydroxyl radical,” Weschler said. “The nitrate radical especially might be important indoors because at anticipated concentrations it reacts quickly with certain indoor pollutants.”
Jason E. Ham of NIOSH is developing instruments for indoor measurements of nitrate radicals using cavity ringdown spectroscopy. The technique is already used in atmospheric chemistry.
Several speakers lamented the lack of field studies in real indoor environments. “Considering that we spend a lot of our time indoors, it seems like there’s missing information,” Iowa’s Grassian said.
Grassian has collaborated with colleagues in public health to make measurements of samples from a factory that manufactures nanomaterials (J. Occup. Environ. Hyg. 2009, 6, 73). With single-particle analysis using electron microscopy and energy-dispersive X-ray analysis, they found a combination of large and small particles. The large particles turned out to be the nanomaterial, whereas the smaller particles were welding materials. “If you didn’t know that, you would start setting standards about these small particles, which you may have thought were coming from the engineered material,” she said.
She would like to do more indoor field studies, but convincing funding agencies in the occupational health field is difficult, she said. “It’s really clear to me that single-particle analysis would be really helpful,” Grassian said. But single-particle methods are expensive, and many review panels are more interested in cheap, easy methods that could be used for routine monitoring. “They’ll just shut the door,” she said.
Missouri’s Morrison is worried that lab tests performed under precisely controlled conditions don’t adequately reflect the real world. “When you go into a building, basically a goo forms on everything,” he said. “The surfaces are coated with all kinds of junk—salt and skin oils. If we’re doing laboratory work, we need to recognize that the chemistry taking place in that goo is going to be pretty important.”
Morrison advocates conducting indoor field studies in parallel with atmospheric field studies. He organized a meeting separate from the symposium to discuss how such coordination might happen.
“We’re going to hash out what it would take to go after a major campaign to understand indoor chemistry in multiple homes during a smog season and do it while a similar campaign is taking place outdoors,” he said. “The potential for discovery is enormous.”
Real indoor field studies will require measuring many species simultaneously, as is done in outdoor experiments. During outdoor field studies, “we try to measure all the emissions, the oxidants, and their products,” said Joost A. de Gouw, an atmospheric chemist with the National Oceanic & Atmospheric Administration. “Typically, when we do a field study, we have 10, 20, up to 30 instruments that look at all these aspects of the air. We’re beyond the stage where a few instruments are going to tell us enough.”
Finlayson-Pitts agrees. “As time has gone on, the outdoor community has recognized that you need to measure a lot of things at once to understand the chemistry fully and to have all the inputs you need to model it,” she said. “Taking that comprehensive approach indoors is a good idea.”
The organizers and others hope the symposium succeeded in bringing the needs of indoor chemistry to the attention of the atmospheric chemistry community, which tends to be much better funded. “So far, all the indoor air community has had is crumbs from the banquet of outdoor air chemistry research,” Nazaroff said. “We really deserve a place at the table.”
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