The Goop in Our Air

Even on an ostensibly clear, sunny day, the outdoor air can be filled with super-tiny pollutant particles. They’re known in the regulatory community as ultra-fine particulates. At less than a micrometer in diameter, these motes are so small that they can remain airborne for a week or longer (time enough to travel from Pittsburgh to the Gulf Coast and back). More importantly, at least as concerns human health, they’re small enough to be inhaled and lodge deeply in the lungs. Increasingly, high airborne concentrations of these particles have been linked to heart ails — even death.

Although studies have suggested that these micropollutants can be mighty toxic, fine particulates have thus far escaped regulation. Not only have they proven hard to measure, but regulators have questioned what should be limited. Particles of a particular size, of a particular chemical composition, or from a particular pollution source?

Complicating the issue is that the chemical nature of these particles morphs from hour by hour, depending on what other chemicals the particulates encounter in the air — neighbors that can vary dramatically during an air parcel’s journey across hundreds of miles or more.

So identifying where a particular particulate came from becomes devilishly hard if it’s not caught in a fairly pristine form, shortly after its birth.

Just how quickly and substantially these particles undergo change was the subject of a lunchtime conversation today (Monday) at Carnegie Mellon University between me and three other reporters (this year’s Steinbrenner Institute media fellows) and our hosts (three CMU particulates specialists).

One of them, atmospheric chemist Neil Donahue, has been probing what triggers fine particles to morph. And a principal finding coming out of his lab and others’ has been that these pollutants are incredibly reactive. Once a small particle is emitted by fossil-fuel combustion, for instance, it begins bumping into other particles and glomming onto them — or interacting with gases in the air. The latter ultimately leave a surface deposit of some ill-defined goop on each particulate they encounter.

And yes, that appears to be the formal scientific term: goop.

What many people — including regulators and policymakers — don’t understand is that some 70 to 85 percent of each fairly mature, tiny airborne particulate represents chemical freeloaders, materials that attached themselves to the original sulfate particle or whatever was released by combustion, Donahue explains. And, he adds, a very high proportion of the accumulated “goop” appears to stem from interactions between the growing particles, airborne oxidants, and organic vapors.

Those organics can come from anywhere and everywhere, he and his colleagues explained. From fossil-fuel combustion, from waxy compounds emitted by trees, from solvents, household cleansers, even an open can of motor oil.

The role of oxidized organics in reforming the chemical makeup of airborne particulates is relatively new, according to CMU’s Allen Robinson. Many of the first papers suggesting their role only emerged in the past two to three years, he says. The new data “flew in the face of the existing conceptual model” of what happens to particles — and added whole new levels of complexity, he says.

For instance, he notes, most toxicity studies that have exposed animals to particulates used fresh pollutants, motes unaltered by organic transformations. And this raises questions, he says, of how relevant the data from such studies are to the particulates we breathe — the ones that have developed a surface coating of goop.

The Environmental Protection Agency has been asking scientists to identify which source of particulates is most toxic, Robinson says; those are the pollutant particles the agency says it would like to focus on. But the chemical composition of a particle at birth may be irrelevant to what we ultimately breathe in, he says. If that’s true, regulators may simply be asking the wrong questions.

Indeed, if the chemistry of these particulates affects their toxicity, he says, it becomes increasingly important to probe the role of organic interactions in refashioning the particles. “Mechanistically, we have to find out why particles are bad for you,” he says. And this is a step toward answering that.

Here at CMU’s Pittsburgh facility, Robinson, Donahue, and their colleague Peter Adams — today’s luncheon hosts — are sharing laboratory space to understand goop science. We toured their facilities in the bowels of a nearly century-old engineering building where cleaned air is injected into 10-square-meter Teflon bags. Into these sacks, they add known quantities of ammonium sulfate, essentially starter cultures of particulates, together with various organic compounds. Then they briefly irradiate the room-size bags of air with ultraviolet light, to initiate reactions.

Samples of air are siphoned out of a bag every few minutes for the better part of a day and analyzed to characterize the changing nature of the particulates and the goop they acquire.

These are not the only scientists pushing the frontiers of goopology. But based on the collective pioneering efforts, we may one day breathe easier.