It’s been a smoky summer in New Mexico and all around the West. Huge wildfires in the Pacific Northwest and Montana were in the news for weeks, spreading a fine haze across the Southwest, raising the obvious question: What’s in that smoke? A big fire gives off a big plume made up of a complicated and constantly changing mix of particles, gases, and other chemical compounds. Explosions — including nefarious detonations — can emit similar plumes.

As this smoke lifts into the atmosphere and spreads downwind, it can change air temperatures over large regions and make people uncomfortable or sick. Health alerts are one obvious outcome. We all know it doesn’t feel good to breathe smoke that contains fine particles. Bigger particles can irritate your throat and reach the lungs, for instance, causing respiratory problems. The smallest particles can even cross the protective blood brain barrier to interfere with brain function.

The threat from these environmental and health problems depends on the plume’s composition. The exact makeup of that smoke also provides forensic clues to the source and cause of an illicit explosion, so there are plenty of good reasons for learning more about smoke.



We still have a lot to learn, but this much we know: In broad terms, soot, aerosols, and thousands of chemical compounds make up a typical wildfire plume. Aerosols are fine particles or droplets of a substance suspended in the atmosphere. Fog, haze and smoke are familiar aerosols. As time passes and the particles interact with sunlight, they age, growing, shrinking and chemically changing into different compounds.

When a fire begins — the flaming phase — it pumps out black-carbon soot, particles of incompletely burned organic matter that don’t vaporize. The later smoldering phase churns out white smoke, fine particles lofted by the hot, buoyant plume high into the atmosphere, where winds can carry them over continental distances. The black soot particles absorb sunlight, which creates a warming effect locally, regionally, and even globally sometimes. Smoke, on the other hand, reflects sunlight, which creates a cooling effect.

To discover the finer points of smoke’s composition, Los Alamos National Laboratory has launched the Center for Aerosol Forensic Experiments — CAFE, for short. The lab has a long history of researching the atmosphere, work that stems from its primary mission as a national nuclear security laboratory and its role in monitoring nuclear activity around the globe. This new suite of instruments, which centers on an aerosol mass spectrometer, provides detailed information on the chemistry of wildfire smoke particles. A mass spectrometer takes in samples, gives them an electric charge, then sorts them by the ratio of their electric charge to their mass. It’s a powerful means of identifying the elements and compounds in a complicated sample like smoke.

With this capability, CAFE can tell a lot about fires, teasing apart their various particle emissions to reveal how those particles are aging and changing in the atmosphere. That also makes the instruments useful for the detective work of explosives forensics — since each detonation (or fire) has its own chemical signature, CAFE can identify the explosives or propellants that blew up.

Most explosives and propellants, such as TNT and RDX, contain nitrogen mixed with binders that burn. The explosive might include charged particles of iron, aluminum or even lead. After a detonation, CAFE analyzes these components in soot and looks at the ratio of carbon to nitrogen to determine what exploded. It can even determine how powerful the explosion was. Of particular importance to national security, CAFE can detect dangerous nuclear material in the detonation of a “dirty” bomb of conventional explosives.

During the lab team’s initial research with CAFE this fall, scientists sampled smoke from the Pacific Northwest fires in real time for a week. They discovered that particles in this three-day old “aged” plume were highly oxidized and characterized by carbon compounds that tend not to vaporize and do not readily take up water. Being less volatile and less likely to be scavenged by raindrops, these aerosols are thus able to survive longer in the atmosphere. As the plume lingers, it is more likely to affect regional or even global air quality, temperatures and precipitation.

In contrast to the aged Pacific Northwest plume, samples taken from a late-September controlled burn in the Jemez Mountains show chemistry more typical of fresh emissions: they contain more hydrogen and less oxygen, and they are more susceptible to evaporation. The team is looking deeper into other chemical and physical signatures in the plume to better understand what happens during biomass burning — forest fire, in this case, but applicable to other combustion sources.

This early work proves the potential for CAFE to bring a new understanding about how smoke and aerosols behave. CAFE will lead to more nuanced models of the atmosphere and earth systems. CAFE also adds yet another tool to the Los Alamos national security toolbox by providing a new forensic capability for sleuthing out the origins and nature of explosions and detonations, which can have major impact on national security and international treaty monitoring. Closer to home, CAFE can help keep an eye on local skies, giving New Mexicans a clearer picture of the fire season’s hazy days.

Manvendra Dubey of the laboratory’s Earth and Environmental Sciences division is a recently named Los Alamos National Laboratory Fellow. He is internationally recognized for his research, which has changed the science community’s understanding of aerosol impacts on planetary temperatures. The CAFE team includes aerosol chemistry expert Allison Aiken of the laboratory’s Earth System Observations group. In 2014, she was named one of “The World’s Most Influential Scientific Minds” by Thomson Reuters.

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