Environmental Transformations and Interactions
Aloft, Some Mysteries of Complex Chemistry
Researchers chase the nature of atmospheric aerosols, with EMSL experts and instruments as leverage
Swarup China, a research scientist at the Environmental Molecular Sciences Laboratory (EMSL), is an expert on some of the smallest things in the world that have the biggest impact.
Not microbes. Though they rate that distinction too. But atmospheric aerosols, the ultrafine liquid and solid particles that influence how clouds are formed and how much solar radiation arrives on the surface of the Earth.
Atmospheric aerosols help set the planet’s thermostat. They also prompt precipitation and steer the power of its storms. In addition, some atmospheric aerosols, breathable deep into the lungs, influence air quality and human health.
In recent years, China has studied aerosols emitted directly into the atmosphere from forested and agricultural landscapes. Aerosol-forming particles include dust, organics, and biological fragments from soil, plant matter, fungi, and bacteria. They make up a complex detritus swept into the air by wind and atmospheric vertical mixing.
EMSL Imaging Resources
In a 2016 paper, China led a study of how atmospheric disintegration of biological spores from fungi (large 2-10 micron particles) become sources of much smaller (less than 1 micron) aerosol particles above and below an Amazonian forest canopy. The highly variable chemical composition of these submicron particles is difficult to characterize and hard to distinguish from other particle types.
Investigation of the chemistry and composition of biological aerosol particles is the heart of China’s work at EMSL.
China and his coauthors conducted a follow-up study in 2018 published in the journal Nature Communications. They found that during the rainy season most cloud-forming sodium salt particles afloat above Amazon rainforests come from fungal spores, not (as long believed) distant ocean sources. To do the 2016 and 2018 work, as well as similar research that followed, China leveraged chemical-imaging resources at EMSL.
Prominent among his workhorse techniques is computer-controlled scanning electron microscopy (CCSEM) coupled with energy dispersive X-ray spectroscopy (EDX)―an approach called CCSEM/EDX. For the 2016 paper, he also used an environmental scanning electron microscope (ESEM). CCSEM/EDX determines the morphology, size, and elemental chemical composition of individual atmospheric particles―factors that can help reveal the sources of aerosols.
China also routinely employs transmission electron microscopy (TEM), which provides images of the internal structure of particles. TEM is especially useful for chemical imaging of smaller particles―those less than 200 nanometers wide. (One nanometer is one billionth of a meter.)
In addition, scanning TEM (STEM) coupled with electron energy loss spectroscopy (EELS) provides chemical bonding information. For example, probing the cell walls of fungal spores using STEM/EELS helps reveal the structural variability between different particles and their susceptibility to rupturing in an environment.
EMSL Users Bring Expertise, Too
China’s research involves far more than technologies and techniques. It also involves scientific outreach and engaging collaborative projects with many scientists outside EMSL. There are 766 worldwide.
In the 2016 and 2018 papers, along with many others, China has explored the chemistry of atmospheric particles in concert with long-time EMSL user Alexander Laskin, an analytical chemist at Purdue University in Indiana.
In 2016, they helped write a memorable paper in Nature Geoscience demonstrating that after a rainstorm as much as 60% of airborne soil organic particles are ejected into the air by raindrops. Before this, it was assumed that such particles entered the atmosphere only as the result of wind erosion or human activities like agricultural tilling.
The paper’s first author was Bingbing Wang, then an EMSL postdoc. He is now a professor at Xiamen University in China. EMSL-enabled scanning electron microscopy (SEM) images helped unlock the nature of these particles, which dry into near-spherical glassy shapes. At high altitudes, these solid glassy surfaces speed up ice formation in clouds.
“EMSL has a range of techniques, staff expertise, and chemical instrumentation that’s used in complementary ways to decipher aspects of individual particles,” said Laskin. “That’s the unique position of EMSL.”
Still, when asked what the most important thing EMSL brings to his research, Laskin has a quick answer: “Swarup China.”
A Burning Research Thread
The China-Laskin collaboration goes back to 2015, when China was a postdoc at EMSL and Laskin was there as a scientist. (His EMSL tenure spanned the years 1999 to 2017.)
Laskin said his work with China “is related to the grand scheme of aerosol chemistry and impact.”
That includes the work with China on biogenic aerosol sources as well as another thread of Laskin research: analyzing the particles emitted from wildfires and controlled agricultural burns, collectively known as biomass burning.
Two papers illustrate the burning-centric research thread. Both relied on measurements made at EMSL and both were a product of lead author and Laskin coauthor Peng Lin, who not long ago was a postdoc at EMSL. He is now a senior air pollution scientist with the California Environmental Protection Agency.
One paper inferred the molecular chemistry and light-absorbing properties of atmospheric brown carbon (BrC) during a biomass burning event in Israel. The other used mass spectrometry to characterize BrC in simulated wildfire smoke.
These studies led to Laskin getting an aerosols chemistry NSF-BSF grant in February 2021 to work with aerosol optical measurement experts in the Yinon Rudich research group at the Weizmann Institute of Science in Israel.
Laskin said the project will “rely on novel combinations of mass-spec analysis at his lab in Purdue and EMSL, complemented with advance broadband optical measurements.”
‘The Toughest’
At the root of both research threads, agricultural and biomass burning particles, lies what Laskin calls “the very complex puzzle” of aerosols.
In the atmosphere aerosols are highly dynamic, shapeshifting, and chemically reactive. Their multiphase chemistry―driven by sunlight, high surface area, and water content―continuously modifies the composition and properties of particles.
Add to all that the complexity of aerosol sources. The primary particles that contribute to aerosols come from the surface of the Earth as mineral-rich dust and sea spray; as soot and organic particles from power plants, combustion engines, wildfires, and other sources; and from fragments of plants and animals.
Others, called “secondary” particles, are formed by gas-particle conversion in the atmosphere itself.
Aerosols also come in a wide range of sizes―as small as a few nanometers across or tens of micrometers in diameter, so big in atmospheric science terms they are the dinosaurs of the aerosol particle world.
Investigating aerosol chemistry means probing reactions of a shifting soup of various particles, which change as they age (evolve) while suspended in the atmosphere. Aerosols float, suspended, because they have negligible terminal fall speeds; that is, they are too light to fall.
Among challenges within aerosols studies, aerosol chemistry “is the toughest,” said Allison Aiken, an aerosols scientist at Los Alamos National Laboratory in New Mexico, who follows Laskin’s work with China and others. “In Alex’s work, you have to approach the problem from (two) sides,” looking for molecular markers of aerosol chemistry changes or measuring bulk amounts of aerosols to look for chemical species.
Single-Particle Aerosol Research
At Purdue, Laskin and his team focus on the molecular scale. They pursue deeper understanding of aerosol chemistry by developing and applying new ways to image and analyze atmospheric particles.
Laskin, active in aerosol chemistry research for more than two decades, not long ago was lead author on a review article about chemical imaging advances. His two coauthors were air-quality analyst Ryan Moffet of Sonoma Technology, Inc. in California and Mary K. Gilles of Lawrence Berkeley National Laboratory (LBNL), who recently retired.
The LBNL connection illustrates another facet of the aerosol chemistry work and the nature of EMSL user dynamics as a whole: many players beyond EMSL are involved.
In the case of both Laskin’s biogenic aerosols research and his biomass burning work, EMSL’s imaging and chemistry-characterization research is augmented by resources at another U.S. Department of Energy (DOE) user facility: LBNL’s Advanced Light Source (ALS). Its synchrotron, the size of a football field, accelerates electrons through magnetic fields to create extremely bright light.
Laskin’s group is also a frequent user of a synchrotron-based instrument called an x-ray microscope, “which requires a unique source of light,” he said.
With ALS spectromicroscopy, scientists get x-ray absorption images from particles as small as a 20-nanometer pixel. This yields data on characterized elements and chemical species.
Aerosols from Ag
China is involved in the thread of Laskin’s research on the chemistry of aerosols that originate from open, non-urban settings that are not on fire.
This thread includes measurements of rainy-season fungal bursts in pristine regions of the Amazon basin, as noted above. It also includes intensive China-Laskin studies on the chemistry of the soil, plant matter, fungi, and bacteria that enter the atmosphere, piecemeal, from agricultural fields.
Farmland emissions involve some of the largest particles in the atmosphere, one of China’s specialties. Such bits of matter have short lives, tend to disintegrate easily, and carry rather indistinct chemical signatures.
“But a unique combination of imaging tools (at EMSL and ALS) allows for a reasonable separation of those particles from other stuff,” said Laskin. “Most of the time these technologies work together.”
ASOP Revisited
Two recent papers illustrate the efficacy of this joint-user facility research.
In October 2020, aided by Laskin and China, Lawrence Berkeley National Laboratory postdoc Matthew Fraund (advised by Ryan Moffet) led a study that looked at the optical properties and composition of high-viscosity organic particles collected from ambient air over the Southern Great Plains (SGP) atmospheric observatory in Oklahoma. SGP is operated by yet another DOE user facility, Atmospheric Radiation Measurement (ARM).
The subjects of study in the Fraund-Moffet paper are the same little-known airborne soil organic particles that figured in the China-Laskin paper on particles ejected from splashing raindrops.
This is a continuation of the investigation these particles begun then, said Laskin. “They had not been observed before. We realized we saw something unusual.”
The trip back to SGP focused on measuring optical properties. “This is not the paper that answered all the science questions we asked ourselves,” said Laskin. “Yet these particles exist at certain (rainy) periods of time. We are calling for additional work.”
Airborne soil organic particles are “solid and they help to make ice clouds,” said China. “Alex (Laskin) and I want to extend this work. We have a manuscript in progress, in collaboration with Daniel Knopf, another EMSL user.” Knopf is at Stony Brook University in New York.
The Fraund-Moffet paper confirmed the bubble-bursting mechanism for emissions that Laskin helped formulate a few years ago. As these bubbles burst, soil organics are injected into the atmosphere.
“Triggering new particles from the ground is completely counterintuitive,” said Laskin, “and I am also not convinced that it is important on a large scale. If you compare this with some major sources like biomass burning, it is tiny peanuts. But there are places where this source of particles might be the dominant one.”
Intensified Emissions
Another China-Laskin look at agricultural aerosol particles came in a November 2020 study led by Purdue PhD student Jay Tomlin, who is in the Laskin research group.
The paper combined two of Tomlin’s interests: getting data from instrumented aircraft and using chemical imaging and molecular characterization techniques to pry open the composition of real-world atmospheric particles.
He tapped EMSL for CCSEM/EDX analysis, which among other things was used to identify the relative atomic fractions of 15 elements. (China also provided his data analysis expertise). ALS assessed their share of field samples by using combined scanning transmission x-ray microscopy and near-edge x-ray absorption fine-structure spectroscopy (STXM-NEXAFS).
Tomlin’s paper takes a close look at primary biological atmospheric particles, distinguishing them from other components of real-world aerosols. These plant fragments, spores, pollen, and other debris are directly emitted into the atmosphere in agricultural settings. Seasonal harvesting intensifies these emissions, which so far remain poorly investigated. Existing observations are scant and current measurement techniques provide little more than ambiguities.
One challenge is that in a setting like this, “you don’t know where the (particles) are coming from,” said China. “You see just these fragments.”
Questions and Needed Answers
While Indiana soybean and corn fields were being harvested, a twin-engine Purdue based aircraft managed by Paul Shepson, now at Stony Brook, collected downwind dust plume samples with a time-resolved aerosol collector (TRAC) of the kind EMSL has too.
At EMSL, the aircraft-collected samples underwent CCSEM-EDX analysis to identify main particle types based on the analysis of around 39,000 individual particles. At ALS, researchers determined the mixing state of carbon within particle-type classes by using complementary STXM-NEXAFS analysis.
Most atmospheric particles contain carbon, “and we always ask the questions: What are the molecular differences in their carbon content and where do certain types of particles come from?” said Laskin. “Unambiguous detection of biological fragment particles is a very challenging task for analytical chemistry. It is better than before, but we still need quite bit of work.”
Such particles typically remain undescribed, said Laskin, and the “ambitious” Tomlin study is “raising more questions than providing answers.”
Still, it’s a newly detailed look at the kind of biological atmospheric particles swept into the air by agriculture, he added. “We hope there will be new studies by other groups.”