New particle formation, aerosol aging, and a long list of related issues captivate a changing cast of collaborative experts
Suspended in the atmosphere, tiny particles of liquids and solids work to make life on Earth possible.
These particles, called aerosols, can be as tiny as 2 nanometers wide, about the diameter of a strand of DNA. Others are ten thousand times bigger.
Big and small, taken together, they make clouds and precipitation possible.
Aerosols are the seeds upon which water rides in the planet’s vast water cycle. When numerous enough, they form clouds. When heavy enough, aerosols form water droplets that fall to Earth as rain or snow.
Across the world, scientists study the origin, shape, chemistry, and evolution of aerosols. Being scientists, they form communities of interest. One such community is the Aerosol Working Group within the Atmospheric System Research (ASR) program at the U.S. Department of Energy (DOE).
To join ASR’s Aerosol Working Group, create an account with ARM. Then, under “Subscriptions,” check “Aerosol Processes” to receive its regular newsletter.
One of the group’s two co-chairs, Markus Petters, an atmospheric scientist at the University of California, Riverside, has a caution: There is no set list of members.
”Anyone interested in aerosol science is welcome to join,” he says, with two co-chairs acting as facilitators.
“We (the chairs) are not that important,” except as scientists who keep the wheels of conversation going, agrees co-chair Nicole Riemer, a University of Illinois at Urbana-Champaign professor.
Petters studies the physical and chemical properties of aerosols. Riemer uses surface and satellite observations to create computer simulations of how aerosols behave in the atmosphere.
Some of the surface data Riemer and others use come from Atmospheric Radiation Measurement (ARM), a user facility in the DOE’s Office of Science. ARM operates six observatories across the world; three are fixed locations, three are mobile facilities. The ARM Data Center has archived more than seven petabytes of remote-sensor measurements related to aerosols, clouds, precipitation, and other atmospheric properties.
The Main Task
The ASR group operates in concert with ARM’s own Aerosol Measurement Science Group (AMSG). (Petters and Riemer are ex officio members.)
The AMSG “is really a different group,” says Petters, with a focus on strategic planning for ARM’s aerosol measurement needs and challenges.
The ARM group, for one, creates periodic aerosol science planning documents. The ASR group does not.
In the ASR space, Petters and Riemer provide feedback to the AMSG regarding aerosol measurements of special value to the modeling community.
The ASR mission is to support observational research that will ultimately help inform and improve climate models.
Petters and Riemer enable ASR-related aerosol conversations that they in turn share with DOE managers and principal investigators (PIs). But they also oversee one main annual task: organizing an aerosol working group session and aerosol-related breakout sessions at the Joint ARM User Facility and ASR PI Meeting. (The last such meeting was in August 2023. The next is scheduled for March 3 to 6, 2025, in Rockville, Maryland.)
In the context of the annual PI meeting, Petters says, “We try to facilitate connections and discussions to solve the largest science questions.”
Those discussions take place among AMSG members, ARM and ASR PIs, and ARM translators──the experts who create value-added products (VAPs) to enhance the utility of data to modelers.
There are many large aerosol science questions, says Petters. One of them, for example, is the ongoing puzzle of how aerosols “flux” into the atmosphere, pulsing skyward to play their critical roles.
A Session Template
At the annual ARM/ASR meeting, the aerosol processes working group session Petters and Riemer oversee usually follows a common template, beginning with topical presentations. After that, participants have just one minute each to present on their current research: “Very fast-paced and quite entertaining for all parties involved,” says Riemer.
In 2023, to give a sense of things, the topical presentations started with John Shilling, an atmospheric chemist at Pacific Northwest National Laboratory. He introduced the ARM translator group he is a part of and presented an update on VAPs related to aerosols.
During the same session, Adam Theisen and Maxwell Grover of Argonne National Laboratory joined Giri Prakash of Oak Ridge National Laboratory to outline a series of topics in open science for the aerosol community.
Such annual working group sessions wrap up with a presentation by the AMSG chair. They also look back and look ahead at ARM field campaigns that yielded (or promise to yield) consequential aerosol data.
In 2023, for instance, presenters discussed two recent campaigns: the Tracking Aerosol Convection interactions ExpeRiment (TRACER) in and around Houston, Texas, and the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in coastal Southern California.
That same year, working group presenters looked ahead by discussing aerosol plans for the Clouds, Aerosol, and Precipitation Experiment at kennaook Cape Grim (CAPE-k) and the ARM mobile facility deployment at what is now known as Bankhead National Forest (BNF).
Regarding future campaigns, “the organizers look for input,” says Riemer, recalling the 2023 session. “Chongai (Kuang, the BNF PI and aerosol processes lead) really wanted to hear from the working group.”
One Possible Agenda
What to discuss about a coming campaign at any annual aerosol working group session “is a long process in the making,” says Riemer.
Anticipated events like CAPE-k or BNF may get a spot on the agenda for a couple of years running.
For the 2025 ARM/ASR meeting, there is no working group agenda yet. However, Petters and Riemer speculate there will be short updates on CAPE-k (still underway) and EPCAPE (now in an intense data-analysis phase).
Also likely, they say, will be an early look at BNF data, as well as presentations on two upcoming ARM field campaigns with important implications for aerosol science.
One is slated to begin collecting data in December 2024: the Coast-Urban-Rural Atmospheric Gradient Experiment (CoURAGE) in and around Baltimore, Maryland. Led by Kenneth Davis of Pennsylvania State University, CoURAGE will investigate aerosol patterns and sources in this urban-rural-coastal environment through November 2025.
Further in the future is ARM’s Desert-Urban SysTem IntegratEd Atmospheric Monsoon (DUSTIEAIM), which will span April 2026 to September 2027 in the Phoenix, Arizona, area. The lead scientist is Allison Aiken of Los Alamos National Laboratory in New Mexico, who plans to study how converging desert and urban atmospheric environments influence storm activity and precipitation. So far, she has assembled a team of 19 co-investigators.
DUSTIEAIM will fill in urban-rural atmospheric knowledge gaps along the Flagstaff-Phoenix-Tucson corridor by operating concurrently with the Southwest Urban Integrated Field Laboratory (SW-IFL).
SW-IFL is one of four DOE Urban Integrated Field Laboratory (UIFL) projects underway since 2023.
Remote Places, Too
The structure of DUSTIEAIM “is fairly mature,” says Petters, but every new ARM deployment raises questions the ASR aerosol working group can help with. “What should we sample? How often? What is needed to solve the larger problem?”
Matters such as a renewed interest in urban climates, he says, relate to “forces within DOE,” while working groups address finer-grained science questions.
Clearly, urban places are important, says Petters. “That’s where people live, and human-caused emissions are large.”
At the same time, says Riemer, in the aerosol science community there is still a strong emphasis in non-urban areas.
At one edge of the Southern Ocean, she says, CAPE-k data will add to what we know about cloud and ice formation in pristine environments, where aerosols are scarce. And ARM’s fixed observatory in the Azores, Eastern North Atlantic (ENA), is a window into another remote environment.
On a grand scale, the impact on climate is manifesting itself in cloud decks in both the Pacific and Southern oceans, says Petters. “Remote places will always be of interest. Aerosols there will have an impact on the earth system.”
Four Themes
Four main themes “that appear again and again,” says Riemer, are of broad concern to the ASR aerosol working group.
One is understanding the radiative effects of absorbing aerosols, which also reflect or scatter incoming solar energy.
Another is studying secondary organic aerosols (SOA): how they work, how they can be measured, and how they can be modeled. Such aerosols are formed when organic particles are transformed in the atmosphere via oxidation with volatile organic compounds, including isoprene from trees. In urban and remote areas, SOAs account for a high percentage of fine particles, so they are important to know more about.
A third big theme, says Riemer, is new particle formation, also called atmospheric nucleation. Such particles generate 50% of the seeds that form cloud droplets, but how are they born? How does nucleation work? In models that simulate a changing climate, the chemistry and physical properties of new particles remain a source of uncertainty. A fourth theme is the question of what happens to aerosols during their lifetimes in the atmosphere.
How do they age? That’s a term for the transformation aerosols undergo as they are transported through the atmosphere from their place of origin. Those origins might be as diverse as a wildfire, a burst of fungal spores during a rainstorm, and many more.
How do aerosols evolve to mix with others? How do their properties change? How do changes during aerosol life cycles impact the climate?
And what is the “removal” process? Riemer asks, a term for how aerosols disappear from the atmosphere. “We need to understand that as well.”
Step by Step
The ASR working group is one feature of the cumulative nature of progress in aerosol science, says Petters, which is rarely the result of “single revolutionary papers. It’s about providing robust data sets and showing in a wide scale of environments that these effects hold. So, it’s about statistics.”
In part, these statistics owe a lot to ARM, he adds, “which has excelled at providing long-term data sets” from its observatories in arctic, continental, and marine environments.
Along the way, these data and other data from long-term field campaigns provide rare looks at full annual cycles of aerosol behavior.
One example is an archive of vast new aerosol data sets from the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, a 13-month, 20-nation investigation of the Arctic’s atmosphere, snow, ice, and ecology that began in September 2019.
Work by MOSAiC fieldwork veteran Jessie Creamean, a research scientist at Colorado State University, is just one example of what annual cycles of aerosol behavior can yield. For example, she led a 2022 ARM- and ASR-supported study on the annual cycle of central Arctic INPs.
On behalf of ARM, Creamean also co-leads efforts to collect and analyze samples of ice nucleating particles (INPs).
‘We Keep Adding Knowledge’
Acquiring long-term aerosol statistics may not be glamorous, says Petters, but in the last 20 years, “we keep adding knowledge. We have a much deeper process-level understanding.”
That deeper understanding, for example, means knowing more about the sensitivity of clouds to greenhouse gases, the chemistry of aerosols, their vertical distribution in the atmosphere, aerosol hygroscopicity (how readily they attract water), and cloud condensation nuclei (CCN).
Water clings to CCN, making it possible for clouds to form.
In turn, a greater understanding of aerosol processes adds to the kind of insights needed to make better earth system models.
Knowing more comes with its own challenges, however, says Petters, including “exploding data streams and the increased complexity of analysis.”
Those challenges require the right people to confront them. The ASR aerosol working group exemplifies what is needed: a cast of multidisciplinary experts.
“Aerosols are diverse,” says Riemer. “So is our group.”
# # #Author: Corydon Ireland, Staff Writer, Pacific Northwest National Laboratory
This work was supported by the U.S. Department of Energy’s Office of Science, through the Biological and Environmental Research program as part of the Atmospheric System Research program.