A senior research duo at Colorado State University will collect and analyze ice-nucleating particles
In early January 2020, the Atmospheric Radiation Measurement (ARM) user facility put out a call for an expert.
ARM was looking for a lead mentor―a designated expert―to manage an expanded effort to collect and analyze samples of ice-nucleating particles (INPs).
INPs are part of a diverse cast of tiny particles swept up from earth and ocean surfaces. They are made of inorganics like mineral dust, soil organic matter, leaf bacteria and fungi, fragments of lichen, marine microbes, and other organic matter. They are tied to cloud formation. They are relatively sparse, are rarely observed, and have a big influence on the reflectivity and lifetimes of clouds.
Despite their importance to the water cycle, INPs are extremely rare, making up a minute proportion of all particles in the atmosphere. Measuring the activity distribution of INPs―the number and type per liter of air active at different temperatures―is a specialized skill. Hence, the call for help from ARM, a U.S. Department of Energy (DOE) scientific user facility.
In the spring of 2020, ARM hired two INP experts instead of one: Jessie Creamean, an atmospheric chemist who specializes in polar-region aerosols and Tom Hill, an environmental microbiologist whose main research interest is INPs of biological origin. Both are veteran atmospheric researchers at Colorado State University.
“Combined, we have a gamut of expertise with the DOE community and field deployments, INP sampling and measurements, including using tethered balloons, and sample processing using our ice spectrometer,” says Creamean, a veteran of many cold-weather field campaigns. “Additionally, the measurement involves field collection, offline sample analysis, and data analysis, which is quite time-consuming. It made sense to have two of us.”
A university technician will help too, she adds.
Clouds are made of micron-size water droplets (up to 100 million in each cubic meter).
In some clouds, especially in polar regions, these droplets are “supercooled”―that is, not frozen despite freezing temperatures from 0 to minus 38 degrees Celsius (32 to minus 36 degrees Fahrenheit).
The droplets enclose tiny hydrophilic (water-loving) particles, which collectively cause condensation. But the droplets are typically the size of fungal spores and are too small to fall by themselves. Each is less than a millionth the mass of a raindrop.
To initiate such falling, and the particle aggregation that leads to rain and snow, the particles within cloud droplets need to enable the droplets to become ice crystals. If a droplet contains an INP, and it freezes, the water vapor—gaseous water molecules—in the cloud piles onto it. The droplet then rapidly grows into a tiny hailstone or snowflake that becomes heavy enough to start falling.
“INPs represent a very small fraction of the total aerosol population,” says ARM Technical Director Jim Mather. “Few aerosol particles are effective INPs―and the physical characteristics of particles that make them good INPs are not well understood.”
Not knowing enough about INP distribution and characteristics, he adds, “leaves a big hole in our understanding.”
“Clouds are a crucial component of Earth’s energy budget and precipitation processes. However, their formation mechanisms are arguably some of the least understood atmospheric processes, especially processes involving aerosol impacts on cloud ice formation.”
“Clouds are a crucial component of Earth’s energy budget and precipitation processes,” she says. “However, their formation mechanisms are arguably some of the least understood atmospheric processes, especially processes involving aerosol impacts on cloud ice formation.”
Another difficulty, says Hill, is the rarity of INPs in the atmosphere, as well as their “diverse range of sources, which we are still trying to understand and identify.”
Two Is One
Creamean and Hill represent the newest mentorship to ARM.
“With the demand for INP measurements, I don’t foresee this going away anytime soon,” says ARM Instrument Operations Manager Adam Theisen, who is based at Argonne National Laboratory in Illinois.
Until now, ARM has relied on requests to prompt periodic collection and analysis of INP distribution samples. Bringing on Creamean and Hill to do it in a coordinated and regular way is a new approach.
“ARM routinely gets requests for INP measurements at the ARM observatories, and we have contracted that out on an as-needed basis,” says Theisen. “However, these requests have become common. ARM decided to create the mentorship to have a more coordinated approach to the collection and sampling of INP.”
“ARM decided to create the mentorship to have a more coordinated approach to the collection and sampling of INP.”
ARM collects data—most of them continuous—from more than 400 instrument systems in three fixed and three mobile observatories across the world.
The new co-led mentoring role, says Theisen, will “open the door for long-term seasonal measurements at (ARM’s) fixed observatories,” including its flagship Southern Great Plains (SGP) atmospheric observatory, which stretches over parts of storm-wracked Oklahoma and Kansas. In the first year of analysis, half the aerosol samples will come from the SGP.
The goal of the INP measurements will be to establish a long-term record of the baseline levels and variations of INPs at the SGP, says Creamean, “as well as provide more spatial and temporal coverage during intensive studies or at unique locations.”
The Long View
INP measuring is a capability that can be deployed at other ARM locations, says Mather.
“There are opportunities for sampling at all the ARM sites” and other field campaigns, agrees Theisen, including the TRacking Aerosol Convection interactions ExpeRiment (TRACER), beginning in April 2021 in the Houston, Texas, area. “Jessie and Tom are putting together a measurement strategy for the upcoming years which will detail the complete plan.”
TRACER, a study of deep convection in an ocean-shore urban environment, represents an interesting way to expand ARM’s INP investigations, says Mather. “With the strong impacts of local dynamics, it is less obvious how INPs might control the evolution of a convective cell, but it seems likely that they play some role.”
First, an Arctic Spin
For the time being, however, Creamean and Hill will focus on collecting and processing about 50 samples from instruments at Oliktok Point, Alaska, where the third ARM Mobile Facility (AMF3) is in place. Samples from Oliktok will make up the other half collected in the first year of analysis.
The samples “will span all seasons, including the winter,” says Creamean. “Wintertime arctic INP measurements are incredibly rare, so we hope these data will shed light on how many and which types of INPs are present during the frigid and dark arctic winters.”
Creamean is a veteran observationalist who specializes in arctic regions. Starting in the fall of 2019, funded by DOE’s Atmospheric System Research, she spent four months out on the ice as part of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC), an international expedition partly underwritten by DOE. She took along with her suitcase-size aerosol samplers from Colorado State.
AMF3 is in its last year of operations at Oliktok before ARM moves the facility to the Southeastern United States. Just to the west of Oliktok, at the edge of the Arctic Ocean, is ARM’s North Slope of Alaska site in Utqiaġvik (formerly Barrow).
“We want to take advantage of the DOE resources at a unique arctic oil field site before it moves,” says Creamean.
“There is particular interest in these measurements in polar regions,” says Mather, “and we know that the aerosol properties are quite different at Oliktok and Utqiaġvik. Ultimately, we want to get measurements at both locations, but we only have a little time.”
Disks and Drops and Membranes
ARM’s choice of Creamean and Hill made sense in another way that pinpointed expertise: The INP analysis technique they have developed over the last few years is by now efficient and productive.
So smooth is their collaborative analytic process, says Hill, “that it’s like polishing a table.”
He calls sampling INPs “at heart, very straightforward,” beginning with filtering aerosol samples though a membrane with pores that are only 0.2 micrometers in diameter. That’s small enough, says Hill, to trap most particles, as well as the bacteria, viruses, and minerals that are the scaffolds of ice nucleation.
The membrane filters, he adds, look like 2-inch-wide disks cut out of plastic shopping bags.
Filtering lasts for one to two days, says Hill, “then we shake the membrane in pure water to make a suspension of the particles.”
After that, from that suspension, they make an array of small aliquots (“portions of a whole,” in the idiom of chemistry), each about the size of a drop from a water tap.
The aliquots are then cooled until they freeze. These portions are not pure water, so they freeze at temperatures warmer than minus 38 C (minus 36 F).
From there, “by measuring the number of frozen wells at each temperature, we can easily calculate the numbers of INPs per liter of air,” says Hill. “The skill comes in sampling and working cleanly. It’s a bit like a medical lab, and we have to work extremely cleanly.”
INPs may be rare in the atmosphere, he adds, “but they are abundant―sometimes alarmingly so―in the dust that covers everything in the field and lab.”
At the rate of only two to four samples a day, the analysis is “fairly time-consuming,” says Hill―but even more time will be spent “carefully preparing filters and other sampling supplies for continuous annual deployments, and preparing the data for the community to use.”# # #
ARM is a DOE Office of Science user facility operated by nine DOE national laboratories.