Nature Geoscience article shows atmospheric organic particulate matter can be liquid as well as solid

A scanning cloud radar was one of the instruments taking measurements during GoAmazon 2014/2015.
A scanning cloud radar was one of the instruments taking measurements during GoAmazon 2014/2015.

When scientists participating in the GoAmazon 2014/2015 experiment measured the physical state of aerosols drifting over the Amazon rain forest, they found that 80 percent of the time those particles were liquid. Their findings were a surprising departure from the results of a previous study of the boreal (pine) forest in Finland when scientists determined that airborne particulates were in a solid or semisolid state.

The study in Finland "turned one of the fundamental assumptions about organic liquids on its head," according to Scot Martin, Harvard professor of environmental chemistry and lead scientist for the GoAmazon research campaign, "because before that hardly anyone thought of these organic particles as solids."

The new findings published in Nature Geoscience on December 7, 2015, swing the scientific pendulum, Martin points out. Knowing that atmospheric organic particulate matter can be liquid as well as solid will help scientists enhance the accuracy of global climate models—because the physical state of atmospheric particulate matter affects particle growth, size and reactivity, as well as population size and composition.

The measurements were taken during GoAmazon, a collaborative project of the U.S. Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility and Atmospheric System Research (ASR) Program with Brazilian organizations and other partners. A strong international collaboration was behind the success of the research, involving Amazonas State University, University of São Paulo, and the National Institute of Amazonian Research, all coordinated through the Brazil Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA).

Size and Distribution

The ARM Mobile Facility aerosol container on the left was a part of the GoAmazon research. The container on the right was the data system container with the W-band radar on top of it.
The ARM Mobile Facility aerosol container on the left was a part of the GoAmazon research. The container on the right was the data system container with the W-band radar on top of it.

The properties and size distribution of particles, whether natural or manmade, influence the Earth’s "energy balance" or the amount of solar radiation reaching the Earth by either directly altering or absorbing sunlight or indirectly affecting cloud formation. Together, those factors can alter the Earth’s temperature and climate over time. The Amazon study focused on particles less than a micrometer in size, starting as small as 50 nanometers (nm) in diameter.

In Finland's boreal forest, alpha-pinene produced by pine trees is the primary precursor to organic aerosol. It reacts with other substances such as ozone or hydroxyl radicals to produce atmospheric organic particulate matter, much the same as isoprene does in Amazonia, which is the primary substance produced and emitted by trees and plants in the Amazon rainforest forest canopy.

Organic particulate matter serves as the main seeds of cloud formation in Amazonia under background conditions. Knowing the size of particles in the atmosphere helps to predict how they will scatter sunlight and affect the attributes of the clouds they form.

"Clouds behave differently, depending on whether they form on populations of solid or liquid particles," Martin explains. "Whether the particles are solid or liquid determines how they can best be treated within climate models, meaning what kinds of parameterizations can be valid.”

The study is interesting because it highlights the importance of Earth’s different biomes. The humidity of a boreal forest is lower (about 30 percent) than that of the Amazon forest (about 80 percent).

Specific Local Conditions

One takeaway for climate modelers is the importance of taking into account specific local conditions before running a simulation, according to Adam Bateman, a Harvard research associate and first author of the study.

"One thing we found with our study versus the study in Finland is the regionality of liquid and dry particles," he explains. "It depends on where you are, the kind of biome (ecological community) you have, and what the trees are emitting. Pine trees in much drier conditions mean semi-solid particles and modeling will be different in that case."

"How particles grow depends on whether they are liquid or solid," Bateman continues.  "If particles are semi-solid, conditions can favor new particle formation rather than grow larger, while liquid particles can keep growing and eventually rain out. That has a lot of implications for cloud formation and lifetime."

Once scientists know the kinds of particulates and whether they are in a liquid or solid state, Bateman says, the next step is to adjust climate models accordingly.

"There was a question in the literature about liquid or solid particulates," he points out. "The fact that they are liquid in the Amazon under background conditions is compelling evidence and there are implications about how to simulate them in a global model. We offer some options modelers can use to factor particles in the liquid state into their models."

"Large liquid particles tend to suppress the growth of smaller ones, so there tends to be fewer but larger liquid particles in the Amazon as compared to a boreal forest," according to Rahul Zaveri, a scientist at the Department of Energy’s Pacific Northwest National Laboratory and contributor to the study.
"Rain formation is more efficient when you have fewer particles nucleating to become relatively larger cloud droplets. In comparison, if you have more particles nucleating to become cloud droplets, each droplet will tend to be smaller as the available water will distribute across all of them and rainfall formation will be less efficient and therefore less likely to occur."

The Amazon study highlights why it is important for climate scientists to consider their location on the Earth before employing models to predict climate change.

Martin agrees, "Why is liquid or solid important? What does this mean for our models and how do we update them? It's about where the particles come from, how big they will get, their final sizes, and what kinds of clouds they will form. That touches on all the topics of climate change that we need to think about, whether particulates are organic or not."

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The ARM Climate Research Facility is a national scientific user facility funded through the U.S. Department of Energy's Office of Science. The ARM Facility is operated by nine Department of Energy national laboratories.