Climate: Discovering the role of aerosols

BOULDER, Colo., Dec. 12 (UPI) -- One of the mystery villains in global climate change is the role of aerosols -- dust and small particle emissions -- on climate and temperature.

In her book "The Secret Life of Dust," science journalist Hannah Holmes wrote that between 1 billion and 3 billion tons of desert dust fly into the sky annually, 3.5 billion tons of salt flecks rise off the ocean and one-third billion tons of organic chemicals are exhaled from trees. Burning trees and grass provide 6 million tons of soot, while fossil fuel burning gives off 100 million tons of sulfur, 100 million tons of nitrogen oxides and 8 million tons of black soot.

"Picture a juice glass sitting on a porch railing in the sunshine," Holmes wrote. "It may look empty, but churning inside that glass are 25,000 microscopic pieces of dust -- at least."

In official science-speak, dust and other small atmospheric particles, such as fossil fuel-generated sulfur and black carbon, are called aerosols, which have an important but largely unquantified effect on climate. They can have either a heating or a cooling effect, depending on factors like size and chemical composition. They also affect the formation of clouds and that affects heat retention in the atmosphere.

"In terms of the chemistry, for the future, both of those sources (black carbon and organic carbon) are more important than we had previously thought, since they tend to stick around longer," Lynn Russell, associate professor at the Scripps Institution of Oceanography at the University of California-San Diego, told United Press International.

Russell and colleagues authored a paper in the journal Science last week that said aerosols may have a slightly greater cooling effect than climate models previously have estimated.

Another paper, in "Geophysical Research Letters," showed while aerosols can have both warming and cooling effects, the effects may be regional, depending on the surface area below -- whether forest, croplands or grasslands.

"The big issue with aerosols has been warming or cooling, but we know that climate is more than temperature change," said Dev Niyogi, North Carolina State University assistant professor. "Hydrology is, for instance ... the biogeochemical cycle leading to how the land use is affected. Those are dynamical features of the changing climate.

"We know that aerosols are partly responsible for holding back the warming. Aerosols have to some degree contributed to cooling," Niyogi told UPI.

The Science article found climate models have estimated an oxidation rate equivalent to 60 percent of organic particle mass per day. Previously, Russell said, data on this oxidation rate have not been available.

Russell and colleagues looked at four different aerosol samples that varied in size and composition. They included African mineral dust, Asian mixed combustion, and U.S. combustion on both foggy and clear days. The researchers found the oxidation rate varied between 13 percent for the African dust and 24 percent for the Asian sample, considerably less than the 60 percent estimated in most models.

What this means is the particles last two or three times longer in the atmosphere, working their magic on the climate. The precise impact depends on the size and composition of the particle. Black carbon, for instance, largely a byproduct of industrial processes, has a powerful direct warming impact.

Other forms have a slight cooling impact, which can be either direct, by absorbing light before it reaches the surface, or indirect, by influencing cloud formation.

A slower oxidation rate means organic aerosols will less readily absorb moisture, which will result in reflecting more radiation in its increased atmospheric lifetime. Overall, Russell told UPI, the net impact is a cooling effect of 0.5 watts per meter square.

Future effects of aerosols will depend upon which emissions increase faster -- black carbon or other organics.

"Both black carbon and organic carbon will increase," Russell told UPI. "You will have some offset of the effects. Different models predict different things about the way the net effect will go, and it depends on what control strategies are adopted. Most of those control strategies are in flux."

Niyogi found changes in levels of airborne aerosols change the terrestrial carbon cycle -- the way in which carbon dioxide is absorbed by plant photosynthesis and emitted by soil. The research showed the type of landscape is important in understanding this effect.

"In the research project, six locations across the United States -- encompassing forests, croplands and grasslands -- were studied," according to the paper. "Increased amounts of aerosols over forests and croplands resulted in surface areas below becoming carbon sinks (they absorbed more CO2 than they emitted) but increased amounts of aerosols over grasslands resulted in surface areas becoming carbon sources."

Niyogi said: "We found aerosols do have a very strong effect on the surface, a net ecosystem effect on CO2. This effect is more important than that due to clouds alone."

The picture remains very complex, Niyogi said, but added, "We do find that aerosols can have a significant contribution.

"With more aerosols in the environment, we definitely are going to have an altered carbon balance," he said. "Depending on whether they are sulfate or carbon aerosols, it could have a radiative or temperature feedback, and a third factor is soil moisture. We don't know what happens when the climate is drought-prone, for instance."

He said the processes being identified are important science issues and how they are included in policies and models "is a whole new level of application and debate."

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Climate is a weekly series by UPI examining the latest findings and potential impact of global climate change. E-mail sciencemail@upi.com.