Acidic aerosols greatly increase secondary aerosol formation

By DAVID WILLIAMSON

UNC News Services

CHAPEL HILL — Atmospheric particles that become acidic through exposure to such pollutants as sulfuric acid can lead to vast increases in the formation of secondary organic aerosols, a new study indicates. Such aerosols are major components of the unsightly haze that hangs over cities and oil refineries and even affects otherwise pristine U.S. national parks.

A report on the research appears in Friday’s (Oct. 25) issue of the journal Science. Authors, all at the University of North Carolina at Chapel Hill, are Dr. Myoseon Jang, research associate; doctoral students Nadine M. Czoschke and Sangdon Lee; and Richard M. Kamens, professor of environmental sciences and engineering at the UNC School of Public Health.

“We think this exciting work is potentially very important and so do other scientists we have discussed it with across the United States,” said Kamens. “What Dr. Jang has done in our laboratory was to discover an acid-catalyzed process that brings about secondary organic aerosol formation.

“She also has found that this under-appreciated reaction may generate five to 10 times more aerosol in the atmosphere than we previously thought,” he said. “It appears to explain a number of different kinds of phenomena that lead to aerosol formation.”
Jang’s “ground-breaking” new research involves testing aerosols in reaction chambers and large outdoor smog chambers and determining what happens to them under varying experimental conditions, Kamens said.

In the new work, scientists introduced fine inert particles known as seed aerosols into Teflon film reaction chambers, he said. Into some chambers they injected identical particles coated with 2 percent to 5 percent sulfuric acid, which is about the same level found on tiny bits of floating diesel soot.

“What they did then was to introduce into the gas phase atmosphere of the chambers aldehydes and alcohols,” Kamens said. “Dr. Jang found that when the aldehydes and alcohols were present, there was a huge increase in the amount of aerosol that formed.”

Studies with a variety of different aldehydes, which are formed in the atmosphere by oxidation of emitted hydrocarbons, revealed that some aldehydes derived from aromatic compounds were far more reactive in producing aerosols than scientists believed. Aromatic compounds come largely from automobile and other exhausts, while trees generate massive amounts of terpenoid hydrocarbons, which also form aldehydes and particles in the atmosphere subject to similar acid-catalyzed aerosol-producing reactions.

Jang’s discovery appears to fill an important hole in scientists’ understanding of atmospheric chemistry, Kamens said. Her data also mirrors natural data collected by a Rutgers University team in the Appalachians’ Smoky Mountains under the direction of Dr. Barbara Turpin.

“People from NOAA — the National Oceanic and Atmospheric Administration — got very excited about this work at a recent aerosol research meeting in Charlotte,” he said. “That was because it seems to explain atmospheric reactions going on over Houston, where refineries produce very large emissions of volatile organic compounds and also sulfur dioxide.

“Using Dr. Jang’s theory and findings, they immediately thought that what has been happening there was that sulfur dioxide was being oxidized as sulfuric acid. Then the sulfuric acid was acid-catalyzing organic reactions in the plume over the petroleum refineries to form huge, huge bursts of particles that nobody really understood before.”

The UNC experiments should lead to new insights into global warming, photochemical reactions and weather and, possibly, some useful manipulation of them, Kamens said. They also could have important implications for pollution control and health.

“Environmental Protection Agency researchers also have said they are very interested in this work, and we’re going to share our information with them soon,” he said

Mathematical models the team is creating will help them predict what would happen in the atmosphere in response to lowering volatile organic emissions and other pollutants from cars, refineries and other sources, the scientist said.

The National Science Foundation’s Atmospheric Chemistry Division and the EPA’s STAR (Science to Achieve Results) program supported the exploratory studies with grants to Kamens’ research group.