when a pine-scented molecule combines with ozone in the surrounding air, some of the free radicals created in the process grab oxygen with unprecedented speed
February 27, 2014

Researchers Reveal The Chemistry Behind Particle Formation In Pine Forests

April Flowers for redOrbit.com - Your Universe Online

For atmospheric scientists, pine forests are magical places where coniferous trees give off pine-scented vapors. These vapors form particles, rapidly and seemingly out of nowhere.

A new study from an international group of scientists illuminates the process by which gas wafting from coniferous trees creates particles that can reflect sunlight or promote cloud formation. Both are important climate feedbacks. The findings, led by the University of Washington, were published in a recent issue of Nature.

"In many forested regions, you can go and observe particles apparently form from thin air. They're not emitted from anything, they just appear," said Joel Thornton, a University of Washington associate professor of atmospheric sciences.

The findings reveal the chemistry behind these particles' formation. The researchers estimate that these particles may be the dominant source of aerosols over boreal forests. Aerosols, according to the Intergovernmental Panel on Climate Change (IPCC), have been named one of the biggest unknowns for climate change.

For decades, scientists have understood that gases from pine trees can form particles that grow from just one nanometer in size to 100 nanometers in approximately 24 hours. These airborne particles can be either solid or liquid in form and may reflect sunlight. At 100 nanometers, they are large enough to condense water vapor and encourage cloud formation.

The research team — including members from the University of Copenhagen, the Institute for Tropospheric Research, Aerodyne Research Inc., and Tampere University of Technology — took measurements in Finnish pine forests and then simulated the same particle formation in an air chamber at Germany's Jülich Research Centre. The researchers were able to pick out 1 in a trillion molecules and follow their evolution using a new type of chemical mass spectrometry.

[ Watch the Video: What is a Forest? ]

According to the results, when a pine-scented molecule combines with ozone in the surrounding air, some of the free radicals created in the process grab oxygen with unprecedented speed.

"The radical is so desperate to become a regular molecule again that it reacts with itself. The new oxygen breaks off a hydrogen from a neighboring carbon to keep for itself, and then more oxygen comes in to where the hydrogen was broken off," Thornton said.

Our current understanding of chemistry would suggest that 3 to 5 oxygen molecules could be added per day during oxidation. However, the research team observed 10 to 12 oxygen molecules being added by the free radical in a single step. Thornton said that this new, bigger molecule wants to be in a solid or liquid state, rather than gas. It also condenses onto small particles of just 3 nanometers. So many of these molecules are produced that they are able to clump together and grow to a size big enough to influence climate.

"I think unraveling that chemistry is going to have some profound impacts on how we describe atmospheric chemistry generally," Thornton said.

The largest amount of these compounds are given off in boreal or pine forests, so the implications of these findings are especially important for the northern parts of North America, Europe and Russia. Thornton said that other forests emit similar vapors, and suggests that the rapid oxidation may apply to a broad range of atmospheric compounds.

"I think a lot of missing puzzle pieces in atmospheric chemistry will start to fall into place once we incorporate this understanding," Thornton said.

As temperatures rise, forests are thought to emit exponentially more of these scented compounds. A better understanding of how those vapors react might help scientists predict how forested regions will respond to global warming, and what role these forests might play in the overall response of the planet.

Last summer, Thornton's group participated in a related campaign to study air chemistry over the Southeaster US, where aerosols formed by reforested areas or from pollution could help explain why that region has not warmed as much as other places.

"It's thought that as the Earth warms there will be more of these vapors emitted, and some fraction of them will be converted to particles which can potentially shade the Earth's surface," Thornton said. "How effective that is at temperature regulation is still very much an open question."

Image 2 (below): This is the reaction chamber at the Jülich Research Centre Plant Atmosphere Chamber. Credit: Felipe Lopez-Hilfiker, UW