Colette Heald: Bridging the Gap Between Observations and Models

Dec 02, 2013
Colette Heald: Bridging the Gap Between Observations and Models

Colette Heald has always been a problem solver. But uninspired by the theoretical approach of engineering physics—her major as an undergraduate at Queen’s University in Canada—Heald began studying atmospheric chemistry to tackle problems that directly impact society. Specifically, she studies gases and particles in the lower atmosphere—black carbon from fires, sulfate from power generation, dust from deserts—and how they impact air pollution, climate change and the Earth’s ecosystems. 

“Working on an earth system problem that links to air pollution, climate change, as well as a variety of other important environmental problems, is a real societal driver behind my research,” Heald, Associate Professor in the Department of Civil and Environmental Engineering and Earth, Atmospheric and Planetary Sciences, says. “While I don’t actually work in the policy world myself, it underpins my research, and that’s a very motivating connection.”

Heald is reminded of this human connection to her work every day by reading the news—from heavy air pollution in China to soot from wildfires in Colorado. It also spurs ideas for new research. 

In the Western U.S., for example, bark beetles are now present in greater numbers because increasingly warmer weather has allowed them to survive winters. The beetles then attack forests, creating dead wood on the ground and leading to the rapid spread of fires— and poor air quality from the resulting smoke. But do the beetles contribute more directly to the poor air quality? Heald and her team dug deeper. 

Leaves and other types of vegetation emit gases and particles into the atmosphere. So when vegetation is decreased, such as through a beetle infestation, one would think there would be a decline in these particle emissions. But Heald’s team found that the increase in emission that occurs when these trees are stressed by an insect attack actually outweighs this effect.  This increase in atmospheric particles can degrade visibility in pristine forests and also play a role in climate change.

Building Better Models

Much of Heald’s work focuses on a challenge that is wracking the brains of atmospheric researchers throughout the world: bridging the gap between observations and models.

“I always tell my students not to have too much faith in models. By definition, a model is wrong, because it’s just a simplified description of what we know now. But it’s important to think about what we can learn from these imperfect models,” Heald says. 

Heald and her colleagues didn’t realize just how wrong the models were in the case of organic aerosols until almost a decade ago when the measurements and the models began painting very different pictures. 

“The whole community got turned on its head and realized that there was a lot of chemical complexity that we did not understand and that we are not treating in models.  That leads us to the question: what are we going to do about it?” Heald says.

Heald, her research group, and the whole atmospheric research community are answering that question by looking at more measurements, studying more satellite images, and doing more lab experiments to include factors they didn’t originally think to include. Then, they’re trying to use all of this information to better inform models.  

“So we’re using whatever we can get our hands on to try to reveal where the gaps are between what we see in these snapshot observations and what models would tell us based on what we think is happening.” 

To give an example, Heald is waiting for more data from observations on aerosols in the Southeastern United States.  Climate records show that this region has actually been cooling over the last 50 years. Some researchers have suggested that this cooling may be associated with aerosol particles. Given that the region has a lot of vegetation, Heald and her team know that the particular types of trees growing there substantially emit a compound that can form these aerosol particles. So they wanted to investigate whether this was a likely explanation. 

They used satellite observations to show that there are indeed a lot more aerosols in that region in the summer time which could scatter radiation and cool the climate.  But even when considering the smog from the South’s big urban centers, the satellites are showing aerosol levels higher than what the trees and cities combined would produce, according to her models. 

Heald thinks there has to be another source. Colleagues have hypothesized that there is an unknown chemistry occurring where the smog from urban areas is working together with emissions from trees—causing them to produce higher levels of aerosols. Or there might be some other source or process that is just not well understood. To investigate this, Heald and her team are waiting for data from a field campaign launched by the U.S. Environmental Protection Agency and National Science Foundation in the region this summer. 

This example demonstrates the extreme complexity of the work. But this is just one example of one type of particle in one place. Atmospheric particles come from many different sources. They form many different types and sizes. And they travel in many different patterns over different lengths of time—though generally they last in the atmosphere for about a week before latching onto precipitation and raining down to earth.

“What that means is that when you make a measurement in one location, it doesn’t tell you about anything other than what that one location is experiencing.  It doesn’t give you a sense of any large scale,” Heald says. “So it is really tough to go from a single point measurement of particles to understanding what is happening in terms of their global budgets.”  

Problem Solving at a Global Scale 

Understanding these “global budgets” is especially important when determining the climate impacts of the gases in our atmosphere. 

 “When it comes to thinking about our climate, it’s not just about how many of these particles there are in the atmosphere,” Heald says. “It’s about what are the properties of these particles.  Do they mix together? Do they chemically change form? Do they scatter or absorb radiation? These are really critical questions to ask to better understand the Earth’s radiation balance. But they’re really open questions.” 

While Heald and her colleagues work to learn more about the properties and impacts of many different pollutants, they’re also working to build simple models that may not include all the details, but will provide an overall description of the behavior of particles that matches more closely with what they are observing in the atmosphere.  

“We’re doing this with the idea that if we can come up with something simple, we can use it in climate models for future and past predictions.” 

For more on Heald's research see:
Climate change and air pollution will combine to curb food supplies
EAPS in Brief: Atmospheric Chemist Colette Heald