Chemistry and Clouds

Researchers look at water droplets and chemical reactions.

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By Marlene Cimons, National Science Foundation

When Pierre Herckes goes to the top of a mountain, it’s not for the view. In fact, the more clouds and fog, the better.

Herckes, associate professor of chemistry at Arizona State University, collects water droplets from clouds and fog in order to study their composition. “Water droplets can act as chemical reactors,” he says. “There are chemical transformations and reactions going on within those droplets.”

Herckes’ research focuses on what happens to gases and particles in the atmosphere, specifically within clouds and fog, with the goal of better understanding their potential impact on air quality and health, and on climate change.

Numerous things are happening within the clouds and fog. Chemical reactions within these droplets, for example, can turn toxic substances, like certain pollutants, into benign substances. Conversely, they can combine individual compounds, harmless by themselves, into something potentially risky.

They also can take in certain types of organic matter, such as soot and smoke, which enter the atmosphere as particles and remain in that state--making them primary emissions--and plant materials, known as terpenes, which enter the atmosphere as gases, then become particles, making them secondary particles.

“We don’t completely understand the way these particles are actually formed in the atmosphere,” he says. “This is a big problem for air quality and climate models since we know the effects of primary emissions in the atmosphere, but don’t understand well the importance of secondary particles, that is, these particles that are formed in the atmosphere from gases.”

Depending on their nature, such particles can produce either a cooling or a warming effect in the atmosphere, important information in understanding climate science.

“We are interested in the chemistry going on, and physically, what particles and gases get into cloud droplets and which don’t,” he says. “We want to better understand the particle/cloud interactions, which are some of the highest uncertainties in climate modeling. We know quite well what gases do. A substantial part of the uncertainties of climate are based on what the particles do, and how they interact with clouds. The particles can be warming, but they also can be cooling and impact the frequency and amount of precipitation.

“Our research will help us understand what’s in the clouds, how it gets changed and how it changes the reflectivity of the clouds and the reflectivity of the particles--how much warming or cooling the particles do,” he adds.  “We are looking at the possibility that clouds contribute to this secondary particle formation, that is, whether clouds take these gases and turn them into new particle material.”

Herckes is studying atmospheric conditions under a National Science Foundation (NSF) Faculty Early Career Development (CAREER) award, which he received in 2009 as part of NSF’s American Recovery and Reinvestment Act. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding his work with $569,000 over five years.

He also is studying chemical reactions within atmospheric water droplets, specifically the formation and effects of nitrosamines, which result when nitrogen oxides combine with amines.

Nitrogen oxides, which include, among other things, pollutants from cars, combine with amines, which are a byproduct of agricultural practices, such as manure emissions.  Nitrosamines, which also form under certain conditions when nitrites are added to food, can cause cancer, but it is as yet unclear whether exposure to them in the air is dangerous, Herckes says.

“We have shown that clouds can take innocent chemicals and transform them into something highly toxic,” he says. “We know these reactions can occur, but we don’t yet know what their health implications are.”