American Scientist

Air pollution isn't static. Different substances react with one another to produce shifting combinations of compounds. Krishna L. Foster, a professor of chemistry at California State University, Los Angeles, and one of Sigma Xi's Distinguished Lecturers, studies how solar radiation changes the composition of particulates such as those from automobile exhaust. She gave an overview of her research to managing editor Fenella Saunders.

You study airborne particulates. What are those, exactly?

Particulates are compounds in the air that are not gaseous; they are made of either solids or liquids, and that’s what makes them different than the rest of the atmosphere. They’re suspended in air, and they can last there for sometimes days or weeks, depending on their size. Dust is a huge piece of particulate matter, larger than 10 microns, about the width of a hair. That’s one class, and we aren’t so concerned about that anymore, because it doesn’t get too deep into our respiratory systems. Current research is looking at particles that are smaller than 2.5 microns in diameter, because when inhaled, you can bring these little particles into your body, so it becomes a health concern. The types of reactions that happen on particles may be the same exact ones that happen in the gas phase, but the outcome—how long it will take for a chemical change or even what the products will be—that’s completely different. I study the reaction of compounds in the atmosphere on particulate matter because they affect how that initial pollutant will change over time.

How are particulates created?

The mechanically generated ones—from seawater hitting the beach, for example—are 10 microns or bigger. When we get down to 2.5 microns, this is the maximum cutoff for something that’s chemically generated. It may have even started out in the gas phase and grown over time, or it can be a particle that exists, say a water droplet, that takes up chemicals.

Why is combustion a particular source of particulates?

The first combustion process you might think of is soot. In the 1800s in London, boy chimney sweeps were getting astronomical rates of cancer as they grew up because of inhaling soot. Automobile exhaust in the modern age has similar composition.

The compound formed in incomplete combustion, whether it’s the chimney sweeps in the 1800s or the automobiles and trucks on the roads today, is called PAH, polycyclic aromatic hydrocarbons. These are simply a series of benzene rings that are tied together to form different structures. Benzene is a hexagon shape of carbon atoms with double bonds on every other carbon. Many of the molecules that are part of our life processes have similar structures. One of the routes of toxicity is just confusion; you have a compound that is so similar to something that your body needs to function that when the foreign compound is ingested, it causes harm.

Exposure to some PAH is natural. It happens in barbecues, that’s definitely incomplete combustion, and our bodies are very good at protecting themselves. But chronic, intense exposure becomes problematic. I love a good piece of barbecued corn and I will eat my PAH comfortably, with a smile. But I choose not to live right off a congested highway.

What is the role of the Sun in reacting with these particulates?

The Sun is a powerful source of energy. When it comes to chemical behavior and transformation, the electrons are everything. In a chemical compound in the presence of the Sun, you have the electrons minding their own business, and then if they are exposed to the right wavelength of energy, the compound will absorb it. This causes the electrons to be excited, and once that happens, the world is opened as far as how that chemical will respond. Photochemistry means that the light and its ability to excite the electrons is an important factor in making that chemical transformation take place. So the energy is stored in the electrons moving stepwise—it’s quantized. Say the electrons were at stage 1, and they absorb the right type of energy and they move to stage 2, but they really want to go back to stage 1. The way an electron releases that extra energy could be in several ways: It simply releases more light, it can let the energy go as radiation, or it can transfer that energy to something else.

In your research, you say the Sun "ages" particles. How does it do that?

What the emissions coming straight out of a car’s tailpipe look like are fairly well characterized. The small particles, 2.5 microns and lower, don’t necessarily settle down to the ground instantly. It takes them hours, in some cases days, or on the order of weeks, and therefore they have time to react with the sunlight. So you started out with an initial pollutant that was well characterized, but now we have the Sun, which is an endless source of energy, it’s moving the electrons around in those pollutants that were emitted, and they’re now in a position to change over time. You also have the power of a mixture. For example, PAH is not going to change all by itself, but it will react with something else. There are several oxidants that will react with PAH. So PAH is exposed to sunlight and oxidants, and time passes. That means that what you had at the tailpipe is different than what you’re going to have five days later.

How many potential reactions are involved for this example of PAH?

When I say PAH absorbs sunlight energy and forms singlet molecular oxygen, I’m already talking about at least a dozen reactions. Just to explain a couple of the transformations, you need at least 300 reactions to even start the conversation.

Why do you do field studies in the Arctic regions?

I studied particulates at the northernmost land location in the entire world. It’s one world, and there’s nowhere that’s completely removed from the things we do as humans, but here we can study particulate matter without little interference from urban systems or human pollutants, it’s much more controlled and pristine. The Arctic also has no sunlight for several weeks at a time, so if you’re dealing with light-sensitive chemistry, it makes sense to go somewhere you have lights off, and then lights on.

What do you think is the best way to advance the study of air pollution?

You can have a few people shouting from the rooftops, but it takes hundreds of people working and believing each other to have a shift in the way things function. Science requires innovation; it requires people to work independently and collaboratively. We are trained to be skeptical. It takes a lot of information and publications before the paradigm will shift. And that’s the main purpose of practicing science: To be a conscientious explorer, to ask meaningful research questions, and then to dive into them.

Your data feeds into climate models. Do you think people could have better appreciation of climate change?

When you talk about what’s happening in the global climate change research community, the vast majority of practitioners believe wholeheartedly that this is taking place. What we can’t do is say that science ever stops, because we’re always discovering. But when you have consensus built to the degree that it’s been built around global climate change, I believe it’s foolish not to take that seriously. It’s a time-sensitive problem. If we don’t create preventive policy, the consequences could be dire.

Back in the 1980s, with ozone depletion in the polar regions, scientists were very forthcoming with politicians. There was a kind of urgency to present their consensus on the source of this problem. It ended with strict legislation against the use of chlorofluorocarbons. Legislation is slower to move because there are economic consequences, there’s always a give and a take. Consensus has been clearly established within the scientific community, and now it’s time for collaboration with the legislators.