ScienceMatters - Season 3, Episode 5 - Clearing the Air About Aerosol Science

October 15, 2019
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Sally Ng is one of the top experts in the world on aerosol science, the study of tiny particles in our atmosphere and what they mean for our climate, and our health. Ng, an associate professor in the School of Earth and Atmospheric Sciences, describes her work testing air quality in the field, and in a special indoor lab that she designed.    

(Upbeat music) 

Renay San Miguel: Hello and welcome to ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m Renay San Miguel. 

Sally Ng grew up in Hong Kong, surrounded by the energy of skyscraper towers and the haze of polluted air. But during her 2nd year in college, a student exchange program brought her to the University of Minnesota.  

That state calls itself “The Land of 10,000 Lakes,” yet it was other parts of Minnesota’s natural landscape that caught Ng’s attention, and those were enough to change the course of her scientific career. 

Sally Ng:  At that time, I was mainly studying on the Minneapolis campus, but every Wednesday or so, I have a class in Saint Paul campus. And before I go to the class in Saint Paul, I have to walk through this forest. And the air quality was so nice. And, you know, I was very young, and I was like this air is so nice to breathe. I was like in Hong Kong it’s always really polluted. And I think that got me started as like, what can I do to help to improve the air quality as a chemical engineer so that more people can breathe this amazing air that is clean and fresh? And so I think when I went back to Hong Kong after my year of exchange in Minnesota, I think that played a role in terms of me deciding, OK this is a direction that I would like to pursue. And it makes a difference, you know everybody breathes, so you want to do some research that is impactful, right?  

Renay San Miguel: Mission accomplished for Ng, an associate professor in the School of Earth and Atmospheric Sciences.  

Ng is now one of the world’s experts in aerosol science, the study of tiny particles that float in the air, and what they mean for the climate, and our health.  

And that forest she walked through in Minnesota? She has put a game-changing research spotlight back on trees, which play such an important role in the planet’s air quality. That’s also why she’s spending more time than ever wondering about what happens to our atmosphere when a lot of those trees..are reduced to ashes.  

NBC Newscast, 2018: I’m Steve Patterson in Paradise, California, where nearly the entire town has been incinerated by towering flames. 

CBS Newscast 2019: As fires continue to burn through vast areas of the Amazon, international criticism of Brazil is heating up. 

Renay San Miguel: As if our air quality doesn’t face enough threats these days, we’re seeing an increase in wildfires. The latest major examples: Northern California in 2018, the Amazon rainforest in 2019.  

Those wildfires have provided another spark for Ng’s research. 

Sally Ng: We’re also working on understanding the emissions from wildfires, right? So, if you read the newspapers, you know, the wildfires are getting more intense or more frequent, and when you look at wildfires you see a lot of emissions. You see these pollutants, right? So they emit a lot of different compounds directly as gas and directly as particles. And we are very interested to know what happened to all these emissions from wildfires, you know, how they get transformed in the atmosphere, and are they going to produce compounds that eventually might be bad for human health, right? 

Renay San Miguel: Aerosols can be naturally occurring, or made by humans. Those two kinds of aerosols can get together to form something called secondary organic compounds, or SOCs. Ng’s research on SOCs has resulted in tighter industry regulations and better air-scrubbing equipment.  

Questions remain: How exactly do aerosols get in the atmosphere? What are they made of? And how do they affect the health of the planet -- and of our lungs? 

Since joining Georgia Tech in 2011, Ng has built a formidable research team trying to answer those questions. She has already added considerable insight into what aerosols can do to the environment. 

She wrote the blueprint for Georgia Tech’s Indoor Environmental Chamber facility, for better controlled laboratory testing of air samples. She combines controlled experiments in this facility with her field studies.  

Before arriving at Georgia Tech, she helped develop aerosol measurement devices for a private company. So she’s equipped her testing chamber with the latest measurement technologies. 

Her published work has been cited more than 10,000 times, and she was one of the most highly cited researchers on Clarivate Analytics Web of Science’s annual list of the world’s top scientists for both 2017 and 2018. Georgia Tech presented her with its Outstanding Achievement in Early Career Research Award in 2019. 

For Ng, it all goes back to her first time breathing clean, clear, Minnesota air. Now it’s about making sure everybody has the same chance. 

Sally Ng: When we talk about these pollution-control policies, I would say in the U.S. the pollution control policy is actually very effective. You can see the sulfur dioxide going down, the nitrogen oxides going down, particulate matter from these human sources going down. But, you can imagine the fires as they increase in intensity and frequency, one day fires will become one of the major players in terms of the pollutants that we’re breathing in. So the community now has a lot of interest in understanding, you know, really what happens to these compounds. As you said, it is hundreds and thousands of species all emitted together, reacting together at the same time. So it’s a very, you know, challenging question, but that makes it also very exciting and interesting. 

(Upbeat music) 

Renay San Miguel: Your field of study is aerosol chemistry. How do you define that? 

Sally Ng: Yeah, so maybe first I would define what aerosol is, right? So a more common term for aerosols is called particulate matter. So when you read newspapers you will be like particulate matter level is high. So these are tiny particles suspended in air, and they are so small that you can actually not see them, and they can be solid or liquid, you know, particles. And when you look at a pollution picture or a very polluted city, then you see the haze and smog, and there are a lot of, you know, particulate matter in the air. So one of the common terms is PM 2.5. So PM 2.5 means particulate matter with a diameter smaller than 2.5 microns. And it might be hard to grasp how small that is. So for our hair, you know, the typical diameter of a hair is about 50 to 70 microns, right? So you can get a sense of that 2.5 micron is very small. 00:01:12 

And so in my group we study how these particles are formed, what are they made of, you know, chemical composition-wise, and what are their impacts on human health. 00:01:30 

Renay San Miguel: And also how that matter, how it might interact and combine maybe with other things floating in the atmosphere, right? 

Sally Ng: Yep. And so like the atmosphere is a very complex mixture of different chemical species, right? So we talk about the criteria pollutants. You have ozone, nitrogen oxides, and now we have these, you know, particulate matter. So they can interact and they can react. So there’s all these chemical compositions that are taking place at any different time, and so that’s kind of fascinating to track this chemistry happening in the ambient air. 

Renay San Miguel: Indeed, there’s a laboratory at work over our heads, in our atmosphere, every second. That’s why the trees that play key roles in our environment, with the gases they emit and absorb, are of such interest to Ng.  

Sally Ng: So there is a very important class of compounds emitted by trees; they’re called the volatile organic compounds, and trees emit a lot of those volatile organic compounds and the short form is VOC, Volatile Organic Compounds. So one example is that if you go into a pine forest, you smell the very nice characteristic pine tree smell, right? So that compound is called pinene and it is one of the major types of compounds that is emitted by trees. And if you go to a citrus farm or peel an orange, you smell the citrusy smell, so that is a compound called limonene, right? So trees also emit a lot of these VOCs and some of it they emit it to protect themselves against heat stress. Sometimes they emit it to attract pollinators, or they emit them to kind of scare off the pests or, you know, the insects.  

The southeast U.S. has this very nice, natural environment. You talk, you know, about and all this chemistry happening outside and it is happening outside the window, and we have a lot of these, I would say tree-emitted compounds compared to the rest of the country, making this a very interesting place to study the chemistry of what happens to these compounds after they got emitted by trees. 

Renay San Miguel: Which makes Atlanta and Georgia Tech the perfect place for Ng to be for her research. 

That includes being able to test all those human-caused VOCs from cars on the Downtown Connector, local utility plants, and other sources. 

Sally Ng: So now these VOCs are not the nice-smelling ones that we are talking from the trees—might the emissions from your tailpipe, right? So these compounds is called—we call the anthropogenic VOCs. Anthropogenic means manmade, so manmade VOCs. And the same thing: These VOCs got emitted into the atmosphere, and they can continue to react in different ways and then form these particles. 

A lot of things we study in our group is how emissions from human activities, such of these manmade sources, interact with emissions from the natural sources such as these emissions from trees and how they interact with each other, react in the atmosphere to create this, you know, at the end, this end product that we are breathing in and in the atmosphere. And we’re trying to understand all these interactions and they’re very non-linear. So it’s not like one plus one goes to two, right? So things interact in a very non-linear way and that makes the problem, you know, complex but interesting to study. 

Renay San Miguel: Ng proved that in 2015 with an influential research paper on secondary organic aerosols, or SOA’s, which is the combination of human-made and natural compounds in the air. Her testing found chemicals like isoprene -- a volatile organic compound, or VOC, that mostly comes from plants – were more prevalent than previously suspected. The study helped determine the effectiveness of pollution-removal devices installed by utilities in the Southeast.  

Sally Ng: One of the things that we set out to understand is that, as I mentioned, you’ve got these emissions from human activities, emissions from trees, how do they interact with each other? So we set out to answer this question. So what we find is that the emissions from cars, which is a group of emissions called nitrogen oxides, and the emission from power plants, which is a group of compounds called sulfur dioxide—so these are manmade emissions—and they can interact with these emissions from trees. And what we show is that these human emissions of nitrogen oxide and sulfur dioxide directly mediate the amounts of particles that can form from these tree emissions. 

So you can think of this as—I always see this as a very win-win situation. So first of all we don’t want these emissions by cars or power plants to start with; we want to keep those emissions down. And what we find is that if we keep those emissions down, we can also reduce the amount of particles made by the trees. So that is really a good thing. You know, trees are great, you don’t want to, you know, cut down the trees, so now by controlling these human activities and sources of emission, we can indirectly also control, you know, the particles made by these trees. So it’s good news. 

Renay San Miguel: How surprised were you by that?  

Sally Ng: There’s this synergy and interaction and it’s non-linear. You cannot always say that oh if I reduce that, that will also reduce, right? But they interact in this very non-linear way, and so now we know the mechanisms. So that allows us to make, you know, more efficient, I would say, policies. Say, if you want to control pollution from particles, right, so you want to reduce that source, so now we actually have a mechanism to show, OK, if you reduce emissions from power plants, then you can also you know reduce these particles that is formed from trees. So that is a very, I would say, a potential regulation that can build on very sound, you know, scientific findings. 

This is good news for, I would say, pollution control in general that, you know, if we continue towards reducing these NOx, or nitrogen oxides, and sulfur dioxides from, you know, human activities then we can also reap the benefits you know of their impacts on tree emissions and particles. 

(Upbeat music) 

(Sounds of machinery in Sally’s lab) 

Renay San Miguel: All Sally Ng needs is the air that we breathe – and the latest technology so she can analyze it.  

That’s what you’re listening to right now, one floor up from Ng’s office: The testing machines in the Indoor Environmental Chamber lab, the one she designed from the ground up. 

(Sounds of machinery in Sally’s lab) 

Sally Ng: The best place to study air pollution is outside because that’s the air we breathe, but the problem is that the weather or meteorology changes every day. So today is actually very sunny when you look out, but if it gets rainy or cloudy then the conditions will change, and it is really hard to tease out the mechanisms and compositions when you have so many things changing at the same time. So this Georgia Tech environmental chamber facility you can think of bringing the outside inside so that you can really control your atmosphere. So I always call it a “simulation atmosphere” or “atmospheric simulator.” So we have 300 UV lights surrounding this chamber facility to simulate solar radiation. And we got two reactors, we call two “balloons” or two “bubbles” that is transparent, and those are two atmospheres. So now we have this controlled environment—we can control the temperature, we can control the humidity, we can control the compounds that we put into these reactors in a very, you know, concise way. So now we can really understand the mechanisms and the composition by changing one parameter at a time. 

(Sounds of machinery in Sally’s lab) 

Renay San Miguel: Ng and other Georgia Tech Researchers received a National Science Foundation grant in 2015 so they could buy an advanced, high-resolution gas particle mass spectrometer.  

She and her team have put that high-tech equipment to good use since then. 

Renay San Miguel: How do you gather the samples that you need? When you’re in the field, what kinds of technology’s involved with getting them out of the air? 

Sally Ng: Yeah, so when we go to the field, we actually move all the instruments, you know, to the field, right? So if you come visit my lab during a field campaign, my lab would be pretty empty, right? So the instrument, or technology, that we use the most is something called “mass spectrometry.” So we actually directly pull these samples into the instrument and analyze it right there, and you can see the results on the computers screen in real-time. So it has a sampling tube and it has internal pump that pulls these samples inside. So the samples will contain gasses and particulate matter, right? And these particles will be vaporized. That means they will be converted from particle phase to a gas compound. And then, you can imagine, there’s an electron beam that now heats these vapors and makes some ions, and they will be detected, you know, as products. And so when we take the instruments out in the field, we’re really doing a real-time measurement. I always tell my students you can look at your computer screen and think about what air you’re breathing because the results are immediately shown on the computer. 

Sally Ng: So the power of this instrument is that it can actually analyze gas and particle samples almost simultaneously, right? So this instrument allows you to look at this molecular level information of both the gas and the particle. And, again, it is real time, so it really gives a lot of information for us to really try to understand how these particles are made, the mechanisms, their composition, and hopefully, using that information, we can learn more about their health impacts. 

Renay San Miguel: And so it sounds like this mass spectrometer has made a difference for your research? 

Sally Ng: Yeah so it has been, you know, heavily used since we got the instrument. So, so far it has been used in the lab to do more of the fundamental studies and it has also been taken out to the field. Actually, we just did a study several years ago close to Atlanta at a place called Yorkville. It’s like several hours Northwest of Atlanta in a farm to try to study, you know, the air pollution there. So this instrument has seen a lot of things, and we continue to use it pretty much every day. 

(Upbeat music) 

Renay San Miguel: So tell me what you’re going to be researching in 2019? What is on your agenda that might be new or related to what you’ve been studying? 

Sally Ng: Yeah so there are a couple like really new, exciting projects that we’re working on this year. So I’ll just maybe go through them one by one. So one of the things as I mentioned, the nitrogen emission, nitrogen oxide emissions from cars, right, so we are directing or following that trail of evidence to really learn more about the mechanisms, how the nitrogen oxides emitted by cars will interfere with these reactions by trees. And what we’re trying to understand is really to do this more quantitatively in a way that what is the yield, what is the mechanism, because the nitrogen cycle, you know, we talk about carbon cycle, but nitrogen cycle is also a very important cycle in the atmosphere, and it also effects ozone formation. So we are doing very fundamental lab studies in the lab now using this environmental chamber to understand how these, you know, non-linear processes happened. we actually are going to collaborate with modelers. So in order to, you know, predict future air quality, its impacts on climate and health, our community uses models to do that, right? But models need inputs, right? So right now, this chemistry is not very well-represented in the models because of the lack of lab data, right? So we’re going to do these experiments and hopefully, we’ll be able to provide inputs for constructing the model. 

(Upbeat music) 

Renay San Miguel: I thank Sally Ng, associate professor in the School of Earth and Atmospheric Sciences, for speaking with me about her aerosol research. Ng also has a joint appointment with the School of Chemical and Biomolecular Engineering. 

My thanks also to NBC and CBS News for use of television news clips. 

Siyan Zhou, a former research associate with the School of Psychology, composed our theme music. 

If you like listening to ScienceMatters, please subscribe to our podcast. We’re on Apple Music and Soundcloud. 

This is ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m RSM. Thank you for listening. 

(Upbeat music)