The lab is a scientist’s sanctuary. It is a place where researchers can isolate systems of interest in an attempt to better understand them. Carnegie Mellon’s Air quality lab is equipped with a number of experimental tools that help us replicate atmospheric aerosol processes in a controlled manner. With tremendous help from our collaborators at the University of Bristol, one setup that we have been working on is the optical tweezers, an instrument that uses a laser beam to trap micrometer-sized particles and hold them in place.

How exactly would a laser beam suspend a small object in air? Modern physics has taught us that light or any form of radiation (ie: light) has momentum. A long time before that, Isaac Newton developed the fundamentals of classical mechanics, and at their cornerstone was the fact that forces are created by changes in momentum. How does this apply to optical tweezers? When the converging rays of a focused laser beam enter a particle’s medium they undergo a change in direction, a phenomenon called refraction. This change in direction results in a change in the momentum of the beam which creates a force! It turns out that this force always acts to restore the particle to a position just above the laser beam’s focal point (the point at which all the rays from the laser converge) creating a stable optical “trap” and inspiring the name of the instrument –  the particle is being tweezed by the beam!

After the particle is trapped or caught, the laser beam interacts with the particle and causes it to emit a tiny amount of radiation that we can detect with our sensitive radiation measuring system. The radiation emitted is not too high to affect the particle’s physical conditions but high enough to carry valuable information about its size, physical properties, and chemical composition. By manipulating the environment around the trapped particle we can truly simulate atmospheric aerosol processes like never before. We can look at things like how two particles collide and their corresponding interactions, how particles made of complex mixtures separate into different phases, and how cloud droplets shrink/grow and crystallize.

These are only a handful of potential projects, and there certainly will be many more atmospheric aerosol conundrums that the optical tweezers will play a significant role in elucidating.   

To improve the quality of the air we breathe, regulations typically limit the mass (i.e. micrograms) of particles smaller than 2.5 micrometers in diameter. In response to mass-focused regulations, atmospheric models often neglect or misrepresent particles that contribute very little to the mass, usually small particles less than 0.1 micrometers in diameter. Research suggests that these smaller particles, however, may pose serious health risks because they travel further into the lungs, potentially carrying condensed toxins. How do we account properly for these potentially harmful small particles when mass-focused models and regulations neglect them? We can also look at the number of particles instead of just the mass of particles as a metric.

To examine the effects of particles less than 0.1 micrometers in diameter, which contribute greatly to particle number but little to particle mass, we use a model of the atmosphere over the Eastern United States that focuses on both mass and number down to 0.0008 micrometers in diameter, the approximate size of a freshly formed particle.

In contrast to previous studies that have focused on the formation of new particles in the atmosphere by condensed gases, our study focuses on direct particle emission. The number of particles emitted in each size range changes depending on where the particle came from (gasoline or diesel engines, wood burning, etc.), so we first improve our emissions input data to more accurately represent the size and number of particles coming from each source. This is the current stage of our study – we are testing the changes that we made to the emissions input data to make sure that they have indeed produced a more accurate picture of the number emissions in the Eastern U.S.

Next, we will implement these emissions in the model to determine the approximate contribution of each source to particle number concentrations. With these results, we can then look at what will happen if we reduce the emissions from an important source, such as diesel engines. Reducing both particle mass and number emissions, however, is pretty complex— if we reduce mass emissions by filtering out larger particles, for example, we may actually create higher particle numbers. This happens because the larger particles provide surfaces for gases to condense onto, but in the absence of these larger particles, more gases will condense to form new particles instead of making older ones grow.

After simulating many emission reduction scenarios, we can then assess the effects of emissions control strategies on particle number in the Eastern U.S.

Smokey the Bear is on to something. Since his “birth” in 1944, he’s been helping us become more aware of how our actions affect the Earth’s forests. Some forest fires are started naturally (e.g., by lightning) while others are started directly or indirectly by human activities. One thing, however, is true no matter how forest fires start: in addition to their destructive nature, they make a lot of air pollution.

How much air pollution do they make, exactly? Some estimates say that if you were to measure all of the black carbon in the world (that’s the black stuff you may see coming out an exhaust pipe), nearly half of it comes from forest fires. In the same fashion, if you were to measure all of the particles emitted in the atmosphere that contain organic carbon (“organic” meaning they contain carbon and hydrogen), the majority of them — nearly three-quarters to be exact — come from forest fires.

That said, it’s extremely important to understand what smoke from forest fires looks like and how it “ages,” or changes over time, as it moves through the atmosphere. This is the focus of our current research study, which is taking place in the Fire Sciences Laboratory in Missoula, Montana.

For the past few weeks, we have been burning small samples of different types of wood prevalent in North American forests and measuring what the smoke looks like. After getting a good idea of the size of the smoke particles and some of the different chemicals that are in them, we’ve been subjecting the smoke to different conditions in an environmental chamber. Things like sunlight (simulated by a bed of UV-lights), nitrogen oxides (gases that are emitted by combustion sources like cars, trucks, and power plants), and ozone have been found to play a role in the “aging” process of smoke from these different types of wood—making the particles look differently than they did before they were subjected to these various conditions.

It will take some time to analyze all of the data we’ve collected from these experiments, but when all is said and done, we hope to have a better understanding of the complex chemistry that takes place in the “aging” process in smoke from forest fires.

"We need to let the American people know that controlling particulate matter is what stops more premature deaths than breast cancer, prostate cancer and HIV/AIDS (prevention) combined. And we don't know how to cure breast cancer, prostate cancer and HIV/AIDS, but we do know how to control particulate matter.”

-Paul Anastas, former EPA Science Advisor and Professor, Yale University

This quote was surprising to most of my friends and family I shared it with. Even air quality researchers, always pointing out how important their work is, can forget how profound this is: tiny particles, mostly invisible in the atmosphere, kill about 100,000 people in the United States every year. This number is similar or larger than many other causes of death that we hear about more frequently on the news: flu (60,000), automobile accidents (30,000), commercial airline accidents (less than 100 per year since 1997). Airborne particulate matter is probably the biggest public health problem you have never heard of. Why?

Perhaps a better question is how do we even know this at all? Every year, 2.5 million Americans die, mostly from heart disease, cancer, respiratory disease, and stroke. Particulate matter kills us by increasing the odds we will suffer one of these common deaths. Our best guess is that levels of particulate matter pollution prevalent in most areas of the United States increase the rate of cardiovascular deaths (e.g. heart attacks) by about 10 percent. This is not exactly a needle in a haystack, but air pollution has an insidious way of making sure its victims blend into the crowd. When someone dies from a heart attack, there is no way to know if that particular person’s life would have been saved if the air were cleaner. No death certificates list airborne particulates as the cause.

Society would be oblivious to the particulate matter (PM) problem if it were not for the skill of epidemiologists, people who look for statistical correlations between health problems and potential causes. The link between airborne particles on premature death shows up in several ways. When you look at day-to-day variations in the number of deaths in a given city, you see it is a bit higher on days with higher concentrations of PM2.5 (“fine” particles that are too small to be effectively filtered out by the upper respiratory system). When you compare cities across the United States, cities with lower PM2.5 concentrations are healthier and enjoy longer life expectancies.

Between 1980 and 2000, US life expectancy increased from 74.3 to 77.0 years. One of the most interesting results is that pollution controls during the 80s and 90s probably played a significant role in this improvement. According to a study by Prof. Arden Pope, improvements in PM2.5 levels during this time period increased life expectancy by 0.4 years. Most strikingly, cities with larger PM2.5 reductions tended to have bigger increases in life expectancy. Data in his paper suggest that more aggressive controls on PM2.5 could increase US life expectancy by another year or so.

Epidemiology depends on statistics, and the refrain is always heard that “correlation is not causation”. Is PM2.5 really the cause? The fact that the association is consistently seen when you compare cleaner versus more polluted days, cleaner versus more polluted cities, and cleaner versus more polluted decades suggests it is not a coincidence. The other “usual suspects” that public health experts worry about (smoking, obesity, socioeconomic status, and so on) don’t vary in the right ways to explain the observed trends.

Toxicologists, on the other hand, try to understand how and why PM2.5 leads to heart and lung disease. There is still a lot to learn in this field, but there is increasing evidence that PM2.5 levels lead to inflammation of the lungs, oxidative stress, obstruction of airways, and changes in heart rhythm. A lot of current research is focused on understanding whether some components of air pollution are worse than others and if the tiniest “ultrafine” particles are especially bad. Current EPA standards regulate PM2.5 on a weight basis, implicitly treating everything in PM2.5 as equally harmful. Because we only have rough ideas about how and why PM2.5 harms people, it’s hard to justify anything more specific.

CAPS researchers usually don’t participate directly in the kinds of studies I just described, which mostly depend on medical and public health experts. We are chemists and engineers trying to understand where PM2.5 comes from and what it’s made out of, information that the EPA needs to decide how to reduce PM2.5 concentrations effectively, but we like to remind ourselves and others that our science can be a matter of life or death. Statistically speaking, if you know 10 people who have died from heart or lung disease, you probably know someone whose life was cut short by airborne particles.

I was watching the movie Sabrina -- the original 1954 version with Audrey Hepburn and Humphrey Bogart, not the 1990s remake – recently, and it made me think about pollutant emissions from automobiles. To be fair, this is not a difficult feat, since I spend an inordinate amount of time pondering pollution. In any case, Sabrina (Hepburn) is the daughter of the chauffeur to the wealthy Larrabee family on Long Island, and she has hopelessly pined for David Larrabee, noted playboy and dilettante, to notice her as more than a servant’s daughter. One night, after witnessing David seduce yet another young socialite at a party, Sabrina, in a fit of pure teenage melodrama, decides to end it all by closing the doors to the Larrabee’s garage and running all of the cars. Luckily, David’s older brother Linus (Bogart) hears the cars and notices the smoke pouring out from the cracks under the garage doors, and saves Sabrina.

The filmmakers weren’t too far off of the mark to depict the cars in the Larrabee’s garage as smoking beasts. Sixty years ago, cars did not have the sophisticated emissions controls systems used now. They emitted a host of pollutants, including carbon monoxide (exposure to which would have led Sabrina to a bad end) and particulate matter (PM), in relatively high quantities. This would not last long. The postwar economic boom, coupled with air pollution in cities such as Los Angeles, led to the first vehicle emissions controls being required in the early 1960s.

Control of vehicular emissions goes hand-in-hand with the testing of new vehicles to make sure that they meet certain emissions standards. In the early days of vehicular testing, the vehicles, like those in the Larrabee’s garage, were so dirty that pollutant emissions at the tailpipe were the primary concern. Eliminating the visible plumes of smoke emitting from people’s tailpipes was job one. Governments and regulatory bodies, led by California, developed what was called a mass-based standard for PM. Simply put, cars would be placed on a dynamometer (basically a large treadmill) and driven. The exhaust would be first diluted with some clean air, and then sucked onto filters. Measuring the weight of the filter before and after sampling would give a simple, gravimetric measure of the PM emissions.

Fast forward a half-century, and think about how often you see a car on the road that is visibly smoking. It’s a rare occurrence, and a testimony to the effectiveness of the mass-based standard. However, today’s cars are so clean that it’s actually hard to measure how much PM is emitted. Furthermore, pollution from automobiles does not end at the tailpipe. The compounds emitted by combustion processes enter the atmosphere and undergo what is called secondary pollutant formation from atmospheric oxidation and processing. As the emissions at the tailpipe get smaller and smaller, this secondary pollution becomes more important to the overall picture of pollution from cars and trucks.

We have recently published two papers in the journal Aerosol Science and Technology (this one and this one) looking at new ways to quantify PM emissions from combustion systems. While the mass-based standard is still king, it is informative from both a scientific and a policy perspective to consider how these emissions, especially the organic portion, are spread out in what we call volatility-space (essentially vapor pressure). Quantifying the emissions as a function of vapor pressure allows us to predict how they will behave under changing atmospheric conditions – evaporating when it is hot or the emissions are very dilute, or condensing when it is colder. It also give us some information about the potential for secondary PM formation, and points us in the direction of tackling the next set of pollution challenges from our cars and trucks.

Bonus time! How much does driving a hybrid reduce your pollution footprint? Hybrids boast better fuel economy, which means less gas consumed, and therefore fewer pollutant emissions. However, cars emit a burst of pollutants during what is called the cold start – the first few seconds after starting up the car in the morning after it sat cold all night. Hybrids, just like standard cars, undergo a cold start.

During the 1980s and 90s we frequently heard about a growing hole in the ozone layer.  The ozone layer consists of a dilute layer of ozone gas in the stratosphere between an altitude of 12 and 19 miles above Earth’s surface.  Ozone, a molecule consisting of three oxygen atoms, blocks harmful high-energy UV light from the sun thereby protecting humans, animals, and plants.  For this reason, we typically refer to stratospheric ozone as “good.”  This doesn’t mean that you can stop slathering on that sunscreen—lower energy and relatively less harmful UV light still makes it down to the earth’s surface.  At one point, the protective layer of stratospheric ozone faced a threat to its own viability in the form of man-made substances that certain industries emitted into the air.  For many years, industrially-produced compounds called chlorofluorocarbons (CFCs) used in aerosol cans and refrigerants had been floating up to the ozone layer.  Each CFC molecule that reached the layer could destroy roughly 100,000 ozone molecules.  This sparked worldwide concern that eventually led to a ban on the production of CFCs with the Montreal Protocol in 1987. 

Even though ozone protects human health by filtering out the harmful rays of the sun, it is dangerous to breathe.  Inhalation of ozone can aggravate lung diseases such as emphysema, chronic bronchitis, and asthma.  It may also cause shortness of breath and damage our airways.  While the ozone layer is high enough that it is not a hazard, ozone can be created at ground-level by reactions between nitrous oxides (created by combustion in industry and vehicles), sunlight, and volatile organic compounds (VOCs).  VOCs are chemicals containing carbon that evaporate readily.  Examples include:  gasoline vapors, chemical solvents, emissions from vegetation, and household cleaning products.  Ground-level ozone is typically worse in the summer when the sun is strongest and can be a major air quality issue in Pittsburgh and all throughout the United States.  We regard this type of ozone as “bad” for society.

The negative effects of “bad” ozone do not stop there.  Ozone reacts with volatile organic compounds to form particulate matter (PM).  PM causes a laundry list of health problems such as cancer, heart disease, asthma, lung disease, bronchitis, and may accelerate cognitive decline.  In the United States alone, researchers estimate that PM kills 50,000 people a year.  If you compare this figure to the rate of fatalities due to car accidents (typically between 30,000 and 35,000 per year), you will see why I refer to PM as the “ugly” side of ozone pollution.  In order to regulate the emissions that contribute to PM formation efficiently, we need to understand the chemistry that generates particulate matter in the atmosphere.  Unfortunately, this chemistry is extremely complicated; reaction of ozone and a single volatile organic compound can yield particles that contain thousands of unique chemicals.  These reactions are multi-step processes and for a particular family of VOC precursors, all start with the same initial step.  Through our work at CAPS, highlighted with two papers in the Journal of Physical Chemistry (1,2), we have studied the first step in these PM forming reactions in a very unique way.  Under atmospheric conditions, the first step happens at blazing speeds that are too fast to study.  However, we developed an instrument that cools the reaction to -173^oC so we can slow down the chemistry to the point that we can observe and quantify the effects of this very important step.  We found that the energy needed to complete this first step is larger for certain types of compounds and gained a better understanding of how this step can influence the products of these ozone reactions.  These experiments increase our knowledge of the origins of particulate matter. Hopefully, this will lead to more accurate air quality models and cleaner air for all.   

Air quality (or lack thereof) is a team effort.  Car exhaust and other man-made pollution sources easily spring to mind, but there are natural emissions too.  People are probably most familiar with pollen, much to their chagrin – this is a natural airborne particle that can cause negative effects in humans.  Most varieties of plants, whether they are trees, bushes, grasses, or crops, emit volatile organic compounds (VOCs) as well - these evaporate quickly, much like nail polish remover.  Some of the most common of these VOC emissions increase when the temperature rises, and are thought to help the plant cope with heat fluctuations.  VOCs will be the focus of this discussion, since how they react in the atmosphere is less known than pollen. 

One of the major differences between VOCs and pollen is size; pollen can be as large as 100 micrometers in diameter, while we are particularly interested in how natural VOCs can react to create particles less than 2.5 micrometers in diameter (PM2.5).  For perspective, these particles are about 1/30^th the diameter of a human hair, as shown in the below graphic from the Environmental Protection Agency.  At this size, particles can penetrate deep into the bloodstream and lungs, potentially causing health effects more severe than allergies.

Though there are recognizable shows of sudden natural pollution in large quantities from volcanic eruptions, dramatic forest fires, and the like, these are often short-lived.  The emissions from individual plants are quite small, but they happen all the time, and add up; VOC emissions from vegetation in the United States are comparable in scale to those from anthropogenic, or man-made, sources.  So although biogenic emissions cannot be controlled by policy, they are significant and should be considered when talking about air quality as a whole.

The real issue is this: the Intergovernmental Panel on Climate Change predicts a 1.5 - 4.5 degree rise in the global average surface temperature for the next century.  In preparation, it would be useful to understand how pollution levels will react.  Will they get worse?  Where?  A number of studies have determined that higher temperatures will increase ozone concentrations in the air we breathe, which isn’t good news, but it has been more difficult to predict what PM2.5 will do, especially on a regional level.  Many studies also keep the biogenic emissions in their models constant, but as discussed earlier, when temperature rises, many natural VOC emissions will also increase in response.  More VOCs mean more vapors that could form particulate matter, therefore perhaps increasing pollution!

In work published in the journal of Atmospheric Environment, a computer model was used to evaluate what happens to particulate concentrations when you uniformly turn the summer temperatures up across the eastern United States.  This model incorporates meteorology, chemistry, human and natural emissions, and a host of simulated atmospheric processes; many things can change when temperature is increased, but our model showed that the increase in VOCs from plants had by far the most important effect.  It increased organic PM2.5 up to ten times more than any other trial, especially affecting southern regions like Atlanta that are surrounded by more dense plant cover.  The next step is to look at how much of an effect vegetation will have on particle formation when other meteorological factors like rainfall are changed in addition to temperature.  For now, though, it looks like climate change will affect not only sea level rise and temperature, but the air we breathe as well.