Science in Seconds: Q&A with Armin Sorooshian

By Stephanie Doster | September 10, 2009

Armin Sorooshian is a self-described “aerosol-cloud-rain fanatic” who loves jumping in the back of a Twin Otter airplane and watching chemistry and physics happen at about three miles above the Earth. After graduating from the UA Summa Cum Laude with a BS in chemical and environmental engineering, Sorooshian received an MS and PhD in chemical engineering from the California Institute of Technology (Cal Tech). His thesis work used instrument development, aircraft and laboratory measurements, and modeling to study the role of aerosols in shaping climate and environmental change. He spent 2008–09 as a postdoctoral research scholar at the Cooperative Institute for Research in the Atmosphere (CIRA) at Colorado State University (with the National Oceanic and Atmospheric Administration in Boulder, Colo.), where he investigated aerosol-cloud-precipitation interactions via satellite remote sensing and cloud models. In a Sept. 3 interview, he described the research that has brought him full circle; he’s back at the UA, this time as an assistant professor in the Department of Chemical and Environmental Engineering.

Armin Sorooshian as flight scientist aboard a Twin Otter. From the back of the plane, he tells the pilots where to fly and operates the instruments for his research.

Q: What are aerosols and why do they matter?
Sorooshian: Aerosols are particles in the air. They include dust, sea spray from waves, and exhaust particles from our cars. They are natural and manmade, and can range from about one nanometer in diameter (10-9 meters) to several micrometers. They are really small, but the effect they have on things like climate, health (including the respiratory system), and visibility in the atmosphere is huge and totally disproportionate to their size. Particles in the atmosphere scatter and absorb light, and that degrades visibility. Aerosols also are important in terms of renewable energy because by directly interacting with light and also influencing cloud formation, they can reduce the amount of energy we can get through solar power.

Q: What about climate?
Sorooshian: Because aerosols scatter light, the light that would have come down to warm us is instead reflected up, so they play a role in cooling the Earth. Another thing, which is of great interest to me, involves clouds. The clouds you see in the sky cannot form without particles. Particles are lifted and water condenses around them, and that forms droplets. Clouds also reflect light back to the atmosphere. So the clouds cool the Earth as well, and particles influence precipitation due to their role in cloud formation. The latest Intergovernmental Panel on Climate Change [IPCC 2007] report identified aerosol-cloud interactions as the largest single source of uncertainty in current estimates of the total anthropogenic radiative forcing, which is why I am motivated to do a better job of understanding these processes.

Q: Related to that, describe the study in which you generated a map showing what ocean regions might be more vulnerable to pollution aerosol.
Sorooshian: Aerosols affect clouds because they are the seeds of water droplets. Pretend you have two clouds that are identical, with the same amount of liquid water, but one cloud grows in more polluted conditions. The more polluted, or “optically thick,” cloud will have more, but smaller, droplets. The smaller the droplet, the more reflective the cloud becomes and cools the Earth more than the cleaner cloud. The way you make precipitation in clouds is through the collision of these droplets. They have to hit each other and become larger. But the smaller the droplets are, the harder it is for them to collide. The result is the polluted cloud will be more reflective and will not want to rain as much, so rainfall is suppressed in those clouds. 

We used measurements from the A-train satellites and different models to show for the first time what parts of the world will lose out on rain because of the pollution they have. In some instances, the pollution has come from somewhere else. We only used data over the oceans because that is where the satellite data are most accurate. The uncertainties over land are much larger. The different models and A-train measurements both showed qualitative agreement that clouds with a finite range of cloud water associated with them are the most sensitive to aerosols with regard to how much they will rain. I think it’s really cool and it’s the first time anyone has been able to show this. There are a ton of other questions now.

Q: Such as?
Sorooshian: We want to look at the data over the land. The clouds we looked at are called warm clouds and have liquid water, but there are other clouds, deeper clouds like the monsoon clouds, that can contain ice. The science of those clouds is not as mature because there are so many complexities. We’ve been focusing mostly on warm clouds, but we want to get into deeper clouds down the road. We also want to look at how different types of particles influence precipitation. Dust particles from the Sahara Desert, for example, are a lot different than sea salt particles over the ocean and industrial pollution particles emitted in Southeast Asia. They have different  physical properties and sizes and can influence clouds in different ways.

Q: How did you get interested in aerosols and clouds?
Sorooshian: When I was an undergrad at the UA, I worked for Jost Wendt, who was the department head. He did a lot of combustion research with particles, and that’s how I got into the particle thing. He recommended I apply to do my graduate work at Cal Tech under the supervision of John Seinfeld and Rick Flagan, two experts in the field of aerosols. Throughout my life I’ve had a deep fascination with planes and I knew the Seinfeld/Flagan research group was involved with aircraft measurements. My introduction to the cloud world was during a field mission in the Ohio River Valley. Because I had just been doing aerosol chemistry measurements, I had no knowledge of clouds, and then this flight came along. A power plant plume fed straight into a deck of clouds, and we observed all this exciting chemistry in the cloud droplets, and that’s how I got on this track of studying clouds.

Q: What do you do on the flights?
Sorooshian: There are two pilots and I am the flight scientist. I sit in back and operate the instruments and tell the pilots where to fly, real-time. I see where the plumes are and tell the pilot to go that way, go higher, go back. Most of my graduate work was done in airplanes. We made measurements in all types of situations, including downwind of power plants, ocean-going ships, cattle feedlots, ocean phytoplankton blooms, and urban downtown areas. Air comes through an inlet at the nose of the aircraft and splits off into about 10 different instruments that measure things like particle chemistry, size and, hygroscopicity (how much water the particles absorb).

Q: What did your study involving feedlots reveal?
Sorooshian: We did some lab studies that suggested a class of species called amines exist in particles, so we tested the hypothesis and they were there. The project stemmed from a massive cattle feedlot along Interstate 5 in California. About 100,000 cows are there at any one time and it stinks. As a PhD student, I told my advisors we should fly over this thing for several hours and measure the emissions. When you have a feedlot like this there are a lot of ammonia and amine emissions. When these gases are emitted there is chemistry in the atmosphere and acids and bases can react with each other and make products that condense into particles. Downwind of one of these things you get a lot of ammonium nitrate. The interesting thing in this study is we actually found these elusive amines species in particles that are kind of like ammonium but health effects are associated with them. In a study in which laboratory test subjects were exposed to particles coated with amines, the instance of blood clots went up as compared to other types of particles.

Q: With the fires around Los Angeles in the headlines, do you measure particulate matter in smoke?
AS: As we speak, my group has an instrument measuring aerosol composition on the rooftop of a building at Cal Tech in Pasadena, Calif., so we’re getting really exciting data right now. For two weeks we’ve had a suite of instruments measuring different fire particle properties, and it’s going to be a really nice data set. We can use the data to learn more about the biomass burning aerosols to improve parameterizations of their properties in models.

Q: What is one of your goals as a UA faculty member?
AS: I’d like to get more involved with public policy work. As I get deeper into the science of how aerosol pollution influences things like precipitation, health, renewable energy, and climate, it would be really awesome to make a difference and to inform people about this important problem. Maybe that would help people get more serious about pollution control.