Aerosols, Clouds, and Climate
Jian Wang
Spotlight: Jian Wang | Dept. of Energy, Environmental, and Chemical Engineering
Contributed by Bennett Rosenberg on May 22, 2022.
In June 2017, Dr. Jian Wang led a research investigation deploying atmospheric measurement instruments over the eastern North Atlantic to study aerosols and clouds in remote marine environments. Researching at the Brookhaven National Laboratory, Wang worked with a team of 25 researchers to design flight plans for their research aircraft, sample aerosol populations and properties, investigate the seasonal variation of cloud condensation nuclei, and more. The project was titled ACE-ENA, and the data is public.
Now, at WashU’s Atmospheric Science & Engineering (ASE) Laboratory and with the support of the US Department of Energy, Wang analyzes that data in his research to quantify the effects of atmospheric aerosols on clouds and climate. Aerosols are solid or liquid particles suspended in the air. Aerosol particles can directly influence climate by scattering and absorbing solar radiation. They can also have a strong but indirect influence on climate by changing the properties of clouds, including albedo (a measure of reflectivity, with white being the most reflective), cloud droplet size, and precipitation. Aerosols are among the greatest contributors to the uncertainty in present understanding of climate change.
However uncertain our knowledge is, we do know that aerosols play important roles in climate systems. Take clouds, for instance, which form when water vapor condenses on aerosol particles that serve as “seeds,” called cloud condensation nuclei (CCN). The more numerous these seeds are, the brighter the clouds tend to be. This changes the albedo, or reflectivity, of Earth’s atmosphere. A higher albedo reflects more sunlight back into space and shades the earth. NASA estimates that even a 5% increase in cloud albedo could compensate for the entire increase in greenhouse gasses from the modern industrial era in the global average. The population of the seeds also influences cloud coverage, which could further impact climate. That’s why studying aerosols and how the CCN population evolves in the atmosphere is critical.
*The above graphic is not scientifically accurate. It implies water condenses and coats an insoluble aerosol particle. In most cases, the species in the aerosol particle dissolves in the condensed water and forms a solution.* From Uppsala Universitet.
From his ACE-ENA experiment, Wang discovered that gas phase molecules frequently nucleate and form new particles over the area 1-2 kilometers above the ocean, called the marine boundary layer. Such particle formation inside the marine boundary layer was previously thought to be very rare. When cold fronts pass over the open ocean, favorable conditions are created inside the marine boundary layer for new particles to form. These newly formed particles grow and become cloud condensation nuclei, thereby providing the “seeds” necessary for low-level marine clouds to develop. This particle formation is a significant source of the CCN population over the open oceans. He and a team of scientists published these new findings in January 2021.
Demonstration of the marine boundary layer and the mechanism of new particle formation after a cold front. From Zheng et al., 2021.
Aside from atmospheric analysis, the ASE lab also develops novel aerosol instrumentation. It has four patents, including spectrometers and a rapid condensation nanoparticle counter. These tools enabled accurate measurements of aerosol properties during ACE-ENA and other studies. For instance, at high altitudes over the Atlantic, Wang’s group used aircraft to measure aerosols. However, traditional instruments collect data over a 1-2 minutes span, and in an airplane that travels 100 m/s, the slow traditional instruments would only record a data point every 6-12 km. So, they developed a Fast Integrated Mobility Spectrometer which provides measurements every second, enabling a data point to be recorded every 100m. More data means less scientific uncertainty, and uncertainty is what Wang is chipping away at with his research on climatic aerosols.
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