Getting Cloud Ice from Pollution

Kulkarni, G., Pacific Northwest National Laboratory

Aerosol Processes

Aerosol Life Cycle

Kulkarni G, S China, S Liu, M Nandasiri, N Sharma, J Wilson, AC Aiken, D Chand, A Laskin, C Mazzoleni, M Pekour, J Shilling, V Shutthanandan, A Zelenyuk, and RA Zaveri. 2016. "Ice nucleation activity of diesel soot particles at cirrus relevant temperature conditions: Effects of hydration, secondary organics coating, soot morphology, and coagulation." Geophysical Research Letters, 43(7), 10.1002/2016gl068707.

Science

Atmospheric soot (black carbon) particles absorb solar energy to warm the atmosphere—yet high in the atmosphere they may instigate cloud ice formation. But when these particles mix with other aerosols and compounds in the atmosphere, their ice-forming (ice-nucleating) ability suffers, thus altering the properties of pure ice clouds (cirrus), vital for their impact to moderate the Earth’s energy balance. This experimental study shows the importance of physical and chemical properties of soot particles toward ice nucleation.

Impact

The research found that the efficiency of soot to form ice particles (ice nucleation efficiency) varies with the aging processes such as humidification and coating and that understanding the shift in the ice nucleation from heterogeneous to homogeneous mechanisms under certain conditions shows the importance of using the appropriate representation in the models to correctly simulate soot particle ice nucleation. Understanding how cirrus (ice clouds) are formed and sustained will improve estimates of Earth’s energy budget under the influence of climate change.

Summary

A research team led by DOE’s Pacific Northwest National Laboratory and comprised of experts from different domains of aerosol-cloud interaction research took freshly generated soot particles from a commercial diesel engine, located outside the Atmospheric Measurements Laboratory at PNNL as part of the Soot Aerosol Aging Study (SASS) campaign in 2013-2014. Some of the particles were aged in the PNNL Environmental Chamber with α-pinene and toluene coatings. Bare, aged, humidified, coagulated, and thermodenuded (coated, then stripped of volatile coatings) particles were also investigated. The experiments were designed to simulate various atmospheric aging processes such as hydration, transformation into organic-coated particles, coagulation, and morphology changes in soot in dry and humid conditions. PNNL’s ice nucleation chamber was used to understand the temperature and humidity conditions necessary to nucleate ice on each particle type. A single-particle mass spectrometer was used for detailed, real-time characterization of the bare soot particles generated by the diesel engine and to size-select the particles. An X-ray photoelectron spectroscope was used to determine the chemical state and surface composition of bare diesel soot particles. The researchers found that bare particles have an ice nucleation efficiency similar to particles that are coated under high-humidity conditions, and thermodenuded (soot stripped of coatings) particles, even though the shape of the latter was different. Results suggest that heterogeneous ice nucleation activity of freshly emitted diesel soot particles is sensitive to some of the aging processes that soot can undergo in the atmosphere. The study provides a comprehensive understanding of atmospherically relevant soot particles that promote ice nucleation.