Ice cloud formation potential by free tropospheric particles at Pico Mountain Observatory, Azores
Mazzoleni, Claudio — Michigan Technological University
Knopf, Daniel — Stony Brook University
Area of research:
Atmospheric aerosol particles affect the radiative properties of clouds. An open scientific challenge is understanding the ice nucleation process in the atmosphere. In particular, analyses and characterizations of the ice-nucleating properties of aerosol transported over remote oceanic regions are still sparse. While transported over long ranges, primary aerosol particles internally mix with other material. The resulting changes in the aerosol morphology and mixing configuration can have a significant impact on their ability to nucleate ice in clouds. These processes need to be better understood to improve the numerical model representation of aerosol and clouds for climate predictions. To this end, we studied the ice-nucleating properties of aerosol particles that were transported over long range across the North Atlantic Ocean in the free troposphere. The samples were collected at the elevated Pico Mountain Observatory in the Azores.
The study found that the ability of long-range transported aerosol particles to nucleate ice is significantly affected by the coating material on their surfaces, independent of the primary aerosol particle type (e.g., dust, sea salt, and soot), their source, and their transport pattern. These findings underscore the potential importance of aerosol aging processes on ice nucleation, which in turn influences the means by which ice nucleation is modeled, especially in remote regions, far from the primary emissions. The study is based on a limited set of samples and further sample collection and analysis should be undertaken at remote locations around the globe (both vertically and horizontally) to fully understand the aging processes and their impact on ice cloud formation.
Pico Mountain Observatory (PMO) is located on Pico Island, in the Azores, an archipelago of small volcanic islands in the Central North Atlantic. PMO sits in the summit caldera of the inactive Pico Volcano at 2225 m a.s.l. Because of its location and elevation—several hundreds of miles away from other orographic systems of comparable elevation—PMO often intercepts aerosol particles that have been transported in the free troposphere from North America or that recirculated over the Atlantic Ocean, with negligible contribution from local sources. As part of a larger field deployment in the summer of 2013, we performed a pilot study and collected a few substrates for offline ice nucleation and electron microscopy analysis performed at Stony Brook and Michigan Technological universities, respectively. Ancillary data to support the interpretation of the ice nucleation experiments included particle concentration, bulk chemical composition, single-particle elemental composition, and retro-plume simulations. The most surprising result was that the ice nucleation temperature and relative humidity varied only by 5 percent between samples, despite the very different transport patterns, aerosol types, and sources. The hypothesis put forward is that the ubiquitous coating material found on the aerosol particles studied here (probably of organic nature) is responsible for the similar nucleating conditions. Our study highlights the need to further study the importance that free tropospheric aerosol might have on cloud glaciation and the critical role that atmospheric processing might play in determining their ability to nucleate ice.