To fully address the indirect effect of aerosol particles on Earth's climate must include an understanding of the processes by which those particles catalyze freezing. Particles may act as heterogeneous nuclei in any of four modes - deposition, immersion, condensation, or contact. Of the four, the least understood is contact nucleation, which is a freezing process initiated by contact of an aerosol particle with the surface of a supercooled droplet of water.
Intellectual merits. In this study, we will improve our knowledge concerning ice nucleation in the contact mode by developing and using two instruments, which will measure contact nuclei, using two separate, but complementary techniques. The first, based on electrostatic precipitation, is suitable for laboratory work and is more sensitive to aerosol-droplet interactions. The second, which employs acoustic levitation of the supercooled water droplets, will be amenable to deployment in the field and will have a wider temperature range. The questions to be answered include:
1) What is the mechanism of contact nucleation? 2) How much more effective is the contact mode than immersion/condensation? 3) How does the heterogeneous nucleation rate in the contact mode depend upon the particle size and type? 4) How does the nucleation rate in the contact mode depend upon the number of particles on the surface? (Or what fraction of particles of a given size and type act as contact nuclei) 5) How much time elapses between particle impact and nucleation? 6) What is the concentration of contact nuclei in the atmosphere?
Broader Impacts. The knowledge about contact nucleation provided by these instruments will reduce our uncertainty of the effect of ice and mixed phase clouds on Earth's radiative budget and hydrological cycle, thus contributing to an improved understanding of Earth's climate system. Development of the instruments in this project would also enhance the nation's scientific infrastructure. To our knowledge, no currently existing instrument can quantify contact nucleation in the way that this project will do. Graduate students will also be involved with the project, both in the design and construction of the instruments and in their use to answer the scientific questions posed above. Graduate training and education in these areas will enhance the nation's intellectual infrastructure. In addition, undergraduate students will participate in the experimental design and measurements during the summer for all the three years of the project, providing a unique educational experience.
