Formation of Nanodroplets in N₂/H₂O/SO₂ under Irradiation of Fast Proton Beams

The droplet formation induced by cosmic ray in the terrestrial atmosphere attract certainattention in recent decades because this process could be important to understand thepossible relationship between cosmic ray and climate on the earth. The role of energetic ionsfor the droplet formation would be in both the ion production and oxidation of SO₂ in the air.Attractive polarization forces between ions and molecules decrease energy barrier in dropletgrowth. On the other hand, the oxidation of SO₂ by radical species in ion irradiation result inthe production of H₂SO₄, which also decrease energy barrier of the droplet growth in thebinary nucleation process of water and H₂SO₄.We have performed irradiation of proton beam on the gas mixture of N₂/H₂O/SO₂ andAir/H₂O/SO₂. The reduction of SO₂ concentration by beam irradiation was monitored usingan SO₂ meter and the size distributions of generated droplets were measured with adifferential mobility analyzer. We found that the mass yield of generated droplets showedlinear dependence on the amount of SO₂ oxidation. This behavior is different from binarynucleation theory of water and H₂SO₄. The difference might indicates importance ofconsidering the droplet formation to be a kind of cooperative phenomena by ion processesand oxidation of SO₂.