Measurement of the collision rate coefficients between atmospheric ions and multiply charged aerosol particles in the CERN CLOUD chamber

Aerosol particles have an important role in Earth's radiation balance and climate, both directly and indirectly through aerosol-cloud interactions. Most aerosol particles in the atmosphere are weakly charged, affecting both their collision rates with ions and neutral molecules, as well as the rates by which they are scavenged by other aerosol particles and cloud droplets. The rate coefficients between ions and aerosol particles are important since they determine the growth rates and lifetimes of ions and charged aerosol particles, and so they may influence cloud microphysics, dynamics, and aerosol processing. However, despite their importance, very few experimental measurements exist of charged aerosol collision rates under atmospheric conditions, where galactic cosmic rays in the lower troposphere give rise to ion pair concentrations of around 1000 cm(-3). Here we present measurements in the CERN CLOUD chamber of the rate coefficients between ions and small (< 10 nm) aerosol particles containing up to 9 elementary charges, e. We find the rate coefficient of a singly charged ion with an oppositely charged particle increases from 2.0 (0.4-4.4) x 10(-6) cm(3) s(-1) to 30.6 (24.9-45.1) x 10(-6) cm(3) s(-1) for particles with charges of 1 to 9 e, respectively, where the parentheses indicate the +/- 1 sigma uncertainty interval. Our measurements are compatible with theoretical predictions and show excellent agreement with the model of Gatti and Kortshagen (2008).