10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
How Increased Ionization Can Boost Aerosol Growth to Cloud Condensation Nuclei
MARTIN BØDKER ENGHOFF, Henrik Svensmark, Nir Joseph Shaviv, Jacob Svensmark, Technical University of Denmark
Abstract Number: 120 Working Group: Aerosol Physics
Abstract In this study (Svensmark et al, 2017) the effect of ionization on the growth of aerosols into cloud condensation nuclei is investigated theoretically and experimentally. We show that the mass-flux of small ions can constitute an important addition to the growth caused by condensation of neutral molecules.
Under atmospheric conditions the growth from ions can constitute several percent of the neutral growth. We performed experimental studies which quantify the effect of ions on the growth of aerosols between nucleation and sizes up to 20 nm and find good agreement with theory. Ion-induced condensation could be of importance not just in Earth’s present day atmosphere for the growth of aerosols into cloud condensation nuclei under pristine marine conditions, but also under elevated atmospheric ionization caused by increased supernova activity.
We propose an addition to the condensation equation based on the additional mass-flux of ions to aerosols. The expression depends on the ratio of the number concentration of ions to the number concentration of neutrals, the ratio of the interaction coefficient between ions and neutral aerosols to the interaction coefficient between neutral molecules and aerosols, the ratio of the ion mass to the neutral mass, and finally the ratio of the amount of neutral aerosols to the total amount of aerosols. While the second and fourth terms are (nearly) constant under most conditions the ion/neutral number concentrations and mass vary depending on atmospheric conditions. In a pristine environment at high ionization levels the effect can account for almost 20% of the growth rate of small aerosols and a significant enhancement up to 20-30 nm.
The theory was tested experimentally in 7 m3 atmospheric reaction chamber, measuring the aerosol size distribution and (for some of the experiments) the sulphuric acid concentration. A total of 3100 hours of data with varying gas and ion concentrations were analyzed. For each set of experimental conditions a number of experiments where ionizing gamma sources were opened and closed with typically 2 hour intervals were performed. We then superposed the individual experiments onto each other to reveal the growth profile with and without increased ionization. Analyzing these superposed growth profiles allowed us to compare growth rates with and without ionization to the theory and we found a good agreement between the two.
The mechanism is favoured by high ionization, low gas concentrations, and low aerosol concentrations. This points to pristine settings, such as over oceans away from continental and polluted areas. Potentially the mechanism can contribute significantly to the production of cloud condensation nuclei, but to fully assess the impact the mechanism should be included in global climate simulations.
Svensmark, H., Enghoff, M. B., Shaviv, N. J., and Svensmark, J. (2017). Nat. Commun., 8:2199.