10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Nucleation and Growth of Iodic Particles in the CERN CLOUD Experiment
XUCHENG HE, Lubna Dada, Siddharth Iyer, Hanna Manninen, Yee Jun Tham, Joao Almeida, Dexian Chen, Dominik Stolzenburg, Changhyuk Kim, Arttu Ylisirniö, Maija Peltola, Theodore Koenig, Henning Finkenzeller, Mario Simon, Andrea Baccarini, Richard Flagan, Neil Donahue, Siegfried Schobesberger, Katrianne Lehtipalo, Rainer Volkamer, Douglas Worsnop, Matti Rissanen, Markku Kulmala, Jasper Kirkby, Mikko Sipilä, University of Helsinki
Abstract Number: 276 Working Group: Aerosol Physics
Abstract Trace iodic vapours have a significant impact on atmospheric chemistry, influencing catalytic ozone destruction and the HOx and NOx cycles. Oxidised iodine species also form aerosols in coastal and polar regions [1], so they play a direct role in Earth’s radiation balance. It was recently shown that iodic acid (HIO3) forms new particles in coastal and polar regions [2] but the chemical pathways leading to HIO3, and the nucleation and growth rates of iodic particles, remain poorly understood.
In a comprehensive series of experiments performed under atmospheric conditions in the CLOUD chamber [3] at CERN, we have investigated how the precursor vapours iodine and diiodomethane are oxidized to HIO3; we have also measured the resulting nucleation and growth rates of iodic particles. Surprisingly, we found that HIO3 forms over a range of relative humidity via a previously-unknown mechanism. We have combined our laboratory experiments with quantum chemical calculations to resolve the exact chemical steps leading to the formation of HIO3 in the atmosphere.
A bromide chemical ionization time of flight mass spectrometer was used to measure the concentration of molecular iodine at atmospheric concentrations while simultaneously measuring iodic acids and oxides for the first time. This instrument enabled us to comprehensively track the role of individual iodine species in the gas-phase chemistry and nucleation processes.
We have measured the nucleation and growth rates of pure iodic particles. By comparing experiments under neutral (ion-free) and GCR (galactic cosmic ray) conditions [3], we find an important contribution of ions to the nucleation and initial growth rates. Using a suite of state-of-the-art high-resolution instruments, we were able to follow the appearance and growth of iodic particles from an HIO3 dimer to sizes above 10nm, with chemical identification throughout the entire size range. The data reveal that several growth mechanisms are involved, depending on the particle size.
Our laboratory findings should help to interpret observations of new particle formation in coastal and other regions in the presence of gas-phase iodic species. And the results indicate there are unrevealed oxidation pathways that can affect the fate of ozone in the atmosphere.
[1] O'Dowd, C. D. et al. (2002). Marine aerosol formation from biogenic iodine emissions. Nature 417.
[2] Sipilä et al. (2016). Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3. Nature 537.
[3] Kirkby et al. (2011). Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation. Nature 533.