Read Full Article (file size: 1332978 bytes)    Cited by

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D10, 4318, doi:10.1029/2002JD002452, 2003

New particle formation from photooxidation of diiodomethane (CH₂I₂)

Jose L. Jimenez

Department of Environmental Science and Engineering and Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

Roya Bahreini

Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, California, USA

David R. Cocker III

Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, California, USA

Hong Zhuang

Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, California, USA

Varuntida Varutbangkul

Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

Richard C. Flagan

Department of Environmental Science and Engineering and Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

John H. Seinfeld

Department of Environmental Science and Engineering and Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

Colin D. O'Dowd

Department of Physics, University of Helsinki, Helsinki, Finland

Thorsten Hoffmann

Institute of Spectrochemistry and Applied Spectroscopy, Dortmund, Germany

Abstract

Photolysis of CH₂I₂ in the presence of O₃ has been proposed as a mechanism leading to intense new particle formation in coastal areas. We report here a comprehensive laboratory chamber study of this system. Rapid homogeneous nucleation was observed over three orders of magnitude in CH₂I₂ mixing ratio, down to a level of 15 ppt (∼4 × 10⁸ molec. cm⁻³) comparable to the directly measured total gas-phase iodine species concentrations in coastal areas. After the nucleation burst, the observed aerosol dynamics in the chamber was dominated by condensation of additional vapors onto existing particles and particle coagulation. Particles formed under dry conditions are fractal agglomerates with mass fractal dimension, D _f ∼ 1.8–2.5. Higher relative humidity (65%) does not change the nucleation or growth behavior from that under dry conditions, but results in more compact and dense particles (D _f ∼ 2.7). On the basis of the known gas-phase chemistry, OIO is the most likely gas-phase species to produce the observed nucleation and aerosol growth; however, the current understanding of this chemistry is very likely incomplete. Chemical analysis of the aerosol using an Aerodyne Aerosol Mass Spectrometer reveals that the particles are composed mainly of iodine oxides but also contain water and/or iodine oxyacids. The system studied here can produce nucleation events as intense as those observed in coastal areas. On the basis of comparison between the particle composition, hygroscopicity, and nucleation and growth rates observed in coastal nucleation and in the experiments reported here, it is likely that photooxidation of CH₂I₂, probably aided by other organic iodine compounds, is the mechanism leading to the observed new particle formation in the west coast of Ireland.

Received 16 April 2002; accepted 4 March 2003; published 30 May 2003.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry.

Read Full Article (file size: 1332978 bytes)    Cited by

Citation: Jimenez, J. L., R. Bahreini, D. R. Cocker III, H. Zhuang, V. Varutbangkul, R. C. Flagan, J. H. Seinfeld, C. D. O'Dowd, and T. Hoffmann (2003), New particle formation from photooxidation of diiodomethane (CH₂I₂), J. Geophys. Res., 108(D10), 4318, doi:10.1029/2002JD002452.

Copyright 2003 by the American Geophysical Union.