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Published May 27, 2003 | Published + Erratum
Journal Article Open

New particle formation from photooxidation of diiodomethane (CH_2I_2)


Photolysis of CH_2I_2 in the presence of O_3 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 CH_2I_2 mixing ratio, down to a level of 15 ppt (∼4 × 10^8 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 CH_2I_2, probably aided by other organic iodine compounds, is the mechanism leading to the observed new particle formation in the west coast of Ireland.

Additional Information

© 2003 American Geophysical Union. Received 16 April 2002; revised 25 November 2002; accepted 4 March 2003; published 30 May 2003. This work was supported by U. S. Department of Energy grant DE-FG03-01ER63099. The authors sincerely thank Stan Sander, Kyle Bayes, and Daniel Milligan of the NASA Jet Propulsion Laboratory, Gordon McFiggans of UMIST, Mitchio Okumura and Paul Wennberg of Caltech, Jim Burkholder and Joachim Curtius of the NOAA Aeronomy Laboratory, and Xiao-Ying Yu of Colorado State University for useful suggestions about iodine kinetics.


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, Correction to "New particle formation from photooxidation of diiodomethane (CH2I2)", J. Geophys. Res., 108, D23, 4733, doi:10.1029/2003JD0042492003

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Erratum - jgrd11070.pdf


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August 22, 2023
October 23, 2023