Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine

[摘要] Renewal of ultrafine aerosols in the marine boundary layer may lead torepopulation of the marine distribution and ultimately determine theconcentration of cloud condensation nuclei (CCN). Thus the formation ofnanometre-scale particles can lead to enhanced scattering of incomingradiation and a net cooling of the atmosphere. The recent demonstration ofthe chamber formation of new particles from the photolytic production ofcondensable iodine-containing compounds from diiodomethane(CH₂I₂), (O'Dowd et al., 2002; Kolb, 2002; Jimenez et al., 2003a;Burkholder and Ravishankara, 2003), provides an additional mechanism to thegas-to-particle conversion of sulphuric acid formed in the photo-oxidationof dimethylsulphide for marine aerosol repopulation. CH₂I₂ isemitted from seaweeds (Carpenter et al., 1999, 2000) and has been suggestedas an initiator of particle formation. We demonstrate here for the firsttime that ultrafine iodine-containing particles are produced by intertidalmacroalgae exposed to ambient levels of ozone. The particle composition isvery similar both to those formed in the chamber photo-oxidation ofdiiodomethane and in the oxidation of molecular iodine by ozone. Theparticles formed in all three systems are similarly aspherical. When small,those formed in the molecular iodine system swell only moderately whenexposed to increased humidity environments, and swell progressively lesswith increasing size; this behaviour occurs whether they are formed in dryor humid environments, in contrast to those in the CH₂I₂ system.Direct coastal boundary layer observations of molecular iodine, ultrafineparticle production and iodocarbons are reported. Using a newly measuredmolecular iodine photolysis rate, it is shown that, if atomic iodine isinvolved in the observed particle bursts, it is of the order of at least1000 times more likely to result from molecular iodine photolysis thandiiodomethane photolysis. A hypothesis for molecular iodine release fromintertidal macroalgae is presented and the potential importance ofmacroalgal iodine particles in their contribution to CCN and globalradiative forcing are discussed.

[学科分类] 大气科学