Marine aerosols and iodine emissions

Abstract

Arising from: C. O'Dowd et al. Nature 417, 632–636 (2002); see O'Dowd et al. reply.  O'Dowd et al.¹ describe the formation of marine aerosols from biogenic iodine and the growth of these aerosols into cloud-condensation nuclei (CCN). Based on chamber and modelling results, the authors suggest that biogenic organic iodine compounds emitted from macroalgae may be responsible for coastal particle bursts and that production of these compounds in the open ocean could increase CCN there too. It has since been shown that coastal particles are more likely to be produced from the photooxidation of molecular iodine². Moreover, I contend that open-ocean particle production and cloud enhancement do not result from emissions of organic iodine at atmospheric levels. For iodine particles to affect cloud properties over the remote ocean, an additional source of iodine is necessary as organic precursors cannot be responsible.

Main

O'Dowd et al. propose that, in the presence of ozone, photolysis of atmospheric iodocarbons emitted from marine algae is a viable mechanism for the coastal production of new particles. This follows directly from observations relating aerosol formation and macroalgal iodocarbon emissions³ to the tidal cycle. However, particles formed by a variety of methods² have the same composition as those formed in the chamber study of O'Dowd et al.¹. The methods tested include exposing various Laminaria species to ambient and higher levels of ozone; mixing ozonized air with ozone-free air that has been passed over macroalgae; or exposing molecular iodine vapour (I₂) to ozone². The Laminaria macroalgae species are among those responsible for coastal iodine emissions.

Iodine-atom fluxes from an inorganic source, such as gaseous iodine⁴, have been shown to dominate² those from the organic compound diiodomethane (CH₂I₂; the most photolabile iodocarbon in the coastal environment and the one used by O'Dowd et al. in their chamber). Iodine-atom flux from atmospheric I₂ greatly exceeds that derived from the lowest CH₂I₂ concentrations used in the chamber experiments of O'Dowd et al., which were more than an order of magnitude higher than atmospheric levels. Coastal iodine-containing particles are therefore much more likely to be formed from I₂ than from CH₂I₂.

Particle bursts have been measured at Mace Head in Galway, Ireland, alongside iodocarbon fluxes. The phenomena are characterized by extremely high concentrations and production rates of 3-nm-diameter particles⁵. Some have deduced⁶ that the precursor vapour sources are biologically rich tidal regions, tens to hundreds of metres from the measuring location (or tens of ‘transit seconds’). On these timescales, average atmospheric CH₂I₂ concentrations, which are two orders of magnitude lower than those in the smog chamber¹, would not be able to form the observed 10⁴ to 10⁶ particles cm⁻³. And yet for the laboratory mechanism to work in the coastal atmosphere, nucleation and growth characteristics must behave in the same way.

O'Dowd et al. used a model to demonstrate nanometre-scale sulphate-cluster stabilization by condensation of condensable iodine vapours (CIVs) and a consequent increase in CCN number. Their concentrations of CIVs are consistent with unrepresentatively low source estimates from a model⁷ that is aimed at explaining open-ocean iodine monoxide observations⁸. By assuming that CH₂I₂ is the coastal atmospheric iodine source, the model successfully replicates coastal observations⁷. However, coastal iodine-atom production from I₂ that is associated with an iodine monoxide mixing ratio of a few parts per trillion is about 5×10⁷ cm⁻³ s⁻¹, and not 1×10⁴ cm⁻³ s⁻¹ as calculated from CH₂I₂.

Assuming a common chemical mechanism, the open-ocean iodine-atom production that is required to explain the observed remote marine iodine monoxide must be much higher than that proposed by O'Dowd et al. To achieve this rate, however, a further remote marine-atmospheric iodine source must be found (abiotic production⁹, for example), as the original flux estimate is consistent with the observed open-ocean organic iodine. At increased concentrations, iodine oxides may nucleate in the absence of pre-existing clusters^1,2, a mechanism that could explain the coastal phenomenon and possibly play a part in the open ocean. There is little evidence to support sulphate nucleation in the marine boundary layer following dimethylsulphide oxidation, hence the existence of pre-existing sulphate clusters should not be relied upon to explain marine-iodine enhancement of CCN. The identity of CIVs and their role in aerosol and CCN formation are unclear, as are their production rates as a function of an enhanced iodine-atom source. Taken together, all this calls for an improved understanding of the key species and processes involved in aerosol nucleation and growth in the marine boundary layer.