Iodine-containing emissions from marine algae can be converted by sunlight into aerosol particles. If this phenomenon occurs on a large scale, it could have significant effects on climate.
The discovery of a previously unrecognized source of aerosol particles is big news to atmospheric scientists. Just such a source is described by O'Dowd et al. on page 632 of this issue1. Building on earlier work, they have unravelled a photochemical phenomenon that occurs in sea air and produces aerosol particles composed largely of iodine oxides. The precursor molecules are organic iodide vapours emitted by marine algae.
One reason for the interest in atmospheric aerosols is their effect on climate and on our understanding of climate change2. In particular, uncertainties about the composition and distribution of fine aerosol particles, no more than a few micrometres in diameter, cause large uncertainties in predictions of global warning driven by the accumulation of greenhouse gases. Depending on their composition, aerosols can have a direct effect on Earth's radiative balance by back-scattering (or absorbing) incoming solar radiation, leading to cooling (or warming). Indirectly, their influence is felt through their action as cloud-condensation nuclei, catalysing cloud formation. The more aerosol particles that can induce droplet formation in a cooling air mass, the smaller the resulting cloud droplets. Smaller droplets produce brighter clouds, which might also be longer lived because they are less likely to precipitate as rainfall. Indeed certain observations indicate that aerosols from forest fires and urban pollution can suppress rain and snow fall3,4.
Aerosol sources are shown in Fig. 1 and can be divided into two types. Primary aerosols are emitted directly, such as smoke from bush or forest fires, soot and ash from factories, motor vehicles, trains, boats and planes, airborne dust, and sea-salt particles produced when sea spray dries out. Globally, however, much of the ambient particulate burden, and most of the fine aerosols, are produced in the atmosphere itself. These secondary aerosols arise from oxidation of precursor gases, such as sulphur dioxide, nitrogen oxides and volatile organic compounds, to form less volatile products. The resulting oxidation products then nucleate to form new particles or condense on pre-existing particles. Figure 1 also indicates the main effects of atmospheric particles.
For many years it was assumed that the primary chemical source of new atmospheric particles was the co-condensation of sulphuric acid vapour and water vapour5. Over continental areas, sulphuric acid vapour is formed primarily by the oxidation of sulphur dioxide produced in burning oil and coal. In clean marine environments it is produced by the oxidation of dimethylsulphide and other reduced sulphur compounds emitted by marine organisms6.
With improved measurement techniques, however, bursts of formation of new particles have been observed when the concentration of sulphuric acid vapour is too low to support its combination with water vapour. In some special cases, atmospheric concentrations of other condensable inorganic species, for instance ammonia or nitric acid vapour, are high enough to account for particle formation and growth through a nucleation mechanism7. And quite recently, evidence has emerged for a completely different source in forested regions: gaseous monoterpenes released from trees can be photo-oxidized to condensable carboxylic acids fast enough to produce bursts of particle formation and growth8,9.
Over the past few years, O'Dowd and co-workers have observed episodes of particle creation in their study areas along the Irish coast. But these episodes could not be explained by nucleation and condensation driven by sulphuric acid or carboxylic acids10,11,12. Knowing that seaweed can emit easily photolysed alkyl iodide compounds, such as CH2I2, and that the resulting gaseous iodine reacts rapidly with ozone and other atmospheric oxidants to produce iodine oxides, they explored this avenue through laboratory experiments. This preliminary research showed that the photolysis of CH2I2 in the presence of ozone produces copious numbers of fine particles13.
In their latest paper1, O'Dowd and colleagues have taken the earlier work a crucial step further. They have extended the laboratory experiments to realistic coastal conditions, reproduced in a state-of-the-art atmospheric smog chamber. They show that the photolysis of CH2I2 at concentrations as low as 0.015 parts per billion by volume, well within levels often found in coastal environments, is a potent source of aerosol particles. Using a suite of instruments for characterizing the dynamics of particle formation and growth, and an aerosol mass spectrometer for determining chemical content as a function of particle size, they charted particle dynamics and confirmed that the particles formed in their chamber were predominantly composed of iodine oxides, the simplest of which may be OIO, HOI and I2O2. The authors' suggested reaction mechanism for the creation of these species, after the photochemical production of iodine from algal CH2I2 emissions, is shown in their Fig. 2 on page 633.
To produce stable new particles in the clean, open-ocean marine atmosphere, concentrations of condensable vapour have to be high enough both to nucleate new nanometre-scale particles and to allow them to grow by agglomeration and vapour condensation to the stable 50–100-nm size range5. If there is too little condensable vapour, new particles don't form or they re-evaporate or agglomerate with pre-existing particles. O'Dowd et al.1 describe modelling calculations which suggest that CH2I2 concentrations over the open ocean might well be high enough for the resulting condensable iodine oxides to allow newly nucleated sulphuric acid particles to become large enough to survive. The authors propose that the resulting particles might be abundant enough to influence the Earth's radiative balance. At the least, their model suggests that iodine oxides produced from volatile organic iodide compounds such as CH2I2 must be added to the list of precursors for secondary aerosol formation.
In retrospect, this might not be too surprising. In pioneering research off Hawaii14 and Puerto Rico15 in the 1970s, it was shown that iodine becomes concentrated in atmospherically processed sea-salt aerosol. In contrast, other halogens — chlorine and bromine — are depleted. These 30-year-old studies further showed that the iodine levels vary inversely with particle size, just as one would expect from a gas-phase condensable source of iodine oxide such as that described by O'Dowd et al.1.
The obvious task that remains is to determine just how widespread this newly identified mechanism of particle growth is. To have a significant influence on climate, it would have to be effective over the oceans as a whole, not just in the coastal environment. The appropriate field-measurement tools and analytical models are already in hand.
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Characterization of iodine species in the marine aerosol: To understand their roles in particle formation processes
Frontiers of Chemistry in China (2006)