Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 36

A significant proportion of atmospheric nanoparticles are generated above the oceans. These are known as Marine Aerosol and are produced by a number of sources. The simplest to understand are sea‐salt particles, which are produced when bursting bubbles at the surface produce a spray of droplets of brine from which the water evaporates to leave salt particles. These have a wide size range but all are small enough to form an aerosol and as with most aerosols, when measured as the number of particles per unit volume, the nanoparticles dominate with most particles having sizes around 30 nm [17]. Bearing in mind how slowly these falls out due to gravity they are easily carried into all levels of the atmosphere by winds and updrafts and a significant proportion of the aerosol over land is sea‐salt particles.

The story of Marine Aerosol becomes much more complicated when life is included since plankton and microorganism on the ocean surface, enable other mechanisms for producing particles. A common example starts with the chemical dimethyl sulfide (DMS), which is produced by phytoplankton³ and released to the atmosphere above the oceans. Phytoplankton is the collective name for the many types of microscopic plants, coming in a variety of shapes that dwell just below the ocean surface. Their name is derived from the Greek phyton or “plant” and plagty or “drifter” and they are sometimes referred to as the “grasses of the sea.” They are similar to land‐based plants, containing chlorophyll, and using sunlight for photosynthesis, which is why they are found close to the surface. Their prevalence is revealed by satellite images such as the one shown in Figure 2.11 from NASA's terra satellite. Huge turquoise colored regions reveal the presence of blooming phytoplankton. The DMS released by the plankton emerges from the sea and oxidizes in the atmosphere. The resulting compounds condense into sulfur‐containing nanoparticles that are carried high into the atmosphere.

Figure 2.11 Phytoplankton bloom in the North Sea. Clouds of phytoplankton are the turquoise colored patches in this image acquired on 27 June 2003 by the MODIS instrument on NASA's terra satellite. The landmass at the top right is Norway and Denmark is on the bottom right. Phytoplankton grows in nutrient‐rich waters, and multiplies very quickly; blooms big enough to be seen from space, like this one, can take only days to appear. Also visible are a number of streaky aircraft contrails.

Source: Image reproduced courtesy of NASA (http://visibleearth.nasa.gov).

Figure 2.12 Composition of particles produced by phytoplankton. (a) Seasonal average over five years of sea‐surface chlorophyll concentrations in winter (top image) and spring (bottom image) obtained by the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) instrument in low earth orbit. The seasonal difference in biological activity is clear. The location of Mace Head where the aerosol composition was measured is shown in the top image. (b) Composition of aerosol in different size ranges. The region from 0.06 to 0.125 μm (60–125 nm) shows the nanoparticle abundance. In winter they are undetectable but during phytoplankton blooms they are abundant. The data is given for the different particle types: Sea salt (produced by the bubble‐bursting mechanism), NH4, non‐sea‐salt (NSS) SO₄, NO₃, water‐soluble organic carbon (WSOC) and water‐insoluble organic carbon (WIOC).

Source: Reproduced with the permission of the Nature Publishing Group from C. O'Dowd et al. [18].

A study of marine aerosol, generated above the North Atlantic and arriving at Mace Head on the West coast of Ireland, quantified the organic (life‐produced) and inorganic contributions to the marine aerosol in different seasons [18]. A marked difference was found between periods of high biological activity (plankton blooms) in the summer and periods of low biological activity in the winter. Figure 2.12a shows the level of chlorophyll across the Atlantic in summer and winter with (appropriately) green representing high chlorophyll levels. Notice how the summer bloom lights up the whole North Atlantic – an event of enormous scale. The bar graphs in Figure 2.12b show the composition of the marine aerosol during the two periods separated into different types of particle in different size ranges. The first bar covering the size range 0.06–0.125 μm (60–125 nm) shows the nanoparticle abundance. In winter these are undetectable when measured as a mass fraction (though they would still dominate if measured as a number density) and the entire diagram is dominated by sea‐salt particles. During the summer periods, water‐soluble, and water‐insoluble organic nanoparticles generated by phytoplankton are prolific.

The fact that life generates aerosol produces an important feedback mechanism in the climate. As described above, an increase in the density of atmospheric aerosol generates more cloud, which reduces the amount of sunlight reaching the sea surface and thus reduces the energy available for phytoplankton. If the phytoplankton is less prolific the nanoparticle production rate via the DMS route is scaled back. In the context of global warming, this is a stabilizing effect since warmer seas encourage phytoplankton growth, which increases the rate of DMS production and thus the amount of cloud, which produces a cooling effect.

Not just the products of life but life itself are thrown out of the sea by the bursting bubble route that produces sea‐salt particles. Among the soup of microscopic organisms that live near ocean surfaces are bacteria and viruses. The ones thrown out of the sea join the general atmospheric aerosol of nanoparticles and act as CCNs. In the arctic, they are thought to be a significant contribution to the CCNs responsible for clouds [19].