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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].

2.5 Effect of Cosmic Rays on Atmospheric Aerosol

It is known that the constant stream of fast charged particles, mostly protons that emanate from the sun affects our climate. The details of the interaction are complex and there may be several different mechanisms but a prominent one involving nanoparticles is that the cosmic rays entering the atmosphere leave a trail of ionized gas molecules that can act as nucleation centers for CCNs. Put simply the cosmic rays encourage cloud formation. As described above this can, in turn, affect the atmospheric aerosol load via, for example, the feedback mechanism involving DMS and phytoplankton. The relatively small flux of cosmic rays incident on the Earth can thus have a disproportionately large effect on atmospheric conditions. The amplifying effect arises from cosmic rays influencing the amount of atmospheric aerosol, mostly in the form of nanoparticles. The solar cosmic‐ray flux shows large variations with conditions in the sun, for example, the 11‐year sunspot cycle. A possible illustration of how influential solar activity can be is the correlation of the period of the Maunder minimum, when very few sunspots were observed, indicating low activity and cosmic ray flux, and the coldest part of the so‐called Mini Ice Age in the seventeenth and eighteenth centuries (Figure 2.13).

Strong evidence for a direct link between atmospheric aerosol and solar activity comes from the Greenland Ice Sheet Project 2 (GISP2), which examines the depth profile of impurities in Greenland ice. This is a convenient way to determine atmospheric conditions in the past as any particular concentrations of chemicals or particles are frozen into the ice at a depth that depends on how long ago they were present in the atmosphere. Analysis of the variations in particle concentration of the top 120 m, corresponding to the last 400 years, shows a correlation of the aerosol load with the sunspot number, that is, with the solar cosmic ray flux [20]. Not all cosmic rays come from the sun and there is a significant flux, especially of higher energy particles, from sources outside our own galaxy. It is sobering to realize that events in the far reaches of the Universe influence our climate.

Figure 2.13 Maunder minimum and the little ice age. The Plot, using historical records, of the observed sunspot number over the last 400 years. The Maunder minimum between 1645 and 1715 coincides with the coldest part of the so‐called Mini Ice Age in the seventeenth and eighteenth centuries in which unusually cold winters occurred.

      Source: NCdave. https://commons.wikimedia.org/wiki/File:Sunspot_Numbers.png, Licensed under CC BY‐SA 3.0 (https://creativecommons.org/licenses/by‐sa/3.0/deed.en).

2.6 Nanoparticles in Space

      Nanoparticles themselves do not stop at the top of the Earth's atmosphere and cosmic particles (referred to as “dust” by astronomers) are spread throughout space from a number of sources. Supernovae (Figure 2.1e) have already been mentioned but others include outflowing material from carbon‐rich stars, which is rich in silicon carbide and titanium carbide particles [21] as well as various forms of pure carbon particles including fullerenes (see Chapter 3). As with the particle populations measured in the Earth's atmosphere, when measured as the number density or surface area, it is nanoparticles (<100 nm) that dominate the distribution (Figure 2.2). Thus nanoparticles provide a significant proportion of the solid surface area in space on which chemical reactions can take place.

      Dust particles accelerate the process of condensation of gas clouds by gravity to form stars and planets thus nanoparticles were an important ingredient in the initial formation of our own sun and its planets, including the Earth. It is interesting to note that the special behavior of nanoparticles compared to the bulk matter discussed in Chapter 1 is also important in this context. For example, a significant fraction of particles produced by supernova explosions contain iron (from the core of the exploding star) and are magnetic. The magnetic interaction between the particles in space, which is orders of magnitude stronger than their gravitational attraction, can significantly accelerate the process of condensation and for this to work the particles must be single‐domains, that is, permanently magnetized. As discussed in the previous chapter this requires that they are smaller than a critical size of about 100 nm. Once stars and planets are formed they produce interplanetary particles by various processes. For example, in our own solar system the Jovian satellite Io, which it has a very high volcanic activity sprays vast quantities of particles into the rest of the solar system [22].

Comets are thought to preserve the pristine dust that was present during the formation of the solar system and this dust along with water is shed in the comet's tail as it is warmed during its closest encounter with the

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