Introduction to Nanoscience and Nanotechnology. Chris Binns

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Introduction to Nanoscience and Nanotechnology - Chris Binns


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fall sufficiently slowly under gravity to be “suspended” (see Advanced Reading Box 2.1) but under certain circumstances can grow large enough to precipitate out as rain.

      CCNs are an example of where it is the number density of particles that is important rather than the mass they contain. Each particle will act as a perfectly good CCN, although until recently, it had been thought that particles smaller than 50 nm, referred to as ultrafine aerosol particles or UAPs by atmospheric scientists, do not make efficient CCNs. It is not just size that has influence but also the material. For example, the growth of cloud droplets is profoundly affected if the CCNs are soluble in water and one mechanism is that soluble CCNs can change the surface tension of the water droplets condensing onto them thus changing the stable droplet size for a given water vapor pressure (see Advanced Reading Box 2.2).

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      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.

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


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