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.
Under certain conditions, the cloud droplet can grow large enough to drop out of the cloud as rain. These raindrops contain the CCNs that started the water drop growing in the first place so, although there is a tendency to regard rainwater as pure it contains the particles that formed the original CCNs. If these contain sulfur the rain will be acidic to a degree and, as described below, there are natural processes that produce sulfur‐containing aerosol so a certain amount of acid rain is inherent in climate processes and has nothing to do with human activities. The relative sizes of CCNs, cloud droplets and raindrops are illustrated in Figure 2.9. Precipitating clouds are a mechanism for removing atmospheric aerosol and thus form a self‐regulating feedback system. An increase in the density of aerosol produces more CCNs, which produce more cloud, which in turn increases the rate at which particles are washed out back to the ground.
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).
Recently an experiment conducted in cloud formation above the Amazon rain forest provided new insight into the role of nanoparticles with sizes below 50 nm [16]. Large areas of the sky above the pristine rain forest have very low densities of aerosol particles, often measuring hundreds per cubic centimeter, which is similar to densities in preindustrial times. Within this environment lies the city of Manaus with 1.8 million inhabitants, which is a significant source of atmospheric nanoparticles with sizes below 50 nm that are carried in a plume by the North‐Easterly trade winds. Thus it was possible for the first time to isolate the effect of nanoparticles injected into preformed clouds.
Figure 2.10 Effect of nanoparticles on DCCs. In clouds that lack nanoparticles with sizes below 50 nm (UAP < 50) (left), the clouds are highly supersaturated above the cloud base with relatively few cloud droplets at high altitudes. With added UAP < 50 (right, red dots), an additional number of cloud droplets are nucleated above the cloud base, which enhances condensation, releasing additional latent heat at low and middle levels, thus intensifying convection, which drives more CCNs to higher altitudes. This increases the intensity of precipitation and electrification.
In particular, the study looked at the characteristics of deep convective clouds (DCCs) that are responsible for storms. Generally, these form in moist air when an atmospheric instability has initiated rising air that carries the moisture up into cooler air to form a supersaturated vapor. This will then condense into droplets in the presence of CCNs and the process releases latent heat into the surrounding air that increases the updraft driving more moisture into supersaturation. In very clean environments, in which CCNs are virtually absent at high altitudes, the process is self‐limiting. The effect of introducing nanoparticles into this situation was found to intensify the storm by the process illustrated in Figure 2.10. With the absence of the nanoparticles and a low density of larger CCNs, at high altitudes, the cloud droplets tend to remain at lower altitudes and form warm rain that reduces the droplet area available for condensation. Above the cloud base there is a very high level of supersaturation and injecting large numbers of nanoparticles with diameters less than 50 nm (UAP < 50) into this situation produces a significant increase in cloud droplets above the cloud base. The enhanced condensation generates latent heat that drives an updraft that carries larger CCNs to high altitudes and increases condensation and energy release, which further increases convection. This drives the cloud to higher altitudes, increases precipitation, and enhances storm electrification. Thus, the study showed that nanoparticles generated by human activity can significantly increase the intensity of storms in the tropics and this is important data to be fed into climate change models.
2.4 Marine Aerosol
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