Chris Binns - Introduction to Nanoscience and Nanotechnology

There are identifiable harmful effects on health from exposure to nanoparticles, especially cardiovascular problems associated with inhaled airborne particles and various guidelines for limits of acceptable particulate densities have been published, for example, in the European Directive on air quality [14]. Current air policies on dust levels only distinguish particle sizes in a broad‐brush manner and focus on all particles smaller than 10 μm (the PM 10fraction) and those smaller than 2.5 μm (the PM 2.5fraction). It is clear from the previous discussion that in the future there will need to be further limits set at PM 0.1and PM 0.05(particles smaller than 50 nm).

The presence of aerosol in the atmosphere has a significant influence on climate, its most important role being in the formation of clouds. Pure water vapor in the atmosphere is invisible but when it condenses into microscopic water droplets, suspended as an aerosol, over a region of sky a cloud is born. The process of cloud formation and how they evolve and precipitate is a complex process but an important fundamental consideration relevant to this book is that without a preexisting aerosol of particles, clouds would not form except in extremes of high supersaturation. In a purely gaseous atmosphere, even one saturated with water vapor, it is not possible for water droplets to start growing, unless there are some initial “seed” particles that water can condense onto. These seeds are referred to as cloud condensation nuclei (CCNs). The reason why pure water vapor will not form droplets is described briefly in Advanced Reading Box 2.2but in a nutshell, although water molecules do stick together, at normal temperatures, and vapor pressures, they do not stay together long enough for a third and fourth molecule to join them and start a droplet growing. However, a water droplet above a critical size that somehow appeared would be stable and in a humid atmosphere would grow. Without CCNs there is no way to achieve the initial water droplet above the critical size. The presence of preexisting CCNs changes that and water molecules can easily condense onto them and grow to a normal cloud droplet size. These fall sufficiently slowly under gravity to be considered as suspended (see Advanced Reading Box 2.1).

The vapor pressure above a flat liquid surface within a closed container is [15]:

(2.2)

where n sis the atomic density near the surface, E fis the enthalpy of evaporation (or the energy required by a molecule to escape from the flat surface), θ is the sticking probability of a vapor phase molecule incident on the liquid surface, k is Boltzmann's constant and T is the temperature. Since the term kT varies slowly compared to the exponential term, for the present purposes, (2.2)can be simplified to:

(2.3)

where A includes all the constants in (2.2). If we now consider a curved liquid surface, say a drop, in equilibrium with its vapor, a molecule near the surface has, on average slightly fewer nearest neighbors because of the curvature. As a result, the enthalpy will decrease and the vapor pressure will be greater than above the flat surface. The enthalpy becomes dependent on the radius of the drop and it can be shown that [15] the enthalpy is (see Problem 3):

(2.4)

where E c( r ) is the radius‐dependent enthalpy for a curved surface, γ is the surface tension of the drop and v is the volume of the departing molecule. This equation is derived by working out how much the surface energy of a drop changes as a result of losing a molecule. The increased vapor pressure, p > p 0of a drop compared to a flat surface is obtained by replacing E fin Equation (2.3)with E c( r ) given by (2.4), that is:

(2.5)

This is known as the Kelvin equation. So now we can consider what happens if we have a vapor with a pressure p > p 0(a supersaturated vapor) containing no liquid drops. If we introduce a drop with a radius r derived from (2.5)into this vapor, it will be stable because the rate of molecules evaporating from it will equal the rate of molecules incident on it from the vapor. If our initial drop is smaller than r however, it will shrink because it will evaporate molecules faster than acquiring them from the vapor. Similarly, a larger initial drop will grow. In a highly pure vapor, getting the initial stable size drops is a bottleneck because the only way they can form is by the simultaneous collision of a sufficient number of molecules (homogenous nucleation), which is a highly improbable event. If there are, however, preexisting particles (liquid or solid) in the supersaturated vapor, it quickly condenses onto these. In the case of clouds, these preexisting particles are called cloud condensation nuclei or CCNs.

Figure 2.9 Relative sizes of particles involved in clouds.Comparison of a CCN, a typical water droplet in a cloud and a raindrop. In order to get a visible comparison, a typical CCN has been compared to a small raindrop, which can be a factor of 10 larger. The CCNs are the preexisting particles that allow cloud droplets to form (see Advanced Reading Box 2.2). Normal cloud drops 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).