Aerosol are present throughout the boundary layer, at number concentrations depending upon factors such as location, atmospheric conditions, annual and diurnal cycles and presence of local sources. The highest concentrations are usually found in urban areas, reaching up to 10⁸ and 10⁹ particles per cc (Seinfeld and Pandis, 1998).

The study of aerosol is interesting for a number of reasons. It is thought that aerosol may be involved in a feedback to global warming. It is certainly important in the Earth’s radiation budget. There are also concerns about the effects of aerosol on human health. Finally it is, in some cases, an important part of the chemical deposition budget for certain chemical species to ecosystems.

The feedback to global warming would tend to cool down the Earth in the event of a rise in temperature, and acts in two ways. With increased temperature would come stronger average winds, suspending more crustal material, thus increasing average aerosol concentrations. The first mechanism is called the direct effect. This is simply aerosol reflecting back incident solar radiation into space. The magnitude of the direct effect is simpler to estimate than that of the indirect effect.

The indirect effect again involves increased average aerosol number concentration. An increased aerosol population means that there are more cloud condensation nuclei, which would lead to more clouds forming. This situation is slightly more complicated, as the effect of the clouds on the Earth’s radiation budget depends upon the cloud height, but increased tropospheric aerosol would also have a cooling effect on the atmosphere.

The smallest aerosol are small enough to get into the human respiratory system. British standards define the respirable fraction as those aerosol smaller than 5 m m, which as we will see, is a significant proportion of the total. Even aerosol composed of benign materials can be irritants (e.g. glass calibration microspheres used in the lab), and some aerosol are partly made of toxic materials (heavy metals, organic chemicals etc.).

The rate at which aerosol can act as a deposition pathway is an important part of total deposition rates for a number of species. Clearly this is more so for species with few other deposition pathways (e.g. heavy metals).

The size of these particles ranges from around 100 m m to a few nm. These are the largest particles which can be suspended in air for a significant amount of time and the smallest clusters of molecules which can be classified as particles respectively. Aerosol has a number of properties such as size, chemical composition, hygroscopiscity, density and shape. Size is normally used to classify aerosol because it is the most easily measured property and because inferences about the other properties can be drawn from size information.

The primary interest in this work is urban aerosol and urban airshed aerosol. These two situations normally yield much higher number concentrations than any other circumstances. For the purposes of this report, we will first look at sources and sinks of aerosol in general, before looking at typical sources and ambient concentrations for three well studied cases (marine, remote continental and urban).

Aerosol can either be produced by ejection into the atmosphere, or by physical and chemical processes within the atmosphere (called primary and secondary aerosol production respectively). Examples of primary aerosol are sea spray and wind blown dust. Secondary aerosol are often produced by atmospheric gases reacting and condensing, or by cooling vapour condensation (gas to particle conversion). Figure 1 shows some of these processes, along with the three size ranges (modes) where high aerosol concentrations are often observed.

Although figure 1 is a plot of surface area against particle diameter, rather than of number concentration (the parameter used in this work), it shows some of the important sources and sinks of aerosol without reference to location. Seinfeld and Pandis (1998) make a strong distinction between fine and course particles (the distinction is shown in figure 1). They note that the sources, chemical and optical properties, transformation mechanisms, effects and deposition pathways are generally very different for the two classes of particles.

It is worth noting that there are no sources shown in figure 1 in the accumulation mode. This is an important property of many aerosol size distributions, which will be discussed in the section on deposition pathways (section 3.2). Table 1 shows estimates of total global aerosol emission. A number of classes of aerosol are shown, and the size of the particles comprising the flux is also estimated.

It can be seen from Table 1 that it is not thought that anthropogenic aerosol sources produce a large proportion of the total aerosol by mass (best estimate gives about 14.5%). However, this is on a global scale, and in the vicinity of major towns and cities the figures would be quite different (see section 9). In the vicinity of large scale biomass burning (either fossil fuels or vegetation for land clearance) the proportion of anthropogenic aerosol would also be much higher.

Table 1 also shows that the sources of course particles tend to be physical. These include volcanic activity, production of sea spray and suspension of surface dust. However, chemical reactions and changes of state of gases produce the majority of fine particles (by mass).

Once aerosol is suspended in the atmosphere, it is altered, removed or destroyed. It cannot stay in the atmosphere indefinitely, and average lifetimes are of the order of a few days to a week. Clearly the lifetime of any particular particle depends on its size and location. Larger aerosol settle out of the atmosphere very quickly under gravity, and some surfaces are more efficient at capturing aerosol than others. We will first examine some removal pathways before looking at how aerosol may be expected to change during the course of its atmospheric residence. Note that the mechanisms described below bring the particle to the surface. Precise details of how the particle adheres to the surface are not discussed here.

Wet deposition is the name given to deposition pathways involving water. They include rainout, washout, sweepout and occult deposition. Brief qualitative explanations of these will be given, as the primary focus of the work referred to in this report is dry deposition.

Rainout describes the removal of a cloud condensation nucleus. As described in section 3, aerosol act as nuclei for the condensation of cloud droplets. In clouds producing rain, some of these drops grow to such a large size that they fall (gravitationally settle) to the surface as rain drops. The aerosol (condensation nuclei) deposited in this way are said to have been rained out.

Washout describes the removal of aerosol by cloud droplets. If an aerosol is incorporated into an already existing cloud drop, and that drop grows large enough to fall as rain, the particle is said to have been washed out. Note that the difference between washout and rainout is the required pre-existence of a collecting drop for washout.

Another fairly closely related wet deposition process is sweepout. Aerosol remaining below the cloudbase of a raining cloud can impact into falling raindrops. If the impact leads to incorporation of the aerosol into the drop, the aerosol is deposited with the raindrop, the condensation nucleus, and any other washed or swept out particles.

Although the final fate of rained, washed and swept out particles is the same, the three processes are distinct because the efficiency of each, and the size and amount of aerosol swept out by each process is calculated differently. The distinction is therefore mainly useful in modelling work where the total deposition due to all three processes is of interest.

Occult deposition is a slightly more complicated concept than the other three wet processes examined. Impaction efficiency is the likelihood that a particle will strike a surface feature encountered in a flow, rather than be deflected around the object. It is a strong function of size, with larger aerosol being more likely to impact on a surface feature than smaller particles.

Aerosol can be incorporated into droplets in clouds making contact with the surface of the ground (e.g. fog, orographic clouds). The impaction efficiency of droplets is higher than that of the aerosol they nucleate on. This produces an enhanced probability of impaction for such aerosol incorporated into drops. Sticking efficiency is the probability that an impacted object will not bounce off and be instantly resuspended. Providing the sticking efficiency of cloud drops is not significantly lower than that of the nucleating aerosol, (it is not) clouds contacting the ground can give rise to an enhanced deposition rate for small aerosol.

Dry deposition pathways are the group of deposition mechanisms that transport pollutants (in this case particles) directly to the surface without the aid of precipitation. Through the boundary layer there are two dry deposition mechanisms. Each will be described briefly here.

This process is possibly the simplest of all the deposition processes to describe. It simply means a particle falling under gravity. Very large particles fall, reaching a terminal velocity, which can be found by equating the force due to gravity by the drag force (from Stokes’ law) and solving for velocity. It falls through the boundary layer at this rate until it strikes the surface.

Gravitational settling is of only secondary interest here, as in this work we are interested in particles too small to be effectively deposited by gravitational settling (settling velocity is directly proportional to particle mass).

Turbulence is the most effective dry vertical transport mechanism in the boundary layer. It is also the major focus of measurements presented in this work. A description of the treatment of turbulence can be found in section 4.1.

The removal mechanisms described previously are all very efficient at depositing coarse particles. Very fine particles, however, are often not removed as such, but are transformed into larger particles before being deposited. Figure 1 shows some transformation mechanisms schematically, but for the purposes of this discussion an aerosol size distribution taken during the SASUA I project is presented in Figure 2.

Figure 2 shows an aerosol number distribution covering most of the "fine" particle range defined in Figure 1. The range covers diameter 3.5 nm > D_p > 450 nm. Aerosol in the very small part of the range (10 nm > D_p) are probably new secondary particles. Aerosol at the larger end of the spectrum are either old particles, or are primary aerosol. The possible dynamics of this spectrum will be discussed in the following sections.

As previously stated, there are no effective deposition pathways for very small particles. Such particles must therefore leave the Aitken Range (< 100nm as shown in Figure 1) by growing before being deposited at larger sizes. One manner in which small particles can rapidly leave the Aitken mode is by coagulation.

Coagulation is the sticking together of two particles. It is the result of particles coming into contact due to Brownian diffusion or some force (electrostatic, phoretic effects etc.). Note that contact does not necessarily lead to coagulation, but must happen as a pre-requisite. This happens more quickly for Aitken mode (also known as nucleation mode) particles with large aerosol than for coagulation of two Aitken mode particles (Seinfeld and Pandis, 1998). Coagulation is also enhanced in shearing or turbulent flows, as these induce fast relative particle motion.

As long as the partial pressure of a compound in the gas phase is higher than the vapour pressure of that compound in aerosol, growth will occur. Of course, the opposite situation is possible, where particles outgas certain compounds, but in the urban environment growth tends to be the dominant process. Condensational growth is another mechanism by which aerosol can leave the nucleation mode.

Seinfeld and Pandis (1998) derive an expression for particle diameter with respect to time for cases of condensation of outgassing:

Where: Particle diameter

Initial particle diameter

Constant

The term A is, in fact, related to the driving force for the condensation, and is only constant in situations like persistent supersaturation of the gas phase species. The interesting point about the equation is that it predicts smaller particles will grow proportionally faster than large particles. In fact, Seinfeld and Pandis comment that it tends to produce monodisperse (one size) aerosol as .

Cloud drops undergo aqueous chemical reactions. Because of their larger surface area, reaction rate per drop is usually higher than reaction rate per condensation nucleus. If a drop that has been involved in such reactions subsequently evaporates, the condensation nucleus is left behind and may be considerably larger than it was before entering cloud. This process is called cloud processing, and can considerably increase the rate at which accumulation mode aerosol grow.

Cloud processing generally affects accumulation mode rather than coarse mode aerosol ,as the former tends to be more hygroscopic.

Marine aerosol is mentioned here for comparison purposes only. Figure 3 shows size spectra measured in the Atlantic and Indian oceans, and one modelled spectrum.

Particles in remote ocean environments are found at concentrations around 100 – 300 per cc. They occupy the three modes described previously, with the course fraction containing most of the total particulate mass, but low number concentration. Coarse marine aerosol are mainly composed of salt from evaporated spray droplets (Seinfeld and Pandis, 1998). The finer aerosol too, are produced in the ocean environment, but from processes such as DMS reaction product condensation (DMS, or Dimethyl Sulphate is produced at sea by plankton (M. Flynn, pers. comm.).

Remote continental aerosol are produced naturally over land. The remote continental case is interesting because it is the background against which anthropogenic emissions have their effect. It shows the same three modes as marine aerosol, but at concentrations around 2000 – 10000 per cc. The majority are composed of dust and pollen or oxidation products derived from ammonia and sulphates (Seinfeld and Pandis, 1998). Figure 4 shows typical number, surface area and volume distributions.

Urban aerosol is the subject of interest in this work. Again, typical spectra have three peaks, visible in the three plots of number, area and volume distribution shown in figure 5. As before, fine and course mode particles have different sources. Coarse particles tend to be mechanically generated, but are composed of materials such as tyre dust as well as sea salt and dust in urban areas. Fine particles (accumulation and nucleation mode) tend to be produced either directly from combustion sources, or by gas to particle conversion involving reaction products of sulphates, nitrates, ammonium and organics.

Figure 5: Typical urban aerosol number, surface and volume distributions

The nucleation mode in urban areas often contains comparatively few particles. Very close to local sources, significant numbers may be found, but in urban areas, there is generally a good supply of species to rapidly condense onto small aerosol, moving them into the accumulation mode. Further, accumulation mode concentrations tend to be very high, so that available precursor gases will condense onto those particles without the need to nucleate fresh particles in most circumstances.

The main difference between the three categories of aerosol distribution examined here is the total number concentration. In urban areas, aerosol concentrations reach around 10⁸ to 10⁹ per cc. Figure 6 shows a comparison between urban, urban influenced and background conditions. Note that the background here is higher than in the remote continental spectra shown in figure 4, as it probably contains some anthropogenic particles (making it a rural continental distribution).

Figure 6: Aerosol number distributions next to a source (freeway), for average urban, for urban influenced background, and for background conditions.

Although numerous trace elements are found in aerosol, Seinfeld and Pandis (1988) give the main constituents as sulphates, nitrates, ammonium, organics, crustal species, sea salt, hydrogen ions and water. Figure 7 shows the proportions of some of these ions as an equivalent number concentration.

From Figure 7 it is worth noting that the sodium ion is concentrated in the coarse mode (sea spray particles), and that the chloride ion seems to have been involved in some reaction, redistributing it into fine particles. Other than that, the graph shows how different species are included in different particle modes.