The particulates that most affect degradation of cultural objects exposed to the outside environment are plant seeds, fungal spores, particulates produced by industrial and transport sources, and marine aerosols.
Within the corrosion science literature, there are a number of studies that provide either empirical evidence or models of marine aerosol distribution over land. The emphasis in this chapter is on marine aerosols, as air pollutants are limited and decreasing in the areas under study. Ohba et al. (1990) model the salt concentration in the air over land and divide salt into two components – salt produced at the shoreline, which shows an exponential decrease with distance from the coast, and a second component that relates to salt produced at sea, which is relatively constant. They relate this difference to the coarser nature of surf-generated salt compared to ocean-generated salt. Gustafsson and Franzen (1996) carried out an extensive monitoring program on Sweden’s west coast and found that dry deposition of sea salt depended on the wind velocity on the coast and the downward distance from the coast. In the field of corrosion science, several surveys (Johnson and Stanners, 1981; Strekalov and Panchenko, 1994; Corvo et al., 1995, 1997; Cole and Ganther, 1996) have been undertaken in different countries to study the variation of salinity on land with distance from the coast. In reviewing some of this literature, Morcillo et al. (1999) found that, in general, a double dependence was evident, with a rapid decrease within the first few hundred meters and then a slower decrease tending towards
K= H C(OH) HCHO
2 2(aq)
(aq)
⎡⎣ ⎤⎦
⎡⎣ ⎤⎦
NH3⋅H O2 WNH4++OH− NH3+H O2 WNH3⋅H O2
Holistic Modeling of Gas and Aerosol Deposition and Degradation 133
rides depended on both the average velocity of total winds (marine and continental) and on the product of wind velocity and duration, which they referred to as wind power.
Morcillo et al. (2000) used data derived from Spain’s Mediterranean coast to significantly extend Strekalov and Panchenko’s concept of wind power, and proposed that certain marine wind directions (which they referred to as saline winds) are critical to the deposi- tion of marine salts across land.
Cole et al. (2003) have integrated marine aerosol generation and transport into the holis- tic model of corrosion. Marine aerosols are produced by breaking waves, both on the shoreline and in the open ocean. In the open ocean, aerosols are produced by whitecaps of ocean waves. Whitecap production varies systematically with longitude and season, being at a maximum in low latitudes in July and at a maximum in high latitudes in December, and low all year round in tropical seas. Thus, tropical seas produce a relatively low volume of marine aerosol, resulting in decreased marine corrosion in near-equatorial regions. Salt production is also controlled by ocean effects such as local wind speed, beach slope and fetch.
Factors controlling the transport of particulates of all types are outlined by Cole et al.
(2003). Aerosol residence times are controlled by convection and gravity, and aerosol scav- enging by cloud drops, raindrops and physical objects on the ground (trees, buildings, etc.). Thus, marine aerosol transport is likely to be higher in dry climates with low rainfall and low ground coverage, while it will be restricted in humid and high-rainfall climates with forest cover. Aerosols produced by surf tend to be coarse (5–20 mm), and those produced by whitecaps are generally smaller (0.5–3 mm).
Thus, surf-produced aerosol rapidly deposits (due to gravity), while ocean-produced aerosol may be transported over considerable distances.
An Australia-wide map of airborne salinity derived in this way (Cole et al., 2004a) is presented in Fig. 5. Coastal salinity depends on latitude, and the salinity inland depends on the distance from the coast. The salinity map highlights the pronounced effect of both ocean state (as defined by whitecap activity) and climate factors in controlling airborne salinity in Australia. For instance, southern Australian coastal zones, where whitecap activ- ity is high, have appreciably higher airborne salinity levels than Australia’s northern coast, where whitecap activity is low.
Aerosol deposition onto exposed objects has been computed using CFD and the equa- tions presented in Section 4. It is primarily controlled by gravity, momentum-dominated impact and turbulent diffusion (Cole and Paterson, 2004). Figure 6 shows the competing influences of momentum-dominated impact and turbulent diffusion on a cylinder of arbitrary diameter at arbitrary wind speed. From Fig. 6, it is evident that turbulence diffusion dominates deposition for small particles (Dp<0.5 mm) and that the turbulence intensity has a dominating influence on the deposition efficiency. At larger diameters, particle diameter is dominant, and deposition efficiency rises strongly with particle diameter.
The size and shape of objects are important. For complex forms such as cultural objects, deposition efficiency will vary across a structure, with deposition being highest at the
Holistic Modeling of Gas and Aerosol Deposition and Degradation 135
Fig. 5. Salinity map of Australia.
Fig. 6. Influences of momentum-dominated impact and turbulent diffusion on the deposi- tion of aerosols on a cylinder of arbitrary diameter at arbitrary wind speed, as a percentage of the aerosol flux that would pass through in the absence of the cylinder. The percent turbulence is the turbulence intensity (rms velocity/mean velocity) upstream.
0 10 20 30 40 50 60 70 80 90 100
0.01 0.1 1 10
Particle diameter/mean diameter
Percentage deposited
Momentum-dominated impact Turbulent diffusion 5%turbulence Turbulent diffusion 10%turbulence Turbulent diffusion 20%turbulence
edges of a structure where turbulence is highest. Figure 7 presents the results of a CFD simulation in which particles impact on a simplified building of dimensions 10 m height and 20 m by 20 m in plan. About 4 million particles were released in an area of 982 m2 upstream. There are 10 colors ranging from blue to red. Red corresponds to 18% or more of the upstream mass flow density, and blue corresponds to 2% or less. The aerosol diam- eter is 10 mm.
5.1. Implication from source transport and deposition models to degradation of objects
A knowledge of airborne salinity and its deposition is, of course, relevant to the preserva- tion of metal objects exposed in the open. The mapping work defined in Fig. 5 provides an indication of environmental severity in a given geographical location. For example, where salinity is greater than 8 mg/m2.day, moderate atmospheric corrosion (of course, the extent of the corrosion rate depends on the material) is probable. However, preventative mainte- nance, primarily regular washing, would be sufficient to protect most metal work (with the exception of uncoated iron or steel). Where salinity is greater than 32 mg/m2.day, severe corrosion is possible for some metals, and more active protective measures such as coatings may be required.
Fig. 7. Aerosol deposition on a building 10 m high and 20 ¥ 20 m in plan. The flow is from right to left. Blue is low concentration, and red is high concentration.
Local factors may, however, change these effects. The effects relate to roughness and turbulence. Surface features, trees, buildings, etc., scavenge salt from the atmosphere, depleting aerosol concentration to roughly the height of the object. Thus, a structure in a landscape with significant features of greater height than itself will have a reduced salt load compared to the same structure in an open area. This can of course be used to one’s advan- tage in landscaping, as trees may be planted to reduce salt loads on structures that are upwind from marine breezes. Structures that rise above the surrounding landscape may show a maxi- mum in salt deposition (and thus possibly corrosion) just above the height of surrounding objects. The second major effect is that of turbulence. This can, of course, lead to differen- tial deposition on an object (as in Fig. 6), and thus additional attention during maintenance should be given to edges on objects. However, turbulence may also affect objects placed on structures. For example, “gargoyles” placed on medieval churches are at a position of high turbulence and will thus suffer from increased pollutant and aerosol deposition. In a more modern context, care should be taken when placing structures near or around sculp- tures or other openly exposed pieces to ensure that they do not produce heightened turbu- lence on the cultural object. This may be negated to some extent due to moisture effects, where increased turbulence will promote drying and lead to decreased corrosion rates, which is especially important when hygroscopic salts are present on a surface. However, increased dryness will also reduce degradation due to biological activity from molds and mildew.