In this chapter, the word “particle” is used for both solid and liquid aerosols, but not for gaseous pollutants. Common dangerous classes of particulate deposits are (Hill and Bouwmeester, 1994):
∑ acidic substances,
∑ oxidizing substances,
∑ soot and tarry particles from burning fuel,
∑ large abrasive particles,
∑ hygroscopic materials,
∑ particles containing traces of metals (that act as catalysts),
∑ wet and oily particles from food preparation,
∑ alkaline particles from new concrete,
∑ salt crystals and dissolved salts (corrosion and microorganisms),
∑ textile fibers and skin fragments as food for insects.
Particulates may arise from exterior sources, either natural (sea salt aerosol, bushfires, windblown dust, pollen and other plant products and insect, arachnid and bird products), agri- cultural (agricultural sprays), transport (oil and soot) or industrial (organics and sulfuric acid).
Particles may also arise from within, either human-related (lint, dirt, hair, skin flakes, droplets and food), microorganism-related (mites, mold and other microorganisms), combustion-related (cigarette smoke, soot and ash from candles, incense, smoke from the kitchen range and frying oil), renovation-related (brick, plaster and asbestos dust and clean- ing products) or resuspended (from vacuuming, walking, dusting and sitting on furniture).
In a study of particulate soiling in museums and historic houses (Yoon and Brimblecombe, 2001), sticky samplers were used to collect particulates. The main components were soil dust, soot and fibers with lesser amounts of human hair and skin, plant and paint fragments and insect parts. Some typical particle size ranges are given in Fig. 2 (Annis, 1991).
2.2. Gases
In the external environment of urban regions, the major acidic and oxidizing gaseous pollutants occur as a result of industrial activity. Sulfur dioxide and nitrogen dioxide are produced from the combustion of fossil fuels (coal, gas and oil) used in energy production and transport. While natural sources may be the primary contributors for these pollutants in remote areas, human activity is by far the major source in the modern built environment.
Although naturally occurring in the stratosphere, ozone, an oxidizing pollutant, is found at ground levels and is most often a result of photochemical smog. Levels of ozone can be directly related to those of SOxand NOx.
The consequences of outdoor materials are (1) acidic and (2) oxidizing. Specific impli- cations of acidic attack exist for calcareous materials, cellulosic materials and particularly ferrous but most metals. Buildings and monuments made of limestone, marble and other alkaline stones are literally dissolved through the neutralizing reaction with acids.
Oxidizing attack occurs through destructive oxidation of chemical bonds within organic materials, e.g. synthetic polymers, natural polymers (cellulose and protein) and dyes.
However, the short half-life of ozone somewhat modifies its powerful oxidizing action.
Acidic activity of any particular acid is indicated by its dissociation constant pKa (see Table 1). The overall acidic effect experienced by a surface is contributed to by the sum of all the atmospheric acids present, including carbonic (from CO2) and organic acids, but is influenced by pKa. Therefore, considering that nitric and sulfuric acids have a pKa much Holistic Modeling of Gas and Aerosol Deposition and Degradation 119
Fig. 2. Size ranges of indoor pollution particles.
lower than carbonic acid, in a solution where the concentration of nitric and sulfuric acids may be much lower, these highly acidic species will contribute much more to the total acidity.
While particulates are generally considered to have adverse effects for both humans and objects, the definition of pollutant gases for collections remains more open to interpreta- tion and reliant on the particular situation. Gases that have measurable health risks for humans are not always dangerous for collections, and the reverse also applies. Those most often associated with harm to collections are SOx, NOx, carboxylic acids (acetic, formic and fatty acids), ozone and some amines. Volatile organic compounds (VOCs) are included as they are often present in association with the short-chain carboxylic acids and their precursors, although the specific effects of most VOCs are still unknown. Larger molecules may photodissociate to produce active smaller molecules or be involved in inter- or intra- molecular chemical reactions. For example, VOCs with primary alcohol groups may react with oxygen to become aldehydes, and then carboxylic acids. Gases can also interact and influence the particulate composition of an environment. Ozone and terpenes have been shown to increase the number and mass concentrations of sub-micron particles, resulting in an undocumented source of indoor particulates (Weschler and Shields, 1999).
It is also worth noting that in studies of concentrations of indoor air VOCs, total VOC (TVOC) concentrations are considerably higher than individually measured VOCs (Brown et al., 1994). This suggests that there are a large number of chemical compounds present, but only a select few are evaluated using current monitoring methodologies.
Acid Formula pKa
Perchloric HClO4 ~-7
Hydrochloric HCl ~-7
Chloric HClO3 ~-3
Sulfuric (1) H2SO4 ~-2
Nitric HNO3 ~-1.3
Oxalic (1) H2C2O4 1.23
Sulfuric (2) 1.92
Chlorous HClO2 1.96
Phosphoric (1) H3PO4 2.12
Nitrous HNO2 3.34
Formic HCOOH 3.75
Oxalic (2) 4.19
Acetic CH3COOH 4.75
Carbonic (1) H2CO3 6.37
Hydrosulfuric H2S 7.04
Ammonium ion 9.25
Hydrogen peroxide H2O2 11.62
(1) and (2) refer to the first and second deprotonation stages of the acid.
NH4+ HC O2 4− HSO4−
In the inside-museum environment, gaseous pollutants arise from both internal and external sources. Internally generated VOCs are generally recognized to occur in far greater concentration indoors compared with the external environment, in the ratio 8:1 for established buildings and greater than 200:1 for new buildings (Brown, 2003). Conversely, pollutants generated outdoors are found to be in much lower concentration indoors, as exemplified by the measurements of SO2, O3, NOxand NO given in Table 2.
Gaseous emissions may occur from the artworks themselves, or from the materials and coatings used in storage, transport and display. Processed wood products are the most often cited sources of VOCs such as acetate and formate. Newer building materials are expected to reduce emissions to an acceptable level with time; however, the persistence of emissions is demonstrated in older wood in a study by Rhyl-Svendsen and Glastrup (2002). It was found that a 15-year-old oak plank had an acetic acid emission of 55.7 (±5.6) mg/m2h and a formic acid emission of 11.1 (±2.5) mg/m2h; and a 12-year-old coin collection drawer with a Masonite board base and maple wood edges had an acetic acid emission of 172.5 (±6.9) mg/m2h and a formic acid emission of 94.1 (±7.2) mg/m2h.