2
Air Pollution and Turbulence: Modeling and Applications
urban air. More recently the popularity of diesel engines has increased the emissions
of fi ne particles in cities. While these changes have raised concerns in terms of
human health there have been parallel worries about the damage it is causing to the
architecture of cities.
1.2 POLLUTION AND ARCHITECTURE
In the early twentieth century the dominant impact of air pollution on stone was the
sulfation of the surfaces through the deposition of sulfur dioxide and its oxidation to
sulfuric acid. The concomitant deposition of soot onto the surfaces led to thick disfi guring black gypsum crusts. Blackened buildings typifi ed the coal-burning cities
of fi n de siècle Europe and led not only to a century of scientifi c concern and a range
of interventions, but also infl uenced the nature of modern architecture. Architects
were forced to abandon detailed moldings in soft light-colored stone and looked to
produce buildings with simpler lines and darker colors constructed in more resistant
materials (Bowler and Brimblecombe, 2000).
The cleaner air found in cities of the second half of the twentieth century gave
some respite to the formation of black crusts on the surfaces of buildings. Although
there was a public debate over the role of acid rain in causing damage, in reality
the corrosion rates of metals and building stones in cities such as London declined
through the last half of the century. These improving conditions allowed a developing civic desire for clean buildings. People were suddenly confronted by architecture
that was very bright and unfamiliar. The buildings were now clean and may have
looked closer to the architect’s original intent, but this change was not free of criticism from those who feared that history had been scraped away (Andrew, 1992; Ball
et al., 2000; Grossi and Brimblecombe, 2004a). Somehow our most loved and valuable buildings had lost their patina.
Despite the overall improvements in air quality, diesel-derived particulate matter
was increasing in concentration in heavily traffi cked areas. Contemporary particulate material is fi ner than that from coal smoke and blacker and richer in organic
matter. Even more problematic for those charged with caring for the urban fabric
was a population accustomed to lower soiling rates. The last years of the twentieth
century saw black crusts on buildings reemerge as an air pollution issue with some
buildings grossly disfi gured (Figure 1.1).
1.3 CHEMISTRY
The traditional view of the way in which stones and metals are degraded is from
acid produced by the oxidation of sulfur dioxide. In the case of limestone and other
calcareous stones this leads to a transformation of the carbonate to a sulfate. Calcium
sulfate or gypsum is more soluble in water such that the carbonate readily dissolves
from the building. Gypsum also has a larger molecular volume than calcium carbonate so the increasing volume of the mineral on the outside of the stone imposes
mechanical stress and disrupts the surface.
In the contemporary atmosphere, sulfur dioxide concentrations are much lower
than in the past. However, ozone and metal ions could act as oxidants or oxidation
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
urban air. More recently the popularity of diesel engines has increased the emissions
of fi ne particles in cities. While these changes have raised concerns in terms of
human health there have been parallel worries about the damage it is causing to the
architecture of cities.
1.2 POLLUTION AND ARCHITECTURE
In the early twentieth century the dominant impact of air pollution on stone was the
sulfation of the surfaces through the deposition of sulfur dioxide and its oxidation to
sulfuric acid. The concomitant deposition of soot onto the surfaces led to thick disfi guring black gypsum crusts. Blackened buildings typifi ed the coal-burning cities
of fi n de siècle Europe and led not only to a century of scientifi c concern and a range
of interventions, but also infl uenced the nature of modern architecture. Architects
were forced to abandon detailed moldings in soft light-colored stone and looked to
produce buildings with simpler lines and darker colors constructed in more resistant
materials (Bowler and Brimblecombe, 2000).
The cleaner air found in cities of the second half of the twentieth century gave
some respite to the formation of black crusts on the surfaces of buildings. Although
there was a public debate over the role of acid rain in causing damage, in reality
the corrosion rates of metals and building stones in cities such as London declined
through the last half of the century. These improving conditions allowed a developing civic desire for clean buildings. People were suddenly confronted by architecture
that was very bright and unfamiliar. The buildings were now clean and may have
looked closer to the architect’s original intent, but this change was not free of criticism from those who feared that history had been scraped away (Andrew, 1992; Ball
et al., 2000; Grossi and Brimblecombe, 2004a). Somehow our most loved and valuable buildings had lost their patina.
Despite the overall improvements in air quality, diesel-derived particulate matter
was increasing in concentration in heavily traffi cked areas. Contemporary particulate material is fi ner than that from coal smoke and blacker and richer in organic
matter. Even more problematic for those charged with caring for the urban fabric
was a population accustomed to lower soiling rates. The last years of the twentieth
century saw black crusts on buildings reemerge as an air pollution issue with some
buildings grossly disfi gured (Figure 1.1).
1.3 CHEMISTRY
The traditional view of the way in which stones and metals are degraded is from
acid produced by the oxidation of sulfur dioxide. In the case of limestone and other
calcareous stones this leads to a transformation of the carbonate to a sulfate. Calcium
sulfate or gypsum is more soluble in water such that the carbonate readily dissolves
from the building. Gypsum also has a larger molecular volume than calcium carbonate so the increasing volume of the mineral on the outside of the stone imposes
mechanical stress and disrupts the surface.
In the contemporary atmosphere, sulfur dioxide concentrations are much lower
than in the past. However, ozone and metal ions could act as oxidants or oxidation
© 2010 by Taylor and Francis Group, LLC