Deposition, Transformation, and Remobilization of Soot and Diesel
7
of the surfaces was related to the area concentrations of EC (Bonazza et al., 2007).
These tended to reach a minimum refl ectivity of around 35% and appeared very
black when EC concentrations in the crust reached more than 10 g m −2 . Such high
loadings of EC were probably reached during the last century, when the airborne
load of EC was higher than at present (Brimblecombe and Grossi, 2005).
1.7 APPEARANCE OF BUILDINGS
The changing color and blackening are key drivers of concern about appearance.
This can be especially troublesome in the case of historic buildings and provides a
challenge for management in some cases. Take for example the White Tower of the
Tower of London, where the name implies a specifi c color, so while managing this
important building one has to consider that visitors may be disappointed to fi nd that
the tower does not appear white (Grossi and Brimblecombe, 2005). More generally
the color of buildings affects our appreciation by creating a sense of care or civic
pride as distinct from neglect.
The issues above suggest that considerations of the darkening process need to go
beyond measurements of refl ectance and examine aesthetic aspects. Public sensitivity to blackening has increased in recent decades in line with improved air quality
and the acid rain debate. Public perception of darkening can be investigated using in
situ questionnaires at historic buildings, which can exhibit varying degrees of blackening (Brimblecombe and Grossi, 2005). This technique can also be used to assess
the success of treatments and choice of replacement stone (Valach et al., 2006).
Such studies of perception have been used to establish aesthetic thresholds by
comparing the proportion of the public fi nding a building discolored to the perception of its lightness or refl ectance. The “perceived lightness” of a façade can be
assessed by inviting visitors to compare the tone of the building with a grey scale
(Brimblecombe and Grossi, 2005). Most respondents experience little diffi culty in
deciding the tone of the building even though they are offered a grey scale and building façades are typically colored. It proved relatively simple to relate the perceived
lightness of the building to their views on whether the building is “dirty” or needs
cleaning. As would be imagined where the building is perceived to be light in color
respondents also believe it to be clean (see Figure 1.6a). Where refl ectance or lightness was low, respondents usually believed the building was dirty and correlated
with a view that it was in need of cleaning (Figure 1.6b).
Aggregating data from a range of buildings enabled us to establish the average
perceived lightness. We were also able to assess approximate concentrations of EC at
the survey sites and thus relate perceived lightness of the façade to amount air pollution (Figure 1.7). A range of approaches allowed us to assess the perceived lightness
that was likely to be acceptable to the public (see Brimblecombe and Grossi, 2005 for
details). This band of acceptability is also shown on the diagram so it would appear
that at 10 μg m −3 the façades built from light-colored stone are typically found to be
unacceptably dark. However, by the time the EC concentrations drop to 2–3 μg m −3
the buildings have a more acceptable appearance.
These ideas offer the possibility for setting acceptable concentrations of EC in the
atmosphere on the basis of building aesthetics. Nevertheless a more detailed analysis
© 2010 by Taylor and Francis Group, LLC
7
of the surfaces was related to the area concentrations of EC (Bonazza et al., 2007).
These tended to reach a minimum refl ectivity of around 35% and appeared very
black when EC concentrations in the crust reached more than 10 g m −2 . Such high
loadings of EC were probably reached during the last century, when the airborne
load of EC was higher than at present (Brimblecombe and Grossi, 2005).
1.7 APPEARANCE OF BUILDINGS
The changing color and blackening are key drivers of concern about appearance.
This can be especially troublesome in the case of historic buildings and provides a
challenge for management in some cases. Take for example the White Tower of the
Tower of London, where the name implies a specifi c color, so while managing this
important building one has to consider that visitors may be disappointed to fi nd that
the tower does not appear white (Grossi and Brimblecombe, 2005). More generally
the color of buildings affects our appreciation by creating a sense of care or civic
pride as distinct from neglect.
The issues above suggest that considerations of the darkening process need to go
beyond measurements of refl ectance and examine aesthetic aspects. Public sensitivity to blackening has increased in recent decades in line with improved air quality
and the acid rain debate. Public perception of darkening can be investigated using in
situ questionnaires at historic buildings, which can exhibit varying degrees of blackening (Brimblecombe and Grossi, 2005). This technique can also be used to assess
the success of treatments and choice of replacement stone (Valach et al., 2006).
Such studies of perception have been used to establish aesthetic thresholds by
comparing the proportion of the public fi nding a building discolored to the perception of its lightness or refl ectance. The “perceived lightness” of a façade can be
assessed by inviting visitors to compare the tone of the building with a grey scale
(Brimblecombe and Grossi, 2005). Most respondents experience little diffi culty in
deciding the tone of the building even though they are offered a grey scale and building façades are typically colored. It proved relatively simple to relate the perceived
lightness of the building to their views on whether the building is “dirty” or needs
cleaning. As would be imagined where the building is perceived to be light in color
respondents also believe it to be clean (see Figure 1.6a). Where refl ectance or lightness was low, respondents usually believed the building was dirty and correlated
with a view that it was in need of cleaning (Figure 1.6b).
Aggregating data from a range of buildings enabled us to establish the average
perceived lightness. We were also able to assess approximate concentrations of EC at
the survey sites and thus relate perceived lightness of the façade to amount air pollution (Figure 1.7). A range of approaches allowed us to assess the perceived lightness
that was likely to be acceptable to the public (see Brimblecombe and Grossi, 2005 for
details). This band of acceptability is also shown on the diagram so it would appear
that at 10 μg m −3 the façades built from light-colored stone are typically found to be
unacceptably dark. However, by the time the EC concentrations drop to 2–3 μg m −3
the buildings have a more acceptable appearance.
These ideas offer the possibility for setting acceptable concentrations of EC in the
atmosphere on the basis of building aesthetics. Nevertheless a more detailed analysis
© 2010 by Taylor and Francis Group, LLC