A fourth economically relevant category is air quality.
While the potential may be great for green roofs to improve
air quality in densely developed areas, the type of
vegetation found on the rooftop largely determines theamount of air-quality improvement. Trees, grasses, and
shrubs both filter pollutants and transpire moisture much
differently than the Sedum plant species commonly found
on modern green roof applications. Cross-applying airquality
improvements from one type of green roof
application to another can be very misleading. For
example, air-quality benefits have been modeled for grass
roofs in Toronto with the authors-finding significant
economic benefits to air quality under grass roofing
scenarios (Currie and Bass, 2005). The Georgia test plot,
however, was designed to be simple and easily replicable
using Sedum plants. These plants do not have the same leaf
area index, photosynthetic activity, or growth pattern as
grasses thus making this particular air-quality benefit
unsuitable for this study.
Other researchers evaluated nitrogen oxide uptake
made by the Crassulaceae plant family of which Sedum is
a member (Sayed, 2001). While this CO2 uptake is well
documented, the air-quality improvements provided by
the function are less certain, but basic estimates for
economic quantification of these improvements are possible
by including Sedum green roofs as part of a cap-andtrade
emissions credit system. Using 2005 market value for
NOx emission credits of $3375/ton, Clark et al. (2005)
estimated the credit for a Sedum green roof to be $0.11/m2
.
This value was applied to the current analysis as the airquality
benefit since it was deemed more appropriate for
the roof system used in this study. Both the private and
public sectors benefit from this technology as green roofs
reduce the pollutant loads in the ambient air of the city
improving social welfare while allowing the private
building owner to receive economic compensation from
providing a service for industries looking to offset their
polluting activities.