Air Quality Management
The following is an influence diagram constructed for the
Visual Quality objective of an air quality management process.
(Click image to enlarge)
The influence diagram shows the relationship between
major pollutant sources and the ultimate effects or
endpoints. The effects are shown at the far right. To the
left of the effects, the major ambient air concentrations
that most directly contribute to the effects are shown. To
the left of concentrations are the major emissions that
cause them, and at the far left, are the broad categories of
major sources. Dominant pathways are shown with bold lines;
very minor pathways are not shown at all. Ovals represent
factors outside the influence of an air quality planning
process. The shadowed box indicates the proposed evaluation
criterion. In this case, Visual Range, reported in
kilometers, represents visual quality, and is proposed as a
proxy for two more fundamental objectives: residents'
quality of life and tourism revenue. Alternatively, it might
be possible to develop constructed scales for these more
fundamental objectives.
Using the Influence Diagram to build common
understanding
The influence diagram clarifies that
there are two distinct types of visibility impairment in
this airshed - a "white haze" in the east and "brown clouds"
in the west. Both are largely caused by the presence of fine
particulate matter in the atmosphere. The finest particles,
(PM2.5), have the greatest "light scattering" and
extinguishing characteristics. In the western part of the
airshed, NOx emissions are a significant factor in
visibility impairment and are largely responsible for the
orange-brown color. In the east, ammonia from agricultural
sources plays an important role in secondary particle
formation and the formation of white haze. The effects of
air emissions are exacerbated by topographic features, which
reduce dispersion of pollutants, and by local weather
conditions, which in warm summer periods produce winds that
drive air pollution inland during the day and back out over
populated areas at night. Temperature inversions, most
frequent in the fall but possible year round, contribute to
pollutant buildup. The combined effect is the occurrence of
visibility episodes, most often during the summer and early
autumn. Poor visibility affects aesthetic quality, which
adversely affects local residents. Studies have also shown
that it is likely to affect tourism and related local
economic development. A more detailed version of the
influence diagram documenting all these interactions was
initially developed, and then simplified. It is possible to
construct influence diagrams to varying degrees of detail
depending on the purpose.
The proposed evaluation criterion
The proposed
evaluation criterion would report the visual range, measured
in kilometers in a) the eastern valley and b) the western
urbanized area. The proposed statistic is the mean of the
worst 20% of days. A representative site would be chosen in
each region based on a review of existing data. Previous
data have demonstrated a strong correlation in visibility
across various sites in the eastern valley, suggesting that
a single site can likely serve as a surrogate for the entire
valley. A loss in visual range represents a loss in
aesthetic quality that affects quality of life for valley
residents; it is also an indicator of direct economic losses
to the region from losses in tourism revenue. The visibility
attribute will be normalized for weather, as will all other
attributes, where relevant. That is, it will evaluate impact
of emissions on visibility (mean of worst 20% of days) under
a pre-defined set of weather conditions. The more detailed
version of this diagram indicated that the estimation
methods for this evaluation criterion would need to be
sensitive to changes in peak and mean annual ozone, PM2.5,
PM10, as well as of ammonia/um concentrations for the
eastern valley and NOx emission in the west.
Other Potential Evaluation Criteria
An
alternative form of the evaluation criterion that could be
considered is the loss in tourism, represented as # person
days or # of visits per year, or the value of this loss in
dollars per year. This would have the advantage of
integrating white haze and brown smog effects into a single
criterion and conveying the monetary implications of
visibility effects, but the disadvantage of losing intuitive
understanding for the true aesthetic effects and
corresponding non-monetary effects, which are important for
evaluating the significance of any losses or gains. Full
monetization of the effects is possible and could provide
important insights if the economic loss associated with
tourism impacts is thought to be significant. Care should be
used in combining true monetary effects with
willingness-to-pay estimates, as the willingness to pay
estimates are often insufficiently transparent and not
trusted by stakeholders.
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In this example, an influence diagram is used to
link the emission sources that are managed by regional
air quality managers (on the left of the diagram) with
the objectives that they are ultimately trying to
achieve.