The following is an influence diagram constructed for the Visual Quality objective of an air quality management process.

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.