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14 1.6.8 Air Quality Goods movement emissions are commonly represented as mobile/line (e.g., trucking) or area/volume (e.g., cargo han- Air quality refers to the level of contaminants in ambient dling equipment) sources. Some sources may be represented air. It is assessed through measurements and/or numerical as point sources (e.g., hotelling OGVs). In air quality model- model applications. Many freight-related air quality impacts ing, the representation of emissions strength, location, size, are assessed by modeling studies that couple freight emis- shape, and temporal profile all influence concentration. Other sions inventories--as discussed throughout this report-- exhaust parameters that may be considered include emission with meteorological and other data to estimate concentrations release height, exit temperature, exit velocity, stack diameter, of pollutants resulting from atmospheric releases from goods and initial plume size. Other indirect parameters (e.g., shape movement activities. of buildings, terrain in the region) will also influence concen- This discussion focuses on how these concentrations are tration. Most of these parameters are not included in a typi- assessed from the emission estimates discussed in this chap- cal emission inventory. ter, and the associated uncertainties. As such, this section Total uncertainty in predicted concentrations from freight does not review the uncertainties in any given model or the movement represented using a dispersion methodology is uncertainties in any other parameter input to these models, due to uncertainty in the following: but rather on the emissions-relevant model parameters and processes. Emission input parameters; Most commonly, one of the two following general meth- All other input parameters (e.g., meteorology); and ods will be employed in air quality modeling: Methodology (e.g., model formulation and choice). Grid Modeling for national and regional scales (typically Total uncertainty is generally unquantifiable. However, for citywide and larger analyses) and principal emission-related drivers of uncertainty include Dispersion Modeling for local/project scales (facility to those shown in Exhibit 1-8. Unlike Sections 3.2 through 3.8, citywide analyses). which are directly related to emissions, the "uncertainty" for all air quality parameters is shown here as "high," due to char- Total uncertainty in predicted concentrations in either acterization of the variation in values. This is because this method is due to uncertainty in the emission inputs as well as variance itself varies greatly between methods, models, and the uncertainties in all other inputs (e.g., meteorology, chem- applications. istry) and model formulations. Total uncertainty is generally unquantifiable for Photochemical Grid Models (PGMs), but 1.7 Conceptual Model sensitivity to individual inputs for specific scenarios may be characterized. For dispersion modeling methods, too, this The Conceptual Model described in Section 4 offers a com- value is generally unquantifiable. However, the uncertainty prehensive representation of freight activity in the United due to calculated emission rate may be characterized directly States, covering all modes and relationships between modes. from the input uncertainty given its linear nature and lack of In order for this model to be effective in improving emissions other complicating factors. Sensitivity to other emission pa- estimates, it captures the factors in freight movements and rameters may be assessed for any particular scenario. freight equipment that most influence emissions. Exhibit 1-8. Emission-related air quality parameters. Geographic Impact on Parameter Parameter Methods/Models Scale Emissions Uncertainty Source Orientation, Size, and Shape All All Low/Moderate High Emission Temporal Profile All All Moderate High Exhaust Temperature/Buoyancy All (If Plume Rise Is All Moderate High Parameters Considered) Initial Plume Size and Shape All All Moderate High Release Height All All Moderate High Source Location All All Moderate High Emission Rate All All Moderate/High High