Sensitivity of AERSURFACE Results to Study Area and Location. Paper 2009-A-127-AWMA

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1 Sensitivity of AERSURFACE Results to Study Area and Location Paper 2009-A-127-AWMA Prepared by: Anthony J. Schroeder, CCM Senior Consultant George J. Schewe, CCM, QEP Principal Consultant Trinity Consultants 1717 Dixie Highway Suite 900 Covington, KY (859) June 18, 2009

2 Sensitivity of AERSURFACE Results to Study Area and Location Paper 2009-A-127-AWMA Anthony J. Schroeder, CCM and George J. Schewe, CCM, QEP Trinity Consultants, 1717 Dixie Highway, Suite 900, Covington, KY ABSTRACT The United States Environmental Protection Agency (U.S. EPA) released a new version of the AERSURFACE tool on January 9, AERSURFACE is used to develop direction-specific estimates of surface characteristics (albedo, Bowen ratio, and surface roughness) for use as one input in AERMET to produce meteorological data sets that can be used in regulatory dispersion modeling analyses using AERMOD. AERSURFACE uses the land use characteristics of the area surrounding the meteorological data collection site or the model study site to determine the appropriate surface characteristics for input into AERMET. In some cases, land use can vary significantly over short distances; therefore, the exact location of the center of the AERSURFACE study region and the radius of the study region can be important inputs that affect the surface characteristics that are output by AERSURFACE. Variations in surface roughness output by AERSURFACE with differences in study center and study radius are examined in this paper. INTRODUCTION The AERSURFACE program is one of the support tools that can be used to generate input data for the AERMOD modeling system, which is the United States Environmental Protection Agency s (U.S. EPA s) preferred dispersion model for near-field (i.e., receptors less than 50 km from sources) applications. AERSURFACE is used to develop estimates of surface characteristics (albedo, Bowen ratio, and surface roughness) for use in AERMET to create AERMOD-ready meteorological data sets. A new version of AERSURFACE was released by U.S. EPA on January 9, 2008, in which publicly available files containing land use information for the area surrounding the meteorological data collection site or model study site, are used to define surface characteristics. The primary input used in AERSURFACE is a single National Land Cover Database 1992 (NLCD92) file for the area surrounding the meteorological tower location or the industrial site being modeled. NLCD92 is a 21-category land cover classification scheme that the United State Geological Survey (USGS) has applied consistently over the conterminous U.S. (See Table 1 for a description of each of the NLCD92 categories.) As discussed in the AERSURFACE Users Guide, the land use in sectors surrounding a center location is used to assign the albedo, Bowen ratio, and surface roughness for each sector, using seasonal values of each parameter derived from available literature. 1 The albedo and Bowen ratio are defined based on a simple unweighted geometric mean for the 10 km by 10 km area centered on the study center location. Therefore, a single value is defined for albedo and Bowen ratio for all wind directions. The 1

3 surface roughness is defined for various sectors surrounding the study center location based on an inverse-weighted geometric mean for a user-defined upwind distance. While the upwind distance is user-defined, in the AERSURFACE Users Guide EPA suggests the use of a default upwind distance of 1 km. Additionally, up to twelve sectors, each at least 30 degrees in length can be defined for the area surrounding the center location, with the intent that areas of similar land use be grouped together in sectors. Table 1. USGS NLCD92 Land Cover Categories Classification Class Number Land Cover Category Water 11 Open Water 12 Perennial Ice/Snow Developed 21 Low Intensity Residential 22 High Intensity Residential 23 Commercial/Industrial/Transportation Barren 31 Bare Rock/Sand/Clay 32 Quarries/Strip Mines/Gravel Pits 33 Transitional Forested Upland 41 Deciduous Forest 42 Evergreen Forest 43 Mixed Forest Shrubland 51 Shrubland Non-natural Woody 61 Orchards/Vineyards/Other Herbaceous Upland 71 Grasslands/Herbaceous Herbaceous 81 Pasture/Hay Planted/Cultivated 82 Row Crops 83 Small Grains 84 Fallow 85 Urban/Recreational Grasses Wetlands 91 Woody Wetlands 92 Emergent Herbaceous Wetlands This study presents a series of analyses testing the sensitivity of the surface characteristics output by AERSURFACE to required inputs. As discussed above, a default study radius of 1 kilometer is recommended by EPA for determination of surface roughness. Exceptions to this rule may be considered on a case-by-case basis in situations with significant discontinuities in land cover just beyond the recommended 1 kilometer upwind distance. 2 However, many airport sites where meteorological data are commonly collected have very sharp discontinuities in land cover very near the observation location, with the land cover type changing rapidly from commercial/industrial/transportation (site at airport) to residential, forest, or cropland at the edge of the airport. Therefore, the differences in AERSURFACE-predicted surface characteristics are examined using both the default 1 kilometer study radius and the 3 kilometer study radius previously recommended for use in the AERMET User s Guide and 40 CFR 51, Appendix W (Guideline on Air Quality Models). 3,4 Another sensitivity analysis that is considered is the exact location of the meteorological observation site input into AERSURFACE. The airport location provided in many sources may accurately represent the location of the airport to several decimal locations for latitude and longitude; however, in some cases the reference location may not represent the exact location of the meteorological observation station at the airport. Therefore, an uncertainty of the exact observation location of tens to hundreds of meters may exist. If a 1 kilometer study radius is 2

4 used, this may mean that an entirely different study area is used to determine surface characteristics in AERSURFACE than actually surrounds the observation location. Therefore, differences in surface characteristics predicted by AERSURFACE using slightly different study center locations at airport and industrial locations are also examined. METHODOLOGY Study locations are defined in three general areas of the United States, the eastern U.S., the central U.S., and the western U.S. The three airports chosen are the Albany Airport (ALB) located in Albany, New York, the Jackson Julian Carroll Airport (JKL) located near Jackson, Kentucky, and the Pocatello Regional Airport (PIH) located in Pocatello, Idaho. To represent typical industrial operations in each of these regions, an industrial facility located in the vicinity of each airport is chosen for inclusion in this study. At each airport, AERSURFACE is run with both a 1 km and 3 km study radius to define surface roughness, with the study center location defined by the airport location found on the National Climatic Data Center (NCDC) website. AERSURFACE is also run at each airport with a 1 km surface roughness study radius and a study center location at an alternative location at the airport. For each plant site, AERSURFACE is run with both a 1 km and 3 km study radius and a study center location defined based on the approximate center of the facility. The land use for each airport and plant site, as well as the different study areas are presented in Figures 1 through 6. The area surrounding ALB, shown in Figure 1, consists of primarily high intensity and low intensity residential and commercial/industrial/transportation land use with smaller areas of deciduous and evergreen forest, pasture/hay, and small grains. The 1 km study area using the NCDC location as the center point contains primarily commercial/industrial/transportation and high intensity residential land use to the northeast and a mixture of deciduous and evergreen forest, small grains, and pasture/hay to the southwest. The 1 km study area using the alternative center point is almost oppositely divided, containing commercial/industrial/transportation and high intensity residential areas to the southwest and a mixture of deciduous and evergreen forest and small grains to the northeast. The area surrounding the NY Plant in the 1 km study radius, shown in Figure 2, consists primarily of the quarries/strip mines/gravel pits and mixed forest land use categories with smaller areas of pasture/hay, commercial/industrial/transportation, and evergreen forest land use categories. In the NY Plant scenario, at distances beyond 1 km, land use categories also include large areas of high and low intensity residential to the southeast, quarries/strip mines/gravel pits to the southwest, and pasture/hay to the northwest. The area surrounding JKL (Figure 3) is much more homogeneous than either ALB or the NY Plant site. The 1 km and 3 km study areas centered on the NCDC location and the 1 km study area centered on the alternative location all contain primarily the deciduous forest land use category. There are also some smaller areas, particularly outside the 1 km study areas, but within the 3 km study area, of pasture/hay, mixed forest, and evergreen forest land use types near JKL. The KY Plant location (Figure 4) is surrounded by a much more complex mixture of land use types compared with JKL. Within the 1 km study area the primary land use type is transitional barren to the northeast, deciduous forest and open water to the southeast, pasture/hay and deciduous forest to the southwest, and deciduous forest pasture/hay, and open water to the northwest. Outside the 1 km study area, but within the 3 km study area, the primarily land use types are deciduous and mixed forest, except to the south of the center location where a mixture 3

5 of pasture/hay, and row crops dominates. In the 1 km study area surrounding the NCDC center location at PIH (Figure 5), land use types include shrubland in all directions, with some additional row crops to the southeast, commercial/industrial/transportation to the southwest, and orchards/vineyards/other to the northwest. The land use categories in the 1 km alternative study area vary from those in the 1 km NCDC study area, with shrubland and commercial/industrial/transportation to the northeast, pasture/hay and orchards/vineyards/other in all other directions. There is also a component of commercial/industrial/transportation land use to the south of the alternative 1 km study area center location. The 3 km NCDC study area contains similar land use types compared with the 1 km study areas, but with different distributions. There are also some areas of woody wetlands and emergency herbaceous wetlands to the northeast of the center location in the NCDC 3 km study area at PIH. The area surrounding the ID Plant (Figure 6) consists of a patchwork of row crops, orchards/vineyards/other, shrubland, pasture/hay, and commercial/industrial/transportation land use categories within both the 1 km and 3 km study areas. In the 3 km study area, there is also a larger area of woody wetlands and emergency herbaceous wetlands to the west of the study center location. Other AERSURFACE user inputs are defined to make these analyses as representative of the different regions of the U.S. as possible. For all scenarios, twelve 30-degree sectors, starting at 0 degrees (North) are used to define surface roughness parameters with varying wind direction. Surface characteristics are defined on a seasonal temporal resolution for all locations. The New York and Idaho airport and plant locations are assumed to experience continuous snow cover for most of the winter. All locations are assumed to be non-arid. Average precipitation compared with climatological average is assumed for all locations. 4

6 Figure 1. Land Use Surrounding the Albany Airport (ALB) Figure 2. Land Use Surrounding the NY Plant Site 5

7 Figure 3. Land Use Surrounding the Jackson Julian Carroll Airport (JKL) Figure 4. Land Use Surrounding the KY Plant Site 6

8 Figure 5. Land Use Surrounding the Pocatello Regional Airport (PIH) Figure 6. Land Use Surrounding the ID Plant Site 7

9 RESULTS The surface roughness, also designated as z o, output by AERSURFACE for each scenario, averaged over all sectors and seasons, is shown in Tables 2 and 3. The correlation coefficient for each scenario and the percent difference between the 1 km radius average and the 3 km radius average are also shown in the tables. The percent difference is calculated by taking the absolute value of the difference between the two averages and dividing by the average surface roughness for the 1 km study radius using the NCDC coordinate as the study center. In this study, only variations in surface roughness are examined and not variations in albedo and Bowen ratio between the various cases. This limitation was chosen for two reasons: 1) in the 1 km versus 3 km study radius scenarios, the albedo and Bowen ratio values are identical because the same 10 km by 10 km area is used to define these parameters regardless of the study radius used to define surface roughness and 2) previous studies have shown that of the three surface parameters output by AERSURFACE, variations in surface roughness have the most significant impact on pollutant concentrations modeled using AERMOD. 5 As shown in the tables, it is not possible to state that in every case using the 1 km radius resulted in lower or higher average surface roughness values compared with using the 3 km radius. Also, comparing the surface roughness data for the 1 km study radius using the NCDC center coordinate with the alternative center coordinate does not yield and consistent trend for all scenarios. However, a few items worth noting can be seen in the data in Tables 2 and 3. In areas with homogeneous land use, such as JKL (see Figure 3),variations in the study radius and the study center location result in relatively low variations in surface roughness output by AERSURFACE, as can be seen by the low percent difference and high correlation for JKL in Tables 2 and 3. In areas with more heterogeneous land use, such as ALB, the NY Plant, and the KY Plant, variations in the study radius and study center result in greater variations in surface roughness, as seen by the higher percent differences and low correlations in for these scenarios in Tables 2 and 3. Table 2. 1 km Versus 3 km All Sector Average Surface Roughness Location 1 km Radius z o (m) 3 km Radius z o (m) Percent Difference Correlation ALB % NY Plant % JKL % KY Plant % PIH % ID Plant % Table 3. NCDC 1 km Versus Alternative 1 km All Sector Average Surface Roughness NCDC 1 km Radius z o (m) Alt. 1 km Radius z o (m) Percent Location Difference ALB % JKL % PIH % Correlation 8

10 As shown in additional detail in Figure 7, in areas where the land use types do not vary significantly between the 1 km study radius and the 3 km study radius, low percent differences and high correlations result. At the ID Plant location, the primary land use types present in both the 1 km radius and 3 km radius include row crops, pasture/hay, and orchards/vineyards/other along with smaller areas of emergent herbaceous wetlands and residential areas (Figure 6). However, for the NY Plant location the land use varies more between the 1 km study radius and the 3 km study radius and there is less agreement between the surface roughness values output by AERSURFACE for these two study radii, as shown in Figure 8. At the NY Plant, the land use types within the 1 km radius consist primarily of quarries/strip mines/gravel pits and mixed forest, as seen in Figure 2. However, in the 3 km study radius added to these land use types are large areas of pasture/hay, row crops, open water, and residential areas. Figure 7. Surface Roughness Comparison for ID Plant Location 3 km Surface Roughness (m) r 2 = ID Plant km Surface Roughness (m) Figure 8. Surface Roughness Comparison for NY Plant Location 3 km Surface Roughness (m) NY Plant r 2 = km Surface Roughness (m) 9

11 The assertion that there is less variability seen in surface roughness when changing study center locations if the land use is more homogeneous is further demonstrated below. Figures 9 and 10 each contain a scatterplot comparing the surface roughness value output by AERSURFACE for each season and sector for the 1 km NCDC study area versus the 1 km alternative study area. Figure 9 shows the data for the JKL study areas and Figure 10 shows the data for the ALB study areas. For the JKL study areas, where the land use consists primarily of deciduous forests in all directions, the data points are located near the one to one line on the scatterplot and the correlation is high. For the ALB study areas, where the land use is a mix of commercial/industrial/transportation, residential, mixed forest, and small grains, there are a number of data points where the surface roughness shows little agreement between the two study areas and the correlation is relatively low. Figure 9. Surface Roughness Comparison for JKL Location Alt 1 km Surface Roughness (m) JKL r 2 = NCDC 1 km Surface Roughness (m) Figure 10. Surface Roughness Comparison for ALB Location Alt 1 km Surface Roughness (m) ALB r 2 = NCDC 1 km Surface Roughness (m) 10

12 CONCLUSIONS The study radius and center location can affect surface roughness parameters output by AERSURFACE using NLCD92 data as input. The degree to which the surface roughness is affected can vary from case to case, but is influenced by whether the land use in the potential study areas is homogeneous or heterogeneous. In areas with very heterogeneous land use, the surface roughness may be affected more by the study radius and center location compared with more homogeneous regions. The affect of these variations in surface roughness (and also albedo and Bowen ratio for the scenarios in which the center location is varied) are examined in a companion presentation (#2009-A-168-AWMA). In their AERMOD Implementation Document, EPA allows for case-by-case discretion in the use of an alternative study radius in a regulatory dispersion modeling study, in situations with significant discontinuities in land cover just beyond the recommended 1 kilometer upwind distance. These discontinuities occur most frequently in areas of heterogeneous land use, such as urban areas and large industrial complexes, which are frequently where airports and/or plants for which dispersion modeling is required are located. Because the study radius can have an impact on surface roughness, and potentially modeled concentrations, if an alternative study radius is thought to be appropriate, the regulatory agency should be contacted as early as possible in the application process and a formal dispersion modeling protocol submitted to receive approval. REFERENCES 1. AERSURFACE Users Guide. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. January AERMOD Implementation Guide. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. Revised January User s Guide for the AMS/EPA Regulatory Model - AERMOD. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. Revised September Guideline on Air Quality Models. Appendix W to 40 CFR Parts 51 and 52. Federal Register, November 9, pp Carper, E. and E. Ottersburg. Sensitivity Analysis Study Considering the Selection of Appropriate Land-Use Parameters in AERMOD Modeling Analysis. Presented at the 2004 AWMA Annual Conference. June KEYWORDS AERSURFACE, AERMOD, Land Use 11