Appendix 7 

Dispersion Model Receptor Revisions and Additions 
Dispersion Model Receptor Revisions and Additions for the Seven Chemical Sector 

Source Categories

To estimate ambient concentrations for evaluating long-term exposures, the HEM-3 

model uses the geographic centroids of census blocks (currently utilizing the 2000 Census) as 

dispersion model receptors. The census block centroids are generally good surrogates for where 

people live within a census block. A census block generally encompasses about 40 people or 10-

15 households. However, in cases where a block centroid is located on industrial property, or 

where a census block is large and the centroid less likely to be representative of the block’s 

residential locations, the block centroid may not be an appropriate surrogate. 

Census block centroids that are on facility property can sometimes be identified by their 

proximity to emission sources. In cases where a census block centroid was within 300 meters of 

any emission source, we viewed aerial images of the facility to determine whether the block 

centroid was likely located on facility property. The selection of the 300-meter distance reflects a 

compromise between too few and too many blocks identified as being potentially on facility 

property. Distances smaller than 300 meters would identify only block centroids very near the 

emission sources and could exclude some block centroids that are still within facility boundaries, 

particularly for large facilities. Distances significantly larger than 300 meters would identify 

many block centroids that are outside facility boundaries, particularly for small facilities. Where 

we confirmed a block centroid on facility property, we moved the block centroid to a location 

that best represents the residential locations in the block. 

In addition, census block centroids for blocks with large areas may not be representative 

of residential locations. Risk estimates based on such centroids can be understated if there are 

residences nearer to a facility than the centroid, and overstated if the residences are farther from 

the facility than the centroid. To avoid understating the maximum individual risk associated 

with a facility, in some cases we relocated block centroids, or added dispersion model receptors 

other than the block centroid. We examined aerial images of all large census blocks within one 

kilometer of any emission source. Experience from previous risks characterizations show that in 

most cases the MIR is generally located within 1 km of the facility boundary. If the block 

centroid did not represent the residential locations, we relocated it to better represent them. If 

residential locations could not be represented by a single receptor (that is, the residences were 

spread out over the block), we added additional receptors for residences nearer to the facility 

than the centroid. 

For these source categories, the table below contains each census block for which we 

changed the centroid location because it was on facility property or was otherwise not 

representative of the residential locations in the block. The table also contains the locations of 

additional receptors that were included to represent residential locations nearer to the facility 

than the block centroid. 
Revised Census Block Centroid Locations and Additional Receptors for Seven Chemical 

Sector Source Categories 

Centroid Revisions

Source

Category

Block ID NEI ID New

Latitude

New

Longitude

Comment

PAI 220479525012021 NA 30.234178  ‐91.108686 on a facility building

PAI 051399509001065 NA 33.200722  ‐92.672774 on a facility building

PAI 540390115004023 NA none none remove ‐ 0 population now

PAI 191390504003018 NA 41.34472  ‐91.088829 on a facility building

PAI 390852053009001 NA 41.750978  ‐81.161068 on plant property

PAI 260639512001013 NA none none remove ‐ 0 population now

PAI 010970057001032 NA 30.937818  ‐88.021147 on plant property

PePo 450830229004075 NA 35.027531  ‐82.073953 on plant property

PePo 220050303003072 NA 30.208337  ‐90.977473 on plant property

PePo 482030206021032 NA 32.429187  ‐94.690602 on plant property

PePo 482012526001019 NA 29.810243  ‐95.109249 on plant property

PR4ABS 541070109013012 NA 39.255265  ‐81.690164 on plant property

PR4PET 450179501001023 NA 33.855418  ‐81.005817 on plant property

PR4PS 290997006014015 NA 38.281962  ‐90.38895 on plant property

Additional Receptors

Source

Category

Block

ID

NEI ID New

Latitude

New

Longitude

Comment

PAI NA nei5992 30.233511 -91.107381 Add User Defined Receptor 

PAI NA nei5992 30.232785 -91.106298 Add User Defined Receptor 

PAI NA nei5992 30.232078 -91.105219 Add User Defined Receptor 

PAI NA nei5992 30.231352 -91.104 Add User Defined Receptor 

PAI NA nei5992 30.230627 -91.10292 Add User Defined Receptor 

PAI NA nei5992 30.229833 -91.101662 Add User Defined Receptor 

PAI NA neiart$10758 33.20031 -92.67115 Add User Defined Receptor 

PAI NA nei8596 30.953298 -88.027869 Add User Defined Receptor 

PAI NA neiwv0390007 38.391216 -81.808432 Add User Defined Receptor 

PAI NA neiwv0390001 38.238993 -81.541725 Add User Defined Receptor 

PePo NA nei32995 37.994614 -86.109426 Add User Defined Receptor 

PePo NA nei32995 37.990975 -86.107339 Add User Defined Receptor 

PePo NA nei32995 37.991707 -86.103371 Add User Defined Receptor 

PePo NA nei32995 37.98713 -86.101016 Add User Defined Receptor 

PePo NA nei32995 37.999448 -86.099087 Add User Defined Receptor 

PePo NA nei6013 30.160047 -90.988949 Add User Defined Receptor 

PePo NA nei12066 30.311408 -95.373946 Add User Defined Receptor 

PePo NA nei12066 30.306872 -95.372125 Add User Defined Receptor 

PePo NA nei12066 30.321833 -95.377722 Add User Defined Receptor 

PePo NA nei7491 29.577102 -95.423985 Add User Defined Receptor 

PePo NA nei7491 29.570455 -95.42235 Add User Defined Receptor 
Additional Receptors

Source

Category

Block

ID

NEI ID New

Latitude

New

Longitude

Comment

PePo NA nei11319 29.829382 -95.087012 Add User Defined Receptor 

PePo NA nei11319 29.82691 -95.076678 Add User Defined Receptor 

PePo NA nei8432 29.999718 -94.958772 Add User Defined Receptor 

PePo NA nei8432 30.003207 -94.958599 Add User Defined Receptor 

PePo NA nei8432 30.005341 -94.953407 Add User Defined Receptor 

PePo NA nei8432 30.007608 -94.95304 Add User Defined Receptor 

PR4ABS NA nei43298 39.25975 -81.664057 Add User Defined Receptor 

PR4ABS NA nei51461 41.343605 -88.755693 Add User Defined Receptor 

PR4ABS NA nei51461 41.341315 -88.746689 Add User Defined Receptor 

PR4PET NA nei44658 35.620626 -79.030713 Add User Defined Receptor 

PR4PET NA nei44658 35.629696 -79.024895 Add User Defined Receptor 

PR4PET NA nei44658 35.622076 -79.036009 Add User Defined Receptor 

PR4PET NA nei44658 35.623715 -79.050571 Add User Defined Receptor 

PR4PET NA nei44658 35.629491 -79.051519 Add User Defined Receptor 

PR4PET NA nei41393 33.852788 -81.011959 Add User Defined Receptor 

PR4PET NA nei41317 34.744187 -82.351083 Add User Defined Receptor 

PR4PET NA nei41302 34.198359 -79.58893 Add User Defined Receptor 

PR4PET NA nei43310 34.264122 -79.818106 Add User Defined Receptor 

PR4PET NA nei43310 34.260011 -79.817459 Add User Defined Receptor 

PR4PET NA nei43310 34.272689 -79.846157 Add User Defined Receptor 

PR4PET NA nei43310 34.273013 -79.821523 Add User Defined Receptor 

PR4PET NA nei43310 34.264714 -79.801435 Add User Defined Receptor 

PR4PET NA nei41309 33.835751 -79.772634 Add User Defined Receptor 

PR4PET NA nei41309 33.839257 -79.776805 Add User Defined Receptor 

PR4PET NA nei41302 34.193849 -79.587596 Add User Defined Receptor 

PR4PET NA nei41302 34.187502 -79.580996 Add User Defined Receptor 

PR4PET NA nei45591 36.270925 -86.669448 Add User Defined Receptor 

PR4PET NA nei45591 36.276675 -86.674268 Add User Defined Receptor 

PR4PS NA nei43287 40.084427 -76.165499 Add User Defined Receptor 

PR4PS NA nei43287 40.082354 -76.180236 Add User Defined Receptor 

PR4PS NA neisc0600-00 35.109847 -81.607599 Add User Defined Receptor