Limitations in the 1999 Estimated Emissions and Ambient Concentrations

The draft 1999 emissions and ambient concentrations should not be used for comparison with 1996 NATA results to perform trends analysis because, in addition to being draft, changes have been made to emission inventory and processing methodologies. The purpose of these results are to provide information to assist in the review of draft version 3 of the National Emission Inventory (NEI) for Hazardous Air Pollutants (HAPs). Revisions will be made to the emissions and modeling approach (e.g., background concentrations and secondary concentrations) for the National Scale Assessment for 1999 to be conducted during the summer of 2003.

Please see https://www.epa.gov/airtoxics/nata/natsalim2.html for details on the limitations associated with the 1996 National-Scale Air Toxics Assessment. The information provided on limitations, variability and uncertainty in the 1996 air toxics assessment also pertains to the draft emissions and ambient concentrations for 1999, noting that the emissions data used here are for 1999 and not 1996.

In addition, please be aware of the following limitations in draft version 3 of the National Emission Inventory for HAPs:

1) Not all of the State submitted data have been incorporated into this version. See for more details:

https://www.epa.gov/ttn/chief/net/1999nei_online.pdf

The above link also provides more information on how the inventory was developed and how you can access more detailed information on the inventory.

2) Data were augmented when the quality assurance routines found issues with the draft inventory's point source locations and stack parameters. These procedures are different from those used for the 1996 NATA. For example, point sources with missing or invalid locations (e.g., geographic coordinates conflict with the county information) were defaulted to the county centroid rather than to different tract centroids in the county. See the files posted under the following link for more details on the augmentation procedures:

https://www.epa.gov/ttn/chief/emch/invent/index.html

The relevant files under this link are: National Emission Inventory QA and Augmentation Memo (PDF 35K), SCC- Based Emission Release Point Physical Parameter Default File (XLS 1.3M), and SIC-Based Emission Release Point Physical Parameter Default File (XLS 403K).

With regards to the ambient concentrations, please note the following:

1. Background concentrations were the same as those used in 1996 (see https://www.epa.gov/airtoxics/nata/backcon.html ). They will be updated when concentrations are modeled with the final version 3 1999 National Emission Inventory for HAPs to account for more recent data and spatial variation of background.

2. Secondary concentrations for acetaldehyde and formaldehyde were estimated using a less refined method than the ASPEN model approach for this draft. For these pollutants secondary contributions were estimated using similar proportions of secondary to primary concentrations as were computed by ASPEN in the 1996 NATA. When the concentrations are modeled with the final version 3 inventory, the current ASPEN methodology or an enhanced photochemical-modeling based methodology will be used.