The National Emissions Inventory
The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of both Criteria and Hazardous air pollutants from all air emissions sources. The NEI is prepared every three years by the USEPA based primarily upon emission estimates and emission model inputs provided by State, Local, and Tribal air agencies for sources in their jurisdictions, and supplemented by data developed by the USEPA. The 2008 NEI was built from emissions data in the Emissions Inventory System (EIS).
                         Version History: (For a detailed list of caveats and changes from version one, please see the Version 1.5 issues file. 
   *                           Version 1.5 (released May 16, 2011): Key updates are to include the nonpoint data and major updates to point sources in Indiana, Maryland, Nebraska, and Lincoln/Lancaster air agencies. 
   *                           Version 1 (released April 4, 2011) 
Detailed documentation for each of the five data categories Point, NonPoint, OnRoad, NonRoad, and Event are available below.

2008 National Emissions Inventory Data
NEI Browser. Note that a public NEI Browser is available that currently retrieves NEI data for only the Criteria air pollutants.
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Sector Summaries - Criteria and Hazardous Air Pollutants by 59 emission sectors
*To use the multiple selection boxes hold CTRL and click on options you would like to include in your query. To select multiples in order click your first option then hold SHIFT + click the last selection to highlight everything in between. To unselect options use the same method as with selecting.
*Sector selections are limited to 12 states to avoid any timeout issues. 
This site uses pop-up windows, click here for help on allowing pop-ups from this site

State / County 
                            Geographic Aggregation 
                                 State County 
                                       

                                  Pollutant 
                                    Sector 
                                       
                                       

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Tier 1 Summaries - Criteria Air Pollutants only by 13 majors tiers 
                                State / County 
                            Geographic Aggregation 
                                 State County 
                                       

                                  Pollutant 
                               Tier 1 Category 
                                       
                                       

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The full detail data files for the Point, Onroad and Nonroad data categories and a Facility-level by Pollutant summary file can be downloaded from the following drop-down menu. A README file is available here.

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Additional Summary Data
   *        Tribal Lands Emissions Summaries 
         o              Sector (Includes both criteria and hazardous air pollutants) 
         o              Tier 1 (Includes criteria pollutants only)
   *        Facility Emissions Summaries 
         o              Lead (Pb) by facility by data source emissions summary 
         o              Lead (Pb) Version 1.5 by facility by data source emissions summary 
         o              Sulfur Dioxide (SO2) by facility by data source emissions summary 
         o              Sulfur Dioxide (SO2) Version 1.5 by facility by data source emissions summary


2008 National Emissions Inventory Documentation
The 2008 NEI is based primarily upon emission estimates and emission model inputs provided by State, Local, and Tribal air agencies (S/L/Ts), supplemented by data developed by the USEPA. In general, the USEPA used State, Local, or Tribal air agency data whenever it was reported and was not flagged as an obvious outlier. A summary of the data submitted by each S/L/T for version 1.5 is provided here.
USEPA datasets were developed and used to fill gaps where pollutants or entire source types were not reported by S/L/Ts. The main gap-filling by USEPA datasets were for certain NonPoint emissions sectors that could be estimated adequately by USEPA using nationally-available data; for Point sources for some hazardous air pollutants where not reported by S/L/Ts; for OnRoad and NonRoad sources where emission model inputs were available to USEPA; and for Aircraft, Locomotive, and Commercial Marine Vessel sources. The dataset selection order that was used for each of the four data categories included in version 1.5 (Point, NonPoint, OnRoad, and NonRoad) is available here. 
In addition to the above-described USEPA developed datasets, the USEPA also created and used three types of datasets based upon the 2008 data submitted by S/L/Ts. The first dataset type was used to split S/L/T reported emissions of total chromium into the Chromium III and Chromium VI valences that are preferred. Any S/L/T reported values for Chromium III and Chromium VI were used. The second dataset type was used to gap-fill the various Particulate Matter size ranges and filterable or condensable components when not all were reported by the S/L/Ts. In a small number of cases this dataset has values to be used in lieu of S/L/T values where the S/L/T values were internally inconsistent, such as where a PM2.5 value was greater than a PM10 value. The third type of dataset are the USEPA Overwrite datasets, which are used to select EPA emissions values (usually zeros) over individual S/L/T values which have been flagged as obvious outliers or are for pollutants and SCC combinations that are inconsistent with EPA's expectations. At this time the inconsistent pollutant-SCC combinations overwrites are limited to the OnRoad and NonRoad data categories, where emission models are used to develop emission estimates.
Description of NEI Data Categories 
The NEI Point data category contains emissions estimates for sources that are individually inventoried and usually located at a fixed, stationary location, although portable sources such as some asphalt or rock crushing operations are also included. Point sources include large industrial facilities and electric power plants, but also increasingly include many smaller industrial and commercial facilities, such as dry cleaners and gas stations, which had traditionally been included in Nonpoint sources. The choice of whether these smaller sources are estimated individually and included as point sources or inventoried as a NonPoint source County or Tribal area aggregate is determined by the separate State, Local, or Tribal air agency.
The NEI NonPoint data category contains emissions estimates for sources which individually are too small in magnitude or too numerous to inventory as individual point sources, and which can often be estimated more accurately as a single aggregate source for a County or Tribal area. Examples are residential heating and consumer solvent use.
The NEI OnRoad and NonRoad data categories contain mobile sources which are estimated for version 1.5 of the 2008 NEI via the MOBILE6 and NONROAD models, respectively, run within the National Mobile Inventory Model (NMIM). Note that emissions data for aircraft, locomotives, and commercial marine vessels is NOT included in the NonRoad data category starting with the 2008 NEI. Aircraft engine emissions occurring during Landing and Takeoff operations and the Ground Support Equipment and Auxiliary Power Units associated with the aircraft are now included in the point data category at individual airports in the 2008 NEI. Emissions from locomotives that occur at rail yards are also included in the point data category. In-flight aircraft emissions, locomotive emissions outside of the rail yards, and commercial marine vessel emissions (both underway and port emissions) are included in the NonPoint data category.
The Events data category includes wildfires and prescribed burns. This data category is not included in version 1.5 of the 2008 NEI. 

