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Support Center for Regulatory Atmospheric Modeling (SCRAM)

Nitrogen Dioxide/Nitrogen Oxide In-Stack Ratio (ISR) Database


Section 5.2.4 of the EPA’s Guideline on Air Quality Models, Appendix W to 40 CFR Part 51 (PDF)(45 pp, 803 K, About PDF), recommends a three-tiered screening approach to estimate ambient concentrations of nitrogen dioxide (NO2):

  • Tier 1 – assume complete conversion of all emitted nitrogen oxide (NO) to nitrogen dioxide (NO2)
  • Tier 2 – multiply Tier 1 results by a representative equilibrium NO2/NOx ratio
  • Tier 3 – perform a detailed analysis on a case-by-case basis

Prior to the April 2010 revision of the NO2 NAAQS, most facilities were able to demonstrate compliance with the NAAQS using the Tier 1 and 2 screening methods. However, with the additional stringency of the new 1-hour NO2 NAAQS, the need for facilities to use a Tier 3 approach has increased. EPA issued guidance, Applicability of Appendix W Modeling Guidance for the 1-hour NO2 National Ambient Air Quality Standard (PDF)(16 pp, 278 K, About PDF) on June 28, 2010, indicating that the three-tiered approach in Section 5.2.4 is generally applicable for the 1-hour NO2 NAAQS, and also clarified that the Ozone-Limiting Method (OLM) and Plume Volume Molar Ratio Method (PVMRM), included as non-default options in the AERMOD dispersion model, are currently considered to be detailed screening methods under Tier 3.

The use of the Tier 3 PVMRM and OLM options in AERMOD requires the specification of an in-stack ratio (ISR) of NO2/NOx for each source (using the CO NO2STACK and/or SO NO2RATIO keywords). The June 28, 2010 guidance emphasized the importance of these in-stack ratios for the 1-hour NO2 NAAQS, recommending that in-stack ratios used with either the OLM or PVMRM options be justified based on the specific application (i.e., there is no “default” in-stack NO2/NOx ratio for either OLM or PVMRM). Additional guidance, (Additional Clarification Regarding the Applicabilition of Appendix W Modeling Guidance for the 1-hour NO2 National Ambient Air Quality Standard (PDF)(27 pp, 443 K, About PDF)) issued on March 1, 2011 allowed for a default ISR of 0.5 in the absence of more appropriate source-specific information. However, the recommended default ISR may still be too conservative for many applications such that there remains a significant need for a widely available and well-documented database of ISRs, which is the impetus for the current data collection effort.


NO2_ISR_database_template.xlsx (XLSX)(400 K) – This file contains the template for submissions of ISR values. The entry fields for the template are discussed in detail in the accompanying readme file (PDF)(3 pp, 98 K). The template has been prepared to accept a maximum of 80 entries. If more than 80 entries are necessary, it is requested that the extra data be entered in a new template. If submissions indicate that more than 80 entries are typical, the template will be updated as appropriate.

NO2_ISR_database.xlsx (XLXS)(411 K) – This file contains the NO2 ISR data that has been submitted via the formal collection initiated by OAQPS. The database is expected to be updated on a monthly basis, though the frequency of updates will be based on the number and frequency of submissions and the availability of OAQPS staff.

NO2_ISR_alpha_database.xlsx (XLSX)(474 K) – This file contains NO2ISR values collected by various Regional, State, and Local air permitting offices prior to the formal collection initiated by OAQPS. While this database contains a large number of entries, none fully satisfy the requirements for the formal collection effort.

Data Usage

Inclusion of data in this database does not imply any pre-approval for usage and no single or collection of datum contained in the database have been reviewed or determined to be complete, correct, or acceptable by OAQPS for modeling applications. As with any Tier 3 NO2 modeling, the use of any single or collection of NO2 ratios can only be used in permit modeling with the approval of the appropriate reviewing authority, whether an EPA Regional Office, State, or local permitting organization. It is encouraged that the user coordinate with the appropriate reviewing authority prior to any application of the ISRs contained in the database.

As noted above, the alpha database (NO2_ISR_alpha_database.xlsx (XLSX)(474 K)) contains entries that do not fully satisfy the requirements for the formal collection effort. As with the primary database, the use of any data contained therein must be coordinated with the appropriate reviewing authority.

Data Entry and Submission

Data should be submitted by emailing a completed template to Chris Owen ( The template may be renamed to suit the filer’s purpose and as many templates may be sent as necessary. However, DO NOT SEND "ZIP" files. All emailed files are automatically scanned by the EPA email server and ".zip" files are automatically deleted from incoming emails and are never seen by the recipient.

There is a wide range of sampling scenarios that may occur (e.g., continuous monitoring, stack tests, etc) and may result in dozens of tests for each emission unit per year. We request that each test be included as an individual entry into the database to maximize the statistical significance of the ISRs for each source type. For example, a facility may be required to perform monthly testing. At each monthly test, 3 30-min samples may be taken, resulting in 36 ISRs collected per year. Ideally, all 36 test results would be included in a submission of the ISR data.

The template has 41 entry fields, 31 of which are marked as “required”. Guidance on each data field is given in accompanying readme file (PDF)(3 pp, 98 K). Data submitted that does not include all required information will not be added to the posted database. Since the data submitted to the database does not undergo any review or QA/QC vetting by OAQPS, the required fields include sufficient information to identify the original submitter and specific tests reports, in case the Regional Office requires additional vetting of supplied NO2 ISRs.