Air Quality Dispersion Modeling (AQDM) | Background and ozone data

Background concentrations

Background concentrations are a critical component of the total air quality concentration in a cumulative modeling analysis which demonstrates compliance with the NAAQS and PSD increments. Guidance for estimating background impacts can be found in Section 8.3 of the U.S. EPA’s Guideline on Air Quality Models (Appendix W to 40 CFR Part 51) and the “Background concentration” section of MPCA’s Air Dispersion Modeling Practices Manual.

Background concentrations include pollutant impacts from nearby sources (not including the facility under consideration).  Nearby sources are individual point sources that cause a significant concentration gradient in the vicinity of the source under consideration.  It is assumed nearby sources are not adequately represented by ambient monitoring data. Their contributions to the total concentration are estimated by explicitly modeling their emissions.  The MPCA provides a GIS tool for identifying potential nearby sources to include in a modeling demonstration on the Air Quality Dispersion Modeling | Tools page.

Background concentrations also include “other” sources of ambient air pollutants such as natural sources, smaller unidentified sources in the vicinity of the project, and contributions from regionally transported pollutants.  This component of the background concentration is developed from ambient air quality monitoring data. For most cases, using the current design value for the applicable NAAQS from a representative monitor is the most appropriate use of monitoring data. Representative monitor selection and proposed background concentrations should be well documented within a modeling protocol and submitted to the MPCA.

If you are in need of monitoring data other than the design values, please send an email request to airmodeling.pca@state.mn.us.

Ozone data

Ozone Monitors

The MPCA has pre-processed hourly ozone data for all of the ozone monitors shown above for use in Tier 3 NO2 modeling. Missing hours have been filled using interpolation for 1-hour gaps, and with max monthly/hourly values for longer gaps. As only Voyageurs and Blaine recorded ozone data in the winter for the current meteorological data period, those two monitors were used to fill in winter values for the remaining ozone monitors.  Voyageurs was used for monitors in the northern half of the state, while Blaine was used for monitors in the southern half. The Filling Missing Ozone Data for OLM and PVMRM Applications working practice memo provides further information.

Hourly ozone data are concurrent with the most recent meteorological data posted on the MPCA’s Meteorological data page. Once a representative ozone monitor has been selected, click on Figure 1 to access pre-processed hourly ozone files. Once the Processed Ozone Data map displays, click one of the green ozone monitor icons to download pre-processed hourly data for that site. Concentrations are in units of parts per billion. Please make a note in the e-Service modeling protocol “Pollutant Based Considerations” section, under “NO2 Tier 3, Additional Information,” that pre-processed hourly data are being used.