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

Background concentrations

Section 8.2 of the U.S. EPA’s Guideline of Air Quality Models (Appendix W to Part 51) states:

“Background concentrations are an essential part of the total air quality concentration to be considered in determining source impacts. Background air quality includes pollutant concentrations due to: (1) Natural sources; (2) nearby sources other than the one(s) currently under consideration; and (3) unidentified sources.”

Background concentrations are developed from ambient air quality monitoring data, and must be included in cumulative NAAQS analyses. The spreadsheet below was developed by MPCA air data staff, and contains design values for air monitoring data from Minnesota for the years 2011-2013.  For details regarding the individual air monitoring sites, please the MPCA’s Air Monitoring Network Plans. If you are in need of monitoring data other than the design values included below, please contact an MPCA air dispersion modeler for more information. 

PDF icon Minnesota 2013 Design for All Criteria Air Pollutants

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. Since only Voyageurs and Blaine record ozone data in the winter, those two monitors were used to fill in winter values for all of the other ozone monitors (Voyageurs North, Blaine South).

Hourly ozone data is concurrent with the meteorological data posted on the MPCA’s website.   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 modeling protocol that pre-processed hourly data are being used.

For more information on the ozone data and methods used to fill missing hours, see the working practice memo on the Guidance, Policies, and Practices webpage.

Class II PSD minor source baseline dates

A feature unique to managing air quality through the PSD program is the concept of an air PSD Increment. A PSD Increment reflects how much air pollution is allowed to increase in a given geographic or baseline area. This approach was designed to keep the air quality in areas where air is “clean” (or below the NAAQS) from significantly deteriorating. The pollutant-specific PSD increment values are designated by federal regulation. Air quality dispersion modeling is used to examine potential air quality compliance issues with the PSD Increments.

A PSD increment modeling analysis is location-specific, meaning that each analysis is created from the pollutant-specific ambient air quality baseline conditions for the area in which the project is located. Increment consumption and expansion are determined by emission changes relative to two different dates: 

Major Source Baseline Date is the date after which increases in air pollution from major sources (as defined in PSD rules) "consume" increment. The pollutant specific dates are:

  • January 6, 1975 for PM10
  • January 6, 1975 for SO2
  • February 8, 1988 for NOx 
  • October 20, 2010 for PM2.5
  • The Major Source Baseline Dates are the same for every county in Minnesota.

Minor Source Baseline Date is the date after which increases in air pollution from all sources “consume” increment. The Minor Source Baseline Dates are pollutant and county specific in Minnesota. The Minor Source Baseline Date for a specific county is set by the date the first complete PSD permit application for that county was submitted to the MPCA. Minor Source Baseline dates for Minnesota counties are shown in the files below