About air quality data

About the Air Quality Index

The Air Quality Index, or AQI, was developed by the U.S. Environmental Protection Agency (EPA) to provide a simple, uniform way to report daily air quality conditions. Minnesota AQI numbers are determined by hourly measurements of five pollutants:

  • fine particles (PM2.5),
  • ground-level ozone (O3)
  • sulfur dioxide (SO2)
  • nitrogen dioxide (NO2)
  • carbon monoxide (CO)

The pollutant with the highest AQI value determines the overall AQI for that hour.

The five pollutants measured for the AQI are good indicators of daily air quality, but are not the only air pollutants which may cause health effects, such as air toxics pollutants. Additionally, the AQI does not account for temperature or pollen levels, which may increase sensitivity to air pollutants.

Calculating the Air Quality Index

The AQI is calculated by converting measured pollutant concentrations to a uniform index which is based upon peer-reviewed scientific evidence of the health effects associated with a pollutant. The health benchmarks used for calculating the AQI are pollutant specific and are established by the EPA through the National Ambient Air Quality Standards. The Clean Air Act requires the EPA to review these standards every five years. The MPCA calculates the AQI for each pollutant, at each monitoring site, every hour. The pollutant with the highest AQI value is used to characterize the overall AQI.

The following formula is used to convert measured pollutant concentrations to an AQI:

  • AQI = the AQI for the pollutant
  • Cp = the rounded concentration of the pollutant
  • BPHi = the breakpoint that is greater than or equal to Cp
  • BPLo = the breakpoint that is less than or equal to Cp
  • IHi = the AQI value corresponding to BPhi
  • Ilo = the AQI value corresponding to BPLo


The AQI for fine particles (PM2.5) is based on a 24-hour average concentration. When PM2.5 levels change quickly, the 24-hour average PM2.5 concentration is slow to respond. To better reflect real-time PM2.5 conditions, the MPCA and EPA AQI websites calculate the PM2.5 AQI using a formula called NowCast.

The NowCast formula estimates real-time PM2.5 conditions by placing greater weight on PM2.5 concentrations measured in the most recent hours. Each hour, the NowCast formula recalculates the hourly AQI for all of the previous hours of the day. Because each hour is recalculated to reflect actual PM2.5 measurements, the real-time AQI value reported at 8 a.m. may be different from the final AQI value for 8 a.m. that day. The real-time AQI value is calculated using the NowCast formula, the final AQI value for an hour is the rolling 24-hour average PM2.5 concentration.

Want the technical details? See the formula: PDF icon AQI: Computing the NowCast

AQI pollutant concentration breakpoints

 
AQI breakpoints
Ozone (ppb)
8-hour
PM2.5
(ug/m3)
24-hour
SO2 (ppb)
24-hour
CO (ppm)
24-hour
NO2
(ppb)
1-hour
 Good

0 – 50

0 - 59

0 – 12

0 - 34

0 - 4.4

0-53

 Moderate

51 – 100

60 – 75

12.1 – 35.4

35 – 144

4.5 – 9.4

54-100

 Unhealthy
 for sensitive
 groups

101 – 150

76 – 95

35.5 – 55.4

145 – 224

9.5 – 12.4

101-360

 Unhealthy

151 – 200

96 – 115

55.5 – 150.4

225 – 304

12.5 – 15.4

361-640

 Very
 unhealthy

201 – 300

116 – 374

150.5 – 250.4

305 – 604

15.5 – 30.4

650-1240

 

AQI monitor locations

The MPCA reports the AQI for areas across Minnesota:

  • Brainerd
  • Detroit Lakes
  • Duluth
  • Ely
  • Grand Portage
  • Marshall
  • Red Lake
  • Rochester
  • St. Cloud
  • Twin Cities
  • Virginia
  • Leech Lake Nation: Cass Lake
  • Mille Lacs

 

Fine-particle pollution is measured year round in all reporting areas. Ozone pollution is monitored from April through October in all reporting areas except Grand Portage and Virginia. The other AQI pollutants, carbon monoxide, sulfur dioxide, and nitrogen dioxide, rarely influence the AQI and are only measured at a few sites.

For a complete listing and map of current AQI monitoring locations, visit the current air quality detailed results page.

The Annual Air Monitoring Network Plan for Minnesota describes all air pollution monitoring activities in Minnesota.

Estimating AQI in areas with no monitors

The AQI results displayed on the current conditions map are generated by the EPA and are based upon real-time air pollution monitoring results from across the country. Air quality conditions in areas without an air pollution monitor are estimated based upon the nearest air pollution monitor. For Minnesota, air quality conditions are estimated using air pollution monitoring results from monitors within Minnesota and from surrounding states.

These estimates are based upon the best available data, but may not reflect highly localized air quality events, such as a fire. Current conditions are not estimated for the northwestern portion of Minnesota because there are not enough monitors to make a reliable estimate for that area of the state.

National AQI results are available from EPA: AQI: AIRNow - National AQI

Daily air quality forecasts

The MPCA produces daily air quality forecasts for eighteen locations across the state of Minnesota, including all monitor locations and the Twin Cities metro area split into three zones. These forecasts use the AQI scale for users to best understand the expected air quality around each location. The primary pollutants forecast are fine particle (particulate matter 2.5 microns) and ozone. These daily forecasts are important for awareness and planning, especially for those most sensitive to air pollution. When the forecast is for AQI values to be near or to exceed 101, an air quality alert is issued. These forecasts are available on many platforms including the MPCA's current air quality page,  the Minnesota Air mobile app for iOS or Android, and Enviroflash emails.

Issuing air quality alerts

The MPCA uses hourly air pollution monitoring results and daily air quality forecasts to determine whether air pollution concentrations have reached air quality alert levels.

An air quality alert is issued when measured or forecasted air quality conditions are expected to be greater than 101 AQI. An air quality alert means that current or forecasted air quality conditions are expected to contribute to adverse health effects for populations that are sensitive to air pollution. Those sensitive to air pollution include:

  • individuals with preexisting lung or heart disease
  • the elderly
  • children
  • those participating in activities that require heavy or extended exertion

Air quality alerts can be issued for anywhere in the state of Minnesota using a combination of air quality and meteorological information. To be notified of air quality alerts, download the Minnesota Air mobile app for iOS or Android, or sign up for e-mail alerts at Enviroflash.

Factors that contribute to poor air quality

Many factors can lead to poor air quality days. Air pollution levels in Minnesota come from both local pollutant emissions from sources such as industries, cars, and homes, as well as pollution that is blown into Minnesota from surrounding areas.

Most poor air quality days are not caused by a short-term increase in pollution emissions (notable exceptions include wildfire smoke or fireworks). Rather, most poor air quality days are driven by changing weather conditions that increase the rate at which air pollutants are formed or accumulate in the air.

Ozone pollution levels tend to rise on very hot and sunny days with little wind. Ozone pollution is not emitted directly from emission sources; it is formed through a chemical reaction between emitted nitrogen oxides and volatile organic compounds in the environment. Hot and sunny weather increase the speed at which this chemical reaction occurs, causing ozone pollution levels to increase. Due to the nature of this reaction, the highest ozone levels tend to occur downwind of the urban core.

Fine particle pollution can be elevated at any time of the year, but in Minnesota, the highest levels are typically measured between November and March. Fine particles are emitted directly from pollution sources and created through reactions between emitted sulfur oxides, nitrogen oxides, and ammonia in the air. Fine particles are microscopic pollutants that are easily trapped in the air. Weather conditions, such as high humidity, high pressure, strong overnight temperature inversions, or low wind speeds can cause fine particle pollution to increase.

Air pollution levels are susceptible to year-to-year variations in meteorological patterns in an area. For example, the number of poor air quality days due to ozone pollution is expected to be higher in years when the number of very-high-temperature days is above normal. Similarly, there will be fewer poor air quality days when the number of very-high-temperature days is below normal.

For more information