'Unhealthy': Highest ozone readings in 11 years in parts of Minnesota

Ozone levels are on the rise around most of Minnesota Tuesday.

The combination of sunlight, warmer temperatures and increased pollutants is accelerating the formation of ground-level ozone.

Air quality is forecast to reach the unhealthy for all (red) range around the Twin Cities Tuesday afternoon (map above). According to the Minnesota Pollution Control Agency, it’ll be the first time since June 2012 that ozone levels have pushed air quality into the red category.

Air quality index readings are already in the orange category in much of central Minnesota Tuesday afternoon including parts of the Twin Cities. The AQI reading as of this post (below) at St. Michael just northwest of the Twin Cities is at 150 for ozone. That’s at the top of the unhealthy for sensitive groups range.

Air quality alert continues through Thursday

The latest air quality alert for most of Minnesota continues through 9 p.m. Thursday. Conditions will remain favorable for the formation of ozone.

Here’s the wording on this week’s alert:

Air quality is expected to reach the orange AQI category in central and southern Minn., which is unhealthy for sensitive groups.

The Minnesota Pollution Control Agency (MPCA) has issued an air quality alert for central and southern Minn., effective from noon on Tuesday, June 20, through 9 p.m. on Thursday, June 22. The affected area includes central and southern Minn., and the tribal nations of Leech Lake, Mille Lacs, Upper Sioux, and Prairie Island.

Ground-level ozone is expected to be high during the afternoon hours on Tuesday, Wednesday, and Thursday across central and southern Minn. Sunny skies, warm temperatures, and low humidity will create an environment favorable for two types of pollutants (Volatile Organic Compounds and Nitrogen Oxides) to react in the air to produce ground-level ozone. These pollutants will be transported by southerly winds from the Chicago area, northeast Iowa, and southeast Minn. Ozone will be highest during the afternoon and early evening hours when sunshine is most abundant, and temperatures are highest. Ozone will be low in the morning, late evening, and overnight.

Ozone levels are expected to reach the orange air quality index (AQI) category, a level considered unhealthy for sensitive groups, across central and southern Minn. The alert area includes the Twin Cities, Brainerd, Alexandria, Albert Lea, Marshall, Worthington, Rochester, Hinckley, St. Cloud, Ortonville, Mankato, and the tribal nations of Leech Lake, Mille Lacs, Upper Sioux, and Prairie Island. In the orange area, sensitive groups should limit prolonged outdoor exertion.

Here’s the latest forecast discussion from the MPCA:

Updated Tue, Jun 20, 2023

Noon update: ozone concentrations have climbed considerably this morning. Ozone in the Twin Cities and St Cloud area are forecast to be in the Red category this afternoon. Red AQI is unhealthy for everyone, and everyone should limit prolonged time outdoors this afternoon and evening in the above areas. -The last time we observed Red ozone AQI in Minnesota was in June 2012.-

Previous discussion: Southerly winds, sunny skies, and warmer temperatures will hold over the Midwest for Tuesday and Wednesday. Southeast flow will bring pollution from the Chicago area into Minnesota, and there will also be some ozone precursors in the light wildfire smoke persisting over the area. The dry and cloud-free conditions combined with a dirty air flow will cause ozone to rise well into the Orange category, which is Unhealthy for Sensitive Groups (USG). There is a chance ozone could reach Red, a level not attained in Minnesota for over a decade. We will keep a close watch on monitor values and will upgrade to Red if necessary. High ozone levels are expected to be widespread over much of southern and central Minnesota Tuesday, Wednesday, and Thursday.

Light southeast winds will continue to pump in pollutants from Chicago and other Midwest cities this week. That air flow will reinforce pollutants that contribute to ground-level ozone formation this week.

Stay tuned.