You’ve probably seen that the Minnesota Pollution Control Agency has upgraded the air quality alert over parts of Minnesota this week to the red category.

But what’s the deal with that donut hole in the air quality alert over the Twin Cities?

It’s sometimes called the ozone paradox. It turns out that ozone concentrations can be higher around cities and even in rural areas than in the inner urban core. The complex chemistry of ozone formation can actually work to reduce ozone in inner cities closer to the pollution sources.

Basically, the same nitrous oxides (NOx) and volatile organic compounds (VOC) that cause ozone to form in cities can also degrade the newly created ozone as time goes by.

Here’s a good explanation of ozone formation in cities and surrounding areas from the Belgian Interregional Environment Agency.

Why are ozone concentrations higher in rural areas than in cities?

Ozone is a secondary pollutant, which means it is not directly emitted by traffic, industry,.. but it is formed on warm summer days by the influence of solar radiation on a cocktail of airborne pollutants. These ozone precursors are nitrogen oxides (NOx) and volatile organic compounds (VOC). Traffic is the main source (>50%) of ozone precursors.

It may sounds curious, but it is a fact that most of the times ozone levels are higher in rural areas than in cities. This is sometimes called the 'ozone paradox'. Ozone can be degraded by the compounds (NOx) by which it is also formed. This degradation occurs more often in cities than in rural areas, because there is more NO in cities (see further).

This does not mean there is less air pollution in cities (other pollutants are present, with higher concentrations than at the countryside), yet there is less ozone. As such, there is no linear connection between the amount of ozone precursors and the amount of ozone itself. A diminution of ozon precursors can even lead to an increase of ozone concentrations. This is what is observed during weekends: although less traffic is present, more ozone is observed (the formed ozone gets degraded less, because there is less pollution). In the scientific world, this is called the 'weekend effect'. The whole process of ozone formation clearly is a complicated story.

To explain why most of the time there is more ozone at the countryside than in cities, some explanation regarding the chemical base reaction concerning formation of ozone is needed. This reaction can be presented as follows (in a simplified manner):

NO2 + O2 (+ solar UV-light, + heat) --> NO + O3

In human language: nitrogen dioxide and oxygen react, which results in nitrogen monoxide and ozone. This reaction is speeded up in warmer conditions and with more UV-light.

This is an equilibrium reaction, which means the reaction also works in the other direction (whereby ozone gets degraded again): NO + O3 --> NO2 + O2.

In chemistry there is a law that describes an equilibrium will try to re-establish itself after a disruption. This is where VOC enter the scene: they react with NO, with formation of NO2 as a result. The NO concentration decreases (because it reacts with VOC). The equilibrium will try to repair this, thus the reaction will form more NO, according to the law mentioned above: the equilibrium is shifted to the side of NO. Trying to form more NO however also means an increase of ozone. In addition, more NO2 is formed, which can form even more ozone, leading to even higher ozone concentrations.

If you still remember that a lot of NO can be found in the exhaust gases from cars and NO has a very short lifespan (a couple of minutes) and is immediately converted (oxidised) into NO2, which has a longer lifespan of hours to even days, which in turn allows it to be transported over larger distances by wind, from the city to more rural areas, you will understand that there is less ozone in cities than at the countryside.

So summarizing:

- in cities, high levels of NO due to traffic reacting with ozone, forming NO2

- NO2-based ozone formation at the countryside (in warm and sunny conditions), but much less NO (because of less traffic) and thus less ozone degradation.

Highest ozone in 11 years

The last time the Minnesota Pollution Control Agency recorded a red (unhealthy for all) level of ozone in Minnesota was in June of 2012. Air quality readings for ozone are as high as 172 in St. Michael Tuesday afternoon.

Air quality alert through Thursday

Here’s the updated text from the MPCA’s latest air quality alert upgrade.

The Minnesota Pollution Control Agency (MPCA) has upgraded the north Twin Cities metro, south Twin Cities Metro, and Saint Cloud areas to the red air quality index (AQI) for Tuesday afternoon and Wednesday afternoon.

An air quality alert for central and southern Minn. remains in effect 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.

Ground level ozone has increased rapidly as of midday Tuesday, and ozone is now expected to reach red AQI in the north Twin Cities metro, the south Twin Cities metro, and St Cloud area. The last time ozone reached the red AQI in Minnesota was June 27, 2012. For the red air quality area, the air is considered unhealthy for everyone. Everyone should reduce prolonged or heavy outdoor exertion. 

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.

And finally, here’s the latest forecast discussion form the MPCA meteorologists.

Updated Tue, Jun 20, 2023

Ozone concentrations have climbed into the Code Red this afternoon in portions of the Twin Cities metro north to St Cloud. The last time we observed Red (Unhealthy) ozone AQI in Minnesota was on June 27, 2012. Across the rest of southern and central Minnesota ozone has reached Orange AQI. Red AQI is unhealthy for everyone, and everyone should limit prolonged time outdoors this afternoon and evening in the TC metro north to St Cloud. Southerly winds, sunny skies, and warmer temperatures will continue over the Midwest for 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) in much of the alert area, and up into the Red category once again in the north and south Twin Cities Metro north to St Cloud. Note that ozone in the core of the Twin Cities will be slightly lower due to a phenomenon called NOx titration, where an overload of NOx can actually reduce ozone. High ozone levels, though slightly lower compared to today and Wednesday, are forecast again for Thursday over much of southern and central Minnesota. A front will arrive on Friday with some widely scattered showers and thunderstorms. Ozone could attain Orange again in parts of the area. The front will stall over the area during the weekend and will be a focal point for scattered showers and thunderstorms. Greater cloud cover and rain changes should bring AQIs down to Green and Yellow.

Stay tuned.