Wednesday, April 12, 2017

Explanation for April 9-10, 2017 Air Quality in Fort Collins

Fort Collins Air Quality Warning:

CDPHE issued this alert on 9 April, 2017:
The Colorado Department of Public Health and Environment has issued an  ACTION DAY ALERT at 4PM on Sunday, April 9, 2017 for the Front Range Urban Corridor from El Paso County north to Larimer and Weld counties, including the Denver-Boulder area, Colorado Springs, Fort Collins and Greeley. A stratospheric intrusion is producing ozone concentrations in the Moderate to Unhealthy for Sensitive Groups range in the Front Range region, particularly from Denver southward to Colorado Springs and westward from those areas to the Continental Divide.” (source:

How did this affect our air quality?

The following figure is a time series of ozone concentrations in Fort Collins, CO (unvalidated data obtained from The figure shows the eight-hour average (i.e., the value shown at 8 am is the average from 1 am to 8 am).

We can see from this figure that surface-level ozone concentrations were higher on April 9th and April 10th, reaching the “Moderate” AQI level.

What is a stratospheric intrusion?

First, an explanation of the stratosphere:

There are different layers of the atmosphere (shown in the following diagram). Humans live in the lowest layer, the troposphere. The troposphere extends up to about 10 km (it can be 7-20 km, depending on where you live), and is where weather occurs. In the troposphere, temperature generally decreases with altitude (shown by the red line in the following diagram). The stratosphere lies above the troposphere and extends up to about 50 km. In the stratosphere, temperature increases with height, because of the ozone layer. Ozone molecules absorb ultraviolet light from the Sun, converting the energy into heat.
Ozone in the stratosphere protects humans from harmful UV radiation, while high ozone concentrations at the surface can be detrimental to human health.

From Randy Russell, UCAR

So, what is a “stratospheric intrusion”?

A stratospheric intrusion is when stratospheric air is folded into the troposphere and extends to the surface. Stratospheric intrusions generally occur in the spring and are associated with storm systems. On Sunday, there was a cold front that passed over Colorado (and provided some snow for the mountains!). We see these intrusions in Colorado more than other places because of the high elevation, which allows the stratospheric air to actually reach the surface. Stratospheric air has higher ozone concentrations and can lead to an increase in surface ozone concentrations, sometimes above the EPA’s National Ambient Air Quality Standards (NAAQS) threshold

How can we tell when there is a stratospheric intrusion?

There are a few characteristics that we look for: higher ozone concentrations, lower carbon dioxide concentrations, and drier air (potential vorticity and beryllium-7 are also used). We can often see the impact using surface measurements (like the ones shown above), but these events are sporadic and sometimes the effect is very localized, making it difficult to attribute if the impact is minor. Therefore, we often also look at atmospheric profiles from radiosondes, ozonesondes and lidars. We can also use satellite observations and model output.