Air Quality Portal
An interesting air quality event may have happened during the morning hours of Friday, May 15. Two high elevation (ridge top) ozone monitors, one in North Carolina and one in Tennessee, spiked into Code Orange range for several hours. While measuring high ozone is not atypical at the ridge tops (more on this later), it was the possible contributing source of ozone that was unusual. It appears that some of the ozone may have come from a place above where our weather occurs: the stratosphere.
A Quick Refresher on Ozone
Before we dig deeper, let’s do a quick refresher on ozone. Ozone (O3) consists of three oxygen atoms and is produced in the stratosphere and the troposphere, the latter being where we live.
In the stratosphere, ozone is naturally formed through the interaction of solar ultraviolet (UV) radiation and oxygen (O2) and is referred to as the “good” ozone, as it helps reduce the amount of harmful UV radiation that reaches the surface.
In the troposphere, ozone is formed through photochemical reactions between certain pollutants (nitrogen oxides and volatile organic compounds) and sunlight. This ozone is referred to as the “bad” ozone, since it forms near the ground and is in the air that we breathe, which can lead to hazardous impacts to human health.
The Ups and Downs of Ozone
Certain weather conditions accelerate ground-level ozone formation. Sunny, warm, and stagnant (calm/light wind) days provide prime conditions for ozone to form. It’s why ozone has its own season in North Carolina that runs from the beginning of March through the end of October, which is when sunlight and heat are most prevalent.
It’s also why ozone concentrations at most locations usually follow a diurnal curve, hitting their highest mark in the afternoon and early evening, when sunlight and temperatures normally reach their peak and ozone production is maximized.
However, there are some locations where this doesn’t always hold true. In fact, it can be the exact opposite, where ozone concentrations increase and peak during the night into the early morning hours and then decrease when sunlight is increasing.
In North Carolina (and other states), this occurs at high elevation locations, where different factors are at play. At night, the ground cools faster than the air above it and this creates what is known as a temperature inversion, or an atmospheric “lid”, where warmer air is layered above colder air. This prevents air from rising and traps pollutants, including ozone, in a layer that is near the ridge tops.
At lower elevation locations, ozone normally drops off at night due to chemical reactions that scavenge ozone and deposition. However, at higher elevations, this happens at a lower rate and more ozone remains. Ozone aloft can also be transported in from an area upstream where high levels of ozone were produced the day (or days) prior and reach our higher elevation locations.
A Case for Stratospheric Ozone
There were two ozone monitors that stood out during the morning of Friday, May 15. North Carolina’s Mount Mitchell monitor, which sits at 6,637 feet, and Tennessee’s Clingmans Dome monitor, which sits at 6,630 feet and is right on the border of North Carolina. Both monitors measured a rise in ozone after midnight that peaked in Code Orange range for a few hours during the early morning hours, before falling around midday Friday.
A quick glance and this may not seem all that unusual. As explained above, high elevation locations are no stranger to increasing ozone at night, with ozone transport and inversions usually to blame. However, after digging a bit deeper, there were several pieces of data that suggested this event was different and could actually have been a result of a stratospheric intrusion event, where air from the ozone-rich stratosphere descends into the troposphere.
Stratospheric intrusions can occur when the jet stream, a river-like feature of high winds aloft, becomes wavy and features strong dips southward. This leads to increased vertical motions and strong sinking of air, which can pull the tropopause and stratospheric air downward, known as tropopause folding. Stratospheric intrusions are not that uncommon and typically occur in the spring but usually remain well aloft in the troposphere. However, on rarer occasions, they can be strong enough to reach the surface.
In order to determine if stratospheric ozone influenced the high ozone readings at the monitors, we first had to rule out ozone transport from upstream. We looked at back trajectories, which are modeled paths that an airmass took to reach a specific point. These are helpful by providing insights into whether or not the airmass came from a region with high levels of ozone.
Back trajectories at Mount Mitchell and Clingman’s Dome from Friday morning show a path from Canada and the Great Lakes southward across the Ohio River Valley before reaching western North Carolina. Ozone maximum 8-hour averages in the source region and along the path during this timeframe were in Code Green range, which suggested that it was unlikely the high ozone came from an upstream low-level source.
With upstream ozone sources ruled out, we then analyzed 500 millibar height and vorticity, which showed a strong upper-level low centered to the north-northeast of North Carolina. This positioned the state on the southwestern edge of this system. Stratospheric intrusions are most common on the western and southwestern flanks of upper-level lows, where large-scale subsidence is usually found.
Another piece of data that we looked at was the modeled 2 PVU (Potential Vorticity Units) heights. Without going into too much detail, 2 PVU heights represent the height of the tropopause, or the thin layer of the atmosphere that is between the troposphere and stratosphere. In the image below, there is a dip in the 2 PVU heights over the mountains in western North Carolina, which suggests that a stratospheric intrusion was modeled and could have reached the mountains.
We also analyzed surface weather observations and one variable stood out Friday morning. Data from a weather station at Mount Mitchell State Park measured a steep drop in relative humidity. This drop in relative humidity lined up quite well with the increase in ozone that was measured. Air from the stratosphere is very dry and large drops in relatively humidity are one of the key indicators of a stratospheric intrusion event.
All of this evidence suggests that a stratospheric ozone intrusion event likely influenced the spike in ozone concentrations that were measured at some of the high elevation monitors Friday morning. As the data stands now, this event did not cause any daily exceedances of the National Ambient Air Quality Standards (NAAQS) for ozone at the monitors.
As a reminder, you can check out our ozone forecast and discussion on the Air Quality Portal during ozone season.