Air Quality Portal
As calendars flipped to November, the 2025 ozone season officially came to an end across North Carolina. The season, which runs from March 1 through Oct. 31 every year and peaks in the summer months, was unusually quiet this year.
Preliminary data shows that the 2025 ozone season did not have any daily exceedances of the National Ambient Air Quality Standard (NAAQS) for ground-level ozone at monitors across the state. Exceedances are days where the highest 8-hour average of ozone reaches Code Orange (71 ppb) or higher.
The data will still need to undergo a certification process, where the state agency will review all monitor data from 2025 and ensure it is complete and accurate and then submit it to EPA for review and concurrence. Agencies must submit a certification letter to EPA by May 1, 2026, and finalized data should be published by July.
If the data is certified without changes, it will be only the second time that the state’s monitors did not have an exceedance of the NAAQS for ozone. The only other year this happened was 2020 and that was largely due to the COVID pandemic when mobile source emissions, which produce precursor pollutants needed for the formation of ozone, were much lower than normal.
To further put this year’s quiet ozone season into perspective, North Carolina averaged 9 exceedances of the NAAQS for ozone from 2014 to 2024 (excluding 2020), and exceedances were even more common in the decade prior.
So, what possibly led to the quiet ozone season across the state? Let’s dig a little deeper …
Record-Breaking Humidity in July
July is usually when North Carolina observes its warmest temperatures of the year. It’s one reason, among several, as to why it is a busier months for ozone.
Ground-level ozone is formed through chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. Warmer temperatures and the strong sunlight in July increase these chemical reaction rates.
The warmer temperatures also lead to a higher demand for electricity as people try to stay cool, which leads to increased electrical generating unit emissions. These emissions add to the pool of pollutants needed for ozone formation.
This past July was warmer than normal. According to a climate summary blog post from the North Carolina State Climate Office (NCSCO), July 2025 was the second-warmest July on record across the state. Such a warm month would seem conducive for yielding at least a few high ozone days across the state, but that was not what we observed.
In addition to the heat, July was also much more humid than normal. We can gauge how humid it was by looking at dew points across the state, which is a variable that indicates how much moisture is in the air. Looking at monthly mean dew point anomalies across the state for the month of July in the figure below, you’ll see that anomalies were positive and much of the state measured monthly mean dew points 3 to 5 degrees F above normal.
In fact, according to the NCSCO, “Raleigh had its highest average dew point – 69.8°F – out of any July since 1946. Among our 39 ECONet stations with at least 10 years of observations, 33 of them had record-high average dew points for the month of July ….”
So, not only was it more humid than usual, it was the most humid July on record for much of the state.
Now, how does this tie to ozone? There are a few angles here.
July was largely dominated by the Bermuda High, a typical high pressure zone that sits over the Atlantic Ocean to our east. This feature brings southerly winds which advect moisture from the Atlantic and the Gulf into the state. Both areas of water were much warmer than normal, which in turn, made the air above them warmer. Warmer air can hold more moisture. What we saw in July was a persistent advection of warmer and more moist air into the state compared to what we typically observe. Persistent maritime airmasses flowing into the state typically produce a cleaner airshed with lower precursor pollutants and residual ozone.
More moisture at the surface favors more vertical mixing, which helps carry pollutants (including ozone) upwards and away from the surface. It also favors the development of more clouds, especially cumulus clouds, which we commonly see bubble up in the summer. Clouds help block some of the incoming sunlight which hinders ozone formation.
These clouds can grow more when the atmosphere is moist and unstable and turn into showers and thunderstorms. In July, there were many days that featured scattered showers and thunderstorms and this helped contribute to a wetter than normal month for much of the state. This rain helped cleanse the airshed by removing ozone and precursor pollutants.
Also, research shows that when humidity is higher, plants increase their intake of ozone through pores, removing it from the airshed in a process called dry deposition. This may have also had a contribution to lowering ozone across the state.
Additionally, an abundance of water vapor can affect the complex chemical processes that produce ozone in such a way that ozone production is actually curtailed. This has to do with the production of different kinds of free radicals, which under certain conditions can destroy ozone faster than it is produced. This is why (generally speaking) higher humidity is associated with lower ozone production.
Summer Disappears in August
Following a hot and steamy July and the summer season still in full swing, it seemed like it would be a while before any prolonged relief from summer conditions would come; however, summer ended up disappearing for most of August.
The average weather pattern for the month featured anomalous high pressure to our north which led to more days with a north, northeast, and east wind across the state.
Using our wind rose tool, we can drill down to the local level to see how winds behaved in August. Looking at the Charlotte area, winds were primarily from the north, northeast, and east during the month, more often than what typically is observed.
This brought cooler Canadian and maritime-influenced air into North Carolina, and that air became wedged into the western half of the state for a few days during the first half of August in a phenomenon known as cold air damming. This type of pattern typically brings cloudy and cool days and that is indeed what we saw. During the coolest period from Aug. 2-11, Charlotte observed 10 straight days of below-average temperatures, with departures for the entire period averaging minus 7.2 F. In fact, the high temperature of 69 F observed on Aug. 5 broke the daily record for the lowest maximum temperature and several other days were within a few degrees of their respective daily lowest maximum temperature records.
While the state did see a warmup during the middle of the month, it was short lived as Hurricane Erin passing close by to the east and a couple of cold fronts toward the end of the month both brought cooler and cleaner airmasses to the state. The month would end up finishing as the 11th coolest August on record across the state, according to the NCSCO.
The first half of August was also very wet for parts of the state. A stalled frontal boundary persisted along the southeastern U.S. coast and wavered northward at times as waves of weak low pressure rode along it. This helped bring cloudy skies and several rounds of heavy rain to the state, especially to the Southern Appalachians and Piedmont.
In hindsight, it is no surprise that August was not a busy ozone month across our state this year. For ground-level ozone to form — especially to levels that are unhealthy — stagnant, sunny, warm, and dry days are needed. August was a cooler-than-normal month and featured many days that were cloudy and rainy. Also, persistent north, northeast, and east winds and a couple fall-like cold fronts helped keep the airshed clean.
North Carolina Avoids Worst Canadian Wildfire Smoke
2025 was an active wildfire season across Canada, coming in as the second worst in Canadian history with nearly 8.8 million hectares burned. While meteorologists with DEQ’s Division of Air Quality actively monitor wildfires across North Carolina and factor the resultant smoke into our air quality forecast, we also intently monitor wildfires happening outside of our state and the impacts they may have on the air we breathe.
As we saw in the summer of 2023, wildfires that are hundreds to thousands of miles away can lead to widespread smoky skies and air quality problems here in our state. This smoke is transported in by stronger winds aloft and mixed down to the surface, leading to elevated and sometimes unhealthy levels of fine particulates.
The smoke also contains nitrogen oxides and volatile organic compounds and when combined with locally produced pollutants, can lead to a sharp uptick in ozone, especially on hot and sunny days in more urban areas.
Despite the numerous wildfires burning in Canada this year, North Carolina did not see many Canadian wildfire smoke intrusions into our state. A large reason for this was due to the average upper-level wind pattern during the summer months that carried the majority of the smoke into the Upper Midwest, Great Lakes, and through eastern Canada.
There were a few days in June and July that the state did see some Canadian wildfire smoke move in overhead. However, these smoke intrusions were not as robust as those that occurred in 2023. While there was likely a slight uptick in ozone due to the presence of wildfire smoke, other meteorological factors helped keep ozone in check and below the exceedance standard.
Ozone Trends in North Carolina
North Carolinians are breathing the cleanest air in decades due to a reduction of air pollution emissions. In fact, the state celebrated a decade of attainment with federal air quality standards this summer. Cleaner and more fuel-efficient vehicles, better industrial emission controls, tighter environmental regulations and other factors have sharply curbed emissions of air pollution. This has resulted in lower levels of precursor emissions available that are needed for ozone to form.
This reduction of air pollution emissions is also likely responsible for a shift in the meteorological pattern that is favorable for our highest ozone days across the state. An analysis completed by DEQ Division of Air Quality meteorologists shows that an “Omega block” pattern, an atmospheric setup where high pressure is surrounded by low pressure on both sides in the form of the Greek letter omega, is what drives our highest ozone days.
Looking at our top four highest ozone days this season, the overall pattern did not align with that optimal high-ozone pattern, so it is not surprising that we did not have any exceedances on those days.
We are now in the “off-season” for ozone until March 1, 2026. In the meantime, we continue to monitor and forecast fine particulates across the state. You can find our forecasts and discussion on the Air Quality Portal.