Winter ozone season creeps into April

By Joy Ufford,
Posted 4/5/23

“This is the first time AQD has extended the winter ozone season,” said DEQ spokesperson Kimberly Mazza, adding the extension to April 15 “is warranted.”

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Winter ozone season creeps into April


SUBLETTE COUNTY – Although the Upper Green River Basin’s “official” winter ozone season ends in March as sagebrush emerges from melting snow, that isn’t the case for April.

Tuesday, the Air Quality Division (AQD) of the Wyoming Department of Environmental Quality released a new UGRB Ozone Outlook to persist from Thursday, April 6, through Saturday, April 8.

“This is the first time AQD has extended the winter ozone season,” said DEQ spokesperson Kimberly Mazza, adding the extension to April 15 “is warranted.”

With unbroken snow covering many local flat lands and meadows, sunlight is more likely to reflect strongly off it and transform the UGRB’s manmade pollutant “precursors” – volatile organic compounds and nitrogen oxides – into ground-level ozone.

The federal standard for ozone is 70 parts per billion; when AQD staff forecast weather combinations of snow-cover, sunlight, temperature inversions and low wind speed to approach that level, AQD sends out public “ozone outlooks” or calls for an Ozone Action Day (OAD).

From January through March, AQD issued six Ozone Outlooks and three OADs.

Higher than usual ozone levels, especially at this altitude, can affect people’s breathing and respiratory health while outside or active, especially in areas of heightened ozone.

Operators, businesspeople and the public are asked to voluntarily reduce or postpone combustion emissions from vehicles, equipment, fires and production.

“Winter Ozone Season Forecasting for the Upper Green River Basin will continue daily through Friday, April 14, and may continue beyond that date depending on prevailing weather conditions and UGRB snowpack,” Mazza said. AQD encourages everyone to continue voluntarily reducing precursor emissions and ozone contingency plans.

March highs

One-hour and 8-hour exceedances over 70 ppb occurred last month at AQD’s UGRB air-quality monitoring stations. One-hour levels at or above 70 ppb were recorded at Big Piney, Boulder, Daniel South, Juel Springs and Pinedale stations.

AQD forecasted these days for potential Ozone Outlooks and also called a handful of OADs, records show. As in past winters, Boulder showed more frequent highs. An exceedance is an 8-hour value (not a one-hour average) that is greater than 70 ppb (i.e., equal to or greater than 71 ppb). Boulder did not have an 8-hour exceedance during the 2023 Winter Ozone Season, but it did meet or exceeded 70 ppb for one-hour values on eight days in March. 

Virtually every station’s highs showed elevated levels before and after peaks.

Daniel South, with one day, was the only one showing an exceedance of the 8-hour rolling averages for March.


On March 1, Boulder’s high was 71 ppb at 4 p.m.

March 5 brought 1-hour highs of 72 ppb from 1-2 p.m. at Boulder and 74 ppb at Juel Spring at the same time. Pinedale had a high of 75 ppb at 5 p.m.

March 7’s highs at Boulder were 73 to 75 ppb at 3, 4 and 5 p.m. On March 8 at Boulder, highs reached 70 ppb at 2 p.m., 74 ppb at 3 p.m. and 75 ppb at 4 p.m.

March 9 records show 81 ppb at Boulder at 2 p.m.; 80 ppb at Daniel South at 5 p.m.; 73 ppb at Big Piney at 4 p.m., 72 ppb at Pinedale at 8 p.m. and 73 ppb at Juel Spring at 1 p.m.

On March 10, Boulder did not exceed 70 ppb but Big Piney reached 71 ppb at 1 p.m. and Juel Spring reported 73 ppb at 4 p.m.

Boulder reported 76 ppb on March 13, 71 ppb on March 19 and 72 ppb on March 21.

Juel Spring posted 74 ppb on March 20 at noon, 71 ppb on March 24 at 3 p.m. and 73 at 1 p.m. on March 30.

Pinedale had 80 ppb on March 13 and leveled out with 70 ppb on March 20, 21 and 30.

Daniel South showed excessive 1-hour levels of 72 ppb on March 24 and 71 ppb on March 27. Daniel South had two 8-hour exceedances on March 9-10.

Big Piney posted peaks of 71 ppb on March 21 and 70 ppb on March 24.

While 1-hour exceedances do not count against the UGRB’s federal ozone non-attainment status, as do 8-hour rolling averages, they are times when susceptible people are advised to remain inside.

To review current and historic data from AQD’s air-quality monitoring stations, go to