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    • Phoenix ozone pollution stretches into fall in 2024
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    Phoenix ozone pollution stretches into fall in 2024

    Harry Posted on

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    While ozone pollution is typically a summer phenomenon in Maricopa County, ozone season stretched well into the fall this year.

    High ozone levels usually drop off in mid-September, but Maricopa County broke federal ozone standards as late as Oct. 12 in 2024. This year was the first year the county has recorded any ozone exceedances in October since new air quality standards went into effect in 2015.

    Phoenix-area ozone levels broke federal air quality standards four times in October. The exceedances happened in Mesa, west Phoenix and Glendale.

    So-called ozone season in the Phoenix area typically runs from April to September. Concentrations of surface-level ozone, an air pollutant that contributes to respiratory and heart issues, repeatedly surpassed federal air quality standards over that period.

    The October exceedances bump Maricopa County up to 46 exceedance days in 2024. A region with more than 12 exceedances over a three-year period falls out of compliance with regulations established under the Clean Air Act. In the three years between 2022 and 2025, Maricopa County has recorded 136 exceedance days.

    Phoenix, which the American Lung Association ranked as the country’s fifth most ozone-polluted city in 2023, is part of a federally designated nonattainment area, meaning it is out of compliance with current federal ozone standards. While Maricopa County’s ozone levels have dropped moderately since 1990, regional efforts to reduce the Phoenix area’s ozone concentrations have not been enough to bring the area into compliance.

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    Ozone forms when sunlight drives chemical reactions between airborne compounds released by plants, gas-powered vehicles, fires and industrial facilities.

    The exceedance days all occurred during a streak of record-breaking high daily temperatures in early October. Experts say ozone does best in hot, sunny weather, with some saying temperature is the biggest driver in ozone creation. Heat amplifies ozone production in multiple, distinct ways. Warmer weather speeds up atmospheric chemical reactions that produce ozone, creates stagnant air and causes some plants to release more of the chemicals in those reactions.

    In general, Maricopa County officials say days with sunny, stagnant weather at 105-110 degrees create the best conditions for the ozone.

    Nevertheless, air quality officials at Maricopa County would not attribute October’s ozone exceedances to the heat wave. Officials cautioned that ozone formation is a complex process with many factors, saying they cannot be sure that the hot fall was the driving factor in October’s exceedances.

    “It is not possible to give a simple accurate reason for high ozone days continuing later into the year,” a spokesperson from the county wrote in an email.

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    That was a heat wave, but what about climate change?

    Researchers agree that temperature is a main driver in ozone formation, largely because of the sunny weather and stagnation that come with it. That said, the future of ozone formation under rising temperatures may not be so simple to predict.

    One complicating factor in ozone’s relationship with heat is “ozone leveling.” In some cases, scientists have noticed that ozone levels can plateau at high temperatures.

    According to Loretta Mickley, co-leader of the Atmospheric Chemistry Modeling Group at Harvard University, clear skies and stagnant air may already be maximizing ozone production in Arizona, meaning simple rises in temperature don’t create much more. Mickley calls that phenomenon “ozone leveling,” though some scientists call it “ozone suppression.”

    “The temperatures keep ramping up, up, up, up, and ozone levels are high, but they’re not budging,” Mickley said. “Suppression sounds like it’s keeping ozone in check, but it’s not really a suppression. It’s more like an ozone leveling.”

    This phenomenon complicates how scientists predict the effects of heat on Arizona’s ozone problem in the future, with some models even showing decreases in ozone concentrations in different parts of the state as temperatures warm. Ongoing scientific debates about the way plants respond to increased temperatures could also change how scientists view the relationship between heat and ozone.

    Scientists agree, however, that wildfires driven by a warmer, drier climate will almost certainly exacerbate ozone levels around the country. Wood smoke contains compounds that react to form ozone under stagnant, sunny conditions. Areas downwind of those fires can suffer from increased ozone-causing pollutants mixing in their local atmosphere, according to Ana Rappold, a pollution researcher at the Environmental Protection Agency.

    “A lot of volatile compounds can get transported across the nation,” Rappold said. “If there’s a fire in Arizona, you could have ozone exceedances as far away as Houston, or even New York.”

    According to Mickley, many of the models testing the direct results of temperature changes on ozone do not account for the effects of wildfires. Those include the models showing decreases in ozone levels in Arizona.

    “A lot of these projections of ozone into the future assume that fire will stay about constant,” Mickley said. “It’s a problem.”

    Austin Corona covers environmental issues for The Arizona Republic and azcentral. Send tips or questions to [email protected].

    Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.

    Sign up for AZ Climateour weekly environment newsletter, and follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook oath Instagram.

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