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As temperatures rise, air quality experts keep an eye on ‘ozone climate penalty’

A view from the foothills of a hazy smog filling the San Joaquin Valley.
Joe Moore
Valley Public Radio

Rising temperatures are likely to result in more smog, even with reductions in emissions of precursor pollutants.

Over the last few decades, air quality in the San Joaquin Valley has greatly improved. But climate change could jeopardize those gains, and researchers are trying to figure out by how much.

One of the Valley’s summertime air pollutants is ozone. High in the atmosphere, the gas protects the Earth, but at ground level it’s a contributor to the summer smog that obscures views of our surrounding mountains and turns the sky a sickly brown. Exposure to ozone isknown to exacerbate respiratory ailments, andrecent research out of UCLA suggests it’s associated with the development of type-II diabetes.

Ozone itself is not directly emitted, but it’s produced when other harmful pollutants like volatile organic compounds (VOCs) and oxides of nitrogen (NOx) combine in the air. Both ozone precursors are emitted by combustion engines and many industrial sources including manufacturers and refineries, although VOCs are also emitted naturally by plants and released during forest fires.

As a result of a changing climate, concentrations of one or both of these compounds could increase, says UC Davis engineering professor and air quality researcher Michael Kleeman. “Hotter temperatures drive higher emissions that are precursors to ozone,” he said, and he also warns that a warming climate could accelerate the reactions that form ozone, which occur more readily in heat and direct sunlight. “Higher temperatures then will speed up some of the chemical reactions that lead to the production of ozone from those precursors.”

Together, these impacts are known as the ozone climate penalty. “We imagine that, if the emissions stayed constant but climate change happened, how much more ozone would we have?” Kleeman said. “That difference between what would have happened with no climate change and what will happen with climate change is what we call the ozone climate penalty.”

Although the concept is well established, the estimate of the penalty is not. According to Kleeman’s research, in a “business as usual” climate scenario, ozone concentrations in the Valley could rise by 0.5 to 1 ppb on average by the year 2050. That penalty may sound modest, but it could rise or fall in the coming decades depending on how effectively VOCs and NOx emissions are controlled.

Air officials—including those in the two most polluted air basins in the country, Los Angeles and the San Joaquin Valley—have put in place measures to limit those precursor compounds, including by limiting industrial sources and penalizing those who exceed those thresholds. They also offer incentives to upgrade engines and other devices that emit VOCs and NOx.

But Kleeman says efforts to combat climate change can also help offset ozone production. That’s because many sources of greenhouse gases targeted for reductions by the Paris Agreement—and recent negotiations at COP26—also emit VOCs, NOx and other harmful pollutants. “If we adopt the clean energy sources that represent the future, in order to reduce the emissions of greenhouse gases, we’ll actually solve some of the air pollution problems that we have,” he said. “There’s a real win-win scenario here where we adopt clean energy sources and we improve our air quality.”

Kerry Klein is an award-winning reporter whose coverage of public health, air pollution, drinking water access and wildfires in the San Joaquin Valley has been featured on NPR, KQED, Science Friday and Kaiser Health News. Her work has earned numerous regional Edward R. Murrow and Golden Mike Awards and has been recognized by the Association of Health Care Journalists and Society of Environmental Journalists. Her podcast Escape From Mammoth Pool was named a podcast “listeners couldn’t get enough of in 2021” by the radio aggregator NPR One.