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Extreme Heat Chips Away at Effectiveness of Ozone Air Standards

July 13, 2021, 10:00 AM

Unrelenting heat waves are boiling cities across the U.S. this summer, spiking ozone pollution levels and threatening the fighting power of air quality safeguards.

Regulations that limit ozone-forming emissions, which migrate into pollution-burdened areas like the coastal Northeast states, have improved air quality since being implemented in the 1970s. But some experts worry that hotter temperatures wrought by global warming could curtail the ability of current rules to limit heat-nourished ozone to health-protective levels.

“The atmosphere is getting supercharged, and the chemistry that makes ozone is going along with it,” said Paul Miller, executive director of the Northeast States for Coordinated Air Use Management. “So it takes less pollution to make the same amount of ozone or more going forward.”

Climate change impacts such as extreme storms and heat exacerbate a variety of air pollution issues, and harmful ground-level ozone thrives during prolonged heat waves.

North American cities have been experiencing “heat domes” in recent weeks—high-pressure weather systems that can trap air pollutants and allow them to accumulate, especially in low-wind situations.

During these peak heat days, power plants and other facilities that belch ozone precursor chemicals work overtime to meet energy demands.

“We don’t release ozone into the air, we only get ozone smog if it’s cooked up from other ingredients,” Rice University associate engineering professor Daniel Cohan said. “Heat and sunlight are the fuel for that oven.”

‘Feature and a Flaw’

The Cross-State Air Pollution Rule, updated in March by the Biden administration, could see its effectiveness blunted as temperatures rise. The rule, which relies on an emissions trading program, gives plants the flexibility to run full steam when the days are hottest— without daily caps on emissions—to meet energy demand.

The rule applies to plants in 12 states: Illinois, Indiana, Kentucky, Louisiana, Maryland, Michigan, New Jersey, New York, Ohio, Pennsylvania, Virginia, and West Virginia.

“If power plants have the flexibility to greatly increase emissions on hot days while dialing back on cooler days, it does potentially decrease the effectiveness of the revised CSAPR update rule even if all the covered power plants are in compliance,” Miller said.

Both Miller and McGuireWoods LLP attorney Aaron Flynn noted that the flexibility afforded by the cap-and-trade program contributes to the traveling pollution rule’s success. Hotter summers, though, are creating a roadblock.

The flexibility “is the feature and the flaw in the program, if you want to think of it that way,” Flynn told Bloomberg Law. “But that’s exactly how it’s supposed to work, and that’s a good thing, but it also has that negative outcome, as well.”

‘Tougher to Do’

The CSAPR program works by allowing plants with less-effective controls to scale back on days with less demand from May to September, and purchase emission allowances through a market to run full-steam on a few high-demand days. Those allowances are created from other facilities that can run pollution controls at lower costs.

Plants running more often on hotter days during the season can snatch up an excess of emissions allowances that discourages running pollution controls on high-demand days, Miller noted.

That flexibility works well for the market, but less so for goals to meet health-protective levels when every reduction counts in the face of a warming planet, Miller said.

“Because fewer emissions still make as much or more ozone because the atmosphere is heated up—and now the standards are more stringent—it makes it tougher to do an emissions trading program,” he said.

New Challenges

Advocates such as Rama Zakaria from the Environmental Defense Fund hailed the CSAPR update as a welcome step forward—the first air action out of the Biden administration—but still call for solutions to the gaps they say persist in ozone regulation.

“There needs to be more done on CSAPR, and there needs to be more done on ozone with the reconsideration,” Zakaria said. “We’re definitely not where we need to be.”

CSAPR, for example, only applies to pollution from 12 states based on the 2008 ozone standard, not the latest 2015 level. And environmentalists still want National Ambient Air Quality Standards for ozone beefed up through a review process like the one announced for particulate matter last month.

Climate change complicating attainment for states facing more stringent ozone NAAQS levels raises questions about what other kinds of regulation options should be on the table if go-to pollution controls aren’t enough, Flynn said.

“We also get into this very tricky situation where it becomes increasingly impractical to meet those standards with simple traditional pollution controls without requiring facilities to shut down,” he said. “That’s a dangerous and complicated situation to be in.”

For now, current control regulations still make a dent in pollutants, but the science is clear that climate change creates the need for more reductions, Miller said.

“We’re playing with those programs that were successful for the times they were created,” he said. “Have they outlived their usefulness? I think that’s the question at least to ask in light of hotter summers, and in light of tighter standards.”

To contact the reporter on this story: Jennifer Hijazi in Washington at

To contact the editors responsible for this story: Rebecca Baker at; Gregory Henderson at