Suncor Energy’s oil refinery in Commerce City exceeded its permitted pollution limits for more than 900 cumulative hours during its three-month shutdown in late 2022 and early 2023, according to a study by an environmental group — and Colorado regulators say they are investigating potential violations during that incident.

350 Colorado, an environmental group that advocates for the elimination of fossil fuels, says in its recently released report that it reviewed Suncor’s self-reported documents connected to the shutdown and determined there was a notable increase in the frequency of permit violations while the refinery was offline.

During that time, the plant released excessive amounts of sulfur dioxide, hydrogen sulfide, carbon monoxide and particulate matter such as soot, according to the report titled “Suncor’s Commerce City Refinery: Looking Back to Plan Ahead.”

The report counted each hour during which at least one of those regulated toxics was emitted. So if all four were released during the same hour, 350 Colorado counted that as four hours’ worth of permit violations.

At the time, people who live in neighborhoods surrounding the refinery hoped that the shutdown would result in cleaner air, but no one had looked into the data, said Heidi Leath, a climate policy analyst with 350 Colorado.

“They just thought they were going to get a break from the pollution,” she said. “We were really curious to see if the air quality would improve.”

The 350 Colorado report also looked at the three-month closure’s impact on gas prices and determined that although gas prices spiked at the time, the refinery does not help Colorado maintain lower gas prices.

Efforts to reach Suncor officials for this story were unsuccessful.

On Dec. 21, 2022, the Suncor refinery’s hydrogen plant malfunctioned during an extreme deep freeze, during which the temperature in Denver dropped 37 degrees in an hour, eventually plunging to -24 degrees. When the hydrogen plant tripped, it caused a cascade of problems for Suncor’s machinery, leading the company to shut down operations for three months to make repairs.

Suncor refines about 98,000 barrels of crude oil per day during normal operations and supplies about 40% of the gasoline used in Colorado. The state’s only refinery also produces diesel, jet fuel and liquid asphalt.