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Juneau's electric utility predicts that by late May or early June, it will exceed some air quality permit limits by powering the city on diesel. State permitters may go easy on the company because the cause is a natural disaster.

Alaska statute and code allow permitters to waive penalties for "unavoidable" violations of their permits or in an emergency. The Department of Environmental Conservation commissioner makes the decision, which does not require the governor to declare a natural disaster.

The provisions have never been invoked, according to Jim Plosay, environmental engineering associate at DEC's Division of Air Quality.

AEL&P is filing a formal request for DEC to consider such circumstances in the event it violates its air quality standards, said Scott Willis, AEL&P generation engineer.

DEC permitters said they were discussing the idea with state attorneys but hadn't made a decision yet.

"We're here to help. We're not here to be heavy-handed," Plosay said.

"Our role is protection of ambient air quality for the public. But on the other hand, the local community needs power," he said.

Alaska Electric Light & Power switched to the generators April 16, when an avalanche knocked out the line supplying hydroelectric power from the Snettisham lakes, 40 miles south of Juneau. Since then, the city has been running on about 90 percent diesel and 10 percent hydroelectric power. It was running the reverse before the avalanche.

While running the city on hydroelectric power, Alaska Electric Light & Power Co. operations are ordinarily well under annual pollutant limits set by the state Department of Environmental Conservation. The generators are merely used as backup.

"Now, all of a sudden, it is an issue," Plosay said.

AEL&P estimates its state-regulated air emissions will jump to 1,469 tons for the year ending in July, from 106 tons for the year ending in December.

All sources in Juneau produced 13,824 tons of those emissions in 2001, the most recent Environmental Protection Agency estimate for the area.

The issue is whether AEL&P will produce emissions that are officially considered to affect air quality.

"The permits are set up so that (emissions) would not impact local air quality if at maximum," Plosay said.

The state Department of Environmental Conservation implements federal air quality standards and issues permits to major sources of pollutants that limit nitrogen oxides, sulfur compounds, carbon monoxide, particles smaller than 10 microns and volatile organic compounds.

Emissions aren't directly measured. They're calculated from the hours a generator is on using data of how much pollution that model of generator is known to emit.

AEL&P's five-year permit dictates how many hours each generator can be on in any consecutive 12-month period.

Each generator is regulated separately. Its limit depends partly on when it was bought: Older units have higher emissions limits grandfathered in from earlier days, even though they're exactly the same models as some newer units, according to Willis.

Deciding which generators are on for how long is a complicated balancing act based on weather, maintenance needs, what time of day it is, and how efficient the machines are, as well as emissions requirements.

AEL&P's smaller generators are more economical, but they're also dirtier, Willis said.

At the moment, he has chosen to rest that balance on the side of consumers, he said.

"We're managing first for cost, where possible," he said. "But we're all breathing this air, and I don't want to violate my environmental permit."

Willis estimates that by early summer, the generators at Auke Bay, though not at Lemon Creek, will have exceeded the limit for at least one of the regulated pollutant categories: nitrogen oxides.

These chemicals react with volatile organic compounds in the atmosphere to produce ozone. Ozone protects life on Earth from ultraviolet rays when it's high above us, but is harmful at ground level.

In 2007 the Auke Bay generators produced three tons of nitrogen oxides. Now Willis anticipates they'll produce 300 tons for the year ending July, which is over the permitted level of 250 tons for any 12 consecutive months.

The Lemon Creek generators have higher limits. Willis estimates total nitrogen oxides production at 984 tons, still under the 1,458 tons permitted for all the generators combined. Last year the generators produced 71 tons of the pollutant, or 5 percent of the permitted level.

• Contact reporter Kate Golden at 523-2276 or e-mail kate.golden@juneauempire.com.