A computer model that the Air Resources Board used to justify historic restrictions on diesel emissions from off-road construction equipment may have attributed twice as much pollution to those heavy trucks as they actually produce, according to interviews with ARB staff.

That error, coupled with the effects of the recession on the construction industry, means that the excavators, backhoes and graders that operate in California are producing only a fraction of the pollutants that the board believed was the case when it adopted the regulations in 2007.

The industry has been pushing the air board to repeal or at least suspend implementation of the rule, which requires contractors to get rid of old, heavily polluting engines and retrofit others with filters to capture the diesel particulate matter before it reaches the ambient air.

From the beginning, construction contractors have contended that the rule was misguided, would force some contractors out of business and had costs that exceeded its benefits.

Now the Associated General Contractors, a lobbying group that is leading the fight against the regulation, has released a report alleging that the ARB model exaggerated the emissions by a factor of about four. Combined with the effects of the recession, the contractors say, emissions today are only one-sixth as high as the board projected they would be at the time the regulation was adopted.

The consequences could be huge for the industry – and for other polluters. If the numbers used by the ARB were wrong, then the construction industry might not have to do much more to meet the emission standards the board adopted through 2025. But since the state is still required by the federal government and its own rules to meet overall goals for reducing pollution, other sources, perhaps on-road trucks and buses, will have to make up the difference.

“What this reveals is that emissions from the off road equipment in the construction industry are far below not only the board’s original estimate, but far below most of its targets,” said Mike Kennedy, general counsel for the contractors’ group. “Without any rule of any kind, the construction industry will exceed the board’s objectives for [Nitrogen oxide] emissions through 2025 and it will exceed the objective for particulate matter emission up to the year 2020.”

The air board is meeting today to begin discussing how to proceed. The board’s staff plans public workshops later this spring to discuss the problem, and plans to make a recommendation to the board this summer. A decision about the future of the off-road truck regulation will probably come in September.

The board’s staff disputes the contractors’ figures and its conclusions about the future of the rule. But they concede that the model was flawed and will need to be rewritten.

“We believe that our previous estimates were a little high,” said Kim Heroy-Rogalski, who manages implementation of the off-road truck rule. “We do believe we need to take a look at it and adjust for whatever inaccuracies might have been in there.”

Michael Benjamin, chief of the board’s mobile source analysis branch, said an internal review of the model has concluded that earlier estimates were off by a factor of between 1.4 and 2. That means the board may have attributed twice as much pollution to the construction trucks as they actually produce.

In addition to the errors in the model, the board’s staff has determined that off-road construction activity in 2009 was only about half what it was in 2006, because of the recession. Correcting for both the error and the effects of the recession could reduce estimates of emissions to levels far below what the board originally required the industry to meet.

The problems with the computer model came to light after a study by UC Berkeley researchers compared the amount of fuel actually used by the trucks to the amount that the ARB model projected they would use. While Heroy-Rogalski and Benjamin said that study oversimplified the problem, they acknowledge that it did prompt them to reexamine their model, a review that uncovered serious flaws.

The model is based on assumptions about the number of construction vehicles in use in California, their age, the size of their engines, how often they operate and the intensity at which they run. This last variable, known as the “load factor” may responsible for much of the error the board is addressing now.

Benjamin said the state uses a formula for the load factor patterned after one used by the US Environmental Protection Agency. The EPA, he said, is revising its model and California hopes to follow suit.

“The load factor is the most problematic,” Benjamin said. “We are hoping we can incorporate the results of their testing programs. But it is not a straightforward and simple process. It’s quite time consuming.”

Heroy-Rogalski said that, despite the problems with the model and the hardships of the recession, the air board still has an obligation to protect public health.

“You can go into this, watch different people debating technical points, but it is important to step back,” she said. “The ARB recognizes that these are really challenging rules to comply with. We recognize there has been this huge recession. Yet the rules are still important from a public health perspective. We’re trying to achieve a balance, making sure we achieve our public health goals and still give as much relief as we can.”

Originally Published at HealthyCal.org.