By Daniel Weintraub

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.

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