A large scrap metal recycler on the upper Minneapolis riverfront is claiming that a state regulator is singling it out over dust emissions and asserting that it has taken every step available to control such particles from its riverside yard.

The assertions came in a letter written for Northern Metal Recycling by Michael Hansel, an engineer and vice president at Barr Engineering. In it, he told the Minnesota Pollution Control Agency (MPCA) that on more than half of the days when particulates measured by a state monitor exceeded standards, the firm couldn't have been responsible for them. The state monitor is located across the street from the firm, the state's largest such metal recycler.

The standard was exceeded for the fifth time in six months on March 19, according to that monitor. The agency posted the infraction on its website but didn't alert the public via e-mail until Friday.

Hansel said Friday that after a company Data Practices Act request, it appears that only one other of the six companies within one-quarter mile of the monitor responded when asked by the agency about their activities. That was GAF, a shingle company on the opposite side of the Lowry Avenue Bridge from Northern Metal. The rooftop monitor is between them.

Hansel asserted to the agency that the wind direction and lack of barge loading operations suggest that the scrap yard didn't contribute to or was a minor factor in all but one of the episodes. He said that unless the agency can show that the firm contributed to the emissions, further controls shouldn't be investigated.

"My client already has every conceivable control measure installed and operating, and is the most tightly controlled and regulated metal shredder in the United States," Hansel said. The firm has three types of dust control mechanisms installed in its shredder building. It also sprays the yard daily and sweeps it twice daily, except in freezing temperatures.

The firm began operating a shredder several years ago to increase the market value of its scrap metal at mills. That followed a long-running controversy in which some area residents sought to block a shredder at a yard where scrap has been processed since 1951.

The MPCA in 2012 modified the company's emissions permit, which a test conducted soon after the shredder began operating in 2009 showed that the company was violating.

The state rejected the need for further environmental studies, sought by area lawmakers, after concluding that the shredder would increase the area's concentration of more dangerous small particles by 2 percent. It installed the monitor shortly afterward. The violations haven't involved the small-particle standard but rather those for overall particles.

Jeff Smith, director of the agency's industrial division, said that because the investigation is continuing, he's limited in what he could say.

He said that Hansel's assertions about the agency not following protocols and location of monitors are based on misunderstandings. He characterized Northern Metal as reluctant to participate in the agency's investigation and said he hopes for more cooperation.