The search for firms contributing to a recent air quality violation in the upper riverfront of Minneapolis is narrowing to a smaller number of firms, according to the Minnesota Pollution Control Agency.

The agency reported in late November that its air quality monitor located just off the river's West Bank south of the Lowry Avenue Bridge had twice recorded violations of the state limit for total airborne particles.

Further examination found that iron particles made up the preponderance of airborne metal particles collected by the agency's sampler, not surprising considering the number of metal recycling firms in the area.

Jeff Smith, director of the agency's industrial division said it is currently looking at six to 10 firms within a one-quarter mile radius of the sampler as potential contributors to the particulates. It's now collecting information from them about their outdoor operations during the late October and early November sampling when the violations were recorded.

"Everyone within that quarter-mile range, we're looking at very closely," Smith said. He said the agency wouldn't discuss specific firms as potential violators during this investigative phase.

The violations were recorded little more than three weeks after the MPCA added monitoring for total particles to its sampling station in October. It previously ad been checking for more harmful finer particles hourly since the start of 2013. The total particle count is a 24-hour sample collected every six days.

Smith said the agency's goal is to reach company-by-company agreements for corrective action that will be transparent to all firms operating in the area.

The two violations haven't been repeated to date, Smith said. But they're already causing changes in monitoring. The initial plan was to stop monitoring for total particles by the end of this year, but Smith said monitoring will continue for at least a year.