A state air pollution monitor on the north Minneapolis riverfront has recorded its fourth incident of high levels of large particles in five months, even as the agency running it has yet to propose corrective action to area businesses for an earlier violation.

The latest violation of the state's large air particle standard was announced Friday and involved Feb. 23 reading.That follows earlier above-standard readings in October, November and January. Any two readings over a standard in a year constitute a violation.

The Minnesota Pollution Control Agency said this week it still is investigating the violation last fall toward the goal of requiring businesses to institute corrective measures. Jeff Smith, director of the agency's industrial division, said that it has determined from the size of the particles that they came from within one-half mile.

Smith said earlier this week that the agency worked with the city and had narrowed its investigation to a half-dozen businesses near the monitor just south of Lowry Avenue N. He said they had been contacted for information about their activities near the monitor that might contribute to the violations. Then the agency will work with contributors on correcting their operations, Smith said.

The monitor is located in the midst of scrap and recycling yards that dominate the area south of Lowry. It monitors overall air conditions in the area rather than which industries contribute to them.

The agency began monitoring air quality in 2013. It followed the agency's decision to loosen an emissions permit for Northern Metal Recycling, which is across the street from the monitor, after a 2009 test showed that the company was violating the emissions permit it had then. The company collects, sorts, pulverizes  and ships scrap metals.

The decision to monitor also followed air modeling that indicated that particles released by area industries potentially exceed national air quality standards for fine particulates, which can be dangerous to breathe.

The monitor has found no violation of the more dangerous small particles to date, but the agency added large particle monitoring last fall and found a violation of the air-quality standard within weeks.

The agency said it can't comment further by law on its investigation until it is concluded. Environmental activist Alan Muller said he believes the Legislature should change the allow greater public input. That would allow those affected to comment on proposed enforcement actions before the agency makes a final decision, he said.