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In recent years, the Arizona city of Nogales has failed to meet federal air quality standards, but a new dust control plan for the city takes into account that some of the dust recorded on sensors there is generated in Mexico.

The situation is unique, says Trevor Baggiore, the deputy director of the air quality division at the Arizona Department of Environmental Quality.

Were it not for the dust created in Mexico, Nogales, Ariz., would meet federal air quality standards, he says.

"The Clean Air Act explicitly has an exemption for those cases where we can demonstrate that ... if those emissions were not being transported into our area, our area would be in attainment with the standards," Baggiore says.

That exemption is recognized in the EPA's new air quality plan for Nogales.

The area was determined in 1990 to be out of compliance with federal air quality standards, though it has intermittently met the standard since then, he says.

ADEQ submitted a plan in 1993, though no federal action was taken. In 2010, Baggiore says the Center for Biological Diversity sued the EPA for not taking action.

"Once we got EPA motivated, shall we say, the issue was resolved quickly," he says.

ADEQ has worked to improve dust on the U.S. side of the international border, Baggiore says. That includes paving roads in the area and making improvements at the port of entry.

"The actions and the steps, mostly paving, and improvements at the port of entry, those actions have made a difference as far as trying to meet the dust standards and the remainder of the dust is outside of our control," he says.

Though it's outside of Arizona's regulatory control, ADEQ has been working with the Mexican government to make improvements on the southern side of the border, he says.

That work includes paving dirt roads in Nogales, Sonora, and installing devices on cargo trucks that reduce diesel emissions from trucks that cross the international border, he says.