(CNN) Last year, cops in Manhattan arrested people for smoking or possessing small amounts of marijuana a little more than 5,500 times. A disproportionate number of those arrested were minorities.

In a few months, most similar cases won't be prosecuted. On Tuesday, Manhattan District Attorney Cyrus Vance Jr. announced his office would decline to take up those cases, effective August 1.

"The dual mission of the Manhattan DA's office is a safer New York and a more equal justice system," Vance said. "The ongoing arrest and criminal prosecution of predominantly black and brown New Yorkers for smoking marijuana serves neither of these goals."

Vance, a Democrat who is in his third term, said his office was discussing with New York City police and Mayor Bill de Blasio what exceptions there should be to the policy.

New York Police Commissioner James O'Neill on Tuesday announced a working group will take the next 30 days to look at the enforcement measures by the department.

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