On today’s episode of The Confluence: The federal Environmental Protection Agency has proposed reducing the standard for fine particulate matter pollution allowed before a regulatory agency steps in, but some advocates say the standard should be lower; the Mendelssohn Choir of Pittsburgh’s music director is stepping down after seven seasons with the group; and a look at the life of Mary Cardwell Dawson, who founded the National Negro Opera Company here in Pittsburgh.Today’s guests include: Kristina Marusic, reporter with Environmental Health News; and Matthew Mehaffey, the Robert Page Music Director of the Mendelssohn Choir Of Pittsburgh.