When I was in Brussels, Belgium they had a wooden wall against a building specifically for graffiti art, ever changing. Make designated areas around Lowell and turn areas into themed art. Make it into a tourist walk. One wall might be themed “Lowell History” or “The Enviornment” etc.
People can walk and see Lowell. Artists can express themselves and create beautiful, ever-changing pieces. In Brussels, it reduced the bad graffiti.