I've been to New Orleans, but it was years ago, before Katrina. It didn't impress me as particularly dirty. Atlanta did---at least in the area near the Underground. I was there with a large group of people, but still felt uneasy. I used to live in San Antonio, which had good air quality and was usually neat and clean. But my impression on just entering Houston via the expressways was that of a dirty city. It seemed to have a green grunge on everything. The humidity was overbearing, as was the heat.
I guess bad impressions are lasting. I never wanted to live there, although probably didn't give it a fair chance.