First responders found about 46 people who appeared to have overdosed on the New Haven Green on Wednesday, with 25 of those overdoses occurring within a three-hour span in the morning and some four to six at a time, officials said.
'New Haven Green' is a park's name. We usually say in the park or at the park. It seems to me that New Haven Green here is referencing to a site. Therefore, they put "on the New Haven Green", just like "on the New Haven Green site". But I am not quite sure if this is the very reason. I'm also wondering if it's still correct to put "in the New Haven Green" in this context? If so, is there any difference?
The full source.