An air quality alert is in place for much of southern New England through 11 p.m. Wednesday night. The alert was issued by the U.S. Environmental Protection Agency in advance of elevated ozone levels. All the areas under the EPA’s alert were far-western Connecticut, central and eastern Massachusetts, southern New Hampshire and higher elevations at Acadia National Park in Maine….
tl;dr: elevated ozone. Article doesn’t directly state why ozone is elevated, but talks about climate change increasing ozone, insinuating it’s the reason in this case.