Air Pollution and National Park Visitation

I read a short news clip in the morning paper today about visitor attendance declining in out national parks due to air pollution.  I tried a scholar google search to find the research but could not find it.

I did find this book titled Air Pollution and Its Impacts on U.S. National Parks and this paper.

I will check into a bit more since I will be teaching Recreation Geography this fall and I do a section on environmental impacts.