Guy, go ahead and post some of the other pictures on this site as well. Looking at the pictures (on the other site) my initial impression was an abiotic injury. That's why I asked if forest trees were affected also. All city trees, even in the boulevards and parkways, can be affected by bizarre weather fluctuations of light and wind from all points of the compass. I have seen sunscald on the bottom of limbs due to reflection from the house.
The Celtis occidentalis, for as tough as it is, also has a low tolerance to salt. I'm thinking beyond just the road salts but all of the salts in fertilizers and such as well. It will be very interesting what you find when you get on site.
Dave