Hi Scott,
I am sorry to see that you are having trouble with your aspen trees. Your photo is very helpful, but I cannot diagnose an issue from a photo alone. We need a physical sample for a diagnosis. I recommend that you bring in a sample of your tree (ideally, bring in a portion of a branch that includes both affected and unaffected leaves). You can bring it to the Master Gardener Extension office located at the Boulder County Fairgrounds.
9595 Nelson Road 
Samples can be brought into the office for diagnosis:
8am-4:30pm, Monday through Friday.
In the meantime, you might want to read the following fact sheets:
https://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/
https://extension.colostate.edu/topic-areas/insects/leafmining-insects-5-548/
Again, we cannot diagnose without a sample, but these fact sheets might be helpful to look at while you are waiting for your diagnosis.