Thank you for your question. I think the issue might be oystershell scale. See link below. You tree looks to be very large and perhaps it might be "older." Info in link says life expectancy of aspens in an urban area is 25 years. Perhaps this tree has reached it's life expectancy. Also, it is recommended that when crown becomes thin, removal of the tree is recommended.
Oyster Shell Scale - http://extension.colostate.edu/topic-areas/insects/oystershell-scale-5-513/