Thank you for including the image with your questions.  Many other people in our region are seeing similar clouds of the same small white flying insects during the past several months.  The insects were recently identified as ash whiteflies, Siphoninus phillyreae, potentially serious pests of woody plants. These insects were first reported to be in the Willamette Valley during 2014. Very little is known at this time about how damaging ash whitefly will be in our region or what sort of management may be needed.

This webpage for commercial growers has some background information and images of infested leaves: http://oregonstate.edu/dept/nurspest/Ash_whitefly.html. (Near the top of the page, notice the link to a local garden show with an interview with a university entomologist.)

It’s not yet been determined if treatment will be needed during the growing season. Further, no treatments are effective against the adults you see flying now.  In places where the swarms are particularly dense, some people are using dust masks which are sold by paint and hardware stores.