Puzzling Purple Plethora

Photo date: July 29, 2015. Photographer: Mark Mahin.

The photo below (taken at 9:02 PM) shows a surge of purple orbs on Fifth Avenue in New York City. This is the full original photo, without any cropping.

 How can we tell that these orbs are not dust near the camera? For one thing, the largest orb is way too big to be a dust particle (as discussed here, outdoor dust particles are only about 1 micron in size, only about 1/15000 of the width just in front of a camera lens).  For another thing, we see almost no orbs in the bottom third of the photo.  If we were seeing particles in front of the camera, they would be randomly distributed around the photo area; but they are not.

A photo of a piece of cardboard at arm's length taken during the same minute (9:02 PM) showed no orbs (indicating that the previous photo was not showing particles near the camera).

There was no fog, mist, rain, or precipitation on this night. The air quality as listed on airnow.gov was "moderate." When I took these photos, there was nothing unusual I could see with my eyes, and I could not smell anything unusual.