Whenever I've had aphids, I used Safer's Insecticidal soap. Note that many of these products suggest that you apply out of the sun or you may get leaf damage or drop. If you have a spray bottle, you could also try holding the branches over a bucket and blast the aphids off with water. The adults fly in and will lay eggs which hatch to produce the aphids that you see, so it doesn't matter that there's no vegetation nearby. I'm up on the 18th floor of a concrete building, way above the trees and get aphids. :-(

Also hope that you start seeing more of these:

{{gwi:3424}}

This is a ladybug larva I took a picture of last evening. These little hungry guys dispatch of aphids like there's no tomorrow. If you see any of these, try to move them to the infestation.