You can generate much faster frequencies with pwm but you end up losing resolution by decreasing the bits/top value...
Normally the atomizer transducers use a self resonant circuit to maximize power transfer.
A Google image search for "ultrasonic atomizing circuit" gives lots of examples like this one:
A few years back I experimented with a 200w ultrasonic traducer, I used phase angle to track the resonant frequency:
http://krazatchu.ca/2008/04/17/ultrasonic-resonant-tracker/