Looks like you may need to detect the inbound frequency and then decide whether you either want to attract it by emitting the opposite frequency, or to emit the same frequency to try and repel it. Either way, looks like going beyond 2kHz is probably a waste of time.
"Male mosquitoes have a wing beat frequency of about 600 Hz, females 400 Hz. Yet, when they mate, they are matching tones not at this fundamental frequency but at a harmonic near 1200 Hz. The problem is that the literature reports that male mosquitoes can hear only up to a few hundred Hz and females are completely deaf! Ron Hoy and his postdoctoral associate, Ben Arthur demonstrate how they determined that both male and female mosquitoes can detect tones up to 2000 Hz."
I captured a mosquito once and tried to bother it by slowly sweeping across the whole spectrum from a couple of Hz to ~300 kHz, using various speaker designs. The mosquito was unfazed.
The only repellents that actually work are chemical ones, but they're not efficient outdoors when it's windy, with the exception of the ones you apply directly on your skin.
