It's actually used for more than aircare traffic. The "9" channels are set aside for Fairfax EMS transport units to talk to all the hospitals in Fairfax County. They're main use is to transmit patient information to the hospital. 9A is the channel for Fairfax hospital. This is the hospital that aircare flies to the most so thats why you often hear aircare traffic.
9C Mount Vernon Hospital
9D Fair Oaks
9E Reston
9F Springfield
9G Access of Fairfax
9H Access of Reston