A CRJ-200 Captain reported receiving a L 10th DUCT message. The QRH procedure called for idle thrust and land as soon as possible; so a divert decision was made; followed by a normal landing.

Narrative

The LH AC (left-hand air conditioning) Pack was deferred so all 10th stage bleed valves were closed on the LH duct. At FL230 in cruise we received a L 10TH DUCT red message. We complied with the QRH; which directed us to close the 10th stage valves on the LH side. Those valves were already closed; so we continued the QRH which instructed us to reduce LH engine power to idle and land as soon as practical. Considering our altitude; the multiple long runways; and the availability of fire/rescue and maintenance in ZZZ; we elected to land there. The First Officer continued to fly the aircraft while the Captain coordinated with ATC; declared an emergency; and coordinated with the Flight Attendants and company. After preparing the aircraft for landing per the QRH; we transferred controls to the Captain prior to intercepting the localizer for an ILS approach. The Captain landed uneventfully and we rolled to the next to last turn off before exiting the runway. After exiting the runway; we noted that the L 10TH DUCT message was extinguished; and that there were no other indications of danger. We informed ATC that we no longer needed assistance and taxied to the gate and deplaned.