B767-300 Captain reported an unsafe CO2 level in the cockpit during preflight; caused by a large amount of dry ice left in the aircraft overnight. The CO2 was reduced to a safe level and the flight departed without any further issues.

Narrative

During preflight duties the crew discovered an unsafe level of CO2 in the cockpit. This was indicated by 3 separate CO2 monitors being used at the time. The flight was conducted using supplemental dry ice procedures. The aircraft was diverted into ZZZ approximately 12 hours prior due to weather in ZZZ1. The aircraft was parked in ZZZ with doors closed and all cargo remained onboard; including dry ice. Crew notified ramp personnel of the problem and coordinated with Maintenance; Operations; and ramp personnel to reduce CO2 levels. It required approximately 2 hours for CO2 measurements to return to acceptable levels. Flight crew completed preflight duties and conducted flight from ZZZ to ZZZ1 without any further problems.Aircraft was completely closed for a considerable amount of time with a large quantity of dry ice aboard. This allowed the dry ice to sublimate and remain within the aircraft. [I suggest] better coordination and adherence to procedures when supplemental operations are being conducted. Large quantities of dry ice must be removed from the aircraft if it the aircraft is to remain on the ground closed for long periods of time.