B767-300 Captain reported upon aircraft arrival a dangerous situation due to Supplemental Dry Ice had been loaded on board and all doors were locked with no ramp or maintenance personnel present. The Captain took control and notified appropriate personnel to complete safety procedures as the CO2 monitors indicated elevated CO2 levels.

Narrative

Upon arrival at the aircraft; all doors were closed and crew stairs were locked. No ramp or maintenance personnel were present. Started the APU and operated both air conditioning packs due to Supplemental Dry Ice loaded on board. During preflight; CO2 monitors were deployed per QRH procedures. All 3 monitors indicated elevated CO2 levels with audible chime alerts and flashing red lights. About this time; the assigned ramp agent arrived at the aircraft. Captain asked the ramp agent to open all cargo doors to assist in ventilating the aircraft. The elevated CO2 levels persisted and the Captain directed the crew to exit the flight deck area. Dispatch; Maintenance; and ramp control were all notified of the situation. Captain requested maintenance to determine the accuracy/calibration of the CO2 monitor readings. Requested dispatch advise the crew when and if the CO2 readings returned to safe levels. Company was advised of the situation and crew actions. Approximately 20 minutes later; Captain was informed CO2 levels returned to safe levels. Maintenance advised the CO2 monitors were functioning properly and accurately calibrated via an Aircraft Maintenance Logbook entry and discussion with the Captain at the aircraft. Crew proceeded with normal pre-flight duties and departed uneventfully.The ramp agent informed the crew that all non-living cargo (including Dangerous Goods (DG)/Dry Ice) was loaded the previous day. It was determined approximately 202 KG of dry ice was on the aircraft without ventilation for almost 24 hours. It is DG policy and procedure to NOT preload any DG on aircraft during holidays or layovers. This oversight presented an unnecessary risk to the flight crew and all other personnel at the aircraft.DG policies/procedures were not followed regarding proper loading of DG/Dry Ice specifically during a holiday/layover period.DG policies/procedures should be reemphasized and load crews properly trained in the loading criteria of DG/Dry Ice. Inaccessible Dangerous Goods (IDG) can be just as hazardous as Accessible Dangerous Goods (ADG) when improperly handled or loaded on the aircraft. Fortunately; the CO2 monitors accurately sensed the elevated CO2 levels to warn the crew and lead to a safe resolution.