A CRJ DEPARTING GRR AFTER DEICING IS UNABLE TO MAINTAIN CABIN PRESSURE DURING CLB. CARGO DOOR WARNING IS NOTED; EMER DECLARED FOLLOWED BY A DIVERSION.

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WE PUSHED BACK AT GRR. THE WX CONDITIONS WERE LIGHT FREEZING RAIN; SNOW AND MIST. WE WERE IN LINE FOR DEICING AND TYPE 1 WAS APPLIED AT XA45 FOLLOWED BY 100% TYPE 4 AT XB02. ACFT TAXIED OUT FOR TKOF AT XB12. ON CLBOUT; IN IMC; WE NOTICED THAT CABIN PRESSURE WAS RISING ABOUT AS FAST AS WE WERE CLBING. SHORTLY THEREAFTER; WE GOT A CARGO DOOR CAUTION MESSAGE THAT CLRED AFTER A FEW SECONDS. SYNOPTIC PAGE SHOWED DOOR CLOSED AND 'GREEN.' CAPT DIRECTED QRH PROCS. PER QRH; MANUAL WAS SELECTED AND SEEMED TO CTL PRESSURE. CAPT HAD LEVELED OFF AT 10000 FT AND HAD NOTIFIED ATC. CHICAGO ASKED WHEN WE COULD TAKE A FURTHER CLB. WITH CABIN PRESSURE SEEMINGLY UNDER MANUAL CTL; WE STARTED TO CLB TO 16000 FT. PASSING 12000 FT; CABIN PRESSURE AGAIN STARTED TO CLB AGAIN AT HIGH RATE AND CARGO DOOR CAUTION AGAIN CHIMED. WE DSNDED BACK TO 10000 FT. DOOR CAUTION NOW REPEATEDLY INTERMITTENT. MAX CABIN PRESSURE 5400 FT/CHANGE IN PRESSURE 2.6 PSI. QRH DIRECTS DIVERSION TO NEAREST SUITABLE ARPT IF PRESSURE IS UNCTLABLE. BASED ON TIME AND WX CONDITIONS. CAPT COORD DIVERSION WITH ZAU. EMER WAS DECLARED. LNDG WAS UNEVENTFUL. CARGO DOOR WAS FOUND TO BE LATCHED BUT SHOWED A 1/2 INCH GP FROM FRAME AT TOP L SIDE. ARPT PERSONNEL TOOK DIGITAL PICTURES. BELIEVE QRH IS CONFUSING AS PRESENTLY WRITTEN. NOT CLR WHEN WE COULD CONTINUE/HAD TO DIVERT. ERRED ON THE CONSERVATIVE SIDE.