REACTION WITH TROPOSPHERIC OH IS THE PRIMARY REMOVAL PROCESS FOR CH4 OZONE-DEPLETING SUBSTANCES (ODSS) AND THEIR REPLACEMENTS AS WELL AS MANY OTHER SHORT-LIVED ATMOSPHERIC CONSTITUENTS. GLOBAL MEAN OH ABUNDANCE AND ITS VARIATIONS ARE IMPORTANT FOR UNDERSTANDING THE OBSERVED LONG-TERM TRENDS AND INTERANNUAL VARIABILITY OF CH4 AND THE SHORT-LIVED AIR POLLUTANTS. IT IS ALSO USED FOR FUTURE PROJECTION OF ATMOSPHERIC ABUNDANCES OF ODSS AND THEIR REPLACEMENTS. THEREFORE AN ACCURATE GLOBAL OH ESTIMATE HAS IMPORTANT IMPLICATIONS FOR FUTURE PROJECTION OF AIR QUALITY CLIMATE AND STRATOSPHERIC OZONE LAYER RECOVERY. METHYLCHLOROFORM (CH3CCL3 OR MCF) OBSERVATIONS (AND THE INFERRED MCF LIFETIME) HAVE BEEN USED TO CONSTRAIN GLOBAL MEAN TROPOSPHERIC OH FOR THE PAST THREE DECADES. DUE TO REGULATION BY THE MONTREAL PROTOCOL (MP) THE ATMOSPHERIC MCF LEVEL HAS RAPIDLY DECLINED MAKING IT DIFFICULT TO SERVE AS AN OH REFERENCE GAS. THEREFORE IT IS CRITICAL TO FIND A MCF-ALTERNATIVE OH REFERENCE GAS.THIS PROPOSED WORK AIMS TO FIND A MCF SUBSTITUTE TO CONTINUE PROVIDE QUANTITATIVE INFORMATION ON TROPOSPHERIC OH ABUNDANCE. WE WILL CONDUCT AN INTEGRATED ANALYSIS USING ATMOSPHERIC OBSERVATIONS OF HYDROCHLOROFLUOROCARBONS (HCFCS) AND HYDROFLUOROCARBONS (HFCS) FROM GROUND-BASED NETWORKS AIRCRAFT MISSIONS AND ACE-FTS ALONG WITH THE NASA GEOS-5 CHEMISTRY CLIMATE MODEL. WE WILL ADDRESS THE FOLLOWING QUESTIONS:AMONG THE MAJOR LONG-LIVED TRACE GASES WHICH ONE (ONES) WILL SERVE AS THE BEST SUBSTITUTE FOR MCF AS AN OH REFERENCE GAS?HOW WELL CAN WE DERIVE THE GLOBAL OH ABUNDANCE USING THE PROPOSED REFERENCE GAS(ES) AS COMPARED TO MCF?HOW CAN WE UTILIZE ATMOSPHERIC OBSERVATIONS AND GLOBAL CHEMISTRY MODELS TO NARROW THE UNCERTAINTY IN THE DERIVED GLOBAL OH ABUNDANCE?IS THE OH ABUNDANCE SYMMETRIC BETWEEN THE NORTHERN AND SOUTHERN HEMISPHERES AS SUGGESTED BY A RECENT MCF-BASED ANALYSIS?