ORGANIC AEROSOL (OA) IS ONE OF THE LARGEST CONTRIBUTORS TO THE GLOBAL AEROSOL BURDEN AND ITS EMISSIONS ARE STRONGLY IMPACTED BY ANTHROPOGENIC BIOGENIC AND BIOMASS BURNING (BB) SOURCES WHICH ARE STRONGLY MODULATED BY GLOBAL CLIMATE. THE THOUSANDS OF ORGANIC COMPOUNDS THAT MAKE UP OA ARE IN A DYNAMIC EQUILIBRIUM WITH THE GAS PHASE AS DEFINED BY A KEY THERMODYNAMIC PROPERTY THEIR VAPOR PRESSURE (VOLATILITY). AS BOTH LABORATORY AND FIELD EXPERIMENTS HAVE SHOWN IN THE PAST DECADE ESPECIALLY NEAR SOURCES A LARGE FRACTION OF ORGANIC COMPOUNDS THAT PARTICIPATE IN OA FORMATION ARE IN THE GAS PHASE. AS AIR MASSES MOVE DOWNWIND OXIDATION OF GAS PHASE COMPOUNDS WILL SHIFT THE EQUILIBRIUM TOWARDS THE PARTICLE PHASE WHILE DILUTION WILL SHIFT EQUILIBRIUM TOWARDS THE GAS PHASE. INCORRECT ASSUMPTIONS ON THE VOLATILITY OF OA COMPONENTS IN MODELS WILL NOT ONLY CAUSE SUBSTANTIAL ERRORS IN SOURCE CONTRIBUTIONS BUT ALSO LEAD TO UNREALISTIC DESCRIPTIONS OF OA AGING. THIS CAN LEAD TO OVERESTIMATING OA CONCENTRATIONS FURTHER DOWNWIND AS MOST CHEMICAL TRANSPORT MODELS (CTMS) STILL DO. TO BETTER ADDRESS THIS PROBLEM RECENT CTM RELEASES NOW INCLUDE VAPOR PRESSURE DISTRIBUTIONS (PARAMETERIZED AS VOLATILITY BASIS SETS VBS) FOR SOA MODELING BUT DIRECT MEASUREMENTS OF VOLATILITY FOR DIFFERENT TYPES OF OA ARE SCARCE AND OFTEN HIGHLY VARIABLE. THIS PROPOSAL LEVERAGES OA MEASUREMENTS FROM 11 NASA AND 3 NSF AIRBORNE CAMPAIGNS TO CHARACTERIZE THE VOLATILITY OF OA CLASSIFIED BY DIFFERENT CHEMICAL SOURCES AND AGES THAT WILL RESULT IN UPDATED CTM-SUITABLE OA PARAMETERIZATIONS THAT WILL SUBSTANTIALLY IMPROVE OA REPRESENTATION IN GLOBAL MODELS. SPECIFICALLY WE PROPOSE TO: -CONSTRAIN OA VOLATILITY DISTRIBUTIONS WITH THERMAL DENUDER MEASUREMENTS CONDUCTED ON 3 OF THESE AIRBORNE CAMPAIGNS. THESE MEASUREMENTS ARE SIMILAR TO THOSE CARRIED OUT IN LABORATORY AND GROUND EXPERIMENTS AND PROVIDE UNIQUE INFORMATION ABOUT THE VOLATILITY OF OA ALOFT. -INVESTIGATE GAS-PARTICLE PARTITIONING AT THE NEAR-MOLECULAR LEVEL USING AIRBORNE EXTRACTIVE ELECTROSPRAY MASS SPECTROMETRY (EESI) MEASUREMENTS WHICH ENABLE US TO SEPARATE THE IMPACT OF VOLATILITY OF INDIVIDUAL OA COMPONENTS FROM THE CHEMICAL EVOLUTION OF BULK OA. -COMBINE THESE THERMAL DENUDER MOLECULAR AND BULK MEASUREMENTS AND VBSS TO EVALUATE THE EXTENT TO WHICH OA EVAPORATION IN AIRCRAFT INLETS IMPACTS THE QUANTIFICATION OF AEROSOL PROPERTIES DURING THESE CAMPAIGNS IN PARTICULAR OPTICAL PROPERTIES USED TO INFORM SATELLITE RETRIEVALS. WE WILL DEVELOP A CORRECTION AND SOFTWARE PACKAGE SUITABLE FOR OTHER IN-SITU AIRBORNE DATA. -USE THE NEAR-GLOBAL SCOPE AND LARGE CONCENTRATION AND TEMPERATURE RANGE OF THE OBSERVATIONS TO REFINE AND VALIDATE THE PARAMETERIZATIONS AGAINST AMBIENT DATA. WE WILL USE SEVERAL PASSIVE TRACERS SUCH AS CO TO ACCOUNT FOR SOURCE STRENGTH AND DILUTION. BOTH GAS-PHASE TRACERS AND AEROSOL SOURCE ATTRIBUTION BY POSITIVE MATRIX FACTORIZATION (PMF) WILL BE USED TO DERIVE SOURCE-SPECIFIC VOLATILITY AND EXPLORE THE EFFECT OF AIRMASS AGE. -IMPLEMENT THE SOURCE-SPECIFIC VBS IN A STATE-OF-THE-ART CTM AND CONDUCT TARGETED SENSITIVITY STUDIES TO QUANTIFY THE IMPACT OF THE REVISED PARAMETERIZATIONS IN OA VOLATILITY ON GLOBAL AEROSOL BURDEN AND RADIATIVE FORCING. THIS PROPOSAL DIRECTLY ADDRESSES THE GOALS OF THE ACCDAM PROGRAM BY ANALYZING AND MODELING DATA PRODUCED AS PART OF MULTIPLE NASA AND NSF AIRBORNE CAMPAIGNS. IT DOES ADDRESS A MAJOR UNCERTAINTY IN THE GLOBAL OA BUDGET WHERE AIRBORNE DATASETS PROVIDE UNIQUE CONSTRAINTS AND ITS RESULTS CAN BE DIRECTLY APPLIED TO IMPROVING CTMS HENCE ADDRESSING GLOBAL ATMOSPHERIC COMPOSITIONAL CHANGE. THE VOLATILITY CORRECTIONS FOR OTHER IN-SITU AIRBORNE AEROSOL DATA THAT THIS WORK WILL DERIVE WILL LEAD TO IMPROVED AOD ESTIMATES AND SATELLITE RETRIEVALS ESPECIALLY FOR BBOA. LASTLY BY IMPROVING THE REPRESENTATION OF THE AGING PROCESSES DOWNWIND OF SOURCES THIS RESEARCH WILL BETTER CONSTRAIN AIR QUALITY MODELING DOWNWIND OF LARGE BB AND URBAN PLUMES.