FIRES ARE ONE OF THE MAIN CONTRIBUTORS TO THE GLOBAL FINE AEROSOL BURDEN ESPECIALLY FOR ORGANIC AEROSOL (OA) AND ITS OPTICALLY ACTIVE FRACTION BROWN CARBON (BRC) BUT ALSO FOR INORGANIC SPECIES. CHANGES IN LAND USE PATTERNS AS WELL AS A CHANGING CLIMATE ARE EXPECTED TO SIGNIFICANTLY INCREASE THE PREVALENCE OF FIRES AND HENCE THEIR AEROSOL EMISSIONS IN THE US AS WELL AS GLOBALLY. HOWEVER THE EMISSIONS OF PRIMARY AEROSOL AND GASEOUS AEROSOL PRECURSORS FROM FIRES AND THEIR SUBSEQUENT ATMOSPHERIC EVOLUTION ARE STILL POORLY UNDERSTOOD. THERE IS HIGH VARIABILITY IN EMISSION RATIOS AND THE OXIDATIVE EVOLUTION OF OA AMONG INDIVIDUAL FIRES IN FIELD AND LABORATORY STUDIES AND HENCE NOT SURPRISINGLY IN GLOBAL MODELS. AS PART OF NASA FIRECHEM THE PROPOSED WORK WILL ADDRESS THE FOLLOWING OBJECTIVES: (A) QUANTIFY THE PRIMARY OA (POA) AND INORGANIC EMISSIONS FROM AGRICULTURAL AND WILD FIRES. THE VOLATILITY OF POA A CRITICAL BUT POORLY UNDERSTOOD PROPERTY THAT AFFECTS AGING WILL BE DIRECTLY QUANTIFIED BY THERMAL DENUDING AS WE DEMONSTRATED IN KORUSAQ AND WINTER. THE RELATIONSHIP BETWEEN EMISSION RATIOS AND FIRE PROPERTIES (INCLUDING MODIFIED COMBUSTION EFFICIENCY MCE) INCLUDING THE NATURE AND VARIABILITY OF THE MOLECULAR AEROSOL COMPOSITION WILL BE INVESTIGATED. (B) STUDY THE EVOLUTION OF THE MASS AND COMPOSITION OF OA AND INORGANIC AEROSOLS DOWNWIND OF FIRES INCLUDING SECONDARY OA (SOA) FORMATION OA OXIDATION AND THEIR VARIABILITY. THE REASONS WHY SOA FORMATION IS OFTEN LOW IN FIELD STUDIES COMPARED TO LARGER FORMATION IN LABORATORY STUDIES WILL BE INVESTIGATED. PRECURSORS AND FIRST-GENERATION PRODUCTS THAT DRIVE OXIDATION OF POA AND SOA FORMATION FROM FIRES WILL BE IDENTIFIED. THE EVOLUTION OF AEROSOL VOLATILITY AND THE IMPACT OF OA EVAPORATION ON TOTAL OA EVOLUTION WILL ALSO BE QUANTIFIED. (C) IDENTIFY MOLECULAR MARKERS IN OA THAT ARE CORRELATED WITH MEASURED BRC LIGHT ABSORPTION AND USE THEM TO INVESTIGATE THE SPECIES RESPONSIBLE FOR PRIMARY BRC AS WELL AS THE POSSIBLE CHEMICAL PATHWAYS FOR BLEACHING OR FORMATION OF BRC DOWNWIND OF FIRES. THE PROPOSED WORK USES A COMBINED INSTRUMENTAL APPROACH TO CHARACTERIZE AND QUANTIFY THE BULK PROPERTIES OF OA AS WELL AS THEIR MOLECULAR CHEMISTRY BOTH AT THE HIGH TIME RESOLUTION REQUIRED FOR MEASUREMENTS NEAR FAST CHANGING SOURCES SUCH AS FIRES: (1) A HIGHLYCUSTOMIZED AERODYNE HIGH-RESOLUTION AEROSOL MASS SPECTROMETER (AMS) THAT HAS MULTIPLE UNIQUE PERFORMANCE ADVANTAGES VS. OTHER AIRCRAFT AMSS. THIS INSTRUMENT WILL QUANTIFY NON-REFRACTORY INORGANIC AEROSOLS AND OA MASS (BOTH BULK AND SIZE SEGREGATED) AS WELL OA PROPERTIES (O/C H/C OSC OA/OC) WITH SAMPLING RATES UP TO 10 HZ (TYPICALLY 1 HZ). (2) A RECENTLY-DEVELOPED INSTRUMENT THE EESI-TOF-MS USING EXTRACTIVE ELECTROSPRAY IONIZATION (THUS AVOIDING THERMAL DECOMPOSITION ARTIFACTS) WILL CHARACTERIZE A LARGE NUMBER OF MOLECULAR COMPONENTS OF OA (INCLUDING E.G. LEVOGLUCOSAN) AT 1 HZ RESOLUTION ALLOWING LINKING BULK AND MOLECULAR-LEVEL OA CHEMISTRY FROM AIRCRAFT FOR THE FIRST TIME. OF PARTICULARLY HIGH INTEREST ARE N-CONTAINING COMPOUNDS PRODUCTS OF OH OXIDATION OF KEY GAS-PHASE PRECURSORS AND PRODUCTS OF OLIGOMERIZATION. THESE RESULTS WILL LEAD TO MORE ROBUST PARAMETRIZATIONS OF THE PRIMARY EMISSIONS FROM FIRES IN PARTICULAR SMALL AGRICULTURAL FIRES WHERE AMBIENT DATA IS SCARCE. THEY WILL ALSO SYSTEMATICALLY ADDRESS WHICH FIRE PROPERTIES AND SPECIES CONTROL OA FORMATION EVAPORATION AND OXIDATION BY TYING TOGETHER BULK OA AND MOLECULAR TRACERS. THIS WORK WILL BE CONDUCTED IN CLOSE COLLABORATION WITH THE NOAA AMS AND UW EESI TEAMS ON THE NOAA P3 AIRCRAFT AS PART OF THE FIRECHEM/FIREX COLLABORATION. THE CONSISTENT AND COORDINATED PAYLOAD ON BOTH AIRCRAFT WILL GREATLY FACILITATE COMBINED USE OF DATA FROM BOTH PLATFORMS. THIS WILL INCLUDE JOINT CALIBRATIONS AND TESTS WITH STANDARDS AND FRESH AND AGED SMOKE SAMPLES FROM DIFFERENT FUELS AT THE CU-BOULDER ENVIRONMENTAL CHAMBER FACILITY.