ISOPRENE MEASUREMENTS FROM SPACE: NEW GLOBAL CONSTRAINTS ON EMISSIONS AND PHOTOCHEMISTRY FROM SYNTHESIS OF CRIS AND OMI DATA

WE PROPOSE A RESEARCH PROGRAM TO ADVANCE SCIENTIFIC UNDERSTANDING OF THE EMISSIONS AND CHEMICAL IMPACTS OF ISOPRENE THE SINGLE MOST IMPORTANT NON-METHANE VOLATILE ORGANIC COMPOUND (VOC) IN EARTH S ATMOSPHERE. EMITTED NATURALLY BY TERRESTRIAL PLANTS WITH A GLOBAL FLUX SEVERAL TIMES GREATER THAN THAT OF ALL ANTHROPOGENIC VOCS COMBINED ISOPRENE SHAPES TROPOSPHERIC COMPOSITION THROUGH ITS IMPACTS ON OZONE AEROSOLS THE ATMOSPHERE S OXIDIZING CAPACITY AND THE NITROGEN CYCLE. SEVERAL FUNDAMENTAL ISSUES LIMIT OUR UNDERSTANDING OF THESE EMISSIONS AND THEIR IMPACTS ON ATMOSPHERIC COMPOSITION: I) BOTTOM-UP EMISSION INVENTORIES ARE DERIVED MAINLY FROM EXTRAPOLATION OF A LIMITED NUMBER OF POINT MEASUREMENTS AND CARRY LARGE UNCERTAINTIES. II) IN SITU MEASUREMENTS ARE ESPECIALLY SPARSE OVER THE TROPICS THOUGHT TO BE THE MOST GLOBALLY SIGNIFICANT SOURCE REGION. III) FORMALDEHYDE (HCHO) MEASURABLE FROM SPACE AND USED AS AN ISOPRENE PROXY IS ALSO PRODUCED FROM OTHER VOCS AND THUS IS NOT A UNIQUE MARKER FOR ISOPRENE. THIS PRESENTS PARTICULAR PROBLEMS OVER REGIONS WITH FIRES AND OVER POPULATED AREAS WITH THEIR SUBSTANTIAL ANTHROPOGENIC VOC SOURCES. IV) ISOPRENE CAN BE AN IMPORTANT SINK FOR THE HYDROXYL RADICAL (OH). IT THUS AFFECTS ATMOSPHERIC OXIDATION (AND ITS OWN LIFETIME) IN A WAY THAT IS NOT YET WELL UNDERSTOOD. V) USE OF HCHO AS AN ISOPRENE PROXY IS HINDERED BY INCOMPLETE KNOWLEDGE OF THE ABOVE CHEMICAL NON-LINEARITIES AND OF THE NOXDEPENDENT CHEMISTRY GOVERNING THE HCHO YIELD AND ITS PRODUCTION TIMESCALE. THIS PROJECT APPLIES NEW SPACE-BASED MEASUREMENTS OF ISOPRENE (FROM THE CROSS-TRACK INFRARED SOUNDER CRIS) WITH OBSERVATIONS OF HCHO AND NO2 (FROM THE OZONE MONITORING INSTRUMENT OMI) TO ADVANCE THE RESOLUTION OF THE ABOVE ISSUES. OUR TEAM HAS RECENTLY DEMONSTRATED THE FEASIBILITY OF THE FIRST-EVER SPACE-BASED ISOPRENE MEASUREMENT. COMBINING THESE OBSERVATIONS WITH THE COMPLEMENTARY CHEMICAL DATA FROM OMI OFFERS A DECISIVE STEP FORWARD FOR OUR ABILITY TO CHARACTERIZE ONE OF THE CENTRAL FACTORS DRIVING TROPOSPHERIC COMPOSITION. RESEARCH WILL BE GUIDED BY THE FOLLOWING CORE QUESTIONS: WHAT IS THE DISTRIBUTION OF ISOPRENE CONCENTRATIONS AND EMISSIONS GLOBALLY AND OVER CRITICALLY UNDERSAMPLED REGIONS? * IS PRESENT UNDERSTANDING OF OH VARIABILITY AND ITS ISOPRENE-NOX DEPENDENCE CONSISTENT WITH THE COMBINED CONSTRAINTS PROVIDED BY SIMULTANEOUS SPACE-BASED MEASUREMENTS OF ISOPRENE HCHO AND NO2? THE PROPOSED WORK WILL ALSO OPEN THE DOOR TO A RANGE OF OTHER INVESTIGATIONS SEEKING TO BETTER QUANTIFY THE DRIVERS AND STRESSORS OF ISOPRENE EMISSIONS AND THE IMPACTS OF ISOPRENE ON AIR QUALITY ACROSS CHEMICAL REGIMES. SPECIFIC PROJECT ACTIVITIES WILL INCLUDE THE FOLLOWING: 1) DEVELOP CRIS ISOPRENE RETRIEVALS AND RIGOROUSLY EVALUATE THE RESULTS USING A COMBINATION OF IN-SITU MEASUREMENTS CHEMICAL TRANSPORT MODELING AND RETRIEVAL SIMULATIONS. 2) GENERATE THE FIRST DIRECT OBSERVATIONS OF THE GLOBAL DISTRIBUTION AND INTRA-ANNUAL VARIABILITY OF ATMOSPHERIC ISOPRENE. 3) COMBINE THE CRIS ISOPRENE MEASUREMENTS WITH CHEMICAL CONSTRAINTS FROM AURA OMI (HCHO NO2) AND THE GEOS-CHEM MODEL TO ADVANCE UNDERSTANDING OF ISOPRENE OXIDATION AND HCHO PRODUCTION ACROSS NOX REGIMES. 4) USING THE GEOS-CHEM ADJOINT INTERPRET THE CRIS ISOPRENE DATA IN AN INVERSE MODELING FRAMEWORK TO DERIVE NEW CONSTRAINTS ON THE SPATIAL-TEMPORAL DISTRIBUTION OF GLOBAL ISOPRENE EMISSIONS AND THE UNDERLYING FACTORS DRIVING THOSE EMISSIONS. COMPREHENSIVE ERROR ANALYSES AND SENSITIVITY STUDIES WILL BE USED TO ASSESS THE POTENTIAL IMPACT OF RETRIEVAL ERRORS AND MODEL BIASES ON THE ABOVE RESULTS. THE PROJECT ADDRESSES KEY GOALS FOR THIS SOLICITATION INCLUDING USING AURA DATA ALONG WITH OTHER SATELLITE TRACE GAS DATA SETS TO QUANTIFY AND MAP EMISSIONS AND USING THE ABOVE-DESCRIBED DATA SETS TO STUDY TROPOSPHERIC AIR QUALITY AND OXIDIZING CAPACITY OF THE TROPOSPHERE.