WE PROPOSE TO DEVELOP A GLOBAL SURFACE N02 DATASET USING TROPOSPHERIC COLUMN N02 OBSERVATIONS FROM THE E0S AURA 0ZONE MONITORING INSTRUMENT (0MI) THAT WILL SIGNIFICANTLY ENHANCE THE RELEVANCE OF NASA SATELLITE OBSERVATIONS TO TROPOSPHERIC COMPOSITION HEALTH AND AIR QUALITY STUDIES. WHILE SATELLITE MEASUREMENTS PROVIDE GREATER SPATIAL COVERAGE THAN ANY OTHER OBSERVING PLATFORM THEIR USE IN TROPOSPHERIC COMPOSITION AIR QUALITY HEALTH AND PARTICULAR CLIMATE ISSUES REMAINS TEPID DUE TO THE NEED TO CREDIBLY INFER A SURFACE CONCENTRATION OR EMISSION FLUX FROM THE ATMOSPHERIC QUANTITY RETRIEVED. INFORMATION FROM A MODEL CAN BE COMBINED WITH SATELLITE RETRIEVALS TO INFER A SURFACE CONCENTRATION A METHOD DEVELOPED BY PI LAMSAL EIGHT YEARS AGO; HOWEVER UNCERTAINTIES ASSOCIATED WITH SUCH ESTIMATES CAN BE LARGE AND HIGHLY VARIABLE OVER SPACE AND TIME. RECENTLY THE NASA EARTH VENTURE-1 DISC0VER-AQ (DERIVING INFORMATION ON SURFACE CONDITIONS FROM COLUMN AND VERTICALLY RESOLVED 0BSERVATIONS RELEVANT TO AIR QUALITY) CAMPAIGN UTILIZED AN OBSERVING STRATEGY OF SYSTEMATIC AND CONCURRENT OBSERVATIONS TO UNDERSTAND THE CORRESPONDENCE OF SATELLITE DATA WITH SURFACE OBSERVATIONS. WITH NEW CONSTRAINTS FROM THESE OBSERVATIONS THERE IS A CRITICAL NEED TO REVISIT ENHANCE AND EVALUATE OUR METHOD TO INFER SURFACE N02. THE FIDELITY OF ANY RETRIEVED PRODUCT (E.G. 0MI TROPOSPHERIC N02 COLUMN) IS DEPENDENT ON THE ASSUMPTIONS (E.G. 3D TRACE GAS DISTRIBUTION) MADE IN THE RETRIEVAL ALGORITHM. THAT IS THE RETRIEVED PRODUCT IS NOT INDEPENDENT OF THESE ASSUMPTIONS. DEFICIENCIES IN MODEL DISTRIBUTIONS OF TRACE GASES MAY BE IDENTIFIED AND IMPROVED THROUGH EVALUATION WITH INDEPENDENT DATA SUCH AS THE SUITE OF DATA COLLECTED DURING THE DISC0VER-AQ DEPLOYMENTS. THEREFORE WE PROPOSE TO (TASK 1) PERFORM EVALUATE AND REFINE HIGH-RESOLUTION MODEL SIMULATIONS FOR USE IN N02 ORBITAL AND SUB-ORBITAL N02 RETRIEVALS AND (TASK 2) PROCESS AND EVALUATE EXISTING ACAM AND GCAS N02 DATA PRODUCTS FROM DISC0VER-AQ TO PRODUCE SCIENCE-QUALITY N02 COLUMN DATA. MANY FACTORS INFLUENCE THE RELATIONSHIP BETWEEN SURFACE CONCENTRATIONS AND THE TROPOSPHERIC COLUMN FOR EXAMPLE ATMOSPHERIC STABILITY AND BOUNDARY LAYER MIXING STRENGTH HEIGHT OF THE BOUNDARY LAYER TRANSPORT FROM EMISSIONS SOURCES AND/OR LONG RANGE TRANSPORT IN THE FREE TROPOSPHERE AND CHEMICAL LIFETIME. USING THE SUITE OF OBSERVATIONS PROVIDED BY DISC0VER-AQ WITH THE ADDITION OF HIGH-RESOLUTION SUB-ORBITAL COLUMN MEASUREMENTS PROVIDED BY TASK 2 WE PROPOSE A RIGOROUS ANALYSIS OF THE OBSERVED AND SIMULATED COLUMN-SURFACE RELATIONSHIP TO QUANTIFY BOTH THE SENSITIVITIES TO THESE FACTORS AND THE POTENTIAL UNCERTAINTIES ASSOCIATED WITH PARTICULAR METEOROLOGICAL AND CHEMICAL CONDITIONS (TASK 3). RESULTS OF THIS ANALYSIS WILL INFORM THE IMPROVEMENT OF OUR CALCULATION OF SURFACE N02 FROM ORBITAL AND SUB-ORBITAL OBSERVATIONS. WE WILL GENERATE AND EVALUATE SEVERAL SURFACE N02 PRODUCTS INCLUDING THOSE DERIVED FROM USING HIGH-RESOLUTION SUB-ORBITAL MEASUREMENTS MADE DURING DISC0VER-AQ TO PROVIDE CONFIDENCE IN THE MISSION-LONG GLOBAL SURFACE N02 PRODUCT THAT WILL BE BASED ON THE 0MI STANDARD PRODUCT (TASK 4).