THIS PROJECT AIMS TO QUANTIFY AND ATTRIBUTE PAST (2005-2018) AIR QUALITY TRENDS IN TERMS OF OZONE AND PARTICULATE MATTER OF AERODYNAMIC DIAMETER SMALLER THAN 2.5 M (PM2.5) OVER THE UNITED STATES (US). AIR QUALITY TRENDS WILL BE ESTIMATED AT THE COUNTY LEVEL FROM HIGH RESOLUTION (12 KM) AIR QUALITY SIMULATIONS BASED ON THE COMMUNITY MULTISCALE AIR QUALITY (CMAQ) MODEL. CMAQ SIMULATIONS WILL BE CONSTRAINED WITH THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA) SATELLITE RETRIEVALS OF ATMOSPHERIC COMPOSITION. THROUGHOUT THE ENTIRE SIMULATION PERIOD WE WILL:. . GENERATE DAILY ANALYSES OF OZONE CARBON MONOXIDE (CO) NITROGEN DIOXIDE (NO2) AND PM2.5 VIA ASSIMILATION OF MODERATE RESOLUTION IMAGING SPECTRORADIOMETER (MODIS) AEROSOL OPTICAL DEPTH (AOD) MEASUREMENT OF POLLUTION IN THE TROPOSPHERE (MOPITT) CO RETRIEVALS AND OZONE MONITORING INSTRUMENT (OMI) TROPOSPHERIC NO2 RETRIEVALS.. . IMPROVE SIMULATIONS OF PHOTOLYSIS FREQUENCIES IN CMAQ BY CONSTRAINING OVERHEAD OZONE COLUMNS USING OMI TOTAL OZONE RETRIEVALS. .THE CMAQ INTEGRATED PROCESS RATE (IPR) ANALYSIS AND INTEGRATED REACTION RATE (IRR) ANALYSIS CAPABILITIES WILL BE USED TO DISENTANGLE THE CONTRIBUTION OF INDIVIDUAL PHYSICAL AND CHEMICAL PROCESSES TO CHANGES IN TROPOSPHERIC AIR QUALITY AND OXIDATION EFFICIENCY FROM 2005 TO 2018. CMAQ AIR QUALITY SIMULATIONS WILL BE EVALUATED AGAINST THE EXTENSIVE IN SITU OBSERVATIONS OF O3 PRECURSOR GASES PM2.5 MASS CONCENTRATIONS AND AEROSOL CHEMICAL COMPOSITION FROM ENVIRONMENTAL PROTECTION AGENCY S (EPA) NETWORK AS WELL AS A SUITE OF NASA SATELLITE OBSERVATIONS INCLUDING THE CLOUD-AEROSOL LIDAR AND INFRARED PATHFINDER SATELLITE OBSERVATION (CALIPSO) THE TROPOSPHERIC EMISSIONS SPECTROMETER (TES) AND THE AEROSOL ROBOTIC NETWORK (AERONET). THE EVALUATED CMAQ AIR QUALITY SIMULATIONS CONSTRAINED WITH NASA SATELLITE OBSERVATIONS ALONG WITH IRR AND IPR ANALYSIS WILL BE MAPPED TO ALL THE U.S. COUNTIES USING THE GEOGRAPHIC INFORMATION SYSTEM (GIS) FOR TREND ANALYSIS AND ATTRIBUTION OF CHANGES IN AIR QUALITY AND OXIDATION EFFICIENCY. .FURTHERMORE WE WILL CALCULATE THE TRENDS USING A CMAQ SIMULATION WITHOUT ASSIMILATION AND COMPARE THOSE WITH THE TRENDS CALCULATED FROM OBSERVATIONS AS WELL AS CMAQ SIMULATION WITH ASSIMILATION TO QUANTIFY THE VALUE OF NASA SATELLITE OBSERVATIONS IN REDUCING UNCERTAINTIES IN ESTIMATION OF LONG-TERM AIR QUALITY TRENDS. AN INTERACTIVE WEB-PAGE WILL BE CREATED TO DISSEMINATE THE COUNTY LEVEL AIR QUALITY TREND INFORMATION TO THE PUBLIC MUNICIPALITIES AND AIR QUALITY MANAGERS. TO ENHANCE THE UTILITY OF THE PROPOSED CMAQ SIMULATIONS BEYOND THIS PROJECT ALL CMAQ SIMULATIONS WILL BE ARCHIVED AT THE NASA ATMOSPHERIC SCIENCE DATA CENTER (ASDC) SO THAT THEY ARE AVAILABLE FOR FURTHER ANALYSIS BY THE COMMUNITY AND CAN BE USED TO DRIVE FINER RESOLUTION (12 KM) FUTURE RETROSPECTIVE AIR QUALITY SIMULATIONS OVER THE US..