WHILE THE STEADY REDUCTION OF NOX EMISSIONS IN THE U.S. FROM 2006 TO 2015 IS CONFIRMED BY BOTH SURFACE AND SATELLITE MEASUREMENTS AN UNEXPECTED SLOW-DOWN OF SUCH REDUCTIONS IN THE RANGE OF 2011-2015 HAS BEEN FOUNDED IN BOTH NO2 SURFACE MEASUREMENTS AND SATELLITE-BASED (TOP-DOWN) ESTIMATES OF NO2 EMISSIONS THAT IS NOT PRESENT IN THE BOTTOM-UP EMISSION ESTIMATES FROM EPA S NATIONAL EMISSION INVENTORY. THE REASON FOR THIS SLOW-DOWN IS HYPOTHESIZED IN PART DUE TO THE GROWING RELATIVE CONTRIBUTIONS OF INDUSTRIAL AREA AND OFF-ROAD SOURCES. TO EVALUATE THE HYPOTHESIS PUT FORTH IN JIANG ET AL. [2018] AND ALSO TO EMBRACE THE OPPORTUNITIES OF NEXT-GENERATION SATELLITE OBSERVATION THIS PROPOSAL WILL CONTINUE STUDYING THE NOX EMISSIONS TREND IN THE U.S. BY NOT ONLY USING OMI AND OMPS DATA TO EXTEND THE PAST ANALYSIS FOR MORE THAN THREE YEARS (UP TO THE END OF 2018 AND BEYOND) BUT ALSO EMPLOYING UNPRECEDENTED SATELLITE OBSERVATIONS OF NO2 AND ARTIFICIAL LIGHTS AT HIGH SPATIAL RESOLUTION FROM TROPOMI AND VIIRS RESPECTIVELY. SPECIFICALLY THESE MULTI-SENSOR DATA WILL BE USED IN CONJUNCTION WITH TWO CHEMISTRY TRANSPORT MODELS: (A) GEOS-CHEM AND ITS ADJOINT WITH NESTED-GRID CAPABILITY AND (B) A REGIONAL MODEL WRF-CHEM WITH NESTED-GRID CAPABILITY TO: QUANTIFY THE AMOUNT AND TREND OF NOX EMISSIONS AT MONTHLY AND REGIONAL SCALES OVER THE CONTINENTAL U.S. DURING 2005-2018 AND BEYOND USING OMI OMPS AND TROPOMI DATA PRODUCTS BY APPLYING THE HYBRID TOP-DOWN INVERSION METHOD WE DEVELOPED IN QU ET AL. [2017]. APPLY VIIRS ARTIFICIAL LIGHTS FOR NOVEL DOWNSCALING OF TOP-DOWN EMISSIONS (~25 KM) TO URBAN SCALE (~4 KM). EVALUATE THE BENEFIT OF UP-TO-DATE DOWNSCALED TOP-DOWN NOX EMISSIONS FOR AIR QUALITY MODELING AND FORECASTING AT URBAN SCALES (~4 KM RESOLUTION) WITH TROPOMI DATA. CONDUCT VALIDATION AND UNCERTAINTY ANALYSIS BY USING IN SITU DATA FROM SURFACE OBSERVATION NETWORK AND FIELD CAMPAIGNS (SUCH AS DISCOVR-AQ SARP OWLETS AND LISTOS) AS WELL AS THROUGH CROSS-VALIDATION.