PARTICLE NUMBER CONCENTRATION IS AN IMPORTANT FACTOR IN THE PUBLIC HEALTH AND CLIMATE IMPACTS OF ATMOSPHERIC AEROSOLS. THE INDIRECT EFFECTS OF AEROSOL PARTICLES ON CLOUD PROPERTIES PRECIPITATION RADIATION AND CLIMATE DEPEND ON THE NUMBER CONCENTRATIONS OF PARTICLES THAT CAN BE ACTIVATED TO BECOME CLOUD DROPLETS. ANTHROPOGENIC EMISSIONS OF SO2 AND NOX IN THE UNITED STATES (US) HAVE BEEN SIGNIFICANTLY REDUCED OVER THE PAST DECADES. HOWEVER A LOGICAL AND IMPORTANT QUESTION "HOW THE EMISSION REDUCTION MAY AFFECT THE IMMEDIATE PRECURSORS OF PARTICLE FORMATION AND HENCE NUCLEATION RATES AND PARTICLE NUMBER CONCENTRATIONS THAT IS IMPORTANT FOR AEROSOL-CLOUD INTERACTIONS?" IS YET TO BE ANSWERED. HERE WE PROPOSE TO STUDY THE PROCESSES CONTROLLING LONG-TERM TRENDS OF PARTICLE NUMBER CONCENTRATIONS IN THE US AND THE ASSOCIATED PHYSICO-CHEMICAL AND CLIMATIC IMPACTS. THE PROPOSED RESEARCH RELIES HEAVILY ON MODEL SIMULATIONS USING A STATE-OF-ART GLOBAL CHEMISTRY TRANSPORT MODEL (GEOS-CHEM) WITH SIZE-RESOLVED ADVANCED PARTICLE MICROPHYSICS (APM) AND GASES AND AEROSOL DATA FROM AURA SATELLITE RETRIEVALS AND NASA AIRBORNE FIELD CAMPAIGNS. SPECIFIC RESEARCH TASKS OF THE PROJECT INCLUDE: (1) EVALUATION AND IMPROVEMENT OF GEOS-CHEM/APM SIMULATIONS OF KEY AEROSOLFORMATION PRECURSORS; (2) KEY PROCESSES CONTROLLING NEW PARTICLE FORMATION CONDENSATION NUCLEI (CN) AND CCN NUMBER CONCENTRATIONS IN THE US; (3) LONG TERM TRENDS OF CONCENTRATIONS OF NUCLEATION PRECURSORS NUCLEATION RATES TOTAL PARTICLE NUMBER AND CCN CONCENTRATIONS IN THE US; AND (4) IMPLICATIONS OF LONG TERM CCN CHANGES FOR CLOUD OPTICAL DEPTH (COD) AEROSOL INDIRECT RADIATIVE FORCING AND TROPOSPHERIC OXIDATION CAPACITY. THROUGH SENSITIVITY STUDIES AND COMPARISONS WITH RELEVANT MEASUREMENTS WE SEEK TO EVALUATE AND IMPROVE THE MODEL PERFORMANCE IN TERM OF CAPTURING BOTH LONG-TERM TRENDS AND SHORT-TERM VARIATIONS OF THE CONCENTRATIONS OF KEY AEROSOL FORMATION PRECURSORS IN THE US. WE WILL ASSESS THE ABILITY OF RECENTLY DEVELOPED NUCLEATION SCHEMES IN EXPLAINING THE CN/CCN NUMBER CONCENTRATIONS AND PARTICLE SIZE DISTRIBUTIONS (PSDS) AT DIFFERENT ALTITUDES MEASURED DURING NASA SUB-ORBITAL FIELD CAMPAIGNS IN DIFFERENT PARTS OF THE US. WE WILL THEN EXPLORE THE IMPACTS OF THE EMISSION REDUCTIONS SINCE 2000 ON PARTICLE NUMBER CONCENTRATIONS AT DIFFERENT ALTITUDES IN THE US AND THE ASSOCIATED PHYSICO-CHEMICAL AND CLIMATIC IMPACTS. THE PROPOSED WORK MATCHES VERY WELL THE FOLLOWING RESEARCH OBJECTIVES OF AURA SCIENCE TEAM AND ATMOSPHERIC COMPOSITION MODELING AND ANALYSIS PROGRAM UNDER SOLICITATION NNH16ZDA001N-ACMAP: (1) USE OF AURA SATELLITE AND NASA AIRBORNE CAMPAIGNS DATA; (2) TROPOSPHERIC AIR QUALITY AND OXIDATION EFFICIENCY; (3) IMPACTS OF POLLUTION-GENERATED AEROSOLS ON CLOUD PROPERTIES; (4) CHEMISTRY-CLIMATE INTERACTIONS; AND (5) LONG-TERM TRENDS IN ATMOSPHERIC COMPOSITION.