AEROSOL PARTICLE CONCENTRATIONS DUE TO ANTHROPOGENIC EMISSIONS INFLUENCE CLIMATE CHANGE IN-DIRECTLY BY ACTING AS CLOUD CONDENSATION NUCLEI (CCN) ALTERING CLOUD PROPERTIES ALBEDO AND PRECIPITATION. AEROSOL ALBEDO EFFECT OR FIRST INDIRECT RADIATIVE FORCING (FIRF) REMAINS TO BE ONE OF THE PRIMARY SOURCES OF UNCERTAINTY IN ASSESSING CLIMATE CHANGE. OBSERVATION BASED ESTIMATES OF FIRF HAVE BEEN MADE POSSIBLE THROUGH THE ADVANCEMENT OF REMOTE SENSING FROM SPACE. COM-PARED TO MODEL-BASED FIRF THE OBSERVATION-BASED VALUES ARE SIMILAR IN MAGNITUDE OVER OCEANS BUT ARE GENERALLY MUCH LOWER (LESS NEGATIVE) IN MAJOR INDUSTRIAL REGIONS. A RECENT STUDY SHOWS AN OPPOSITE AEROSOL-CLOUD CORRELATION BETWEEN LAND AND OCEAN NAMELY CLOUD DROPLET EFFECTIVE RADIUS (CER) IS POSITIVELY CORRELATED WITH AEROSOL INDEX (AI) OVER THREE MAJOR INDUSTRIAL REGIONS BUT NEGATIVELY CORRELATED OVER THEIR ADJACENT OCEANS. THE POSITIVE CERAI SLOPE ALSO KNOWN AS ANTI-TWOMEY EFFECT MAY EXPLAIN THE WEAKER OBSERVATION-BASED FIRF. WHILE THE ANTI-TWOMEY EFFECT HAS BEEN REPORTED IN A NUMBER OF PREVIOUS PUBLICATIONS THE PHYSICAL REASONS BEHIND THIS REMAIN UNCLEAR. ALL PREVIOUS STUDIES ON AEROSOL-CLOUD INTERACTIONS THROUGH ANALYSIS OF THE CONNECTION BETWEEN CER AND AI (OR AEROSOL OPTICAL DEPTH (AOD)) LARGELY RELY ON THE ASSUMPTION THAT AI OR AOD CAN BE USED AS A PROXY FOR CCN. HOWEVER AI OR AOD IS A COLUMN INTEGRATED PARAMETER WHILE CLOUDS ARE MAINLY INFLUENCE BY AEROSOLS SURROUNDING THE CLOUDS. CONSIDERING THAT AOD OR AI IS GENERALLY DOMINATED BY AEROSOL PARTICLES WITHIN THE LOW BOUNDARY LAYER OF ATMOSPHERE AND THE CLOUD PROPERTIES OBSERVED FROM SATELLITES ARE MORE ASSOCI-ATED WITH THOSE AT CLOUD TOPS IT IS NECESSARY TO EXAMINE THE IMPACT OF AEROSOL VERTICAL DISTRIBU-TIONS ON THE RELATIONSHIP BETWEEN AEROSOL AND CLOUD PROPERTIES. IN THIS PROJECT WE PROPOSE TO EM-PLOY CLOUDSAT AND CALIPSO DATA PRODUCTS COMBINED WITH MODEL SIMULATIONS TO STUDY PARTICLE VERTICAL DISTRIBUTIONS AND IMPLICATIONS FOR CLOUD PROPERTIES AND AEROSOL FIRST INDIRECT EFFECT. OUR FIRST TASK IS TO INVESTIGATE AEROSOL VERTICAL DISTRIBUTION AND IMPLICATION OF USING AOD OR AI AS A PROXY FOR CCN. OUR SECOND TASK IS TO LOOK INTO VERTICAL DISTRIBUTIONS OF CLOUD DROPLET NUMBER CONCENTRATION (CDNC) AND CER AND THEIR CORRELATIONS WITH VERTICAL PROFILES OF AEROSOL EXTINCTION COEFFICIENTS AND CCN. OUR THIRD TASK IS TO DERIVE AEROSOL FIRF VALUES BASED ON CALIPSO AND CLOUDSAT AND COMPARE THEM WITH THOSE BASED ON MODIS AND MODEL SIMULATIONS. WE SEEK TO RECONCILE SOME OF DIFFERENCES IN FIRF VALUES DERIVED FROM OBSERVATION AND MODEL SIMULATIONS AND DELINEATE THE UNDERLYING MECHANISMS BEHIND THE POSITIVE OR NEGATIVE CORRELATIONS BETWEEN AOD AND CER IN DIFFERENT REGIONS. THE PROPOSED WORK WILL LEAD TO A BETTER UNDERSTANDING OF AEROSOL-CLOUD INTERACTIONS AND REDUCE THE UNCERTAINTY IN AEROSOL INDIRECT CLIMATE FORCING. OUR PROPOSED STUDY THAT UTILIZES CALIPSO/CLOUDSAT AND OTHER A-TRAIN DATA PRODUCTS TO ADDRESS THE UNCERTAINTY IN AEROSOL INDIRECT EFFECTS ARE HIGHLY RELEVANT TO THE RESEARCH AND STRATEGIC OBJECTIVES OF THE CLOUDSAT AND CALIPSO SCIENCE TEAM RECOMPETE.