show in situ measurements from balloon-borne instruments, aircraft and satellite observations, combined with trajectory and chemical transport model (CTM) simulations to explore the origin, composition, physical and optical properties of aerosols in the ATAL. We show balloon-based observations from our BATAL-2015 field campaign to India and Saudi Arabia in summer 2015, including in situ measurements from COBALD back scatter sondes, and some of the first observations of size and volatility of aerosols in the ATAL layer using optical particle counters (OPCs). Back trajectory calculations initialized from CALIPSO observations point to deep convection over North India as a principal source of ATAL aerosols. Available aircraft observations suggest significant carbonaceous contributions to the ATAL, which is supported by simulations using the GEOS-Chem CTM. Source elimination studies conducted with the GEOS-Chem indicate that 70-80% of ATAL aerosols originate from South Asian sources, in contrast with some earlier studies.