Submicron aerosols have important impacts on climate, human health, and atmospheric chemistry, but their sources, properties, and evolution remain poorly understood. The impact of secondary OA (SOA) formation on human health in urban areas can be better predicted by using recent experimental results, and accounts for ~400,000 deaths / year. The NASA ATom project provides a unique characterization of remote aerosols worldwide pole-to-pole and across the vertical extent of the troposphere. Results suggest the importance of fast OA removal by photolysis and/or heterogeneous reactions. Global models show reasonable agreement for OA, however partially for the wrong reasons as there is a strong overestimation of primary OA (POA) in most models. There is a trend of decreasing submicron aerosol pH from urban to remote regions. Remote aerosols are very acidic with a typical pH ~ 0, significantly lower than predicted by models. Finally, changes of SOA for future climate and emissions will be discussed.