Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high-time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models. ; Jimenez, J. L. Canagaratna, M. R. Donahue, N. M. Prevot, A. S. H. Zhang, Q. Kroll, J. H. DeCarlo, P. F. Allan, J. D. Coe, H. Ng, N. L. Aiken, A. C. Docherty, K. S. Ulbrich, I. M. Grieshop, A. P. Robinson, A. L. Duplissy, J. Smith, J. D. Wilson, K. R. Lanz, V. A. Hueglin, C. Sun, Y. L. Tian, J. Laaksonen, A. Raatikainen, T. Rautiainen, J. Vaattovaara, P. Ehn, M. Kulmala, M. Tomlinson, J. M. Collins, D. R. Cubison, M. J. Dunlea, E. J. Huffman, J. A. Onasch, T. B. Alfarra, M. R. Williams, P. I. Bower, K. Kondo, Y. Schneider, J. Drewnick, F. Borrmann, S. Weimer, S. Demerjian, K. Salcedo, D. Cottrell, L. Griffin, R. Takami, A. Miyoshi, T. Hatakeyama, S. Shimono, A. Sun, J. Y. Zhang, Y. M. Dzepina, K. Kimmel, J. R. Sueper, D. Jayne, J. T. Herndon, S. C. Trimborn, A. M. Williams, L. R. Wood, E. C. Middlebrook, A. M. Kolb, C. E. Baltensperger, U. Worsnop, D. R.