James F. Pankow. The thermodynamic principles according to which organic particulate matter (OPM) forms in the atmosphere have become well identified because of research progress made since about the mid 1990s. These are, ahem, known knowns. However, many unknowns exist regarding the concentrations and chemical characteristics of the biogenic and anthropogenic compounds present in the atmosphere that are important in OPM formation. In this context, since we know what we need to know more about, these are, well, known unknowns. Other known important unknowns are parameterizations according to which chemical activity coefficients for partitioning species (including ions within OPM) can be predicted for the complex particulate mixtures that form in the atmosphere. Have I been perfectly clear? For more information about this seminar, please contact: Hugo Destaillats(510) 486-5897