The Chemical Evolution and Physical Properties of Atmospheric Organic Aerosol: A Molecular Structure Based Approach

Tropospheric aerosol plays an essential role in atmospheric chemistry, global climate and air quality. Recent measurements indicate that the organic aerosol consists of approximately half of total fine mass concentration globally. The complexity of the organic aerosol often frustrates attempts to completely characterize this material in that the oxidation of volatile organics in the atmosphere leads to thousands of stable compounds in the aerosol phase. Development of a tractable framework to consider the chemical and physical evolution of the organic aerosol is crucial for modeling the effect on global climate.

In this work, we built a 3-demensional coordinate system defined by molecular descriptors of molecular weight, heteroatom mass, and double bond equivalents (D.B.E) to better describe the key properties of the organic aerosol. Coupling with high resolution mass analyzers, unambiguous assignment of molecular formula can be allowed. The scheme maintains sufficient complexity to be compatible with quantitative structure-property relationships (QSPRs) used to predict chemical and physical properties that govern aerosol behavior. From available data, both ambient organic aerosol and laboratory generated organic aerosol frequently occupy the region characterized by < 10 D.B.E. < 600 M.W. and < 200 heteroatom mass. A QSPR analysis conducted illustrates spatial trends within the 3D space for both volatility and chemical reactivity for 31000 organic compounds considered. These trends suggest that similarity between ambient and lab-generated aerosol is a consequence of sufficient molecular lifetime, available reaction pathways, and sufficiently low volatility. Additionally, combining these results and another QSPR recently developed in our laboratory allows constraint on the real refractive index of SOA materials.