The Effect of Hydrophobic Gas-phase Organics on Nucleation, Mass Loading, Volatility, Viscosity, and Oligomer Content of SOA Particles

A number of field measurements provided evidence that significant fraction of atmospheric secondary organic aerosol (SOA) mass forms from biogenic precursors through pathways that are either driven by or enhanced by anthropogenic pollution, in a phenomenon that has been termed anthropogenic-biogenic interactions. Thus far, the laboratory studies attempting to reproduce this phenomenon of focused on the effect of acidic sulfate seeds, NOx, and SO2 exert on SOA formation mechanisms and yields.

We have previously shown that when SOA particles are formed from biogenic precursors in the presence of the vapors of polycyclic aromatic hydrocarbons (PAHs), which are common atmospheric anthropogenic pollutants, these PAHs become incorporated and trapped inside the formed SOA particles. While PAHs comprise only a small fraction of the total particle mass, their presence during particle formation, drastically changes the SOA particle properties. Compared with pure SOA particles, particles formed in the presence of PAHs evaporate significantly slower, and have higher viscosity, and oligomer content.

Most recently, we found that the presence of PAHs during SOA formation also significantly increases particles number concentrations and mass loadings. For example, ozonolysis of α-pinene in the presence of pyrene yields significantly higher loadings and particle number concentrations compared to pure α-pinene SOA, especially at low precursor concentrations. For 20 ppb α–pinene, the presence of pyrene increases mass loading by a factor of 1.7 and particle number concentration by a factor of 1.2, while for 5 ppb α–pinene, the presence of pyrene increases mass loading by a factor of 5 and particles number concentration by a factor of 200.

We will discuss the chemical reactions responsible for these new findings, which provide direct evidence related to field data indicating that biogenic-anthropogenic interactions could be responsible for an increase in SOA loadings.