Globally, a large fraction of particulate matter is comprised of secondary organic aerosol (SOA), formed from atmospheric oxidation of organic gases. The carbon content is mostly modern, implying that biogenic emissions are an important source. However, field studies have repeatedly shown strong correlations between anthropogenic pollutants and observed SOA. In this work we examine the hypothesis of enhanced biogenic SOA formation in the presence of anthropogenic pollutants. Atmospheric models commonly assume semivolatile partitioning: primary organic aerosol from urban areas is thought to provide an organic phase for partitioning and enhance biogenic SOA formation. We conducted laboratory experiments with alpha-pinene ozonolysis in the presence of a wide variety of organic compounds, including citric acid, pinanediol, erythritol, tetraethylene glycol (TEG), and hexadecanol. Yield enhancement is observed only with TEG, which is liquid under room temperature. The lack of enhancement for other seeds are attributed to their high viscosity and the viscous nature of SOA. In addition to the phase state, mixing is also dependent on the mutual solubility, and we developed a method using Hansen solubility parameters to predict yield enhancements. We repeated the experiments with ambient particles from Toronto and again saw no signficant enhancement. Our results highlight the need to fully understand SOA phase state in order to understand the biogenic-anthropogenic interactions of SOA formation.
Another potential means of enhancing biogenic SOA is the acidity of the seed. Aerosol acidity catalyzes heterogenous and particle phase reactions, leading to low-volatility compounds. Here we investigate the gas phase interactions between anthropogenic pollutants, such as NOx and SO2, and identify the critical steps in the oxidation mechanisms that lead to acidic aerosols in biogenic environments. Our results show that anthropogenic SO2 can be a major source of aerosol acidity.