Exploring the composition and volatility of secondary organic aerosols in mixed anthropogenic and biogenic precursor systems

Abstract

Abstract. Secondary organic aerosol (SOA) formation from mixtures of volatile precursors may be influenced by the molecular interactions of the products of the components of the mixture. Here, we report measurements of the volatility distribution of SOA formed from the photo-oxidation o-cresol, α-pinene and their mixtures, representative anthropogenic and biogenic precursors, in an atmospheric simulation chamber. The combination of two independent thermal techniques (thermal denuder and the Filter Inlet for Gases and Aerosols coupled to a high resolution time of flight chemical ionisation mass spectrometer) to measure the particle volatility, along with detailed gas and particle phase composition measurements provides links between the chemical composition of the mixture and the resultant SOA volatility. The products that were only present in the SOA of the mixture had higher O:C and lower volatility compared to those deriving from the individual precursors. This suggests that new product formation can reduce the volatility in mixtures. At the same time, some of the larger molecules with lower volatility produced in the single α-pinene and o-cresol system were not present in the mixture leading to an increase of the average volatility. These opposite effects resulted the volatility distribution of the SOA of the mixture to be between those of the individual precursors. For example, compounds with effective saturation concentration less or equal than 0.01 μg m−3 represented 28, 39 and 37 % of the SOA mass in the α-pinene, o-cresol and mixed precursor experiments, respectively. We further explore the sensitivity limitations of our technique to the reported results and we show that the particle volatility can be qualitatively assessed, while caution should be held when linking the chemical composition to the particle volatility. These results provide the first detailed observations of SOA particle volatility and composition in mixed anthropogenic and biogenic systems and provides an analytical context that can be used to explore particle volatility in chamber experiments.