Reply on RC2

Abstract

In this study, we investigate the influence of isoprene on the volatility of secondary organic aerosol (SOA) formed during the photo-oxidation of mixtures of anthropogenic and biogenic precursors. The SOA particle volatility was quantified using two independent experimental techniques (use of a thermal denuder and FIGAERO-CIMS) in mixtures of α-pinene/isoprene, o-cresol/isoprene and α-pinene/o-cresol/isoprene. Single-precursor experiments at various initial concentrations and results from previous α-pinene/o-cresol experiments were used as reference. The oxidation of isoprene did not result in the formation of detectable SOA mass in single-precursor experiments, however isoprene-derived products appeared in mixed systems, likely due to the increase in the total absorptive mass. Addition of isoprene resulted in mixture-dependent influence on the SOA particle volatility. Isoprene made no major change to the volatility of α-pinene SOA particles though changes in the SOA particle composition were observed, with volatility predicted reasonably based on the additivity. Isoprene addition increased o-cresol SOA particle volatility by ~5/15 % of the total mass/signal, respectively, indicating a potential to increase the overall volatility that cannot be predicted based on the additivity. The addition of isoprene to the α-pinene/o-cresol system (i.e., α-pinene/o-cresol/isoprene) resulted in slightly less-volatile particles than those measured in the α-pinene/o-cresol systems. The measured volatility in the α-pinene/o-cresol/isoprene system had ~6 % higher LVOC mass/signal compared to that predicted assuming additivity with a correspondingly lower SVOC fraction. This suggests that any effects that could increase the SOA volatility from the addition of isoprene are likely outweighed from the formation of lower volatility compounds in more complex anthropogenic-biogenic precursor mixtures. Detailed chemical composition measurements support the measured volatility distribution changes and showed an abundance of unique-to-the-mixture products appearing in all the mixed systems accounting for around 30−40 % of the total particle phase signal. Our results demonstrate that the SOA particle volatility and its prediction can be affected by the interactions of the oxidised products in mixed precursor systems and further mechanistic understanding is required to improve their representation in chemical transport models.