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The influence of the addition of isoprene on the volatility of particles formed from the photo-oxidation of anthropogenic–biogenic mixtures

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
(using a thermal denuder and the Filter Inlet for Gas and Aerosols iodide high-resolution time-of-flight Chemical Ionisation Mass Spectrometer – 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 a reference. The oxidation of isoprene did not result in the formation of detectable SOA particle mass in single-precursor
experiments. However, isoprene-derived products were identified in the mixed
systems, likely due to the increase in the total absorptive mass. The
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 and the volatility was reasonably predicted 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 fewer volatile particles than those measured in the α-pinene/o-cresol systems. The measured volatility in the α-pinene/o-cresol/isoprene system had an ∼6 % higher low volatile organic compound (LVOC) mass/signal compared to that predicted assuming additivity with a correspondingly lower semi-volatile organic compound (SVOC) fraction. This suggests that any effects that could increase the SOA volatility from the addition of isoprene are likely
outweighed by 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 oxidized products in mixed-precursor systems, and further mechanistic understanding is required to improve their representation in chemical transport models.
