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The behaviour of secondary organic aerosols (SOA) in the atmosphere is highlydependent on their thermal properties. Here we investigate the volatility ofSOA formed from alpha-pinene, beta-pinene and limonene upon ozone- andOH-induced oxidation, and the effect of OH-induced ageing on the initiallyproduced SOA. For all three terpenes, the ozone-induced SOA was less volatilethan the OH-induced SOA. The thermal properties of the SOA were described usingthree parameters extracted from the volatility measurements: the temperature atwhich 50 per cent of the volume has evaporated (TVFR0.5), which is used as ageneral volatility indicator; a slope factor (SVFR), which describes thevolatility distribution; and TVFR0.1, which measures the volatility of theleast volatile particle fraction. Limonene-derived SOA generally had higherTVFR0.5 values and shallower slopes than SOA derived from alpha- andbeta-pinene. This was especially true for the ozone-induced SOA, partiallybecause the ozonolysis of limonene has a strong tendency to cause SOA formationand to produce extremely low volatility VOCs (ELVOCs). Ageing by OH exposuredid not reduce TVFR0.5 for any of the studied terpenes but did increase thebreadth of the volatility distribution by increasing the aerosols heterogeneityand contents of substances with different vapour pressures, also leading toincreases in TVFR0.1. This stands in contrast to previously reported resultsfrom smog chamber experiments, in which TVFR0.5 always increased with ageing.These results demonstrate that there are two opposing processes that influencethe evolution of SOAs thermal properties as they age, and that results fromboth flow reactors and static chambers are needed to fully understand thetemporal evolution of atmospheric SOA thermal properties.

Ozone and OH-induced oxidation of monoterpenes: Changes in the thermal properties of secondary organic aerosol (SOA)