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

The behaviour of secondary organic aerosols (SOA) in the atmosphere is highly dependent on their thermal properties. Here we investigate the volatility of SOA formed from α-pinene, β-pinene and limonene upon ozone- and OH-induced oxidation, and the effect of OH-induced ageing on the initially produced SOA. For all three terpenes, the ozone-induced SOA was less volatile than the OH-induced SOA. The thermal properties of the SOA were described using three parameters extracted from the volatility measurements: the temperature at which 50% of the volume has evaporated (T_VFR0.5), which is used as a general volatility indicator; a slope factor (S_VFR), which describes the volatility distribution; and T_VFR0.1, which measures the volatility of the least volatile particle fraction. Limonene-derived SOA generally had higher T_VFR0.5 values and shallower slopes than SOA derived from α- and β-pinene. This was especially true for the ozone-induced SOA, partially because the ozonolysis of limonene has a strong tendency to cause SOA formation and to produce extremely low volatility VOCs (ELVOCs). Ageing by OH exposure did not reduce T_VFR0.5 for any of the studied terpenes but did increase the breadth of the volatility distribution by increasing the aerosols’ heterogeneity and contents of substances with different vapour pressures, also leading to increases in T_VFR0.1. This stands in contrast to previously reported results from smog chamber experiments, in which T_VFR0.5 always increased with ageing. These results demonstrate that there are two opposing processes that influence the evolution of SOAs’ thermal properties as they age, and that results from both flow reactors and static chambers are needed to fully understand the temporal evolution of atmospheric SOA’ thermal properties.