Temperature Affects Composition and Cloud Formation Activity of Secondary Organic Aerosol from β-Caryophyllene Ozonolysis

Secondary organic aerosol (SOA) constitutes the most important type of ambient particles and strongly affects tropospheric chemistry and air quality. SOA also affects climate, directly by scattering light and indirectly by acting as cloud condensation nuclei (CCN). SOA mostly forms within the atmosphere by oxidation of volatile organic compounds (VOCs) with tropospheric oxidants, such as ozone (O₃) and nitrate radicals (NO₃). The most important classes of VOCs in the troposphere are monoterpenes and sesquiterpenes. Many previous studies have focused on SOA generated from oxidation of monoterpenes, such as α-pinene, and investigated SOA properties. In contrast, much less is known about SOA formed from oxidation of sesquiterpenes, denoting the second most important class of tropospheric VOCs. In addition, these previous studies were mostly performed at room temperature (T) and there have been very few studies at T < 293 K, despite tropospheric temperature typically ranging from 220 K to 300 K. Studies with realistic SOA formed at T < 293 K are urgently needed to confirm conclusions from previous work and to better understand SOA’s impact on tropospheric chemistry and climate.

Here, we studied SOA generated from oxidation of β-caryophyllene, the most abundant sesquiterpene in the troposphere. SOA was formed in the Aarhus University Research on Aerosol (AURA) atmospheric simulation chamber via dark ozonolysis of β-caryophyllene. Experiments were performed as a function of temperature between ~258 K to 297 K, covering common tropospheric conditions. Gas- and particle-phase chemical composition was monitored online, using high-resolution mass spectrometry, while simultaneously determining SOA’s phase state and CCN activity, using a printed optical particle and cloud condensation nuclei counter, respectively.

Our results demonstrate that the reaction of β-caryophyllene with O₃ and the properties of the resulting aerosols are sensitive to the temperature at which SOA was formed. Temperature impacts the SOA composition and phase state. Interestingly, β-caryophyllene SOA formed at room T showed CCN-activity, while SOA formed at low T showed no CCN activity. We attribute this, at least in parts, to changes in SOA composition. However, changes in the phase state, observed during the experiments, of the SOA formed at different temperatures may help explain the observed changes in CCN activity. Overall, our results suggest that parameterizations based on room temperature SOA measurements frequently used to estimate SOA’s CCN ability in atmospheric models could be more uncertain than previously assumed, with possibly important implications for climate.