Cloud condensation nuclei (CCN) activity and oxygen-to-carbon elemental ratios following thermodenuder treatment of organic particles grown by α-pinene ozonolysis

Abstract

The effects of thermodenuder treatment on the cloud condensation nuclei (CCN) activity and elemental composition of organic particles grown by α-pinene ozonolysis were investigated. The secondary organic material (SOM) was produced in a continuous-flow chamber, with steady-state organic particle mass concentrations M_org ranging from 1.4 to 37 μg m⁻³. Particles exiting in the outflow were heated to temperatures T of up to 100 °C in a thermodenuder. The oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) ratios were measured by on-line mass spectrometry. The observed elemental ratios were fit by a linear function, given by (H:C) = −0.8 (O:C) +1.8 for 0.38 < O:C < 0.50. This fit included the dependence on both M_org and T, meaning that the single variable of post-thermodenuder M_org was sufficient as an accurate predictor for O:C(M_org(T)) and H:C(M_org(T)). This result suggests that equilibrium partitioning theory largely governed the initial volatilization in the thermodenuder. By comparison, the CCN activity had a different dependence on thermodenuder treatment. At 25 °C, the CCN activity was independent of M_org, having an effective hygroscopicity parameter κ_org of 0.103 ± 0.002. At 100 °C, however, κ_org varied from 0.105 for M_org = 1.4 μg m⁻³ to 0.079 for M_org = 37 μg m⁻³, indicating that for high mass concentration the CCN activity decreased with heat treatment. The interpretation is that the oligomer fraction of the SOM increased at elevated T, both because of particle-phase reactions that produced oligomers under those conditions and because of the relative enrichment of lower-volatility oligomers in the SOM accompanying the evaporation of higher-volatility monomers from the SOM. Oligomers have high effective molecular weights and thereby significantly influence CCN activity. The production rates of different types of oligomers depend on the types and concentrations of functional groups present in the SOM, which in turn are strongly influenced by M_org. We conclude with a hypothesis, which is supported by a detailed molecular kinetic model, that the changes in κ_org at high T were more significant at high compared to low M_org because particle-phase SOM at high M_org contained a mix of functional groups favorable to oligomerization, such as carbonyl groups.