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

The effects of thermodenuder treatment on the cloud condensation nuclei (CCN) activity and elemental composition of organic particles grown by alpha-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 mu g m(-3). Particles exiting in the outflow were heated to temperatures T of up to 100 degrees 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 degrees C, the CCN activity was independent of M-org, having an effective hygroscopicity parameter kappa(org) of 0.103 +/- 0.002. At 100 degrees C, however, kappa(org) varied from 0.105 for M-org = 1.4 mu g m(-3) to 0.079 for M-org = 37 mu g 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 kappa(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.