Nonlinear optical studies of terpene-functionalized silica and its interactions with ozone as models for tropospheric aerosol chemistry

Terpenes emitted from vegetation can become oxidized and form molecular films on tropospheric aerosols. These greasy olefinic coatings can be oxidized by ozone and may influence the microphysics of cloud formation and the earth's climate. Using a laboratory approach that combines organic synthesis with nonlinear optical spectroscopy, we utilized vibrational broadband sum frequency generation (SFG) to survey a number of terpene-modified glass surfaces and track their interactions with ozone in real time. Exposure of these surfaces to tropospherically relevant amounts of ozone at 1 atm total pressure and 296 K yield initial reactive uptake coefficients that are significantly higher than those measured in corresponding gas phase reactions and correlate with the accessibility of the C=C double bonds at the surface. The intensity changes in the olefinic =C-H stretch and aliphatic C-H stretching region of surface vibrational spectra were used to characterize surface-bound product species. Combined with a histogram analysis of contact angle measurements carried out before and after ozonolysis, our kinetic and spectroscopic studies suggest a reaction pathway involving vibrationally hot Criegee intermediates that strongly compete with pathways that involve thermalized surface species, a chemical insight which may help reduce uncertainties associated with aerosols when included in global climate change models.