Atmospheric Aerosol Particles and Their Precursors Studied by Sum Frequency Generation

Secondary organic aerosol (SOA) particles are formed from volatile organic compounds (VOCs), like isoprene and alpha-pinene, which are emitted and oxidized in the troposphere. They contribute to radiative forcing by reflecting or absorbing the sun's radiation, but despite their importance, the chemical processes that form these particles and determine their climate-relevant properties are poorly understood. In this work, we sought to gain a molecular-level understanding of SOA formation.;Vibrational sum frequency generation (SFG) was used to assess, with surface specificity, the chemical identity of aerosol-forming constituents and particle samples. Vapor-phase alpha-pinene and isoprene SFG spectra were compared to those of synthetic alpha-pinene- and isoprene-derived SOA particles prepared in an environmental chamber. We found a match between spectra of alpha-pinene and alpha-pinene-derived particles but not between isoprene and isoprene-derived SOA. These results were used to benchmark the analysis of natural particle samples collected during major field campaigns in boreal, tropical, and anthropogenically influenced pine forests. We found that the hydrocarbon component of particles collected in Southern Finland, where alpha-pinene dominates gas-phase emissions, is similar to that of synthetic alpha-pinene-derived SOA.;Many VOCs, including alpha-pinene, are chiral, and chirality is known to influence materials properties. We explored the possibility of stereochemical transfer from VOC precursors to SOA particles. SFG spectra of the synthetic SOA particles formed from different stereoisomeric mixes of alpha-pinene showed that stereochemical configurations may be transferred. The possibility of a chiral tracer to connect the gas phase and particle phase in time was assessed.;To elucidate the molecular origin of the SFG signals produced by particle samples, we synthesized products of isoprene photo-oxidation and of alpha-pinene ozonolysis. The spectral responses from these compounds were compared to synthetic and natural particles, to predict the molecular constituents that may be present at the particle surfaces. We found that trans-beta-IEPOX provided the best match to isoprene-derived SOA.;We analyzed sea spray aerosol (SSA) particles collected from a wave flume. Researchers have hypothesized that surfactant-like coatings may be present in the topmost layer of SSA particles. We confirmed that C--H oscillators associated with alkyl chains are present on SSA particle surfaces.