Atmospheric aerosol is known to have a direct effect on the radiative forcing of the climate by absorbing and scattering light and an indirect effect by acting as cloud condensation nuclei. Organic aerosol (OA), sometimes the major contributor to atmospheric aerosol, contains highly oxidized, multifunctional, low vapor pressure organic compounds. Carbonyls play a significant role in the photochemistry of secondary organic aerosol (SOA) as the near-UV absorption spectra of SOA are dominated by the C=O π*←n transition. SOA photochemistry can be expected to be driven in part by the well known photochemical reactions of carbonyls such as Norrish and Yang mechanisms. Therefore, investigating a model carbonyl, such as a linear chain aldehyde, in an environment that mimics SOA should provide valuable information on the mechanism and rate of photochemical processes occurring in SOA. The pure form of an aldehyde will act as its own SOA-like organic matrix. The quantum yield of photolysis may be suppressed in the condensed-phase, but might still be significantly high to make photolysis relevant. A C₁₁ aldehyde, undecanal, is investigated in this thesis as a model for this carbonyl photochemistry in OA. Undecanal photolysis was investigated at room temperature in liquid and gas phases. Products were analyzed with gas chromatography mass spectrometry or proton transfer reaction mass spectrometry. The products, quantum yields, and rate constants of undecanal in each environment were compared. Results suggest that the loss of carbonyls due to photolysis in the condensed-phase should be just as important as the photochemistry in the gas-phase.

Aerosols can substantially impact human health, atmospheric chemistry, and climate. The composition and photochemistry of a variety of anthropogenic and biogenic primary and secondary organic aerosols (POA and SOA) have yet to be fully characterized. The composition of organic aerosols is extremely complex - they contain a variety of highly oxidized, multifunctional, low vapor pressure organic compounds. The primary focus of this thesis is on the molecular characterization of organic aerosols that are not well understood or have not been studied before, such as primary emissions from electronic cigarettes, iron (III) mediated SOA, and photooxidized biodiesel and diesel fuel SOA. Another focus of this dissertation is the effect of direct photochemical aging on the composition of organic aerosol. Direct photolysis experiments were first applied to a system that is known to have a photolabile composition, alpha-pinene ozonolysis SOA, such that characterization of a photochemical effect would be possible to quantify. Photolysis of more complex SOA that have not been studied before, photooxidized biodiesel and diesel fuel SOA, were also investigated in this thesis. Advanced high resolution mass spectrometry techniques were used in the molecular characterization of organic aerosols, including nano-Desorption Electrospray Ionization Mass Spectrometry (nano-DESI) and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR). An additional suite of online instrumentation was used to measure gas-phase composition, particle-phase composition, particle size and concentration, and absorption properties: Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS), Aerosol Mass Spectrometry (ToF-AMS), Scanning Mobility Particle Sizing (SMPS), and UV-vis spectroscopy. The molecular analysis of these aerosols provides valuable insight to the formation and photochemical behavior of unexpected, polymeric, light absorbing, and unique organosulfur species.