Optical Properties, Chemical Composition, and Aqueous Photochemistry of Secondary Organic Aerosol

A large fraction of organic aerosol particles are formed as secondary organic aerosol (SOA) resulting from the condensation of partially oxidized biogenic and anthropogenic volatile organic compounds (VOCs) with gas phase oxidants such as O3, OH, NOx, and NO3. An additional pathway for SOA formation is by the photochemical aqueous processing of VOC occurring inside cloud and fog droplets, followed by droplet evaporation. Once formed, SOA can age through heterogeneous oxidation and fog photochemical processes involving the hydroxyl radical (OH) as well as various other oxidants in the atmosphere. In addition to condensed phase oxidation, SOA can also age in the atmosphere upon exposure to radiation, for many of these organic compounds are photolabile and can degrade through direct photolysis, wherein the compounds absorb radiation and break into products, and indirect photolysis, wherein absorption of solar radiation initiates chemistry through the production of non-selective oxidants such as OH. These photochemical aging processes have the potential to be on time scales that are comparable to the typical lifetimes of droplets (hours) and particles (days), making them relevant to study further for both climate and health reasons. This dissertation presents a systematic investigation of the optical properties, molecular composition, and the extent of photochemical processing in different types of SOA from various biogenic and anthropogenic VOC precursors. Chamber- or flowtube-generated SOA is made and then analyzed using high-resolution mass spectrometry (HR-MS) to observe the extent of change in the molecular level composition of the material before and after aqueous photolysis. Significant differences in the molecular composition between biogenic and anthropogenic SOA were observed, while the composition further evolved during photolysis. To study the optical properties and lifetimes of organic aerosol, spectroscopy tools such as UV-Vis is utilized. Results of this study suggest that the condensed phase photolysis of SOA can occur with effective lifetimes ranging from minutes to hours, and therefore represents a potentially important aging mechanism for SOA. The outcome of this dissertation will be improved understanding of the role of condensed-phase photochemistry in chemical aging of aerosol particles and cloud droplets

High-Resolution Mass Spectrometry and Molecular Characterization of Aqueous Photochemistry Products of Common Types of Secondary Organic Aerosols

This work presents a systematic investigation of the molecular level composition and the extent of aqueous photochemical processing in different types of secondary organic aerosol (SOA) from biogenic and anthropogenic precursors including α-pinene, β-pinene, β-myrcene, d-limonene, α-humulene, 1,3,5-trimethylbenzene, and guaiacol, oxidized by ozone (to simulate a remote atmosphere) or by OH in the presence of NO_<i>x</i> (to simulate an urban atmosphere). Chamber- and flow-tube-generated SOA samples were collected, extracted in a methanol/water solution, and photolyzed for 1 h under identical irradiation conditions. In these experiments, the irradiation was equivalent to about 3–8 h of exposure to the sun in its zenith. The molecular level composition of the dissolved SOA was probed before and after photolysis with direct-infusion electrospray ionization high-resolution mass spectrometry (ESI-HR-MS). The mass spectra of unphotolyzed SOA generated by ozone oxidation of monoterpenes showed qualitatively similar features and contained largely overlapping subsets of identified compounds. The mass spectra of OH/NO_<i>x</i>-generated SOA had more unique visual appearance and indicated a lower extent of product overlap. Furthermore, the fraction of nitrogen-containing species (organonitrates and nitroaromatics) was highly sensitive to the SOA precursor. These observations suggest that attribution of high-resolution mass spectra in field SOA samples to specific SOA precursors should be more straightforward under OH/NO_<i>x</i> oxidation conditions compared to the ozone-driven oxidation. Comparison of the SOA constituents before and after photolysis showed the tendency to reduce the average number of atoms in the SOA compounds without a significant effect on the overall O/C and H/C ratios. SOA prepared by OH/NO_<i>x</i> photooxidation of 1,3,5-trimethylbenzene and guaiacol were more resilient to photolysis despite being the most light-absorbing. The composition of SOA prepared by ozonolysis of monoterpenes changed more significantly as a result of the photolysis. The results indicate that aqueous photolysis of dissolved SOA compounds in cloud/fog water can occur in various types of SOA, and on atmospherically relevant time scales. However, the extent of the photolysis-driven change in molecular composition depends on the specific type of SOA

Aqueous Photochemistry of Secondary Organic Aerosol of α‑Pinene and α‑Humulene Oxidized with Ozone, Hydroxyl Radical, and Nitrate Radical

Formation of secondary organic aerosols (SOA) from biogenic volatile organic compounds (BVOC) occurs via O₃- and OH-initiated reactions during the day and reactions with NO₃ during the night. We explored the effect of these three oxidation conditions on the molecular composition and aqueous photochemistry of model SOA prepared from two common BVOC. A common monoterpene, α-pinene, and sesquiterpene, α-humulene, were used to form SOA in a smog chamber via BVOC + O₃, BVOC + NO₃, and BVOC + OH + NO_<i>x</i> oxidation. Samples of SOA were collected on filters, water-soluble compounds from SOA were extracted in water, and the resulting aqueous solutions were photolyzed to simulate the photochemical aqueous processing of SOA. The extent of change in the molecular level composition of SOA over 4 h of photolysis (approximately equivalent to 64 h of photolysis under ambient conditions) was assessed with high-resolution electrospray ionization mass spectrometry. The analysis revealed significant differences in the molecular composition between SOA formed by the different oxidation pathways. The composition further evolved during photolysis with the most notable change corresponding to the nearly complete removal of nitrogen-containing organic compounds. Hydrolysis of SOA compounds also occurred in parallel with photolysis. The preferential loss of larger SOA compounds during photolysis and hydrolysis made the SOA compounds more volatile on average. This study suggests that aqueous processes may under certain conditions lead to a reduction in the SOA loading as opposed to an increase in SOA loading commonly assumed in the literature

Absorption spectra and aqueous photochemistry of β-hydroxyalkyl nitrates of atmospheric interest

<div><p>Molar absorption coefficients were measured for select alkyl nitrates and β-hydroxyalkyl nitrates in methanol. The presence of the β-hydroxyl group has a relatively minor effect on the absorption spectrum in the vicinity of the weak <i>n</i> → π* transition, which is responsible for photolysis of organic nitrates in the atmosphere. For both alkyl nitrates and β-hydroxyalkyl nitrates, there is an enhancement in the absorption coefficients in solution compared to the gas-phase values. The effect of the β-hydroxyl group on the spectra was modelled with molecular dynamics simulations using an OM2/GUGA-CI Hamiltonian for ethyl nitrate and β-hydroxyethyl nitrate. The simulation provided a qualitatively correct shape of the low energy tail of the absorption spectrum, which is important for atmospheric photochemistry. The role of direct aqueous photolysis in removal of β-hydroxyalkyl nitrates in cloud and fog water was modelled using a relative rate approach, and shown to be insignificant relative to gas-phase photochemical processes and aqueous OH oxidation under typical atmospheric conditions.</p></div