UC Irvine

After secondary organic aerosol (SOA) forms, its composition changes through various aging processes. In the laboratory setting, aging is sometimes studied in dilute aqueous systems as models for chemistry occurring in cloud and fog droplets. While pure water is often a good model for cloud and fog droplets, these droplets always contain small concentrationsof inorganic salts such as ammonium sulfate or ammonium nitrate. Additionally, the aging of SOA may also occur in aerosol particles, which can include areas of concentrated aqueous solutions of these salts or semi-solid organic and inorganic phases containing less water. These differences in aging matrices can have important effects on the reaction mechanisms and kinetics of SOA aging, and it is important to understand the effects relative to dilute aqueous solutions for a comprehensive understanding of SOA evolution in the atmosphere. Atmospheric cloud water contains low concentrations of inorganic salts (0.01 to 10 mM). The first study discussed in this thesis looks at the effect of ammonium and nitrate ions on the photochemical and dark aging of α-pinene or α-humulene ozonolysis SOA in cloud water mimics. SOA particles were produced from dark ozonolysis of α-pinene or α-humulene and collected on filters, then extracted and aged for 4 h in the dark or with photolysis in pure water or aqueous solutions containing 0.15 mM sodium nitrate or ammonium nitrate. Changes in the chemical composition of the SOA during aging were monitored by direct infusion high-resolution electrospray ionization mass spectrometry. The presence of nitrate ions decreased the average carbon atom number of compounds, even in the dark, in α-pinene SOA but not α-humulene SOA. This work demonstrated that inorganic salts in cloud water-type matrices may affect aging for certain SOA types. Water in deliquesced aerosols has much higher concentrations of inorganic salts than cloud water, with ionic strengths from 1 M up to greater than 10 M. The second study in this thesis examines the effects that ammonium-containing salts at these high ionic strengths have on the decarboxylation of 3-oxocarobxylic acids. Solutions of oxaloacetic acid (OAA) – a representative 3-oxocarboxylic acid – and ammonium salts were prepared, and decarboxylation was monitored using UV-visible spectroscopy. Increasing concentrations of ammonium linearly decreased the decarboxylation lifetime of OAA. Comparing decarboxylation lifetimes and OH lifetimes suggests decarboxylation will be the dominant loss pathway for OAA and likely other 3-oxocarboxylic acids in the atmospheric aqueous phase. Additionally, the inorganic ions in the aging matrix of deliquescent aerosols can have strong effects on the aging of organics under these conditions. Both the high ionic strength conditions of deliquesced aerosols and the low water content of dry organic particles are considered in the third project discussed in this thesis. Toluene SOA was prepared in a smog chamber by photooxidation in the presence of NOx to investigate the effects of various matrices on the photodegradation of SOA that absorbs UV and visible radiation. The SOA was collected and photolyzed under different conditions: directly on the filter, dissolved in pure water, and dissolved in 1 M ammonium sulfate. UV-Visible spectroscopy and electrospray ionization high-resolution mass spectrometry coupled to liquid chromatography separation were used to observe changes in mass absorption coefficient (MAC) and composition during irradiation. The MAC changes over the irradiation time were used to determine photobleaching rates, which were fastest in water, slightly slower in 1 M ammonium sulfate, and much slower directly on the filter. Conversely, the mass spectrometry analysis revealed an efficient photodegradation of nitrophenol compounds, which are expected to be major chromophores in this type of SOA, on the filter but not in the aqueous phases. This study demonstrated that the SOA absorption coefficient lifetime with respect to photobleaching and lifetimes of individual chromophores in SOA with respect to photodegradation will depend strongly and possibly independent of each other on the matrix during aging. In the atmosphere SOA particles exist under a range of different aqueous or dry conditions, providing a range of matrices for aging. This thesis demonstrates that both dark and photolytic aging can be affected by the differences in these aging matrices compared to dilute aqueous solutions, with changes in both the aging mechanisms and rates. Understanding these effects is crucial in determining the fate of individual compounds and complex SOA particles in the atmosphere.