UC Irvine

Atmospheric particles adversely affect climate, visibility, and health and thus the need to understand their formation and growth mechanisms is essential. However, the lack of complete understanding of these mechanisms may be a result of the complexity of the interactions between gas and particle phase species. Model systems of atmospherically relevant species are often utilized to better understand the chemistry of these reactions in the atmosphere.

This dissertation work explores uptake and reaction of various atmospherically relevant reaction systems to determine if the chemical properties of the gas and condensed phases affect the mechanism of uptake. In the first approach, attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopy was used to measure the uptake of various gas phase species on different low viscosity liquids and on secondary organic aerosol (SOA) generated from the ozonolysis of α–pinene. Uptake of gas phase isobutyl nitrate (IBN) into squalane and Fomblin ® was rapid, with equilibrium established quickly. A higher concentration of gas phase IBN was needed to see uptake into Fomblin ®, which may be due to the increased number of polar groups compared to squalane. Uptake of IBN onto laboratory generated SOA was much slower than squalane, which may be indicative of the much higher viscosity of SOA. Polar compounds were readily taken up onto the clean ATR crystal, even with a self–assembled monolayer fixed to the crystal surface.

The second system of interest was the reaction between gas phase amines and solid dicarboxylic acids (diacids), which was carried out using two different techniques. The first technique utilized a flow reactor coupled to a quadrupole mass spectrometer. The second method involved a Knudsen cell interfaced to the mass spectrometer. Uptake coefficients for n–butylamine on solid C2 – C5 diacids were on the order of 10-3 – 10-2 using the flow reactor. Experiments conducted using the Knudsen cell had measured uptake coefficients from 10-1 to ≤ 10-6, depending on the number of carbon atoms in the diacid chain. Those experiments performed using the flow reactor were operated at time constants that may have been too slow to capture the initial uptake of the amine onto the diacid. This manifest itself into a lack of difference in uptake coefficients between diacids, as well as signs of surface saturation. Further exploration of this reaction using the Knudsen cell concluded that uptake coefficients for odd carbon number diacids are orders of magnitude larger than the even carbon number diacids. This difference is due to the dissimilarities in packing of the crystal structures, as well as the formation of an ionic liquid layer on the surface that is able to provide replenishment of the diacid. Subsequent studies of different primary amines, as well as dimethylamine, trimethylamine, and ammonia show that structure of the amine also affects the kinetics and formation of the ionic liquid layer.

These reaction systems show that the nature of the gas and condensed phase species both play a large role in uptake and reaction of the gas phase. Results from these experiments show that overall assumptions in the interactions between the gas and condensed phases may not be accurate in predicting concentrations of atmospheric particles. Experiments carried out on these reaction systems highlight the benefits of studying model systems in order to gain a better understanding of one component of atmospheric interactions between two species. Ultimately, these and other model systems will help build a better understanding on a molecular level of the chemistry behind particle growth in the atmosphere.