Interaction of water, nitric acid and hydroxyl radicals with self-assembled monolayers: A model for understanding heterogeneous processes on atmospherically relevant surfaces

Heterogeneous reactions involving surface organic films and organic aerosols are not well understood. Heterogeneous reactions e.g. the hydrolysis reaction of nitrogen dioxide (NO2) to produce nitrous acid (HONO) and nitric acid (HNO3), have been shown to depend on water adsorbed on surfaces. The molecular level understanding of water interaction on surfaces can give insight into the role of water in these heterogeneous reactions. This is critical since it can be extended to understand the effect of water uptake on the growth and optical properties of particles. In addition to this, the mechanisms and products for the interaction of atmospheric oxidants such as hydroxyl radicals OH with urban surfaces and particles is still not well understood. In this thesis, experiments are reported that targeted the following topics: (1) the amount and nature of water on atmospherically relevant surfaces as a key to understanding the chemical and physical impacts of water films on the atmosphere; (2) the form of nitric acid on surfaces in the presence of water, and (3) the kinetics and mechanism of oxidation of surface organics with hydroxyl radicals. All these studies were conducted on self-assembled monolayers (SAMs) used as a mimic for organics adsorbed on environmental urban surfaces and particles. A number of techniques were utilized including: transmission Fourier transform infrared (FTIR) spectroscopy, attenuated total reflectance (ATR) -- FTIR spectroscopy, temperature programmed desorption (TPD) mass spectrometry and molecular dynamic simulations (MD). Water adsorption studies showed no evidence for increased hygroscopicity of organic SAMs upon oxidation. Our data also showed that there is some water uptake at low relative humidities even on hydrophobic SAMs. We also demonstrated a difference in the IR spectral signature of the adsorbed water as a function of increased relative humidity. The nitric acid studies showed that the behavior of nitric acid and water on organic surfaces is different from silica surfaces; in addition, infrared spectra showed the presence of different forms of hydrated nitric acid, hydrated nitrate complexes, and fully dissolved nitrate anions. Hydroxyl radical reactions with alkene SAMs in the presence of NOx revealed the formation of carbonyls, organic nitrates and alcohols with relatively high yields. The data also show that 40% of the organic film is lost, suggesting volatilization to the gas phase. The atmospheric implications of these studies are discussed.