Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
Margaret A Tolbert
David O De Haan
Jose L Jimenez
A high vacuum Knudsen flow reactor was used to determine the reactive uptake coefficient, γ, of isoprene on sulfuric acid films as a function of sulfuric acid weight percent, temperature, and relative humidity. No discernible dependence was observed for γ over the range of temperatures (220 − 265 K) and pressures (10−7 Torr -10−4 Torr) studied. However, the uptake coefficient increased with increased sulfuric acid concentration between the range of 78 wt % (γi 10−4) and 93 wt % (γi 10−3). In addition to the Knudsen Cell, a bulk study was conducted between 60 and 85 wt % H2SO4 to quantify uptake at lower acid concentrations and to determine reaction products. After exposing sulfuric acid to gaseous isoprene the condensed phase products were extracted and analyzed using gas chromatography/mass spectrometry (GC/MS). Isoprene was observed to polymerize in the sulfuric acid and form yellow/red colored monoterpenes and cyclic sesquiterpenes. Finally, addition of water to the 85 wt % sulfuric acid/isoprene product mixture released these terpenes from the condensed phase into the gas phase. Together these experiments imply that direct isoprene uptake will not produce significant SOA; however, terpene production from the small uptake may be relevant for ultrafine particles and could affect growth and nucleation.
Several laboratory and field studies have suggested that the simple aldehyde glyoxal could be a significant source of secondary organic aerosol (SOA) in the lower troposphere. However, recent studies have found that particles in the upper troposphere also contain significant amounts of organic material, with average organic mass fractions as high as 70%. We have examined whether glyoxal could be a source of SOA in the upper troposphere. The uptake of glyoxal to aerosols generally requires the presence of liquid water. Aerosols in the upper troposphere that could have supercooled liquid on the surface are cirrus ice and particles containing hygroscopic organic material. Several studies indicate cirrus ice may be coated with supercooled liquid HNO3/H2O. Thus we have utilized a high vacuum Knudsen Cell to measure the uptake of glyoxal on ice exposed to nitric acid at temperatures and pressures relevant to the upper troposphere. Here we present kinetic and spectroscopic data that indicates uptake of glyoxal is efficient on these films and irreversible. Spectroscopic data indicates the glyoxal is oxidized to glyoxylic acid, and the presence of glyoxylic acid has been confirmed via derivatization of the products followed by gas chromatography mass spectrometry. We have used the glyoxylic acid products from the ice experiments to test whether hygroscopic organic material exposed to water and nitric vapour would also uptake glyoxal. We have found that the uptake of glyoxal and oxidation to glyoxylic acid occurs in the presence of supercooled HNO3/H2O liquid on glyoxylic acid even at water pressures below the saturation pressure of ice.
The critical saturation ratio required for heterogeneous nucleation of ice was studied on ammonium nitrate films in the presence of nitric acid. Under these conditions deliquescence of the particles was not observed; however, prior to deliquescence the uptake of H2O was observed. Fourier transform infrared reflectance absorbance spectroscopy (FTIR-RAS) indicates the water absorbed water is liquid like in nature, and the film exists as a mixed solid/liquid phase prior to ice nucleation. The films were observed to freeze prior to the predicted saturation ratio for homogeneous freezing. At temperatures lower than 200 K and nitric acid pressures greater than 10-7 Torr, nitric acid trihydrate (NAT) nucleation was observed if the saturation ratio was held below the critical saturation ratio for heterogeneous ice nucleation. The growth rate of the NAT and ice films was measured under similar conditions. NAT growth is considerably slower, and laboratory evidence indicates the growth is regulated by the pressure of nitric acid. The critical saturation ratio for ice nucleation on NAT was determined to be higher than the mixed phase films, and approached the ratio required to nucleate ice on the hydrophobic gold surface.
Connelly, Brandon Michael, "Laboratory Studies of Heterogeneous Chemistry" (2010). Chemistry & Biochemistry Graduate Theses & Dissertations. 23.