Birgit G. Koehler

Infrared optical constants of H2O ice, amorphous nitric acid solutions, and nitric acid hydrates

We determined the infrared optical constants of nitric acid trihydrate, nitric acid dihydrate, nitric acid monohydrate, and solid amorphous nitric acid solutions which crystallize to form these hydrates. We have also found the infrared optical constants of H2O ice. We measured the transmission of infrared light through thin films of varying thickness over the frequency range from about 7000 to 500 cm−1 at temperatures below 200 K. We developed a theory for the transmission of light through a substrate that has thin films on both sides. We used an iterative Kramers-Kronig technique to determine the optical constants which gave the best match between measured transmission spectra and those calculated for a variety of films of different thickness. These optical constants should be useful for calculations of the infrared spectrum of polar stratospheric clouds.

Fourier transform‐infrared studies of thin H2SO4/H2O films: Formation, water uptake, and solid‐liquid phase changes

Fourier transform-infrared (FTIR) spectroscopy was used to examine films representative of stratospheric sulfuric acid aerosols. Thin films of sulfuric acid were formed in situ by the condensed phase reaction of SO{sub 3} with H{sub 2}O. FTIR spectra show that the sulfuric acid films absorb water while cooling in the presence of water vapor. Using stratospheric water pressures, the most dilute solutions observed were >40 wt % before simultaneous ice formation and sulfuric acid freezing occurred. FTIR spectra also revealed that the sulfuric acid films crystallized mainly as sulfuric acid tetrahydrate (SAT). Crystallization occurred either when the composition was about 60 wt % H{sub 2}SO{sub 4} or after ice formed on the films at temperatures 1-4 K below the ice frost point. Finally, the authors determined that the melting point for SAT depended on the background water pressure and was 216-219 K in the presence of 4 x 10{sub {minus}4} Torr H{sub 2}O. Their results suggest that once frozen, sulfuric acid aerosols in the stratosphere are likely to melt at these temperatures, 30 K colder than previously thought. 30 refs., 10 figs.

Characterization of model polar stratospheric cloud films using Fourier transform infrared spectroscopy and temperature programmed desorption

This study combines Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) measurements of nitric-acid/ice films representative of type I polar stratospheric clouds (PSCs). Using this combination of techniques, we are able to correlate the FTIR spectra with measurements of the film stoichiometry. The results confirm the assignments for amorphous nitric-acid/ice films and for crystalline nitric acid trihydrate (NAT), dihydrate, and monohydrate proposed by Ritzhaupt and Devlin (1991). In addition to these films, we observe a new high temperature nitric-acid/ice film which we attribute to a more stable structure of NAT with fewer defects. When low temperature crystalline NAT is heated, such as during TPD or slow annealing, the IR absorption spectrum irreversibly changes around −88° to −75°C. Future IR absorption measurements of PSCs in the atmosphere should be compared with IR spectra of both amorphous and crystalline nitric acid/ice, including both forms of NAT.

Fourier transform infrared studies of the interaction of HCl with model polar stratospheric cloud films

Heterogeneous reactions involving hydrochloric acid adsorbed on the surfaces of polar stratospheric clouds (PSCs) are postulated to contribute to polar ozone loss. Using Fourier transform infrared (FTIR) spectroscopy to probe the condensed phase, we have examined the interaction of HCl with ice and nitric acid trihydrate (NAT) films representative of types II and I PSCs, respectively. For HCl pressures in the range of 10−7 to 10−5 Torr our FTIR studies show that a small amount of crystalline HCl·6H2O formed on or in ice at 155 K. However, for higher HCl pressures we observed that the entire film of ice rapidly converted into an amorphous 4:1 H2O:HCl mixture. From HCl-uptake experiments with PHCl = 8 × 10−7 Torr, we estimate roughly that the diffusion coefficient of HCl in ice is around 2 × 10−12 cm2/s at 158 K. For higher temperatures more closely approximating those found in the stratosphere, we were unable to detect bulk HCl uptake by ice. Our experiments also detected no bulk uptake of HCl by α-NAT or β-NAT under various temperature and pressure conditions. Indirect evidence suggests that HCl adsorption onto the surface of model PSC films inhibited the evaporation of both ice and NAT by 3–5 K.

Formation of model polar stratospheric cloud films

Fourier transform infrared spectroscopy was used to examine the competitve growth of films representative of polar stratospheric clouds. These experiments show that either crystalline nitric acid trihydrate ([beta]-NAT) or amorphous films with H[sub 2]O:HNO[sub 3] ratios close to 3:1 formed at temperatures 3-7 K warmer than the ice frost point under stratospheric pressure conditions. In addition, with higher HNO[sub 3] pressures the authors observed nitric acid dihydrate (NAD) formation at temperatures warmer than ice formation. However, their experiments also show that NAD surfaces converted to [beta]-NAT upon exposure to stratospheric water pressures. Finally, the authors determined that the net uptake coefficient for HNO[sub 3] on [beta]-NAT is close to unity, whereas the net uptake coefficient for H[sub 2]O is much less. 13 refs., 4 figs., 1 tab.

A Fourier transform infrared study of the adsorption of SO2 on n‐hexane soot from −130° to −40°C

This paper focuses on the uptake of SO2 on soot at temperatures below room temperature. Oxidation on soot may provide a mechanism for the oxidation of atmospheric SO2 under conditions when the standard gas-phase and aqueous-phase mechanisms cannot explain the rapid rate of H2SO4 production. An understanding of the uptake of SO2 under dry conditions provides a useful step toward understanding the uptake and oxidation of SO2 on soot under wet conditions. We find that rapid, reversible SO2 adsorption on soot occurs within a few seconds, presumably by adsorption on the outer surfaces of the spherical soot particles. Subsequently, uptake continues slowly for over an hour, presumably by diffusion into micropores within the soot particles. We focused only on the rapid adsorption. An isothermal analysis of rapid SO2 uptake revealed that a small fraction (<1%) of adsorption sites have a strong binding affinity (ΔHdes = 42±4 kJ/mol), while the majority of adsorption sites bind SO2 more weakly (26±4 kJ/mol). The lower-limit saturation coverage of SO2 on soot is 0.3 monolayer, but the more likely value is 0.7 monolayer. The uptake coefficient is 0.002 (plus or minus factor of 2) at low coverages.

Spectroscopic Studies of PSCs

Heterogeneous reactions on polar stratospheric clouds (PSCs) have been implicated recently in Arctic and Antarctic ozone loss. The most important heterogeneous process on PSCs is thought to be reaction (1) C1ONO2 + HC1 → C12 + HNO3 (1) This reaction converts reservoir chlorine (C1ONO2, HC1 into a photochemically active form (C12). Upon photolysis of C12, chlorine radicals are released to participate in catalytic ozone destruction cycles. Laboratory, field, and modeling studies have all provided strong evidence supporting the importance of this reaction in promoting polar ozone loss. Although the occurrence of heterogeneous chemistry is well established, there are still uncertainties regarding the chemical composition of the PSCs. Knowledge of the PSC composition is important for predicting both the cloud formation frequency and the rates of subsequent heterogeneous reactions on PSCs.

Spectroscopic studies of model polar stratospheric cloud films

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric-acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid: nitric-acid monohydrate (NAM), dihydrate (NAD) and trihydrate (NAT). We also observe two distinct crystalline forms of the trihydrate, which we denote (alpha) - and (beta) -NAT. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to study the interaction of HCl with model PSC films. In this work we find that for HCl pressures in the range 10-5 - 10-7 Torr, HCl is taken up by ice at 155 K to form a thin layer of HCl (DOT) 6H2O. At 193 K, the uptake of HCl by ice was consistent with <EQ monolayer coverage. Uptake of HCl by (alpha) - and (beta) -NAT at 175 K was also consistent with <EQ monolayer coverage.