Nitric acid trihydrate

Peroxonitrite is beHeved to be present in the crystals of nitric acid trihydrate that form in the stratosphere and in Martian soil (see Extraterrestrial materials). Peroxonitrous acid may be present in mammalian blood and other biochemical systems. However, peroxonitric acid, HNO, is not known. Before the chemistry of peroxonitrous acid was understood, these two acids were sometimes confused. 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

The presence of PSCs also leads to the removal of nitrogen oxides (NO and NO2) from the gas phase. As long as there are significant amounts of NO2 it will react with chlorine monoxide (CIO) to produce chlorine nitrate (reaction 11). This species subsequently reacts with HQ on PSC surfaces to produce nitric acid (reaction 13), which remains in the condensed phase. Also, nitric acid directly condenses with water to form nitric acid trihydrate particles, hence it is not available to regenerate NO2 by photochemical processes, as it does when it is in the gas phase. 

Quinlan, M. A., C. M. Reihs, D. M. Golden, and M. A. Tolbert, Heterogeneous Reactions on Model Polar Stratospheric Cloud Surfaces Reaction of N205 on Ice and Nitric Acid Trihydrate, J. Phys. Chem., 94, 3255-3260 (1990). 

From DeMore et al. (1997) and Ravishankara and Hanson (1996) and references therein. h NAT = nitric acid trihydrate (solid) SAT = sulfuric acid tetrahydrate (solid). 

Bertram, A. K., and J. J. Sloan, The Nucleation Rate Constants and Freezing Mechanism of Nitric Acid Trihydrate Aerosol under Stratospheric Conditions, J. Geophys. Res., 103, f326f-f3265 (1998b). 

Hanson, D and K. Mauersberger, Laboratory Studies of the Nitric Acid Trihydrate Implications for the South Polar Stratosphere, Geophys. Res. Lett, 15, 855-858 (1988a). 

Pitari, G., V. Rizi, L. Ricciardulli, and G. Visconti, High-Speed Civil Transport Impact Role of Sulfate, Nitric Acid Trihydrate, and Ice Aerosols Studied with a Two-Dimensional Model Including Aerosol Physics, J. Geophys. Res., 98, 23141-23164 (1993). 

Richwine, L. J., M. L. Clapp, R. E. Miller, and D. R. Worsnop, Complex Refractive Indices in the Infrared of Nitric Acid Trihydrate Aerosols, Geophys. Res. Lett., 22, 2625-2628 (1995). 

Tabazadeh, A., and R. P. Turco, A Model for Heterogeneous Chemical Processes on the Surfaces of Ice and Nitric Acid Trihydrate Particles, J. Geophys. Res., 98, 12727-12740(1993). 

Toon, O. B and M. A. Tolbert, Spectroscopic Evidence against Nitric Acid Trihydrate in Polar Stratospheric Clouds, Nature, 375, 218-221 (1995). 

Type Ia-PSCs are solid particles composed of nitric acid/ice (approximately 50-50 wt %), especially in the form of the most thermodynamically stable nitric acid trihydrate, (NAT, HNO,.3I O) with size ranging between 0.3 and 3 pm [21-23]. The condensation temperatures for NAT under stratospheric conditions are typically 5-7 K higher than for... 

Toon, O.B., and Tolbert, M.A. (1995) Spectroscopic evidence against nitric acid trihydrate in polar stratospheric clouds. Nature 375,218-221. 

Hanson, D.R and Maneisberger, K. (1988) Laboratory studies of the nitric acid trihydrate implications for the south polar stratosphere, Geophys. Res. Lett. 15,855-858. 

During the winter season in the South Pole, a strong circumpolar wind develops in the middle to lower stratosphere. Such strong winds, known as the polar vortex, isolate the air over the polar region. In the winter there is no sunlight and the air contained in the vortex becomes very cold (i.e., temperatures below — 80°C) leading to the formation of the polar stratospheric clouds (PSC). These PSC are composed not only of water but also of nitric acid trihydrate. 

As noted in Sections 3.1 and 4.1, the atmospheric relevance of the two acids often extends beyond the issue of their surface dissociation in the conditions that we have considered here. Thus, for example, the widely occurring sulfate aerosols are concentrated sulfuric acid solutions and nitric acid trihydrate (NAT) is also clearly fairly concentrated. Elucidation of the acid ionization state of such aerosol surfaces—and thus the theoretical characterization of the mechanisms and rates of chemical reaction occurring on them—requires other tools to deal with the multiple proton transfer possibilities present in these systems. One such tool, a generalization of available reaction Monte Carlo methodologies [72] to treat such multiple proton transfers, is under development [73]. 

Theoretical studies of the coexistence of gas-phase hydrates of HNO3 under the stratospheric conditions have been carried out by Al Natsheh with co-authors [96,97]. The family of nitric acid trihydrates (NAT), which dominate the composition of the la PSCs, are considered to be thermodynamically stable. However,... 

Heterogeneous reaction rates on particles in polar stratospheric clouds (PSCs) are more difficult to estimate because of the uncertainties in the type of PSC particles present in the lower stratosphere. The particle composition, volume density and radius must be derived from a thermodynamic model (e.g., Carslaw cl at, 1994 1997). Condensation of HNO3 occurs when the partial pressure of nitric acid in air exceeds the equilibrium vapor pressure Pjjnq F°r example, in the case of nitric acid trihydrates (NAT), if partial pressures are expressed in Torr and the temperature T in Kelvin, we have (Hanson and Mauersberger, 1988)... 

Nitric acid trihydrate HNO3 3 H20 (NAT) solid crystals T < 196 K confirmed... 

Voigt, C., J. Schreiner, A. Kohlmann, P. Zink, K. Mauersberger, N. Larsen, T. Deshler, C. Kroger, J. Rosen, A. Adriani, F. Cairo, G. Di Donfrancesco, M. Viterbini, J. Ovarlez, H. Overlez, C. David, and A. Dornbrack, Nitric acid trihydrate (NAT) in polar stratospheric cloud particles. Science 390, 1756, 2000. 

As noted in Chapter 5, Toon et al (1986) and Crutzen and Arnold (1986) pointed out that the PSC particles might be composed not only of water ice but also of solid nitric acid trihydrate (NAT). Both studies noted that such composition could affect the impact on ozone in two ways (i) by reducing the amount of nitrogen oxide that could be present (i.e., not only by forming nitric acid but by removing it from the gas phase) and (it) by raising the temperature at which clouds could form, since thermodynamic analyses suggested that NAT could... 

Voight, C. et al. (2000) Nitric acid trihydrate (NAT) in polar stratospheric clouds, Science 290, 1756-1758. 

Polar stratospheric clouds have been classified into two broad types, so-called Type I and Type II (Table 4.1). Type I PSCs have been further subdivided into Type la and Type Ib. Type la PSCs have traditionally been identified as crystals of nitric acid trihydrate, HNO, 3 H2O, denoted NAT, that form once temperatures fall below about 195 K. Type lb PSCs consist of supercooled ternary solutions of HNO3/H2SO4/H2O, also forming at about the same temperature threshold. Type II PSCs are largely frozen water ice, nonspherical crystalline particles, that form at temperatures below the ice frost point. The ice frost point, for example, at 3 X 10 Torr H O is 191 K. Despite the above classification, the composition of PSCs is still uncertain (Toon and Tolbert, 1995). 

The symbols in Fig. 13 designate the compositions that were used in our experiments. They follow the calculated compositions with the exception of one droplet which was chosen to resemble pure nitric acid trihydrate (NAT). 

Work in the Atmospheric Chemistry Group at SRI and elsewhere has shown that these reactions occur efficiently on water ice and nitric acid trihydrate, the materials believed... 

The Knudsen cell reactor has been used successfully to measure reaction and uptake rates on solid and liquid surfaces, including ice, nitric acid trihydrate, soot and concentrated sulfuric acid [8,9,16,26,27]. Recent measurements of uptake and reactivity on soot surfaces are particularly intriguing. In these experiments, funded by NASA s Subsonic Assessment Program, we are investigating the impact of solid particles found in the exhaust of aircraft, i.e., soot, on stratospheric chemistry. 

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