Nitric stratospheric chemistry

In addition to ice formation, salts also precipitate as these solutions are lofted to higher altitudes. A consequence of the formation of these solid phases (ice and salts) and the low-temperature eutectics of strong acids (Fig. 3.5) is that the atmospheric solutions become more and more acidic with altitude (Fig. 5.8). For example, the final elevation (temperature) examined is 11.54 km (—50 °C). At this point, the calculated concentrations of the Hubbard Brook solution are H+ = 7.55m with acid anions (Cl-, NO3, SO4-, HSOJ) = 7.91m. Similarly, for the Mt. Sonnblick solution, H+ = 6.50 m and acid anions = 6.90 m. These acidic trends are in line with stratospheric chemistries, which are predominantly sulfuric/nitric acid aerosols (Carslaw et al. 1997). For example, the total acid concentration at 20.7km in the stratosphere is 10.17m (calculated from fig. 7 in Carslaw et al. 1997), which is in line with our lower atmospheric concentrations. 

While the sulfuric acid is key nucleation precursor in the low troposphere, its contribution to the polar stratospheric chemistry is a lot more modest. Another strong acid-nitric-plays a major role as the dominant reservoir for ozone destroying odd nitrogen radicals (NOj) in the lower and middle polar stratosphere. Nitric acid is an extremely detrimental component in the polar stratosphere clouds (PSCs), where nitric acid and water are the main constituents, whose presence significantly increases the rate of the ozone depletion by halogen radicals. Gas-phase hydrates of the nitric acid that condense and crystallize in the stratosphere play an important role in the physics and chemistry of polar stratospheric clouds (PSCs) related directly to the ozone depletion in Arctic and Antarctic. 

An important consequence of reaction [3.42] is that NzO plays a certain role in the chemistry of ozone formation. Although a small part of the nitric oxide formed in this way returns into the troposphere by slow diffusion (see later Fig. 15), the majority of NO molecules takes part in stratospheric chemistry as discussed in Subsection 3.4.3. This suggests that N20 arising from the use of nitrogen containing fertilizers may pose a threat to the stratospheric 03 layer. 

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. 

Nitric oxide NO is a major oxides of nitrogen formed by the photolysis of N2O in the stratosphere, but it was not paid much attention in the past since the photolysis rate is not very large. However, the photolysis of NO forms N atoms that leads to the loss of odd nitrogen by the reaction N + N0 N2 + 0, so that it is now recognized to be important in the stratospheric chemistry. 

Paul continued to make major contributions to stratospheric chemistry. For example, he explained how nitric acid clouds cause the Antarctic ozone hole. At the same time, he also turned his attention to the troposphere, which is the air layer that connects with the biosphere and where weather and climate take place. The troposphere is also prone to air pollution, while it is cleaned by oxidation reactions. The self-cleaning capacity relies on the presence of reactive hydroxyl radicals that convert pollutant gases into more soluble compounds that are removed by rain. The primary formation of hydroxyl radicals in turn is from ozone. While most ozone is located in the stratosphere, protecting life on Earth against harmful ultraviolet radiation from the Sun, a small amount is needed in the troposphere to support the self-cleaning capacity. While previous theories had assumed that tropospheric ozone originates in the stratosphere, Paul discovered that much of it is actually chemically formed within the troposphere. The formation mechanism is similar to the creation of ozone pollution in photochemical smog . 

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). 

Most of the research to date has focused on aerosols and PSCs containing inorganic species such as nitric and sulfuric acids. While CH4 is the only hydrocarbon that is sufficiently unreactive in the troposphere to reach the stratosphere, it is oxidized to compounds such as HCHO that can be taken up into sulfuric acid particles (Tolbert et al., 1993). The effects of such uptake and subsequent chemistry are not well established. 

There is therefore concern that the ever-increasing use of synthetic nitrate fertilizers may result in further depletion of the ozone layer. Eventually, stratospheric NO is returned to the Earth as nitric acid (see Section 8.4.2), but the overall dynamics of the complex atmospheric chemistry are still not fully understood. 

Society is facing several crucial issues involving atmospheric chemistry, Species containing nitrogen are major players in each. In the troposphere, nitrogen species are catalysts in the photochemical cycles that form ozone, a major urban and rural pollutant, as well as other oxidants (references 1 and 2, and references cited therein), and they are involved in acid precipitation, both as one of the two major acids (nitric acid) and as a base (ammonia) (3, 4). In the stratosphere, where ozone acts as a shield for the... 

The reason for the dehydration and denitrification of the Antarctic stratosphere is the formation of the PSCs, whose chemistry perturbs the composition in the Antarctic stratosphere. Polar stratospheric clouds can be composed of small (< 1 pm diameter) particles rich in HNO3 or at lower temperatures (<190 K) larger (10 pm) mainly ice particles. These are often split into two categories, the so-called Type 1PSC, which contains the nitric acid either in the form of liquid ternary solutions with water and sulfuric acid or as solid hydrates of nitric acid, or Type II PSCs made of ice particles. The ice crystals on these clouds provide a surface for reactions such as... 

Globally, the oxides of nitrogen, NO (nitric oxide), NO2 (nitrogen oxide), and N2O (nitrous oxide), are key species involved in the chemistry of the troposphere and stratosphere. NO and N2O are produced mostly by microbial soil activity, whereas biomass burning is also an important source of NO. Nitric oxide is a species involved in the photochemical production of ozone in the troposphere, is involved in the chemical produaion of nitric acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in stratospheric ozone depletion and is an important greenhouse gas, with a global warming potential more than 200 times that of CO2. 

The largest variability in solar output is generally observed at the shortest wavelengths. Extreme ultraviolet radiation varies by a factor of two or more over an 11-year solar cycle, and produces nitric oxide in the thermosphere. As already mentioned, NO can be transported to the mesosphere and, to some extent, in winter even to the upper stratosphere where it can perturb upper stratospheric ozone. This is an indirect mechanism linking solar activity at short wavelengths (which do not penetrate below the lower thermosphere) to chemistry at lower altitudes. 

It is worthwhile noting here that Reactions (7.20) and (7.21) are the only definitely established chemical mechanism for producing ozone in the troposphere the other source for tropospheric ozone is downward transport from the stratosphere. Together these two sources maintain a background concentration of ozone in the troposphere of about 0.03 to 0.05 parts per million of the air by volume (ppmv). Ozone is of critical importance in the chemistry of the troposphere because, not only is it a powerful oxidant itself, it is the primary source of the hydroxyl radical (OH), which is highly reactive and of paramount importance in tropospheric chemistry. Also, as can be seen from Eq. (7.23), the concentration of ozone in the air determines the ratio of [N02(g)l to [NO(g)]. Nitric oxide, NO(g), is also a very reactive gas and of great importance in atmospheric chemistry. ... 

The research of Paul Crutzen, the third recipient of the Nobel Prize for Chemistry in 1995, involved the effect of nitric oxide (NO) on the destruction of stratospheric ozone. Unlike CFCs, which may take 50 to 100 years to diffuse into the upper atmosphere, nitric oxide is introduced directly to the stratosphere in the exhaust of high-altitude aircraft. Early in the 1970s, the United States considered construction of a large fleet of supersonic transport airplanes (SSTs), similar to the Concorde. Environmentalists argued, based in part on the work of Paul Crutzen, that to do so would significantly endanger the ozone layer. 

This phenomenon will clearly be more important at low temperatures, a regime of great current interest in atmospheric chemistry in connection with the Antarctic "ozone hole" where temperatures as low as 160 K are encountered in the lower stratosphere in winter. It is exceptionally difficult to study many important reactions in the laboratory at such temperatures because of the low vapour pressure of the species involved. Indeed, an important factor in the chemistry of the Antarctic lower stratosphere may well be die condensation of nitric acid on ice crystals at temperatures below 180 K which could result in heterogeneous reactions which would again be very hard to study in the laboratory. Furthermore, our understanding of the temperature dependence of the rates of simple bimolecular reactions with small activation energies is not such that we could confidently extrapolate measurements to temperatures down to, say, 250 K as far as 160 K. Past experience of shorter extrapolations to 220 K have exposed the risks involved. 

Ozone, in turn, can be destroyed by interaction with another photon that breaks it into an oxygen molecule (O2) and an oxygen atom (O). Stratospheric ozone also can be destroyed by reaction with other species, such as nitric oxide (NO), as shown in Eq. (4.42), and halogen atoms, such as chlorine and bromine. Chlorine and bromine atoms are released into the stratosphere from the photodegradation of haloalkanes, often called halons. Classes of haloalkanes that impact ozone chemistry include CFCs and hydrochlorofluorocarbons (HCFCs). The net concentration of ozone in the stratosphere is established by the rates of both the production and the destruction reactions. 

With regard to direct NOx formation in nuclear explosions, we consider two nuclear war scenarios. Scenario I is Ambio s reference scenario [3]. In this scenario bombs having a total yield of 5750 Mt are detonated. The latimdinal and vertical distributions of the 5.7 x 10 molecules of nitric oxide produced in these explosions are determined by the weapon sizes and targets projected for this scenario. Since most of the weapons have yields less than 1 Mt, most of the NOx is deposited in the troposphere, and the effect on the chemistry of the stratosphere is much less than if the bomb debris were deposited mainly in the stratosphere. The assumed NO input pattern for the Scenario I war is provided in Table 5.1. 

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