Reservoir species

Several authors (8,9) suggested that PSCs could play a major role in the depletion of ozone over Antarctica by promoting the release of active chlorine from its reservoir species, mainly by the following reaction ... 

One issue of interest with respect to reaction (59) is whether there is a second channel that produces the reservoir species HBr, rather than the reactive HOBr, which would have a major effect on the partitioning of reactive bromine in the stratosphere (Lary, 1996). Larichev et al. (1995) place an upper limit of 1.5% on this potential path over the temperature range 233-298 K, which is consistent with indirect estimates of an upper limit of 0.01% (Mellouki et al., 1994) as well as with measurements of HBr and upper limits for the HOBr concentration in the stratosphere (D. G. Johnson et al., 1995). Cronkhite et al. (1998) observe no dependence of the overall rate constant on pressure between 12 and 25 Torr, indicating that a third possible channel producing an H02 BrO adduct is not important. 

Similarly, the absorption cross sections for the reservoir species BrONQ2 are much larger than those of... 

Thus, the effect of heterogeneous bromine chemistry is primarily to amplify the chlorine-catalyzed destruction of ozone through the more rapid conversion of the reservoir species HC1 back into active forms of chlorine (Lary et al., 1996 Tie and Brasseur, 1996). This becomes particularly important under conditions of enhanced aerosol particles, e.g., after major volcanic eruptions. 

An example of the use of this technique is the measurement of NO and N02 made near 50°N latitude by Ridley and co-workers. Summertime measurements of NO, N02, 03, temperature, and the photolysis rate of N02 showed that NO and NO were in photochemical steady state (65). However, the abundances of NOx (NO + N02) were observed to be ten times smaller in winter than in summer at altitudes between 20 and 28 km (63). At altitudes above 28 km, the abundances of NOt were similar in both winter and summer. Considering the trajectories of air at different altitudes, they were able to determine that N205 must be the wintertime reservoir species, as was predicted in a number of previous studies. 

By far the dominating ozone losses, however, occur in the lower stratosphere, at altitudes around and below 25 km. In this height region active chlorine is tied up in the inactive reservoir species HC1 and C10N02 formed in the reactions... 

Thus due to the fact that bromine reactions are faster than chlorine reactions, and bromine sequestration in reservoir species is looser than that of chlorine, bromine, on a per molecule basis, is about SO times more efficient in destroying ozone in the lower stratosphere than chlorine. 

The Airborne Submillimeter SIS Radiometer (ASUR), operated on-board the German research aircraft FALCON, measures thermal emission lines of stratospheric trace gases at submillimeter wavelength. Measurement campaigns with respect to ozone depletion in the Arctic winter stratosphere were carried out in yearly intervals from 1992-97 to investigate the distributions of the radical chlorine monoxide (CIO), the reservoir species hydrochloric acid (HC1), the chemically inert tracer nitrous oxide (N20), and ozone (O3). The high sensitivity of the receiver allowed to take spatially well resolved measurements inside, at the edge, and outside of the Arctic polar vortex. This paper focuses on the results obtained for CIO from... 

For the situation of February 28, 1996 the chemical transport model calculates low CIO mixing ratios inside the vortex at about 20 km corresponding to a deactivation of chlorine into the reservoir species CIONO2 (M. Chipperfield, personal communication), which is in contrast to the ASUR measurement showing clearly a large CIO volume mixing ratio peak of 1.7 0.25 ppbv in the lower stratosphere of the Arctic vortex. 

Reactions (1), (2) and (4) convert stable chlorine reservoir species, CIONO, and HC1, into the more easily photolyzable species Cl, HOC1, and C1NO, (nitryl chloride), respectively. This unique chemistry of CIONO, and N,0, on the cold surfaces of the PSC-surfaces is taking place due to the low temperatures of 180 to 200 K encountered in the lower stratosphere at altitudes between 15 and 25 km in the polar vortex. At sunrise, after the polar winter, these photolabile species release Cl atoms that initiate the chain destruction of ozone according to the mechanism, which is responsible for the fast ozone depletion event occuring within a few days to several weeks [34,35] ... 

Kraabol A.G. and Stordal F. (2000). Modelling chemistry in aircraft plumes 2 The chemical conservation of NOx to reservoir species under different conditions. Atmospheric Environment, 34(23), 3951 -3962. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

The extent of the influence of NO in any given atmospheric situation depends on its sources, reservoir species and sinks. Therefore, an important atmospheric quantity is the lifetime of NO . If nitric acid formation is considered to be the main loss process for NO i.e. NO2), then the lifetime of NO ( no ) can be expressed as the time constant for reaction (2.23), the NO2 to HNO3 conversion. 

Another key feature with respect to the effectiveness of catalytic cycles is the formation of reservoir species via holding cycles. At any given time about 99% of active Cl is held as reservoir species. 

These reservoirs are of great importance to the chemistry of the stratosphere as they act to divert potential catalytic species from active to inactive forms, but they remain available to release the active catalysts again. HCl is the longest-lived and most abundant Cl reservoir species having a lifetime of about one month in the lower stratosphere. It is returned to active Cl largely via reaction with OH... 

The lifetime of CIONO2 is approximately 6 h in the lower stratosphere (< 30 km) decreasing to about an hour at 40 km owing to the increase in UV light. Figure 25 shows the key chemical interconversions of the chlorine chemistry in the stratosphere, delineating the molecules into source, active and reservoir species. 

Figure 25 Inorganic chemistry involved in the interconversion chlorine species in the stratosphere. The relationship between source gases, active species and reservoir species is illustrated...

N2O5 acts as a reservoir species in atmospheric chemistry making NO3, formed by the dissociation of N2O5, the major nighttime oxidant in the stratosphere. 

Chlorine nitrate and HCl are considered to be the most important chlorine reservoir species in the stratosphere. Iodine nitrate has also been considered as a reservoir species for iodine radicals that could destroy tropospheric ozone, but photodissociation of IONO2 to form iodine radicals is only effective at temperatures below 290 K, at higher temperatures thermal decomposition takes place which does not yield iodine radicals. ... 

Field measurements of reservoir species in the lower stratosphere and further laboratory investigations of kinetics governing catalytic chain termination processes must be pursued in order to assess with confidence the effect of further anthropogenic emissions upon the stratosphere. 

Methane is the most abundant hydrocarbon in the atmosphere. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Stratospheric oxidation of CH4 provides a means of introducing water vapor above the tropopause. Methane reacts with atomic chlorine in the stratosphere, forming HCl, a reservoir species for chlorine. Some 90% of the CH4 entering the atmosphere is oxidized through reactions initiated by the OH radical. These reactions are discussed in more detail by Wofsy (1976) and Cicerone and Oremland (1988), and are important in controlling the oxidation state of the atmosphere. Methane absorbs infrared radiation in the troposphere, as do CO2 and H2O, and is an important greenhouse gas (Lacis et al., 1981 Ramanathan et al., 1985). 

For biological threats, enviromnental transport will result in the exposure of populations of various organisms to the threat. These populations can provide a mode for subsequent long-term transport of the threat. This is a particular concern for zoonoses, diseases that normally exist in wild animals but are transmissible to humans. The populations of interest may include host or reservoir species, such as birds, mammals, reptiles, or humans, and vector species, such as arthropods. This chapter accordingly considers the use of models to understand the interactions between biological threats that have become established in natural host populations and human populations. Approaches for placing these models in a spatially explicit context in order to predict transport are also discussed. 

Reactions with water remove reactive nitrogen oxides from the gas phase and, moreover, transform nitrogen oxides into reservoir species, nitric acid and its anions. 

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