Stratospheric chemistry, halogenated

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. 

The residence time for retention of compounds within the stratosphere increases very rapidly from a low and highly variable quantity in the region just above the tropopause, to a period approaching a decade in the lower-middle stratosphere. Thus the altitude at which CFG photolysis occurs can have important consequences concerning the transfer of halogen compounds into the chemical inventory of the stratosphere, which is in turn a measure of the effectiveness of converting industrially produced halogen into active participants in the stratospheric chemistry. 

As was discussed earlier, ozone plays an important part in the chemistry of the troposphere, where its excess is harmful, and in stratospheric chemistry, where its shortage is also detrimental. Ozone can decompose by several mechanisms thermal, photochemical, homogeneous catalysis reactions and under the action of solid surfaces. In the laboratory, the latter effect can be controlled by a suitable treatment of the reactor walls, as well as by a study of the rate of reaction as a function of the surface/volume ratio. In order to eliminate photochemical and homogeneous catalysis reactions, the chemical reaction must be carried out in the absence of radiations and catalytic additives, such as halogenated substances. The mechanism put forward to interpret the thermal reaction, can be written as follows ... 

The reactions of halogen atoms and radicals are of fundamental importance in stratospheric chemistry (see Sects. 4.4 and 8.2, 8.3, and 8.4), and the halogen cycle is also of interest in the marine boundary layer in the troposphere (see Sect. 7.5). In this section, among the atmospheric reactions of halogen atoms and radicals, fundamental homogeneous reactions of Cl atoms and CIO radicals are described, and the reactions of bromine and iodine atoms and radicals are discussed in the more phenomenological discussions in Chaps. 7 and 8. 

Stratospheric chemistry is described in detail by Brasseur and Solomon (2005), including the mesosphere, and is also given in the textbooks by Wameck (1988), Brasseur et al. (1999), Finlayson-Pitts and Pitts (2000), Wayne (2000), McElroy (2002), Seinfeld and Pandis(2006). A review on the reactions of halogen radicals in the stratosphere has been given by Bedjanian and Pullet (2003), and an updated review on stratospheric ozone depletion has been provided periodically by WMO (2011). In the present chapter, chemical reaction system is described exclusively among the stratospheric chemistry in which transport and reaction are coupled together. 

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions. 

The chemistry involving NOx is closely intertwined with that of the halogens (CIO,. and BrOx) and of HO, so that the predicted effects of a given set of emissions from the HSCT depend on these species as well. Because halogen chemistry is treated in more detail in later sections, we shall focus here primarily on the reasons for the different effects of NO, emissions at different altitudes. How closely these chemistries are intertwined will be apparent in the treatment below of destruction of stratospheric ozone by chlorofluorocar-bons (CFCs) and brominated compounds. 

As a result, increasing NCI, emissions does not have a significant direct effect at lower altitudes as it does at higher ones but rather has indirect effects on the halogen and HOx cycles, which reduce the ozone destruction due to these species. The net result, then, is interference in these other ozone-destroying cycles, leading to an increase in ozone at these altitudes as seen in the model predictions in Fig. 12.7. (In the very low stratosphere, NOx can also produce 03 through the VOC-NO,. chemistry discussed in Chapter 6.)... 

In short, the chemistry of the halogens, NOx, and HOx is intimately connected. As we saw earlier with respect to the HSCT, effects on one of these can affect the other cycles significantly as well, and indeed, the overall effects on stratospheric ozone may be due mainly to these secondary interactions involving other families of compounds. 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

In Section C.3, we saw that gas-phase chlorine chemistry in the stratosphere is inextricably intertwined with bromine chemistry. Because of this close interrelationship, altering the concentrations of only one of the halogens (e.g., through controls) may not have the proportional quantitative result that might be initially expected. We explore in this section in more detail the role of brominated organics in stratospheric ozone destruction and the interrelationship with chlorine chemistry. 

Halogens are very reactive chemicals that are known to play an important role in anthropogenic stratospheric ozone depletion chemistry, first... 

Interest in the chemistry of atmospheric halogens took a steep upward surge after it was postulated that the release of industrially produced halocarbons, in particular the chlorofluorocarbons (CFCs), CFCI3, and CF2CI2, could cause severe depletions in stratospheric ozone (Molina and Rowland, 1974) by the reactions involving the CFC photolytic product radicals. Cl and CIO, as catalysts. The first stratospheric measurements of CIO did indeed show its presence in significant quantities in the stratosphere so that by the end of the 1970s USA, Canada, and the Scandinavian... 

After the involvement of halogen species in O3 destruction was shown for both the stratosphere and the Arctic boundary layer, another potentially very important region where active halogen chemistry can occur received attention the marine boundary layer (MBL). The MBL is the lowest, 500-1,000 m deep part of the troposphere that is in direct contact with the sea surface. It is separated from the free troposphere by a temperature and... 

Ozone in the lower stratosphere may in principle be affected by iodine chemistry (Solomon et al, 1994). The abundance of total iodine in the troposphere is believed to be of the order of pptv, but the fractional partitioning of iodine free radicals (I and IO) is much higher than in the case of other halogens (chlorine and even of bromine see Section 5.6.3). 

Long-term Trends and Halogen Chemistry in the Upper Stratosphere... 

Section 5.6.3 discussed the chemistries of the halogens chlorine and bromine, and outlined their interactions with one another and with ozone. More than two decades after the pioneering prediction of possible ozone destruction through humankind s use of halogenated chemicals, upper stratospheric ozone observations began to reveal systematic depletion indicative of a changing chemical state (see SPARC, 1998, and references therein). 

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