Heterogeneous reactions on PSC

Figure 5.57. Meridional distribution of the CIO mixing ratio (ppbv) in July based on the measurements by the UARS/MLS instrument. Elevated levels in the lower stratosphere in winter at high latitudes (here south of 60°S between 15 and 28 km) are associated with heterogeneous reactions on PSC particles. (Courtesy of W. Randel, NCAR).

A unique textbook focused on physics and chemistry in the stratosphere and mesosphere. Dynamics, transport, radiation, and chemical composition are explained in each chapter. Particularly regarding the ozone layer, data from models and observations are explained in detail including gas phase and heterogeneous reactions on PSCs in the chapters of chemical composition and ozone perturbation. 

As for the heterogeneous reaction on PSC, the review have been made in the NASA/JPL panel evaluation No. 17 (Sander et al. 2011), and the IP AC subcommittee report Vol. V (Crowley et al. 2010) and data sheet (WaUington et al. 2012), and recommended values for many reactive uptake coefficients are given. Table 6.3 shows the values of reactive uptake coefficients for Cl-compounds on PSC based on these evaluation reports. 

As other heterogeneous reactions on PSCs, the reactions of N2O5,... 

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. 

CIO and decrease in HCI as the temperature fell, even though PSCs were not present (although it is possible that they were present at some earlier time). These observations were shown to be consistent with heterogeneous reactions on liquid binary and ternary solutions, with the temperature dependence reflecting the increased reaction probability for HCI + C10N02 due to increased solubility of HCI at the lower temperatures. 

It is important to note that gas-phase BrON02 can be converted into HOBr by heterogenous reaction on the surface of sulfate aerosol and PSC particles. 

While the many heterogeneous reactions in the troposphere so far described plays a complementary role to the homogeneous gas phase reactions, heterogeneous reactions on polar stratospheric cloud (PSC) are of primary importance for the formation of stratospheric ozone hole. 

Br-containing species (Finlayson-Pitts and Pitts 2000), only the Cl-containing species are described in this chapter as heterogeneous halogen reactions on PSC. 

The most important heterogeneous reaction with PSCs is the CI2 release into the gas phase by the reaction of C10N02(g) in the gas phase and HCl(s) on the solid surface (Sect. 6.5.5). 

For the heterogeneous reactions of HCl on PSCs and aerosols to be important, there must be mechanisms to continuously provide HCl to the surface. This could occur, for example, if HCl is sufficiently soluble in ice and if it diffuses at a sufficient rate from the bulk to the surface. However, the solubility and diffusion rates have been shown to be sufficiently small that these processes are not expected to be important under stratospheric conditions (see Wolff and Mulvaney, 1991 Domine et al., 1994 and Thibert and Domine, 1997). 

As discussed earlier, it is now well established that a variety of heterogeneous reactions can occur on such aerosols as well as on PSCs. These include the following ... 

Tables on the "reaction probalility or "uptake coefficient" have been summarized for various heterogeneous reactions in a recent review article [87], and by the IUPAC [88] and NASA-JPL [86] evaluation teams. For the purpose of this article, a rough comparison is made of the uptake rates for the reactions (1) to (5) on the different type surfaces. Three major type of surfaces have been considered a) NAT, or Type I PSC, b) Water ice, or Type II PSC and c) sulfuric acid aerosol, which is normally a liquid surface generally composed of 60-80 wt % H,S04 and 40-20 wt % H,0 also considered is the solid form SAT (sulfuric acid tetrahydrate) with a composition of 57.5 wt % H,S04. The importance of chlorine activation on sulfuric acid solutions has been demonstrated in a recent article [89]. Halogen activation on seasalt material will shortly be reviewed as part of the tropospheric processes.

A critical step in the annual depletion of polar ozone is the activation of chlorine on the surface of PSC particles, i.e., the conversion of non-ozone-destroying chlorine forms such as CIONO2 into reactive, ozone-destroying chlorine compounds (Abbatt and Molina 1993 Anderson 1995). Critical heterogeneous reactions, for example, are ... 

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)... 

Gas-phase chemistry associated with the ClOj, and NO cycles is not capable of explaining the polar ozone hole phenomenon. Heterogeneous reactions occurring on PSCs play the pivotal role in polar ozone depletion (McElroy et al., 1986 Solomon et al., 1986 Molina, 1991). The ozone hole is sharply defined between about 12 and 24 km altitude. Polar stratospheric clouds occur in the altitude range 10 to 25 km. Ordinarily, liberation of active chlorine from the reservoir species HCl and CIONO2 is rather slow, but the PSCs promote... 

PSCs therefore provide the surface on which the heterogeneous reactions occur. If the PSC is a solid particle, then its surface area is the relevant quantity for the reaction if a supercooled liquid, then the volume of the entire particle is accessible for the reaction. In either event, the heterogeneous reaction is fast, and, furthermore, the exact chemical composition of the PSC does not appear to exert an important influence on the reaction rate. However, the abundance of PSCs, their location and persistence, is likely a function of PSC composition. As noted above, denitrification is a necessary component of the heterogeneous catalytic cycle that is, the HNO3 must be removed from the system. Denitrification is accomplished if the PSC particles are sufficiently large to fall out of the stratosphere, carrying the absorbed HNO with them. The extent to which such fallout actually occurs is still uncertain. 

As for the photolytic process, reaction (8.47) is known to be the main rout but reaction (8.48) also proceeds partly under the stratospheric conditions (Atkinson et al. 2004 Sander et al. 2011). The heterogeneous reactions of N2O5 on ice particle of polar stratospheric clouds (PSC) (Sects. 6.1.5 and 6.5.1),... 

This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

Besides the direct effects of heterogeneous reactions, equally important indirect effects of PSCs on gas phase chemistry were revealed. It was suggested that HNO3 condenses onto the aerosol particles allowing them to grow at temperatures above those at which pure ice crystals can exist ... 

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