Heterogeneous reactions, aerosols

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

Surface Chemical Analysis. Electron spectroscopy of chemical analysis (ESCA) has been the most useful technique for the identification of chemical compounds present on the surface of a composite sample of atmospheric particles. The most prominent examples Include the determination of the surface chemical states of S and N in aerosols, and the investigation of the catalytic role of soot in heterogeneous reactions involving gaseous SO2, NO, or NH3 (15, 39-41). It is apparent from these and other studies that most aerosol sulfur is in the form of sulfate, while most nitrogen is present as the ammonium ion. A substantial quantity of amine nitrogen also has been observed using ESCA (15, 39, 41). 

Static aerosol chambers have been developed for studying slower heterogeneous reactions (e.g., Zetzsch and Behnke, 1992 Anthony et al., 1995 De Haan et al., 1999). Figures 5.26 and 5.27 show one such system (De Haan et al., 1999). It consists of a 561-L stainless steel... 

Arctic at polar sunrise. The mechanism likely involves regeneration of photochemically active bromine via heterogeneous reactions on aerosol particles, the snow-pack, and/or frozen seawater. The source of the bromine is likely sea salt, but the nature of the reactions initiating this ozone loss remains to be identified. For a review, see the volume edited by Niki and Becker (1993) and an issue of Tellus (Barrie and Platt, 1997). 

Notholt et al. (1992) and Andres-Hernandez et al. (1996) measured HONO, NO, N02, and aerosol surface areas at both urban and nonurban locations. They observed that at Ispra, Italy, HONO concentrations tended to correlate with N02, NO, and aerosol surface areas. Such studies support the formation of HONO from heterogeneous reactions of N02 at the surfaces of aerosol particles, fogs, buildings, and the ground. 

Furthermore, because these reactions result in the effective removal of NOx from ozone production, by removing N02, the model also predicts that O, concentrations will decrease. Figure 7.17, for example, shows the model results for the ratio of O, (R0i) with the heterogeneous removal of N03 and N2Os included to that without these aerosol reactions. In some locations, the 03 concentrations are predicted to be as much as 30% lower than they would have been in the absence of the heterogeneous reactions. Because 03 is also the major OH source on a global scale, via its photolysis to electronically excited oxygen atoms, O( D), which react in part with gas-phase water, this also decreases the predicted OH levels. 

Mamane, Y., and J. Gottlieb, Heterogeneous Reactions of Nitrogen Oxides on Sea Salt and Mineral Particles—A Single Particle Approach, J. Aerosol Sci., 21, S225-S228 (1990). 

Michelangeli, D. V., M. Allen, and Y. L. Yung, Heterogeneous Reactions with NaCl in the El Chichon Volcanic Aerosols, Geophys. Res. Lett., 18, 673-676(1991). 

Wahner, A., T. F. Mentel, M. Sohn, and J. Stier, Heterogeneous Reaction of N205 on Sodium Nitrate Aerosol, . /. Geophys. Res., 103, 31103-31112 (1998b). 

Britton, L. G., and A. G. Clarke, Heterogeneous Reactions of Sulfur Dioxide and S02/N02 Mixtures with a Carbon Soot Aerosol, Atmos. Environ, 14, 829-839 (1980). 

We discuss in this section four key aspects of heterogeneous reactions (1) theoretical and experimental structure and reactivity relationships (2) held measurements of relative and absolute PAH decay rates in near-source ambient air and during downwind transport (3) laboratory studies of the photolysis/photo-oxidation and gas-particle interactions with 03 and NOz of key 4- and 6-ring PAHs adsorbed on model substrates or ambient aerosols and (4) environmental chamber studies of the reactions of such PAHs associated with several physically and chemically different kinds of combustion-generated aerosols (e.g., diesel soot, wood smoke, and coal fly ash). Where such data are available, we also briefly consider some toxicological ramifications of these reactions. 

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. 

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 ... 

Laboratory studies of the uptake of CIO into sulfuric acid (Martin et al., 1979, 1980), taken in light of a deficit in the inorganic chlorine budget at 17 km after the Mount Pinatubo eruption, led Jaegle et al. (1996) to propose that a heterogeneous reaction of CIO on sulfuric acid aerosols to form perchloric acid, HC104, may also occur. 

Of these heterogeneous reactions, the hydrolysis of N,Os is particularly important in midlatitudes. For example, Fig. 12.30 shows the measured NO (NO = NO + N02) to NO,. (NO,. = NO + HN03 + N2Os + ) ratio at different latitudes compared to the predicted ratio using a gas-phase model as well as to a model that incorporates the N205 hydrolysis on aerosol particles (Fahey et al., 1993). Clearly, the inclusion of this reaction is necessary to bring the measurements... 

Keim et a.l. (1996) have shown that there is a significant reduction in NO and increase in NO. and CIO in a layer above the tropopause that has increased aerosol surface areas (Fig. 12.31). They attribute this to increased heterogeneous reactions of C10N02 on particles to form HN03 and active chlorine. 

Borrmann, S S. Solomon, J. E. Dye, D. Baumgardner, K. K. Kelly, and K. R. Chan, Heterogeneous Reactions on Stratospheric Background Aerosols, Volcanic Sulfuric Acid Droplets, and Type I Polar Stratospheric Clouds Effects of Temperature Fluctuations and Differences in Particle Phase, J. Geophys. Res., 102, 3639-3648 (1997b). 

The last years attention has been also paid on the importance of heterogeneous reactions in the troposphere. On the basis of the extremely limited laboratory studies and numerous assumptions it has been calculated that scavenging of compounds such as N03, N205 and HOBr followed by reactions in clouds or/and onto aerosols can be crucial for the oxidant levels in the troposphere [24 - 27]. However, contrary to the gas phase tropospheric chemistry, it does not exist a reference scheme for heterogeneous chemistry nor any consistent compilation of recommended relevant kinetic data. Finally, although CTMs have nowadays more or less sophisticated parameterizations of heterogeneous chemistry,... 

Active chlorine capable of reacting with ozone is liberated from HC1 and C10N02 in heterogeneous reactions on the surface of sulfate aerosol and polar stratospheric cloud particles. The most important ones are... 

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.

Heterogeneous reactions on the surface of seasalt aerosols have been suggested as a potential source of atomic chlorine in the marine boundary layer [10,105]. The tropospheric relevance of the reaction of nitrogen oxide species N203 with Nad and NaBr was demonstrated in a smog chamber experiment on dry and deliquiescent NaCl aerosol and on salt solutions [74,78,106,107] ... 

Hanson, D.R., Ravishankara, A.R., and Solomon, S. (1994) Heterogeneous reactions in sulfuric acid aerosols a framework for model calculations, J. Geophys. Res. 99,3615-3629. 

Usually urban aerosol in fine fraction is neutral. However, in the smoke plumes observed in August 2006 in Helsinki, there was an excess of cations, mainly ammonium [18]. These particles can be good sites for various heterogeneous reactions involving acidic trace gases. In August 2006 smoke particles had transported only a short distance, and therefore there were probably still plenty of gaseous smoke compounds that had not yet reacted with the particles. 

---