In-cloud reactions

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

The rate of aqueous-phase in-cloud reaction can be evaluated, for specified reagent concentrations (including pH) and physical conditions, by means of the following assumptions ... 

An in-cloud reaction of importance at night and possible also during the day is the uptake of nitric acid by gas-phase reactions NO2 + O3 —> NO3 + O2 NO3 + NO2 —> N2O5... 

Techniques are at hand to evaluate the rates of aqueous-phase acid formation reactions in clouds. Such evaluations indicate that oxidation of SO2 by H2O2 and O3 can be important in-cloud reactions for assumed representative reagent concentrations and other conditions. Rapid aqueous-phase reactions do not appear to be indicated for oxidation of nitrogen oxides to nitric acid. 

In continental clouds, hydrogen peroxide is the most important oxidant of suhur dioxide dissolved in the aqueous phase, contributing about 80% to the total oxidation rate. Ozone and peroxynitric acid oxidize up to 10% each, and the gas-phase reaction of SO2 with OH radicals adds about 3%. Clouds are estimated to occupy about 15% of the airspace in the lower troposphere. In-cloud reactions thus oxidize 70-80% of SO2 in the troposphere, the remaining 20-30% of SO2 is oxidized in the gas-phase by reaction with OH radicals in cloud-free air. 

Altieri KE, Carlton AG, Lim HJ, Turpin BJ, Seitzinger SP (2006) Evidence for oligomer formation in clouds reactions of isoprene oxidation products. Environ Sci Technol 40 4956 960... 

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylo1 melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple deterrnination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

SO2 oxidation to H2SO4 on aerosols, in cloud droplets, and by gas phase reactions following attack by OH. 

Gas-phase ion chemistry is a broad field which has many applications and which encompasses various branches of chemistry and physics. An application that draws together many of these branches is the synthesis of molecules in interstellar clouds (Herbst). This was part of the motivation for studies on the neutralization of ions by electrons (Johnsen and Mitchell) and on isomerization in ion-neutral associations (Adams and Fisher). The results of investigations of particular aspects of ion dynamics are presented in these association studies, in studies of the intermediates of binary ion-molecule Sn2 reactions (Hase, Wang, and Peslherbe), and in those of excited states of ions and their associated neutrals (Richard, Lu, Walker, and Weisshaar). Solvation in ion-molecule reactions is discussed (Castleman) and extended to include multiply charged ions by the application of electrospray techniques (Klassen, Ho, Blades, and Kebarle). These studies also provide a wealth of information on reaction thermodynamics which is critical in determining reaction spontaneity and availability of reaction channels. More focused studies relating to the ionization process and its nature are presented in the final chapter (Harland and Vallance). 

However, for other reactions an opposite trend is observed. There are undoubtedly several factors involved, which include F forming the strongest bridge but I being the best "conductor" for the electron being transferred because it is much easier to distort the electron cloud of I- (it is much more polarizable and has a lower electron affinity). Therefore, in different reactions these effects may take on different weights, leading to variations in the rates of electron transfer that do not follow a particular order with respect the identity of the anion. 

Both models apply the same chemical scheme of mercury transformations. It is assumed that mercury occurs in the atmosphere in two gaseous forms—gaseous elemental HgO, gaseous oxidized Hg(II) particulate oxidized Hgpart, and four aqueous forms—elemental dissolved HgO dis, mercury ion Hg2+, sulphite complex Hg(S03)2, and aggregate chloride complexes HgnClm. Physical and chemical transformations include dissolution of HgO in cloud droplets, gas-phase and aqueous-phase oxidation by ozone and chlorine, aqueous-phase formation of chloride complexes, reactions of Hg2+ reduction through the decomposition of sulphite complex, and adsorption by soot particles in droplet water. 

The iodide salt is used as a source of dietary iodine in table salt and animal feed in cloud seeding as a coating in cathode ray tubes as a temperature indicator and as a catalyst in organic reactions. 

In addition to gas-phase chemistry, aqueous-phase chemistry discussed in Chapter 8.C.3 taking place in clouds can also be important in remote regions. For example, modeling studies by Lelieveld and Crutzen (1990) suggest that clouds may decrease the net production of 03 by uptake of H02, dissociation to H+ + 02, and reaction of 03 with 02 in cloud droplets. 

Jaegle et al., 1998b Brunner et al., 1998 Dias-Lalcaca et al., 1998), or by uptake of HN03 in clouds. Another possibility is errors in the kinetics for NOx and NOy reactions in the models. For example, those for the OH + N02 and OH + HN03 reactions have been recently revised and bring the models and measurements into better agreement (see Chapters 7.B.1 and 7.E.2 and Problem 7.9). 

As a result, the initial SOz-OH reaction does not lead to the net loss of OH and a chain oxidation of S02 can result. Perhaps more important, the generation of H02 leads to increased H202 production. As discussed in Section C.3.e, this highly soluble gas is a major oxidant for S(IV) in the aqueous phase so that reaction (5) can affect not only gas-phase processes but also the oxidation in clouds and fogs (e.g., Stockwell, 1994). 

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the OH-SOz gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole. 

While the Henry s law constant for ozone is fairly small (Table 8.1), there is sufficient ozone present in the troposphere globally to dissolve in clouds and fogs, hence presenting the potential for it to act as a S(IV) oxidant. Kinetic and mechanistic studies for the 03-S(IV) reaction in aqueous solutions have been reviewed and evaluated by Hoffmann (1986), who shows that it can be treated in terms of individual reactions of the various forms of S(IV) in solution. That is, S02 H20, HSOJ, and SO2- each react with 03 by unique mechanisms and with unique rate constants, although in all cases the reactions can be considered to be a nucleophilic attack by the sulfur species on 03. 

It is interesting, however, that the HONO-HSO-,-reaction has been shown to form a nitrene (HON ), which Mendiara and co-workers (1992) suggest could contribute to free radical formation in clouds and fogs. 

Igawa, M., J. W. Munger, and M. R. Hoffmann, Analysis of Aldehydes in Cloud- and Fogwater Samples by HPLC with a Postcolumn Reaction Detector, Environ. Sci. Technol, 23, 556-561 (1989). 

Tliis model is simpler that the Kunii-Levenspiel model and eliminates the unsubstantiated expression for cloud-to-emulsion transfer employed by Kunii and Levenspiel (Grace, 1984). Furthermore, compared to the previous models, the introduction of the parameter yb in the model leads to better results as the assumption that there is no solids in the bubble phase may lead to the underestimation of conversion in fast reactions. For slow reactions, the value of yb is of minor importance. However, for fast reactions the model may become sensitive to this parameter and the actual conversion should be bounded between the predicted ones using the upper and lower limits of yh, i.e. 0.01 and 0.001, respectively (Grace, 1984). 

However, more than one reaction pathway may exist, in which case the rate equation will contain sums of terms representing the competing reaction pathways. For example, one of the oxidation reactions that convert the atmospheric pollutant sulfur dioxide to sulfuric acid (a component of acid rain) in water droplets in clouds involves dissolved ozone, O3 (see Sections 8.3 and 8.5) ... 

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