Water droplets, atmospheric chemistry

Brandt and van Eldik30 review of the chemistry of sulfur(IV) oxidation, with emphasis on the catalytic role of metal ions such as Fe2+. We consider here only a simplified summary of the principal atmospheric oxidation processes. It is likely that oxidation is effected primarily through the action of hydrogen peroxide or ozone in water droplets in clouds, through the photochemical effect of ultraviolet light, or by heterogeneous catalysis of the S02-02 reaction by dust particles 9,30,31... 

As indicated in Figure 2.10, many kinds of chemical reactions occur in the atmosphere. These reactions take place in the gas phase, on atmospheric particle surfaces, within particulate water droplets, and on land and water surfaces in contact with the atmosphere. The most significant feature of atmospheric chemistry is the occurrence of photochemical reactions that take place when molecules in the atmosphere absorb energy in the form of light photons, designated hv. (The... 

It would, therefore, be interesting to examine how important the newly measured accommodation coefficients would be in the conversion of S(IV) to S(VI) in a water droplet. A simple model is set up, which considers only aqueous chemistry and gas-phase mass transfer of O3 and SO2 to a cloud droplet. At t = 0, the droplet is exposed to an atmosphere containing constant concentrations of SO2 and O3. The aqueous concentrations of S(IV) and S(Vl) are then calculated as a function of time. 

Describe a cloud chemistry simulation facility to emulate atmospheric aqueous phase interactions among gases, particles, and liquid water droplets. 

Several areas in which chemical measurement technologies have become available and/or refined for airborne applications have been reviewed in this paper. It is a selective review and many important meteorological and cloud physics measurement capabilities of relevance to atmospheric chemistry and acid deposition (e.g., measurement of cloud liquid water content) have been ignored. In particular, we have not discussed particle size spectra measurements for various atmospheric condensed phases (aerosols, cloud droplets and precipitation). Further improvements in chemical measurement technologies can be anticipated especially in the areas of free radicals, oxidants, organics, and S02 and N02 at very low levels. Nevertheless, major incremental improvements in the understanding of acid deposition processes can be anticipated from the continuing airborne application of the techniques described in this review. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

As a first step in assessing the potential importance of nanoparticle reactions, we compare the volume and surface areas of these particles with the same values from other condensed phases with known chemical effects. We first consider nanoparticle volumes. As an upper limit, we consider an urban air parcel containing 20-nm diameter nanoparticles at a number concentration of 10 cm. Under this scenario, the nanoparticle volume is a small fraction (10 of the total air parcel volume. Thus the nanoparticle reaction rate (in units of mol m -air s ) would have to be ca. 10 times as fast as the equivalent gas phase reaction (mol m -air s ) to have a comparable overall rate in the air parcel. For comparison, clouds typically have liquid water contents of 10 to 10 (volume fraction) and can have significant effects upon atmospheric chemistry (Seinfeld and Pandis 1998). For simplicity of argument, if the medium of the cloud droplets and nanoparticles are assumed similar (e.g., dilute aqueous), then the fundamental rate constants in each medium are similar. Under this condition, reactant concentrations in the nanoparticles would need to be enhanced by 10, as compared to the cloud droplets, to have equal rates. Based on this analysis, it appears unlikely that reactions occurring in the bulk of nanoparticles could affect the composition of the gas phase. 

Water droplets and particulate matter often influence the rates of chemical transformations in the atmosphere. Whereas homogeneous reactions involve only gaseous chemical species in the atmosphere, reactions involving a liquid phase or a solid surface in conjunction with the gas phase are called heterogeneous reactions. Reactions that occur much more rapidly in water than in air may occur primarily in droplets, even though the droplets constitute only a small fraction of the total atmospheric volume. Solid surfaces also can catalyze reactions that would otherwise occur at negligible rates specific examples are discussed in the following sections on acid deposition and stratospheric ozone chemistry. 

Natural sources of many common air contaminants make a contribution to the overall atmospheric pollutant loading. Oceans contribute large masses of saltwater spray droplets to the air as a result of wave action. As the water evaporates from these droplets very fine particles of salts are left suspended in the moving air, contributing to sea smell and atmospheric chemistry. Some 13 million tonnes of sulfate ion and similar masses of chloride are contributed to the atmosphere annually in this manner. 

Quantitative analysis of different reaction pathways for the transformation of aquated sulfur dioxide in atmospheric droplet systems has been a major objective of the research conducted in the principal investigator s laboratory for the last four years. Available thermodynamic and kinetic data for the aqueous-phase reactions of SO2 have been incorporated into a dynamic model of the chemistry of urban fog that has been developed by Jacob and Hoffmann (23) and Hoffmann and Calvert (39). The fog and cloud water models developed by them are hybrid kinetic and equilibrium models that consider the major chemical reactions likely to take place in atmospheric water droplets. Model results have verified that... 

At the National Institute of Chemistry (NIC), in the frame of CMD subproject of EUROTRAC-2, experimental studies of the role of soluble constituents of atmospheric aerosols in the aqueous-phase autoxidation mechanisms of S(IV) was studied. The research focused on atmospheric water droplets (clouds, fog), where soluble constituents of atmospheric particles may be important in aqueous SO2 oxidation under non-photochemical conditions. In the frame of CMD project laboratory experiments in a semi-batch continuous stirred tank reactor under controlled conditions (T, air flow rate, stirring), were made in order to study the autoxidation of S(IV)-oxides catalyzed by transition metal ions (Fe(III), Fe(II), Co(II), Cu(II), Ni(II), Mn(II)). These studies were carried out at the National Institute of Chemistry. 

Relative to the levels of the species we have been considering, water vapor is at a high concentration in the atmosphere. Liquid water, in the form of clouds and fog, is frequently present. Small water droplets can themselves be viewed as microscopic chemical reactors where gaseous species are absorbed, reactions take place, and species evaporate back to the gas phase. Droplets themselves do not always leave the atmosphere as precipitation more often than not, in fact, cloud droplets evaporate before coalescing to a point where precipitation can occur. In terms of atmospheric chemistry, droplets can both alter the course of gas-phase chemistry through the uptake of vapor species and act as a medium for production of species that otherwise would not be produced in the gas phase or would be produced by different paths at a lower rate in the gas phase (Fig. 10). Concentrations of dissolved species in cloud, fog, and rain droplets are in the micromolar range, and therefore one usually assumes that the atmospheric aqueous phase behaves as an ideal solution. 

Concentrations of substances dissolved in water droplets and present in particulate matter are of great importance in atmospheric chemistry. A commonly used unit of concentration in the chemical thermodynamics of solutions is molality, mole of solute per kilogram of solvent (mol kg-1). One advantage of the use of molality is that the value is unaffected by changes in the density of solution as temperature changes. 

Figure 2.10 Micro-grafting using atmospheric pressure plasma. (A) Activation of a fluoropolymer foil throi h a silicon stencil mask using cm atmospheric helium plasma jet. (B) After grafting of 4 VP anti reaction with iotio-methane to form a strong polyelectrolyte, distinct differences in surface chemistry are evident from differences in wetting in water vapor. (C) After immersion in water, droplets remain only in the grafted areas.

Table 5.1 classifies how chemical regimes meet in the climate system. We see that almost normal conditions occur and extreme low and high temperatures border the climate system. The chemistry described in the following chapters concerns almost these normal conditions of the climate system. We focus on the troposphere and the interfaces. For example, aqueous phase chemistry in cloud droplets does not differ principally from surface water chemistry (aquatic chemistry) and much soil chemistry does not differ from aerosol chemistry (colloidal chemistry). Plant chemistry, however, is different and only by using the generic terms (Chapter 2.2.2.S) of inorganic interfacial chemistry can we link it. The chemistry of the atmosphere is widely described (Seinfeld and Pandis 1998, Wameck 1999, Finlay-son-Pitts and Pitts 2000, Wayne 2000, Brasseur et al. 2003) and the branches in atmospheric chemistry are well defined (Fig. 5.2). 

Nahir, T. M. and G A. Dawson (1987) Oxidation of sulfur dioxide by ozone in highly dispersed water droplets. Journal of Atmospheric Chemistry 5, 373-383 Nakamura, T. (2005) Post-hydration thermal metamorphism of carbonaceous chondrites. 

In some cases when oxidising conditions are required, milder oxidants may be needed, because the hydroxyl radical can react with the solute forming adducts as well as via electron transfer. Hydroxyl radicals can be converted into milder (one-electron) oxidants by the addition of halides, thiocyanate or azide ions (reactions 8.15-8.17). In fact, halide radical reactions occur in atmospheric chemistry, particularly in urban cloud droplets, as well as in marine water radical reactions [29]. 

As discussed in Section 6.5, water vapor and the heat released and absorbed by transitions of water between the vapor state and the liquid or solid state are strongly involved in atmospheric energy transfer. Condensed water vapor in the form of very small droplets is of considerable concern in atmospheric chemistry. The harmful effects of some air pollutants—for instance, the corrosion of metals by acid-forming gases—requires the presence of water, which may come from the atmosphere. Atmospheric water vapor has an important influence on pollution-induced fog formation under some circumstances. Water vapor interacting with pollutant particulate matter in the atmosphere may reduce visibility to undesirable levels through the formation of very small atmospheric aerosol particles. 

Most of our previous studies have been devoted to reactive oxygen species (ROS) at the air-water interface because such species are ubiquitous and play a crucial role in atmospheric chemistry, in environmental processes, water treatment technologies and biochemical reactions. The complex chemistries associated to these species, and their interconnection across different reaction media, have recently been reviewed [53]. The stability of ozone, molecular oxygen, hydrogen peroxide, hydroxyl and hydroperoxyl radicals, and other related compounds at the air-water interface had been established through classical molecular dynamics simulations [54—56]. Those studies, in particular, suggested that many of the compounds could accumulate at the surface of cloud water droplets, influencing in this way the overall chemistry of the troposphere. Recenfly, combined QM/MM MD simulations have confirmed the marked aflSnity of ROS species such as HO2 [27] and ozone [30] for the air-water interface. 

To calculate gas solubility in natural geochemical systems, basic thermodynamic properties such as the Henry s law constant and, in the case of weak electrolytes the dissociation constant, must be combined with a thermodynamic model of aqueous solution behavior. An analogous approach has been used to predict mineral solubilities in concentrated brines (1). Such systems are also relevant to the atmosphere where very concentrated solutions occur as micrometer sized aerosol particles and droplets, which contain very small amounts of water relative to the surrounding gas phase. The ambient relative humidity (RH) controls solute concentrations in the droplets, which will be very dilute near 1(X)% RH, but become supersaturated with respect to soluble constituents (such as NaCl) below about 75% RH. The chemistry of the aerosol is complicated by the non-ideality inherent in concentrated electrolyte solutions. 

The chemistry that occurs in cloud and fog droplets in the atmosphere has been shown, in the last decade or so, to be highly complex. Most atmospheric species are soluble to some extent, and the liquid-phase reactions that are possible lead to a diverse spectrum of products. The aspect of atmospheric aqueous-phase chemistry that has received the most attention is that involving dissolved S02. Sulfur dioxide is not particularly soluble in pure water, but the presence of other dissolved species such as H202 or 03 displaces the dissolution equilibrium for S02, effectively... 

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