Sulfate cloud processing

Van Dingenen, R., F. Raes, and N. R. Jensen, Evidence for Anthropogenic Impact on Number Concentration and Sulfate Content of Cloud-Processed Aerosol Particles over the North Atlantic, J. Geophys. Res., 100, 21057-21067 (1995). 

Chloride ions from supermicron sea-salt particles are replaced by sulfate, nitrate and minor contributions of oxalate, malonate and succinate. The principal mechanisms causing accumulation of sulfate in sea-salt particles are cloud processing and, to a lesser degree, heterogeneous reactions taking place in deliquescent particles. Mechanisms for the chloride replacement by nitrate are not clear [54]. 

Sulfate aerosols are important for cloud processes and climate because they act, like sea salt aerosol, as CCN. The total number and size distribution of CCN determine the microphysical and, therefore, also radiative properties of clouds. In Section 4.02.4.4 we already discussed the possible involvement of iodine in seeding the formation of sulfate CCN. DMS is the most important precursor for SO2 and sulfate aerosol in the remote MBL. A possible link between iodine and sulfur chemistry was investigated by Chatfield and Crutzen (1990), using a reaction rate coefficient of the lO -p DMS reaction available at that time. They concluded that at that rate this reaction could play a significant role for the oxidation of DMS. In an alternative scenario with a slower rate coefficient, they found the iodine and sulfur cycles to be decoupled, in better agreement with field observations. Later the reaction turned out to be even slower than their lower limit (DeMore et al., 1997 Knight and Crowley, 2001). 

Cloud processing is a major source of sulfate and aerosol mass in general on regional and global scales. Walcek et al. (1990) calculated that, during passage of a midlatitude storm system, over 65% of tropospheric sulfate over the northeastern United States was formed in cloud droplets via aqueous-phase reactions. The same authors estimated that. 

What percentage do cloud processes contribute to sulfate formation in the upper... 

Oxidation of sulfur dioxide in aqueous solution, as in clouds, can be catalyzed synergistically by iron and manganese (225). Ammonia can be used to scmb sulfur dioxide from gas streams in the presence of air. The product is largely ammonium sulfate formed by oxidation in the absence of any catalyst (226). The oxidation of SO2 catalyzed by nitrogen oxides was important in the eady processes for manufacture of sulfuric acid (qv). Sulfur dioxide reacts with chlorine or bromine forming sulfuryl chloride or bromide [507-16 ]. 

Dutch State Mines (Stamicarbon). Vapor-phase, catalytic hydrogenation of phenol to cyclohexanone over palladium on alumina, Hcensed by Stamicarbon, the engineering subsidiary of DSM, gives a 95% yield at high conversion plus an additional 3% by dehydrogenation of coproduct cyclohexanol over a copper catalyst. Cyclohexane oxidation, an alternative route to cyclohexanone, is used in the United States and in Asia by DSM. A cyclohexane vapor-cloud explosion occurred in 1975 at a co-owned DSM plant in Flixborough, UK (12) the plant was rebuilt but later closed. In addition to the conventional Raschig process for hydroxylamine, DSM has developed a hydroxylamine phosphate—oxime (HPO) process for cyclohexanone oxime no by-product ammonium sulfate is produced. Catalytic ammonia oxidation is followed by absorption of NO in a buffered aqueous phosphoric acid... 

DMS has been observed in the marine atmosphere since the early 1970s, but it was not until the mid-1980s that there was interest in this gas as being a natural source for sulfate CCN. Sulfate aerosols are, in number terms, the dominant source of CCN. The major role clouds play in the climate system leads to possible climatic implications if changes to DMS production occurred. Furthermore, the dependence of this production on environment conditions means that scope for a feedback process arises this feedback is called the Charlson hypothesis. ... 

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. 

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

Table 16-2 presents what might be termed the minimum set of constituents that must be considered in the case of cloud/rainwater. If we consider the amount of water, L, to be fixed by atmospheric physical processes, the minimum number of input components that can vary are SO2, NH3, CO2, and whatever solute is present from the CCN, often one or another sulfate compound between H2SO4 and (NH4)2S04. Occasionally, salt particles from the ocean surface may be sufficiently abundant to provide enough solute to influence the pH via the inherent alkalinity of seawater, and we will consider that as a second, somewhat more complicated possibility. 

Fig. 16-4 pH sensitivity to SO4- and NH4. Model calculations of expected pH of cloud water or rainwater for cloud liquid water content of 0.5 g/m. 100 pptv SO2, 330 ppmv CO2, and NO3. The abscissa shows the assumed input of aerosol sulfate in fig/m and the ordinate shows the calculated equilibrium pH. Each line corresponds to the indicated amoimt of total NH3 + NH4 in imits of fig/m of cloudy air. Solid lines are at 278 K, dashed ones are at 298 K. The familiar shape of titration curves is evident, with a steep drop in pH as the anion concentration increases due to increased input of H2SO4. (From Charlson, R. J., C. H. Twohy and P. K. Quinn, Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Atmosphere." NATO Advanced Research Workshop Acid Deposition Processes at High Elevation Sites, Sept. 1986. Edinburgh, Scotland.)... 

FIGURE 9.10 Modified particle modes and growth processes for sulfate particles involving aqueous-phase reactions in low altitude fogs and in higher altitude clouds upon advection of boundary-layer air upwards. (Adapted with permission from Ondov and Wexler, 1998. Copyright 1998 American Chemical Society.)... 

A simple calculation of the lifetime of MSA in cloud water can be made using model estimates of the free radical chemistry of cloud droplets. The OH concentration in cloud water is a complex function of both the gas and aqueous phase chemistry and the dynamics of gas/liquid exchange. A recent model (21) estimated cloudwater OH concentration as 2-6 x 10 M for droplets of 5 -30/im radius. Using the rate constant measured here (4.7 x 107 M 1 s 1), this yields a lifetime of 1.2 3.5 hours. Considering that the lifetime of a nonraining cloud is on the order of a few minutes to an hour, some fraction of the MSA present could react with OH, presumably being converted to sulfate. While such a process may lower the concentratron of MSA in the droplet, it would only have a minor effect on the cloudwater sulfate levels because of the typically low MSA non-sea-saIt sulfate ratio in the aerosol entering the cloud. 

Removal of sulfate particles is dominated by wet removal processes, mostly by dissolution into cloud water concurrent with cloud formation, followed by deposition in precipitation (16). Dry... 

The processes by which clouds incorporate sulfuric and nitric acids are conveniently distinguished into two categories depending upon whether oxidation takes place in the gas phase or in the aqueous phase, as illustrated schematically in Figure 1. For an examination of gas-phase atmospheric oxidation of SO2 and NO2 see (1,2). Products of this oxidation, aerosol sulfuric acid and sulfat and nitrate salts, and gas-phase nitric acid, are expected to be rapidly and to great extent incorporated into cloud droplets upon cloud formation 0,4). 

The characterization of the factors which control the accuracy, precision, and validity of measurements made in a simulation facility for studying in-cloud chemical processes was described. An analysis of a large number of experimental data collected under widely varying conditions was performed. Cloud liquid water content, an observable principally dependent on cooling rate and reaction time, was found to be the most influential of the physical factors controlling the resultant chemistry. In order to precisely control and reproduce the physical conditions in the simulation facility, standard operating procedures and computer control were instituted. This method reduced the uncertainty of the SO2 to sulfate transformation rate by a factor of 4.4. 

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