SO2 uptake

SO2 uptake was measured at total system pressures in the range of 20 to 50 Torr, consisting of 17.5 Torr H2O vapor with the balance either helium or argon. The observed mass accommodation coefficients, 74, are plotted in Figure 2 as a function of the inverse of the calculated diffusion coefficient of SO2 in each H20-He and l O-Ar mixture. The diffusion coefficients are calculated as a sum of the diffusion coefficients of SO in each component. The diffusion coefficients for SO in He and in Ar are estimated from the diffusion coefficient of SO2 in H 0 (Dg p = 0.124 (101) by multiplying this value by the quantity (mH-/mH Q)V2, anti (mAr/m 2o) 2> respectively. The curves in Figure 2 are plots ofEquation 7 with three assumed values for 7 0.08,0.11 and 0.14. The best fit to the experimental values of is provided by 7 = 0.11. Since gas uptake could be further limited by liquid phase phenomena as discussed in the following section, 7502 = 0.11 is a lower limit to the true mass accommodation coefficient for SO2 on water. 

Other clear-air, gas-to-particle conversion processes include uptake of acidic gases by basic aerosol (e.g., SO2 uptake by sea salt or carbonate dust) and uptake of ammonia by acidic aerosol. Some such processes are reversible. An example is the co-condensation of HNO3 and NH3 to form ammonium nitrate, for which the equilibrium constant is rather strongly temperature dependent. An example of the release of species from the condensed to the gas phase is the uptake of HNO3 by sea salt, resulting in release of HCl into the gas phase. 

The solubility of SO2 in water is strongly dependent on its pH, becoming limited below pH = 4. The presence of other pollutants can be important as they affect the pH of the liquid layer on the surface, which may also be buffered by corrosion products per se. Nitric acid deposits quite readily, for example, and could lower the pH and thus inhibit SO2 uptake. On the other hand, many atmospheric particulates are basic, and the limited literature on dew chemistry (Cadle and Goblicki) (22) does not indicate acidic dew composition, (it should be noted that these data were all taken in low SO2 environments). 

For less soluble gases like SO2, uptake is a complex function not only of species properties and cloud droplet distribution, but also of other species that may participate in aqueous-phase reactions (e.g., H2O2) or control the cloud droplet pH (e.g., NH 3). 

Activated carbon obtained from coal and its ash-content-reduced derivatives (obtained by a combined treatment with acidic solutions) were used to investigate SO2 adsorption capacities [348]. The activated carbons with reduced ash content showed better SO2 uptake than the initial carbon, even at lower burn-off values. This particular behavior of low ash activated carbons was linked to the alteration in pore size distribution, and to the redistribution of part of the mineral matter in the carbonaceous matrix, which occurs during the initial acidic treatment of the raw coal [348]. 

The uptake of SO2 on activated carbon is related to the degree of its conversion to SO3, which requires the presence of oxygen. It was found that oxidation of sulfur dioxide to sulfur trioxide occurs mainly in the 0.7 nra pores [26, 27]. With an increase in the size of pores in the carbon adsorbents less SO2 is converted, which results in smaller uptake of sulfitr dioxide. The controversial results are reported regarding the effect of pore volume and SO2 uptake in the presence of oxygen. While Bagreev and coworkers reported the importance of porosity [34], Raymundo-Pinero and coworkers could not find any correlation between volume of micropores and the amount of SO2 adsorbed in the presence of oxygen [26]. 

Pig. 6.10 Idealized pore model to predict the SO2 uptake of CaO. ABFE tuid CDHG are the original pore walls before sulphation. The grey areas represent the build-up of CaS04 on the walls. Adapted from Ref. [28], Copyright 1986, with permission from Elsevier... 

Besides understanding the fundamental chemistry of small-molecule gaseous interactions, a reason to study the reactivity of transition metal thiolates with SO2 is the possibility of developing a chemical sensor for SO2. There are several ways in which chemical sensors can respond to the presence of a specific analyte, but the most practical way is a resulting color change that is easily perceptible to the naked eye. In this experiment, stodents will use color changes to detect SO2 uptake by Ni(BME-DACH) at levels as low as 100 ppm of SO2 in air. 

The rate of SO2 uptake by aj8S-[Co(tetren)OH] (23) has also been studied by stopped flow. The rate and activation parameters for SO2... 

AS = -21.0 cal moF Retardation by SOs and SO4 is observed and is ascribed to the formation of nonreactive ion pairs. The product of SO2 uptake is the O-bonded sulfito complex. At 10°C the protonated form of such a species a/3S-[Co(tetren)OS02H] [pK = 3.3) eliminates SO2 with k = 550 s", NH = 7.2 kcal mol, and AS = -20.4 cal K mol. The oxygen-bonded sulfito intermediate shows no tendency to undergo internal redox, but slowly isomerizes to its sulfur-bonded analog. 

Species sensitivitv to SOo - Plemt species vary in their sensitivity to chronic and acute effects of SO2 uptake (Last, 1982 Guderian, 1977). The relative sensitivity of plant species has been investigated using experimental determinations and observation of impacted sites. Much of the information is based on experiments using acute doses of SO2 and the correlation between sensitivity to acute and chronic effects is poor (Guderian, 1977). 

How would you expect the uptake by Co(NH3)5H20 of SO2 to compare with that of CO2 regarding rates and products ... 

Fig.6 The organo-platinum(II) complex containing N,C,N tridentate ligands reversibly binds gaseous SO2 in the solid state. The reversible reaction occurs at the Pt(ll) complex by Pt-S bond formation and cleavage. Uptake and release of SO2 does not destroy the crystalline ordering... 

Apparent photosynthetic rates in plants subjected to SO2 or NO exposures with constant pollutant concentrations, as illustrated in Figure 1, characteristically dropped rapidly upon initiation of treatment to new depressed equilibrium levels which could be maintained for several ho irs. Hydrogen fluoride, conversely, caused CO2 uptake rates to decline more gradually during fumigation. Chlorine, O3 and NO2 exposures induced inhibition rate responses which were intermediate between these... 

Although stomata in plants treated with HF and SO2 showed some tendency to close as a result of exposures which depressed apparent photosynthetic rates, these phytotoxicants inhibited CO2 uptake rates more by affecting biochemical processes within the leaves than by impeding gas transfer by inducing stomatal closure. 

Of the phytotoxic air pollutants and mixtures tested, O3 or combinations of SO2+NO2 are most likely to occur in ambient atmospheres in sufficiently high concentrations to acutely depress apparent photosynthesis. Ambient HP concentrations of the magnitudes which inhibited CO2 uptake rates in an acute, reversible manner would be rare. Studies into longer-term exposures (several days or weeks) to HP concentrations in the low ppb range have suggested that reduced photosynthesis under these conditions correlated with the amount of necrosis that developed (, ). 

In concentrations approximating present air quality standards (Table III), O3 or SO2 in combination with NO2 could measurably suppress CO2 uptake rates of sensitive plants if exposed under favorable growing conditions. In the controlled environmental chamber studies, 1-hr exposures to 10 pphm O3 (which is slightly above the primary and secondary standards — i.e., 8 pphm for 1 hr) for example, depressed alfalfa CO2 absorption rates by approximately four percent. Exposures to 15 pphm hr SO2 in combination with an equal amount of NO2 reduced uptake rates by 7 percent. Alfalfa, barley or oat canopies exposed to these pollutants singly required higher concentrations (i.e., 1- to 2-hr treatments with more than 20 pphm SO2 or 40 pphm NO2) to measurably reduce canopy uptake rates. 

In order to model the kinetics of such a process one must know the rate of gas uptake for SO2 gas as well as for the gaseous species involved in subsequent oxidation. 

The applicability of this system in the development of a crystalline SO2 sensor was studied in greater detail. While binding of gaseous substrates by nonporous crystalline materials may lead to the destruction of the long crystalline order, this did not happen in the case of 26. The crystallinity of the sensor material is maintained during the uptake and release of SO2, as was shown by time-resolved powder diffraction experiments (Figure 8). 

Therefore, local dissolution and recrystallization seem to play an important role in the gas uptake mechanism in these type of sensor materials. The coordination of SO2 to the platinum center (and the reverse reaction) is therefore likely to take place in temporarily and very locally formed solutes in the crystalline material, whereas the overall material remains crystalline. The full reversibility of the solid-state reaction was, furthermore, demonstrated with time-resolved solid-state infrared spectroscopy (observation at the metal-bound SO2 vibration, vs= 1072 cm-1), even after several repeated cycles. Exposure of crystalline samples of 26 alternat-ingly to an atmosphere of SO2 and air did show no loss in signal intensities, e.g. due to the formation of amorphous powder. The release of SO2 from a crystal of 27 was also observed using optical cross-polarization microscopy. A colourless zone (indicative of 26) is growing from the periphery of the crystal whereas the orange colour (indicative for 27) in the core of the crystal diminishes (see Figure 9). 

Figure 9.20 displays the differential enthalpy of SO2 adsorption at 353 K as a function of the probe uptake on samples with various vanadium contents and on pure Y-AI2O3. 

Here, we only show two pulses of O2 (m/c=32) and CO (m/e=28) from a long string of essentially idenlical, alternating pulses. A complete discussion of this data is given in the original reference however, simple observation indicates that much more CO2 (m/e=44) is formed upon the introduction of CO at -1500 sec on the SO2-poisoned catalyst compared to the unpoisoned catalyst. In this example, approximately 1000 mol/g of oxygen could be removed from the poisoned catalyst compared to only 700 fimol/g on the unpoisoned catalyst. This increased, reversible oxygen uptake on the S02-poisoned catalyst is due to oxidation and reduction of... 

The pH of sea salt aerosol is an important property as many important aqueous phase reactions are pH dependent. For example, oxidation of S(IV) (SO2 + HSOs + SO ) by O3 is only important for pH of more than 6. Sea salt aerosol is buffered with HC03. Uptake of acids from the gas phase leads to acidification of the particles. According to the indirect sea salt aerosol pH determinations by Keene and Savoie (1998, 1999), the pH values for moderately polluted conditions at Bermuda were in the mid-3s to mid-4s. The equilibrium model calculations of Fridlind and Jacobson (2000) estimated marine aerosol pH values of 2-5 for remote conditions during ACE-1. Using a one-dimensional model of the MBL which includes gas phase and aqueous phase chemistry of sulfate and sea salt aerosol particles, von Glasow and Sander (2001) predicted that under the chosen initial conditions the pH of sea salt aerosol decreases from 6 near... 

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