Removal of SO2 from Simulated Flue Gases Using Non-Thermal Plasma-Based Microgap Discharge

Abstract

The removal of sulfur dioxide (SO₂) from simulated flue gases streams (N₂/O₂/H₂O/SO₂) was experimentally investigated using microgap discharge. In the experiment, the thinner dielectric layers of aluminum oxide (Al₂O₃) were used to form the microgap discharge. With this physical method, a high concentration of hydroxyl (OH·) radicals were produced using the ionization of O₂ and H₂O to further the conversion of SO₂ into sulfuric acid (H₂SO₄) at 120° C in the absence of any catalysts and absorbents, which were captured with the electrostatic precipitator (ESP). As a result, the increase of discharge power and concentrations of O₂ and H₂O increased the production of OH· radicals resulting in enhanced removal of SO₂ from gas streams. With the test and analysis, a number of H₂SO₄ droplets were produced in experiment. Therefore, a new method for removal of SO₂ in semidry method without ammonia (NH₃) additive was found.     

Use of Plants for Air Pollution Monitoring

Leaf injury data from acute and chronic exposure studies of Dare soybean were regressed against the logarithms of exposure time and O₃ and SO₂ concentrations to develop a new two-pollutant leaf injury model (which explains 88% of the variance) and to calculate the parameters of best fit for this new model and a previously developed one-pollutant model. Using the calculated parameters, the percentage of leaf surface Injured over a growing season by O₃, SO₂, or both simultaneously was estimated for an ambient air sampling site located 2 miles from a coal burning power plant. For this site, the one- and two-pollutant models predicted that SO₂ effects would be negligible If SO₂ concentrations never exceeded the National Ambient Air Quality Standard (NAAQS) of 0.50 ppm, averaged over 3 h. However, calculations suggest that O₃ may injure up to 24% of Dare soybean leaf surface over a growing season even though the O₃ NAAQS of 0.12 ppm, averaged over 1 h, is never exceeded. Because the 3 h SO₂ standard is exceeded at very few places, the O₃ model is usually sufficient to estimate Dare soybean leaf Injury. Leaf injury is estimated by taking the logarithm of the summation of each daytime hour’s exponentiated O₃ concentration (c) measured at an ambient air sampling site over a growing season. This is expressed as: z = -0.0828 + 0.4876 in (Σco₃ ^2.618), where z is the Gaussian transform of percent leaf injury. The methods developed in this paper, using Dare soybean data as an example, may apply to other plants.     

Estimated washout coefficients for sulphur dioxide,nitric oxide,nitrogen dioxide and ozone

The washout coefficient of a gas in air is the fraction of it removed in unit time by rain below cloud base. The ‘apparent’ coefficients were estimated by statistically comparing hourly ground-level concentrations just before and at the onset of heavy, non-frontal rain. The concentrations were obtained from 5 y of continuous monitoring at a rural site.The coefficient (s⁻¹) estimated for SO₂ was (2.61 ± 0.14) × 10⁻⁵ times the rate of rainfall (mm h ⁻¹). This is completely consistent with a previously published value derived from the data from 10 rural sites for one year, and both estimates are consistent with published values of dissolved SO₂ in rainwater at another rural site, giving some confidence in the technique.The values of the coefficients estimated for NO and NO₂ were about 40 and 80%, respectively of that for SO₂. Some nitrite is found in rainwater, but not enough to explain the washout found. However, the analysers used would measure HNO₃ aerosols as NO₂, and these are very soluble. In addition, fast reactions are known which can convert oxides of nitrogen into soluble nitrates, present in sufficient concentration in rainwater.The statistical process leads to the coefficient estimated for O₃ being negative, implying that O₃ is produced at the time of rain. This is most probably due to the strong winds and turbulence which accompany heavy rain, giving replenishment of low-level O₃ from upper air levels where O₃ is normally produced. No systematic changes in the other gas concentrations accompany the changes in O₃ values.     

Evaluation of a CMAQ simulation at high resolution over the UK for the calendar year 2003

A comprehensive ‘operational’ evaluation of the performance of the Community Multiscale Air Quality (CMAQ) modelling system version 4.6 was conducted in support of pollution assessment in the UK for the calendar year 2003. The model was run on multiple grids using one-way nests down to a horizontal resolution as fine as 5 km over the whole of the UK. The model performance was evaluated for pollutants with standards and limit values (e.g. O₃, PM₁₀) and species contributing to acidic and nitrogenous deposition (e.g. NH₃, SO₄^2–, NO₃^–, NH₄⁺) against data from operational national monitoring networks. The key performance characteristics of the modelling system were found to be variable according to acceptance criteria and to depend on the type (e.g. urban, rural) and location of the sites, as well as on the time of the year. As regards the techniques that were used for ‘operational’ evaluation, performance generally complied with expected levels and ranged from good (e.g. O₃, SO₄^2–) to moderate (e.g. PM₁₀, NO₃^–). At a few sites low correlations and large standard deviations for some species (e.g. SO₂) suggest that these sites are subject to local factors (e.g. topography, emission sources) that are not well described in the model. Overall, the model tends to over predict O₃ and under predict aerosol species (except SO₄^2–). Discrepancies between predicted and observed concentrations may be due to a variety of intertwined factors, which include inaccuracies in meteorological predictions, chemical boundary conditions, temporal variability in emissions, and uncertainties in the treatment of gas and aerosol chemistry. Further work is thus required to investigate the respective contributions of such factors on the predicted concentrations.     

Intercomparison of two techniques for the preparation of gaseous sulfur calibration standards in the low to sub-ppb range

Calibration gas standards for H₂S, CH₃SH, CS₂ and SO₂, independently prepared by two separate research groups from the University of Idaho Department of Chemistry and the NOAA Aeronomy Laboratory, were directly intercompared using a gas chromatographic-flame photometric measurement procedure for all four species and a fluorimetric measurement procedure for H₂S. The NOAA gas standard system used gravimetrically-calibrated ‘low loss’ permeation tubes in conjunction with a three-stage dynamic dilution system. The University of Idaho (UI) system used single-stage dilution of the effluent from ‘ultra-low loss’ permeation tubes, which were calibrated by a metal foil collection/flash desorption/flame photometric detection procedure using aqueous sulfate standards. Comparative measurements between UI and NOAA gas standards in the 1–2 ppbv range showed differences no greater than 12 % for CS₂, SO₂ and CH₃SH, and 21 % for H₂S.     

Effects of air pollutants on agriculture and forestry

Historically, studies of the effects of the main phytotoxic gases (SO₂, O₃, NO_x and HF) have focused on determining the threshold for onset of visible foliar injury. The current U.S.A. air quality standards to protect vegetation (500 ppb SO₂ for 3 h and 120 ppb O₃ for l h not to be exceeded more than once per annum) are good examples of the use of this information in the regulatory process. More recently, research has focused on determining the thresholds for effects on economically important yield parameters irrespective of foliar injury. The implication is that long-term seasonal or annual standards may be required to prevent yield losses particularly for the primary pollutants in diffuse-source regions and for secondary pollutants. This paper reviews the literature on thresholds for yield effects of SO₂ and O₃ and concludes that

• 1.(a) the current EEC standard for SO₂ is adequate to protect most crops and trees and
• 2.(b) more work is required to determine whether the U.S.A. threshold for O₃ effects are applicable to the climate and crops of Europe.

Recent results suggest that yield responses vary so much with climatic factors that broad regional standards may not be acceptable. In addition, the effect of one phytotoxic gas must now be assessed against the background of the other gases.Future research on effects of SO₂ and O₃ in particular, will be increasingly influenced by the use of cost-benefit analysis in the regulating process and the consequent demand for dose-response relationships. This approach is fraught with difficulty and particular problems arise

• 1.(a) when ‘hypothetical’ relationships are assumed in the absence of good data and
• 2.(b) when the linearity of dose-response relationships are presumed to justify the extrapolation from effects at high concentrations to lower ambient concentrations.

The evidence for nutritional effects of low levels of SO₂ and NO_x abrogates this assumption and suggests that for some gases at least, there should be a threshold below which no detrimental effects occur. This paper reviews the recent work aimed at producing dose-response relationships for economically important yield parameters.     

Co-occurrence patterns of gaseous air pollutant pairs at different minimum concentrations in the United States

The frequency of co-occurrences for SO₂NO₂, SO₂/O₃ and O₃/NO₂ at rural and remote monitoring sites in the United States was characterized for the months of May-September for the years 1978–1982. Minimum hourly concentrations of 0.03 and 0.05 ppm of each gas were used as the criteria for defining a ‘co-occurrence’. The objectives of this study were to:

• 1.(1) identify the types of co-occurrence patterns and their frequency;
• 2.(2) identify whether the frequency of hourly simultaneous co-occurrences increased substantially when the minimum concentration was lowered (e.g. from 0.05 to 0.03 ppm) for each pollutant; and
• 3.(3) determine whether the frequency of co-occurrences showed large year-to-year variation.

For all pollutant pairs and co-occurrence thresholds (i.e. 0.03 and 0.05 ppm), the frequency of daily and hourly co-occurrences was low for most sites. Year-to-year variability was found to be insignificant; most of the monitoring sites experienced co-occurrences of any type less than 12% of the 153 days. Based on our observations, researchers attempting to assess the potential effects of SO₂/NO₂, SO₂/O₃ and O₃/NO₂ in the United States should construct simulated exposure regimes so that

• 1.(1) hourly simultaneous and daily simultaneous-only co-occurrences are fairly rare and
• 2.(2) when co-occurrences are present, complex-sequential and sequential-only co-occurrence patterns predominate.

     

Two extreme types of mixing of dust with urban aerosols observed in Kosa particles: ‘After’ mixing and ‘on-the-way’ mixing

Besides well-known episodic Kosa during spring, high concentrations of Ca²⁺ in aerosols were observed early in summer as well as in the semi-continuous data of the aerosols at the summit of Mt. Fuji. We further analysed the data to study the chemical characteristics of the high calcium event during early summer. The back trajectory analyses of the event indicated that Ca was transported from arid and semi-arid regions (e.g. the Taklamakan desert) through the westerly-dominated troposphere higher than the height of the summit of Fuji. The amount of SO₄^2? was always equivalent to that of NH₄⁺ unlike the case of the normal Kosa period where SO₄^2? is in excess with respect to NH₄⁺. This shows the ‘after’ mixing of unreacted CaCO₃ of Kosa origin with (NH₄)₂SO₄, which was only realized by the downward injection of Kosa particles from higher altitudes to the air masses of different origin. In the case of normal Kosa, the air bearing Kosa particles passed through the polluted area to absorb unneutralized acids (‘on-the-way’ mixing), whereas in the case of the Kosa-like phenomena in summer, the acids from the polluted area have been neutralized by NH₄⁺ and become inactive before mixing with CaCO₃ (“after” mixing). We have simplified the chemistry of aerosols using their three major components, Ca²⁺, SO₄^2? and NH₄⁺, and introduced a new triangle diagram with the three assumed end-members of CaCO₃, CaSO₄ and (NH₄)₂SO₄ to quantify the contribution of the ‘after’ mixing to the aerosols (AMI; ‘after’ mixing index). Based on the back trajectories of some high AMI cases, CaCO₃ in Kosa particles was transported through the middle troposphere (5000–7000 m) and descended to meet another air mass where SO₄^2? had been already neutralized by NH₃.     

The uptake of O3 by poplar leaves: the impact of a long-term exposure to low O3-concentrations

Poplar shoots were exposed for 3–4 weeks to filtered air, ambient (maximum values 50–60 nl ℓ^-1) or two times ambient O₃-concentrations under controlled environmental conditions in fumigation chambers. A sensitive (Populus nigra ‘Brandaris’) and a tolerant (P. euramericana ‘Robusta’) cultivar were used. At regular intervals the uptake of O₃, transpiration and CO₂ assimilation rate (P_n) of full-grown leaves were measured with leaf cuvettes. For unaffected leaves, the measured flux of O₃ into the leaves appeared to be larger than can be calculated using the stomatal conductance for O₃ (g_s,o) estimated from the transpiration rates of the same leaves. Resistance analysis revealed that the difference was partly a result of a reaction with the external leaf surface. However, when the O₃ flux into the leaf was corrected for this reaction, it was still larger than can be estimated using g_s,o. As a consequence, negative residual or internal resistances (r_i) for O₃ transport into the leaves were assessed. It is postulated that O₃ molecules moving into the leaf follow a shorter pathway than effluxing H₂O-molecules. P. ‘Brandaris’ leaves showed a reduction in P_n after 12 days of exposure to ambient O₃-concentrations, whereas for P. ‘Robusta’ a reduction in P_n was only observed at two times ambient concentrations. A simultaneous decline in the O₃-flux was found in both cases. The decline occurred before a decrease in g_s,o was observed suggesting that the O₃ flux into the affected leaves was first hindered by internal factors. The measured flux of the affected leaves became smaller than the flux estimated using g_s,o and, consequently, positive r_i-values were estimated. The change in r_i suggests that O₃ molecules not only penetrated deeper into the leaf, but also were accumulating at a prolonged exposure. Our results indicate that r_i may be a potentially important component of the overall resistance for O₃-uptake, which may have important consequences for estimating O₃ uptake from water vapour flux data.     

Synthetic SO2 Sorbents for Fluidized-Bed Coal Combustors

The information presented in this paper is directed to engineers who are involved with environmental emissions from coal conversion plants. Synthetic sorbents were investigated as an alternative to natural sorbents (limestone) for the removal of SO₂ from the combustion gas in a fluidized-bed coal combustor. The sulfation rate of a synthetic sorbent, CaO in α-AI₂O₃, was determined as a function of gas composition, temperature, and calcium concentration in the sorbent. The reaction was found to be diffusion-controlled above 850°C and kinetically controlled at lower temperatures. The physical characteristics of the support material have a major effect oh the sulfation kinetics. Porosity measurements indicated that supports containing large pores (>0.2 µm) produced sorbents having high sulfation rates and that pores with diameters less than 0.2 µm did not contribute significantly to the capture of SO₂. The sorbents SrO in α-AI₂O₃ and BaO in α-AI₂O₃ had lower SO₂ capture rates than did CaO in α-AI₂O₃. The alkali metal oxide sorbents K₂O and Na₂O in α-AI₂O₃ captured SO₂ much faster than did the alkaline earth metal oxides.     

Effects of SO2 and/or NO2 on Native Plants of the Mojave Desert and Eastern Mojave-Colorado Desert

Ten species of plants, five perennials and five annuals, native to the Mojave desert were grown in pots and fumigated in open top plastic greenhouses, 25 hours/week, with SO₂ and/or NO₂. Three levels of SO₂: 2.0, 0.67, and 0.22 parts per million (ppm); three levels of NO₂: 1.0, 0.33, and 0.11 ppm; three treatments with 2.0 ppm SO₂ + 1.0 ppm NO₂, 0.67 ppm SO₂ + 0.33 ppm NO₂ and 0.22 ppm SO₂ + 0.11 ppm NO₂ plus untreated control plants were used in the fumigations. The perennials were fumigated for 16 weeks in 1977 and 32 weeks in 1978. Three species of annuals were grown and fumigated for 17 weeks, a fourth for 16 weeks, and a fifth for 12 weeks. A second crop of the first three annuals were grown; one for 12 weeks, a second for 8, and a third for 9 weeks.

Individual species differed widely in their particular responses to the fumigants. The fumigations of perennials with 2.0 ppm of SO₂ or NO₂ at 1.0 ppm caused extensive leaf injury, and reduced growth or dry weight of Larrea divaricate Cav., Chilopsls linearis Cav., and Ambrosia dumosa (Gray) Payne. The combined fumigants had additive effects. No suggestion of synergism was noted. These fumigants at lower concentrations stimulated lateral growth of Encelia farinosa Gray ex Torr. and Erodium cicutarium (L.) L’Her., dry weight of Atriplex canescens (Pursh) Nutt. and Plantago insularis Eastw. and increased flowering of Balleya pleniradiata Harv. and Gray, thus indicating beneficial effects.

Annual species were more severely affected by 2.0 ppm SO₂ than the perennials and extensive injury or death of plants occurred in all annuals. At the 0.67 ppm level severe leaf injury occurred. NO₂ at 1.0 ppm was less injurious than SO₂ and addition of NO₂ to SO₂ suggested an antagonistic effect. Plant survival and flowering was increased by adding NO₂ to plants being treated with SO₂

Comparison of perennial species showed Larrea sensitive, Chilopsis, Encella and Ambrosia intermediate, and Atriplex resistant. The annual species showed Erodium cicutarium and Plantago Insularis to be extremely sensitive, Phacelia crenulata Torr very sensitive and Baileya pleniradiata sensitive. Chaenactis carphoclinia Gray grew poorly and no valid rating was possible.     

A synoptic climatology for surface ozone concentrations in Southern Ontario, 1976–1981

The synoptic climatology of ozone (O₃) for S Ontario has shown that, over the 1976–1981 period, average summer O₃ concentrations follow a relationship similar to that reported for event analysis during periods of high O₃ concentration. Highest average concentrations, 36 parts per billion (ppb), occur with ‘back of the high’ situations while lowest average concentrations (20 ppb) occur with ‘front of the high’ situations.With similar weather events in the winter, the pattern is reversed with highest average O₃ concentrations on the ‘front of the high’ (19 ppb) and lowest average concentrations on the ‘back of the high’ (13 ppb). Concentration of O₃ in the ‘front of the high’ sector is due in part to the intrusion of O₃ in the vicinity of storms from the stratosphere. The seasonal variation of average concentrations in these situations is low, ranging from 14 to 26 ppb.The very low average concentration during the winter and fall for the ‘back of the high’ situation may be the result of scavenging by NO_x from the urban/industrial areas around the Great Lakes. During the spring and summer, solar energy and warm temperatures cause the photochemical production of O₃ from NO_x and HCs precursors. In the fall and winter, photochemical production of O₃ is either very low or absent, and the NO_x consume O₃ rather than produce it. Thus, average O₃ concentrations for winter ‘back of the high’ situations are one-third of those in the summer months.The synoptic climatology of events during the months from May to September with maximum O₃ concentrations in excess of 80 ppb indicates that 78 % of these events occur under synoptic weather classes generally indicative of back or centre of the high situations.     

The use of chemical and statistical methods to identify sources of selected elements in ambient air aerosols in Karachi,Pakistan

Concentrations of 23 elements plus NO₃⁻, SO₄²⁻ and Cl⁻ were determined for samples collected continuously every 3 or 6 h during 22–27 July, 1985 at a suburban site in Karachi, Pakistan. Concentrations of lithophilic elements and several anthropogenic elements were ~ 80 % higher during daytime than at night. These elevated levels were attributed to daytime increases in wind velocity and anthropogenic activity. Factor analysis showed that ~ 50% of the variance was associated with soil, and ~ 14 % each with oceanic Na and Cl⁻, anthropogenic Sb and Pb, and Zn, Se and SO₄²⁻. A cement (limestone/dolomite) factor was not separated even though Ca and Mg concentrations were unusually high and a cement factory was located nearby. This led to an investigation of a chemical approach to determine the sources. Concentrations of Na, Mg and Ca were determined in water-extracts of the samples. Assuming soluble Na (~ 92 % of total) to be sea derived, marine components of Mg, SO₄²⁻, Ca and Br were determined. Solubility considerations were then used to reveal the cement source and apportion the aerosol sources. On the average, approximately 48 % of the total aerosol mass could be accounted for by the ‘cement’ and ‘soil’ components; about 12 % by the ‘sea salt’, about 3% by the ‘fossil fuel’ SO₄²⁻ (as (NH₄)₂SO₄); about 1 % by the NO₃⁻ (as NH₄NO₃); the remaining 36 % of the aerosol mass was unassigned.     

Cumulative effects of low levels of so2 on o3 sensitivity in bean and cucumber

Cucumber and bean plants were pretreated with different SO₂ concentrations for various lengths of time and subsequently exposed to an injurious O₃ exposure. In cucumber, increasing levels of SO₂ and increasing length of pre-exposure progressively enhanced O₃ injury. In bean O₃ sensitivity progressively decreased with both increasing SO₂ concentration and length of pre-exposure. The effect of SO₂ was not related to measurable increases in tissue S content. SO₂ concentrations as low as 20 ppb can increase O₃ sensitivity in cucumber.     

Biochemical and Physiological Detection of Sulfur Dioxide Injury to Pea Plants (Pisum sativum)

Biochemical and physiological experiments were conducted on pea plants (Pisum sativum) continuously exposed in growth chambers to SO₂ gas for 18 days. S0₂ gas concentrations were 0.1, 0.15, and 0.25 ppm. In plants exposed to 0.1 and 0.15 ppm it was clearly demonstrated that there was a greater accumulation of inorganic sulfur, a reduced buffer capacity of the cells relative to H-ions, and a stimulation of glutamate dehydrogenase activity. The only macroscopic symptom seen was slight chlorosis of the older leaves. There was only a slight decrease in fresh and dry weights of these plants compared to the control plants whereas in the group of plants exposed to 0.25 ppm SO₂ foliage necrosis was considerable. In addition, there was a marked reduction in the fresh and dry weights of the latter plants. However, the relationship among accumulated inorganic sulfur, reduced buffer capacity, and increased glutamate dehydrogenase activity as seen for the lower S0₂ concentrations was close. Accordingly, if might be possible to use these three parameters to diagnose S0₂ injury before any significant symptoms appear. In the case of severe SO₂ injury there was a marked increase in glutamine and ammonia concentrations suggesting that these factors in addition to the above could be used in diagnosing severe SO₂ injury. There was no significant difference between plants treated with 0.1 or 0.15 ppm SO₂ and control plants in the contents of K, Ca, P, and N fractions. Therefore, these factors would not be useful in the early detection of SO₂ injury.     

Photo-Oxidation of Sulfur Dioxide

SO₂/O₂ mixtures were photolyzed at 3130 Å and in the range 2500–4000 Å at room temperature. The only product of photolysis was SO₃. Attempts to estimate ф(S0₃) using mass spectrometry, l.R. spectroscopy and pressure change measurements were unsuccessful, because it was not possible to obtain reproducible quantitative estimates of SO₃. ф(SO₂) values were determined by monitoring the 3130 Å absorption for its concentration measurements. ф(SO₂) was independent of SO₂ (11.6 to 50.4 torr) and O₂ (50.0 to 390.6 torr) pressures. At 3130 Å, ф(SO₂) varied between 1.5 × 10^?2 and 2.2 X 10^?2. Over the integrated range 2500–4000 Å ф(SO₂) values of 2.1 X 10^?3 to 2.9 X 10^?3 were obtained. The differences in ф(SO₂) values are explained in terms of wavelength dependence of the rate constants for the two primary reactions: ¹SO₂ + SO₂ → 2SO₂(1) and ¹SO₂ + SO₂ → ³SO₂ + SO₂(2); (k₂/k₁) _{3130 Å} ≈ 10(k₂/k₁)_{2500–4000 Å}.     

The contribution of in-cloud oxidation of SO2 to wet scavenging of sulfur in convective clouds

The sensitivity of in-cloud oxidation of SO₂ in corrective clouds to a number of chemical and physical parameters is examined. The parameterization of precipitation growth processes is based on the work of Scott (1978) and Hegg (1983). A chemical model predicts gas and aqueous phase distributions of soluble gases and in-cloud uncatalyzed oxidation of SO₂ by O₃ and H₂O₂. Sulfate aerosol and SO₂, CO₂, NH₃, H₂O₂ and O₃ gases and their aqueous phase dissociation products are treated.The results indicate that in-cloud conversion is an important removal mechanism for SO₂ and accounts for a significant fraction of the precipitation sulfate. However, except at low SO₂ concentrations, the precipitation sulfate concentration is insensitive to the initial SO₂ concentration; the sulfate concentration is most sensitive to the initial H₂O₂ and NH₃ concentrations. At low SO₂ concentrations, the precipitation sulfate concentration is determined primarily by the initial sulfate aerosol concentration. The feedback between sulfate production and pH is important in limiting SO₂ oxidation by O₃. If gas phase H₂O₂ of order 1 ppb is the major source of aqueous phase H₂O₂ for S(IV) oxidation, it is likely that the oxidation reaction is oxidant limited. The sulfate concentration is a decreasing function of the precipitation rate. At low rainfall rates (< 1 mm h⁻¹), ice phase growth decreases the sulfate concentration. However, the results are insensitive to an ice phase origin at moderate and high rainfall rates.     

Lidar measurements of sulfur dioxide and ozone in the boundary layer during the 1983 Fos Berre Campaign

Studies of the dispersion of pollutants (SO₂, O₃) in the planetary boundary layer have been performed during the Fos Berre 1983 International Campaign using ground-based DIAL lidar operating in the u.v. wavelength range. Multidimensional measurements of both SO₂ and O₃ were obtained with a spatial resolution better than 500 m, a temporal resolution of 3 mn and a maximum horizontal range of 3 km. The overall accuracy as assessed by various comparisons with other measurement systems was always better than 20%. Examples of application to studies of SO₂ dispersion in the planetary boundary layer, of plume evolution and of horizontal inhomogeneities in the ozone field are given.     

Exposure to moderate concentrations of tropospheric ozone impairs tree stomatal response to carbon dioxide

With rising concentrations of both atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃), it is important to better understand the interacting effects of these two trace gases on plant physiology affecting land-atmosphere gas exchange. We investigated the effect of growth under elevated CO₂ and O₃, singly and in combination, on the primary short-term stomatal response to CO₂ concentration in paper birch at the Aspen FACE experiment. Leaves from trees grown in elevated CO₂ and/or O₃ exhibited weaker short-term responses of stomatal conductance to both an increase and a decrease in CO₂ concentration from current ambient level. The impairement of the stomatal CO₂ response by O₃ most likely developed progressively over the growing season as assessed by sap flux measurements. Our results suggest that expectations of plant water-savings and reduced stomatal air pollution uptake under rising atmospheric CO₂ may not hold for northern hardwood forests under concurrently rising tropospheric O₃.     

Thiosulfate as an Oxidation Inhibitor in Flue Gas Desulfurization Processes: A Review of R&D Results

Sodium thiosulfate (Na₂S₂O₃) has been tested in a pilot plant as an oxidation inhibitor in flue gas desulfurization by lime and limestone slurry scrubbing with and without MgO and adiplc acid additives. The effectiveness of thiosulfate is proportional to the inhibitor product, defined as the product of thiosulfate concentration (M), calcium concentration (M), and the moles of SO₂ absorbed per hour per liter of hold tank volume. Gypsum saturation was less than 100 percent and scaling was eliminated when the inhibitor product exceeded 0.3 × 10^?6(gmol/L)³/h. Thiosulfate was relatively more effective in systems with chlorides and less effective in systems promoted by MgO. An inhibitor product greater than 10^?6(gmol/L)³/h significantly enhanced dewatering of solids from limestone scrubbing. SO₂ removal and/or limestone utilization were increased in systems that started with less than 10 mM dissolved calcium.