The importance of liquid water concentration in the atmospheric oxidation of SO2

The concentration of condensed water available for aqueous chemical reactions is viewed as a fundamental parameter of the heterogeneous conversion of gaseous SO₂ to particulate sulfate. New results from a series of dispersed-phase experiments in a cloud chamber, in which the magnitude of this parameter was allowed to vary widely, demonstrate that the heterogeneous SO₂ conversion rate in hazes is generally limited by the small concentration of condensed water. This limitation precludes the heterogeneous oxidation pathway from being important in the atmosphere during haze episodes except under extreme conditions of high humidities and aerosol loadings. In clouds, on the other hand, the liquid water concentrations are relatively large, permitting chemically related factors, such as pH-dependent equilibria and oxidant abundances, to limit the SO₂ conversion rate.     

Oxidation of SO2 in fog droplets by primary oxidants

Experiments described in this paper demonstrate that incomplete combustion is a source of gas-phase oxidants. These species, when dissolved in water, are manifested predominantly as H₂O₂. Experiments involving a fog chamber show that these primary oxidants oxidize SO₂ to sulfuric acid or sulfate in fog droplets. Drying such fog droplets results in suspended particles. For a constant SO₂ concentration and amount of fuel combusted, the amount of SO₂ oxidized and the amount of oxidant produced depend critically on the combustion conditions, being highest for incomplete combustion and negligible for stoichiometric flames.     

A Study of Sulfur Dioxide in Photochemical Smog I. Effect of SO2 and Water Vapor Concentration in the 1-Butene/NOx/SO2 System

There is an appreciable chemical interaction between SO₂ and photochemical smog which depends on the concentration of SO₂ and water vapor. The rate of decay of SO₂ concentration is greatly increased in the presence of photochemical smog. With 0.75 ppm SO₂, a light-scattering aerosol is produced in dry systems and systems at 22 and 55% relative humidity (RH). Aerosol is not observed until after the NO₂ peak has been reached and the NO concentration has fallen to a very low value. The formation of aerosol corresponds in time to the region of most rapid decrease in the SO₂ profile. In systems at 65% RH or with smaller amounts of SO₂, no light scattering is observed, but the percentage of SO₂ disappearing is greater. In relatively dry systems the presence of SO₂ results in a general slowing down of the photochemical smog reactions. In systems containing water vapor concentrations comparable to those found in the atmosphere, the inhibiting influence of SO₂ on the smog reaction is less pronounced. However, the maximum concentration of oxidant produced by the photochemical smog reactions is significantly lower when SO₂ is present.     

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A continuous method for the measurement of SO₂ in ambient air at trace levels is described. The principle of detection is based on the anodic oxidation of SO₂ in a galvanic cell. A differential measuring technique with a cell with two anodes and one cathode is used; background and noise current are low and stable. A feature of the measuring system is that the air sample is passed through a porous anode against a head of electrolyte on the other side of the anode. The minimum values for the time constant and the lower detection limit were 3 seconds and about 3µg/m³, respectively. The selectivity of the method was compared with those of other techniques in field testing. Trace concentrations measured by the galvanic method in the range from 12 to 135 µg/m³ were in good agreement with those of the flame photometric and the West-Gaeke method. With a Pt/Ni₂B/graphite anode the mean oxidation efficiency was 94 ± 2% on the basis of 2 faraday/mol of SO₂ within the tested concentration range of 50 to 560 µg/m³. During a field test no decrease in the anode activity was observed over a period of ten months. Examples are given for the galvanic detection of other compounds using porous measuring electrodes.     

SO2 emission measurement with the European standard reference method,EN 14791, and alternative methods – observations from laboratory and field studies

EN 14791 is a European Standard Reference method for the measurement of SO₂ in emissions. This standard is based on a wet-chemical method in which SO₂ present in flue gases is absorbed into an absorption solution containing hydrogen peroxide, and analyzed as sulfates after sampling. This study presents the results obtained when three portable automated measuring systems (P-AMS), based on Fourier-transform infrared (FTIR) spectroscopy, non-dispersive infrared (NDIR) and ultraviolet-fluorescence (UV) techniques, were compared to the Standard Reference Method for SO₂ (EN 14791) in order to verify whether they could be used as alternative methods (AM) to EN 14791. In the case of FTIR, the measurements were performed from hot and wet gas, without any conditioning. UV-fluorescence analyzers were equipped with dilution probes and one NDIR applied a permeation dryer, whereas the other had a chiller. Tests were carried out at concentration ranges from 0 to 200 mg/m³(n) and from 0 to 800 mg/m³(n) for testing of equivalency according to CEN/TS 14793 using a test bench. Equivalency test criteria were met for all tested P-AMS except for NDIR at the lower range. The SO₂ results measured with NDIR and the chiller were lower compared to the set-up with NDIR and permeation. This was most probably due to the chiller causing absorption of SO₂ in the condensate. Tests were also carried out at field conditions, measuring the SO₂ emissions from a boiler combusting mainly bark. The same phenomena were observed in these tests as during the test bench study, i.e. the measurement set-up with NDIR and the chiller gave the lowest results. These data demonstrated that the tested alternative methods (FTIR, UV-fluorescence, and NDIR) could be used instead of the standard reference method EN 14791, thus providing real-time calibration of automated measuring systems. It must however be emphasized that when measuring water-soluble gases, such as SO₂, the choice of suitable conditioning technique is critical in order to minimize losses of the studied component in the condensate.

Implications: Portable automated measuring systems (P-AMS) provide real-time information about emissions and their concentrations, thus offering significant advantages compared to wet-chemical methods. This study presents results which can be used as a validation protocol to show that the tested P-AMS techniques (FTIR, NDIR, UV-fluorescence) could be used instead of EN 14791 (CEN 2017a) as alternative methods (AM), when paying attention to the selection of an appropriate conditioning technique.     

Do We Understand Trends in Atmospheric Sulfur Species?

ABSTRACT

Anomalies appear to exist in our understanding of atmospheric sulfur compounds, specifically as evidenced in the time trends of the different chemical forms of these compounds. Trends determined at a number of locations by several different groups seem to indicate that, responding to emission reductions across North America, the concentration of SO₂ in the atmosphere is declining more rapidly than that of aerosol SO₄ ^2-. A number of possible reasons for this discrepancy are examined, but it is not possible to provide a definitive answer at this stage. The intent is to stimulate debate, because shortcomings in our understanding of the processes involved could have profound implications for the credibility of abatement strategies and policies for both acid deposition and fine particulate matter (PM).     

Source and Budget of Sulfate in Precipitation From Central Alberta,Canada

Rain, hail, and snow samples collected in central Alberta have been analyzed for sulfate and chloride content using a conductometric titration method. The mean values of sulfate concentration in rain and hail collected in the region of sulfur extraction gas plants were 2.7 mg/l and 2.9 mg/l respectively. The mean value of the sulfate content of a large number of hail samples collected from one severe storm well removed from a major SO₂ source was only 0.6 mg/l. Several snow samples collected in Alberta and southern British Columbia had a mean sulfate content of less than 0.5 mg/l. These results are discussed in terms of the efficiency with which SO₂ is removed from the atmosphere by the different precipitation processes. The results strongly suggest that most of the sulfate found in central Alberta precipitation is of local industrial origin.

By comparing the sulfate deposition in precipitation around one isolated gas plant with the known SO₂ emission rate, a local atmospheric sulfur budget is derived. This budget indicates that the summertime convective storms are a very efficient mechanism for removing the SO₂ from the atmosphere, with between 32 and 46% of the sulfur emitted as SO₂ arriving at the ground as sulfate sulfur within a radius of 25 miles of the source. In contrast snow is a very inefficient removal mechanism, since in winter less than 2% of the sulfur emission is deposited in the snowfall near the source.     

The change in O3, SO2 and NO2 concentrations in Lithuania

Due to the dynamic nature of the atmosphere, substantial amounts of gaseous and particulate pollutants are transported to the areas distant from their sources. In order to determine the regional concentration levels of atmospheric pollutants in Lithuania, concentrations of gaseous O₃, SO₂, NO₂ and other pollutants have been measured at the Preila background station (55°20′ N and 21°00′ E, 5 m a.s.l.) since 1981. The long-term concentration data set enabled us to get temporal trends, both on a seasonal and longer time scale, to identify source areas of pollutants and to relate them to the emission data. Based on the data obtained, the different tendencies in the pollutant concentration changes were revealed. Positive trends for ozone (of 2.9% per year during 1983–2000) and a distinct negative trend for both sulphur dioxide (of 3.8% per year during 1981–2000) and nitrogen dioxide (of 3.8% per year during 1983–2000) were found. The air mass back-trajectory analysis was used to assess the source region of air pollutants transported to Lithuania. The pollutant concentration levels were compared with their emission changes in Europe and Lithuania. The general trends in SO₂ as well as in NO₂ concentrations observed are consistent with changes in SO₂ and NO₂ emissions in Europe and Lithuania.     

Absorption of Gaseous Air Pollutants By a Standardized Plant Canopy

Concentration profiles for hydrogen fluoride(HF), sulfur dioxide(SO₂), ozone (O₃), nitrogen dioxide(NO₂), and nitric oxide(NO) generated in a standardized alfalfa canopy are presented. Wind, light, temperature, and carbon dioxide(CO₂) profiles, canopy pollutant uptake rates, and canopy structural data are also given. Canopy pollutant concentration profile characteristics were studied to evaluate the relative potentials for major air pollutants to penetrate into canopies. The study was conducted in an environmental growth chamber equipped to control automatically environmental conditions and monitor continuously gas exchange rates. HF, SO₂, and NO₂ profiles suggested that these gases were removed efficiently by the upper portion of the canopy as well as by the immediate subsurface vegetation. The steady state HF profile showed the greatest displacement within the canopy. The NO profile was displaced the least. The uptake rate of NO by plants was apparently too slow in comparison with gas transport and mixing within the canopy to affect the internal profile substantially. O₃ appeared to be readily deposited on the surface tissues, but the deeper tissues in the canopy had less effect on the concentration profile. Data are also presented to show the relationship between NO₂ concentration within the canopy and changes in the air concentration above the vegetation. The results indicated that gas transport between the atmosphere and canopy interior was rapid. The data presented should be of current interest to agriculturists, researchers, administrators, and environmental planners concerned with effects of air pollutants on plants and on the fate of pollutants in the microenvironment.     

Elevated biogenic sulphur dioxide concentrations over the North Atlantic

Elevated biogenic SO₂ from the oxidation of dimethylsulphide (DMS) in the marine atmosphere was measured over the North Atlantic Ocean in spring and summer 2003. Stable isotope apportionment was used to distinguish between anthropogenic and biogenic SO₂ in the marine atmosphere south of Greenland. Atmospheric DMS measurements were within range of previous studies. Biogenic SO₂ concentrations were as high as 82 nmol m^?3 (spring geometric mean: 4 nmol m^?3, σ = 17; summer geometric mean 7 nmol m^?3, σ = 19) and are the highest reported values for marine biogenic SO₂ in the literature. Elevated biogenic SO₂ was found in air masses influenced by anthropogenic pollutants during the summer. This indicates that anthropogenic pollutants may affect the fate of oceanic emissions of sulphur compounds in the atmosphere favoring the formation of cloud condensation nuclei.     

High Volume Sampling of Acid Mist

Many nonferrous metals are produced from sulfur-bearing minerals. When pyrometallurgical processes are used, sulfur dioxide gas is formed as a byproduct. In many cases, a small but significant portion of the SO₂ is oxidized further to SO₃ within the pyrometallurgical reactor system. Upon cooling of the gases, as would occur when such gases contact the atmosphere, SO₃ will absorb moisture and condense as a sulfuric acid aerosol, typically referred to as acid mist.     

Humidity effects on photochemical aerosol formation in the SO2-NO-C3H6-air system

In order to investigate the effects of humidity on the gas-phase oxidation of SO₂ in polluted air and on the subsequent aerosol formation process, photoirradiation experiments were carried out by means of a 4-m³ chamber, in which mixtures containing SO₂, NO and C₃H₆ with concentrations in the ppm range were exposed to simulated solar radiation in different relative humidity (r.h.) conditions. The total amount of oxidized SO₂ was quantified from the SO₄²⁻ yield determined by the chemical analysis of the aerosol product, and a part due to the oxidation by the OH radical was evaluated by estimating the OH concentration from the decay rate of C₃H₆. The remaining part was assigned to the oxidation by the Criegee intermediate, as it had a good correlation with the progress of the O₃ + C₃H₆ reaction. The contributions of the two oxidizing species to the total conversion and the oxidation rate of SO₂ were measured as functions of r.h. As a result, experimental evidence was obtained for the prediction of Calvert and Stockwell's (1983, Envir. Sci. Technol. 17, 428A–443A) simulation that the oxidation due to the Criegee intermediate was retarded by the increase in humidity. The OH contribution, on the other hand, was almost independent of r.h. It was observed consequently that the total oxidized amount of SO₂ considerably decreased as r.h. was higher.The humidity effect on the aerosol formation process was found to be more complicated than the effect on the gas-phase chemistry. The maximum rate of increase in the particle number concentration rose linearly with increasing r.h., but the number concentration itself measured at its maximum or at the end of the irradiation reached a ceiling value around r.h. = 30% and went down for higher r.h. The average panicle size in the final stage of the reaction showed a minimum around the same r.h. at which the number concentration was maximum. The H₂SO₄ concentration in the mist particles, however, decreased monotonically as r.h. got higher. It was suggested that these different responses against the increase in humidity resulted from the cooperation of several processes such as the H₂SO₄ monomer formation, the H₂O condensation, the particle coagulation, etc., which had different dependences on r.h.     

The Absorption of Atmospheric Sulfur Dioxide by Water Solutions

Results of a laboratory study indicate that the rate of solution of atmospheric sulfur dioxide in distilled water, over the range of atmospheric concentrations of 0.81?8.73 mg SO₂/M³, is a function of the concentration of SO2 in the atmosphere, with saturation being reached more rapidly at the higher concentrations. This would indicate that rain water, with constantly renewed surfaces, can be very effective in the removal of atmospheric SO₂. The pH of the exposed water samples reached values of 4.0 or less, comparable to values observed in fog and cloud water near large industrial areas. Overall solubility of sulfur dioxide in distilled water did not follow the law of partial pressure. At the atmospheric concentrations used it was found that over 98.5% of the sulfite in solution was in the form of the bisulfite ion with, the remainder present as unionized sulfurous acid. Computations using the concentration of unionized sulfurous acid in the solution showed that the solubility of this portion of dissolved sulfite did follow the law of partial pressure.     

Evaluation of the Iron-o-Phenanthroline Procedure For Determining S02 in Turbine Exhaust Gases

Several wet chemical methods have been used or suggested for the determination of SO₂ concentrations in air pollution work. These include the iron-O-phenanthroline procedure reported by Stephens and Lindstrom, the Scaringelli-modified West-Gaeke method and the Schulze method. This paper describes a laboratory study to evaluate the usefulness of the iron-o-phenanthroline procedure and is directed to individuals concerned with the analysis of gases from the exhaust of gas turbine engines and other combustion processes, including stationary power plants. The variables considered were: range of usefulness in terms of concentration of SO₂, efficiency of collection, effect of contaminants, specifically oxides of nitrogen, olefin and aldehyde and effect of storage prior to spectrophctometric measurement. The Stephens-Lindstrom method was found to be suitable for measuring higher levels of SO₂ concentrations. It can accurately measure amounts totalling 6000 µl of SO₂ and above whereas the other mentioned methods are generally used for lower levels. Collection efficiency was satisfactory. Contaminants, particularly oxides of nitrogen, are a problem only at low levels of SO₂. NO₂ interference may be eliminated by absorption of the NO₂ on Ultraport S impregnated with ANEDA/H₂SO₄ solution. Temperature control during SO₂ addition is necessary. Storage of exposed reagents prior to measurement produce only small errors if stored at 0°C or at room temperature.     

Simulation of Stack Plume Opacity

ABSTRACT

The visual impact of primary particles emitted from stacks is regulated according to stack opacity criteria. In-stack monitoring of the flue gas opacity allows plant operators to ensure that the plant meets U.S. Environmental Protection Agency opacity regulations. However, the emission of condensable gases such as SO₃ (that hydrolyzes to H₂SO₄), HCl, and NH₃, which may lead to particle formation after their release from the stack, makes the prediction of stack plume opacity more difficult.

We present here a computer simulation model that calculates the opacity due to both primary particles emitted from the stack and secondary particles formed in the atmosphere after the release of condensable gases from the stack. A comprehensive treatment of the plume rise due to buoyancy and momentum is used to calculate the location at which the condensed water plume has evaporated (i.e., where opacity regulations apply).

Conversion of H₂SO₄ to particulate sulfate occurs through nucleation and condensation on primary particles. A thermodynamic aerosol equilibrium model is used to calculate the amount of ammonium, chloride, and water present in the particulate phase with the condensed sulfate. The model calculates the stack plume opacity due to both primary and secondary particles. Examples of model simulations are presented for three scenarios that differ by the emission control equipment installed at the power plant: (1) electrostatic precipitators (ESP), (2) ESP and flue gas desulfurization, and (3) ESP and selective catalytic reduction. The calculated opacity is most sensitive to the primary particulate emissions. For the conditions considered here, SO₃ emissions showed only a small effect, except if one assumes that most H₂SO₄ condenses on primary particles. Condensation of NH₄Cl occurs only at high NH₃ emission rates (about 25 ppm stack concentration).     

The dry deposition of sulfur dioxide on a loblolly pine plantation

A concentration gradient/resistance model approach was used to determine the flux density, deposition velocity, and transport resistances for sulfur dioxide (SO₂ ) between the atmosphere and a loblolly pine (Pirms taeda L.) plantation. Measurements were made on 54 clear to partly sunny days during the period from June 1982 to May 1983. For this stand and these days, the average daylight flux density was 0.052 μg m⁻²s⁻¹ and the deposition velocity for SO₂ was 0.72±0.65 cms⁻¹. The average transport resistance for SO₂ includes the aerodynamic resistance (r_a), canopy resistance (r_c), and internal resistance corrected for solubility (r_ir). The values for these resistances were 15 ±4, 127 ±94 and 14+-39 s m⁻¹, respectively.     

A Device for Measuring Volatile Organic-Carbon Emissions from Cooling-Tower Water

A manual method for measuring reduced sulfur compounds in kraft pulp mill and sulfur recovery plant emissions was evaluated. The method involves removing SO₂ from the gas stream (if present) with a citric acid-potassium citrate buffer that passes reduced sulfur compounds; thermal oxidation of all reduced sulfur compounds to SO₂; collection of the SO₂ in H₂O₂; and a titrimetric analysis of the H₂O₂ for SO₄ ^2?. A heated filter removes alkaline particulate matter that would produce a negative interference if absorbed by the buffer. When used at kraft pulp mills, the method agrees closely with Reference Method 16, provided that nonregulated reduced sulfur compounds, such as carbonyl sulfide, are not present in the emissions. At sulfur recovery plants, nonregulated reduced sulfur compounds, such as thiophene, are likely to be present in the emissions and will produce a positive bias in the results obtained with this method. The precision of the method ranges from 1 to 7 percent relative standard deviation.     

The APCA Exhibition

On June 5 and 6 of 1980, two parallel plume oxidation studies were carried out in the vicinity of the Tennessee Valley Authority's Colbert Steam Plant. One study was performed in a smog chamber into which stack gases were injected and mixed with ambient air. The other study included direct airborne sampling of the power plant plume. Atmospheric oxidation rates for the conversion of SO₂ to SO₄ ^2- and the removal rates of NO _x (which is presumably the rate of NO₃ ^- formation) were estimated for both studies. The SO₂ to SO₄ ^2- rate coefficients were found to be 0.022 ± 0.009 h^-1 for both chamber experiments and the first airborne sampling day. For the second day, a rate constant of 0.041 ± 0.052 h^-1 was estimated from the aircraft data. The large deviation in this value is explained by the fact that the plume from the power plant combined and reacted with the urban plume from the city of Florence, AL. The formation of a very large "O3 bulge" on this day is also attributed to the mixed plumes. The first order rate coefficients for NO _x removal were estimated to be 0.27 ± 0.14 h^-1 for both chamber experiments and the first airborne sampling day. A NO _x removal rate could not be determined for the second airborne sampling day.     

Measurement of the vertical profile of atmospheric SO2 during the heating period in Beijing on days of high air pollution

This paper reports altitude-resolved concentrations of sulfur dioxide (SO₂) and particulate matter up to 10 microns in diameter (PM₁₀) in the planetary boundary layer of major urban centers during extreme pollution episodes. The concentration of SO₂ was observed continuously from November 24, 2004, to December 4, 2004, in Beijing during the heating period. Fluorescence SO₂ analyzers were used to measure the atmospheric SO₂ concentrations. Four SO₂ analyzers were placed at 4 different levels (8 m, 47 m, 120 m, and 280 m) of the 325-m high meteorological observation tower of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences. A maximal SO₂ concentration of 172.3 ppb was measured during this pollution episode, and SO₂ concentration increased with altitude and reached its maximal value at ～50 m. The study also analyzed the meteorological situation before, during, and after the pollution episode.     

Laboratory investigations of the impact of dry deposition of SO2 and wet deposition of acidic species on the atmospheric corrosion of galvanized steel

A series of short-term laboratory experiments were conducted in which galvanized steel samples were exposed to sub-ppm levels of SO₂. Dew was produced periodically on the test panels, and, at the end of some experiments, panels were sprayed with solutions of various pH levels. Both dew and rain rinse samples were analyzed for SO₃²⁻, SO₄²⁻ and Zn.The laboratory results suggest that as a first approximation the damage to galvanized steel induced by the dry deposition of SO₂ can be calculated by equating the dry SO₂ flux to the Zn corrosion flux. SO₂ will deposit onto a fresh dry surface until an amount similar to that of a monolayer has formed. Under wet conditions, the dry deposition flux is controlled by the gas-phase resistance of the atmosphere. Wet deposition of ammonium bisulphate induces corrosion which depends not only on the pH of the incident rain, but also on the exposure history of the samples.