Reducing Interference Effects in the Chemiluminescent Measurement of Nitric Oxides from Combustion Systems

Experiments were carried out to determine the relative chemiluminescence quenching efficiencies as a function of third body concentration for each of the common combustion products, H₂O, CO₂, CO, H₂, O₂ and Ar. These results are compared with those of other investigators. The effect of reaction chamber pressure on analyzer response and the development of an analyzer design which incorporates an adjustable sample capillary inlet capable of maintaining a constant molar flow rate of sample gas to the reaction chamber are discussed. The effect of carbon monoxide interference on chemiluminescent NO_x measurement has been isolated and found to be significant. A means of correcting NO, measurements for these CO interference effects is described. Quantification of NO and NO₂ absorption in liquid water in NO_x sampling systems has been made. Recommendations for sample system designs to handle the presence of water in the sample gas are made.     

A new approach for chemical oxygen demand (COD) measurement at high salinity and low organic matter samples

Background, aim, and scope  

Chemical oxygen demand (COD) is used as a discharge standard parameter in wastewater treatment plant design, environmental modelling and many other applications. Chloride interference is an important problem of COD measurement for wastewaters containing low organic matter and high chloride concentrations. In case of chloride concentrations up to 2,000 mg/L, mercury sulphate addition at a ratio of 10:1 (HgSO₄:Cl⁻) can adequately mask the interference. When chloride concentration exceeds 2,000 mg/L, this ratio becomes ineffective to hinder the interference. At this point, it is proposed to use a greater and constant ratio of mercury sulphate addition. However, this application sometimes results in extra mercury sulphate addition which is not necessary. Even in some cases, greater addition of mercury sulphate alone is not a solution to erroneous measurement results. The purpose of the study is to determine optimum HgSO₄:Cl⁻ ratios according to the chloride concentrations of the samples and to show the importance of the strength of the digestion solution for the correct determination of the COD parameter.     

Uptake of nitrous acid and nitrogen oxides by nylon surfaces: Implications for nitric acid measurement

The interference in HNO₃ determination due to HNO₂ and NO_x retention on nylon filters has been evaluated in laboratory and field conditions. Nitrous acid is retained on nylon filters with efficiencies varying from 25% at 12ℓ min⁻¹ to 80% at 2ℓ min⁻¹, yielding NO₂⁻ ion. In ambient sampling performed during photochemical smog episodes, NO₂⁻ is oxidized to NO₃⁻ with conversion factors up to 100%, resulting in a positive bias in HNO₃ determination.NO₂ reacts heterogeneously with H₂O on nylon surfaces according to the reaction 2NO₂ + H₂O → HNO₂ + HNO₃ with a removal constant of about 1 × 10⁻⁴ ms⁻¹ at a H₂O concentration of 20,000 ppm. The resulting nitrite and nitrate are independent of the sampling flow rate, while NO₂ concentration, sampling time and exposed nylon surface area play a directly proportional role. Accordingly, the relative interference of NO₂ with respect to HNO₃ determination is almost negligible for nylon filters, usually run at relatively high flow rates, while it may be significant for nylon denuders, which are characterized by larger exposed surfaces and lower operating flow rates.     

Simultaneous DOAS and mist-chamber IC measurements of HONO in Houston,TX

Nitrous acid is an important component of nighttime N-oxide chemistry, and provides a significant source of both OH and NO in polluted urban air masses shortly after sunrise. Several recent studies have called for new sources of HONO to account for daytime levels much higher than are consistent with current understanding. However, measurement of HONO is problematic, with most in-situ techniques reporting higher values than simultaneous optical measurements by long-path DOAS, especially during daytime. The discrepancy has been attributed to positive interference in the in-situ techniques, negative interference in DOAS retrievals, the difficulty of comparing the different air masses sampled by the methods, or combinations of these.During August and September 2006, HONO mixing ratios from collocated long-path DOAS and automated mist-chamber/ion chromatograph (MC/IC) systems ranged from several ppbv during morning rush hour to daytime minima near 100 pptv. Agreement between the two techniques was excellent across this entire range during many days, showing that both instruments accurately measured HONO during this campaign. A small bias towards higher LP-DOAS observations at night can be attributed to slow vertical mixing leading to pronounced HONO profiles. A positive daytime bias of the MC/IC instrument during several days in late August/early September was correlated with photochemically produced compounds such as ozone, HNO₃ and HCHO, but not with NO₂, NO_x, HO₂NO₂, or the NO₂ photolysis rate. While an interferant could not be identified organic nitrites appear a possible explanation for our observations.     

Dissociation of Sulfur Hexafluoride Tracer Gas in the Presence of an Indoor Combustion Source

ABSTRACT

As an odorless, nontoxic, and inert compound, sulfur hexafluoride (SF₆) is one of the most widely used tracer gases in indoor air quality studies in both controlled and uncontrolled environments. This compound may be subject to reactions with water vapor under elevated temperature to form acidic inorganic compounds such as HF and H₂SO₄. Thus, in the presence of unvented combustion sources such as kerosene heaters, natural gas heaters, gas log fireplaces, candles, and lamps, the SF₆ dissociation may interfere with measurements of the emissions from these sources. Tests were conducted in a research house with a vent-free natural gas heater to investigate these potential interferences. It was observed that the heater operation caused about a 5% reduction of SF₆ concentration, which can be an error source for the ventilation rate measurement and consequently the estimated pollutant emission rates. Further analysis indicates that this error can be much greater than the observed 5% under certain test conditions because it is a function of the ventilation flow rate. Reducing the tracer gas concentration has no effect on this error. A simple theoretical model is proposed to estimate the magnitude of this error.

The second type of interference comes from the primary and secondary products of the SF₆ dissociation, mainly H₂SO₄, SO₂, HF, and fine particulate matter (PM). In the presence of ~5 ppm SF₆, the total airborne concentrations of these species increased by a factor of 4-10. The tests were performed at relatively high SF₆ concentrations, which is necessary to determine the interferences quantitatively. The second type of interference can be significantly reduced if the SF₆ concentration is kept at a low ppb level.     

Evaluation of the Sodium Arsenite Method for Measurement of NO2 in Ambient Air

The sodium arsenite method for measurement of nitrogen dioxide in ambient air was evaluated. The method has a constant-high collection efficiency (82%) for nitrogen dioxide, and is insensitive to normal variations in operating parameters. Nitric oxide and carbon dioxide are positive and negative interferents, respectively. The combined average effect of these interferents, over ambient levels, is a positive bias of 9.9 µg/m³. This bias, although statistically significant, is minor ( 10 % ) in relation to the ambient air standard of 100 µg NO₂/m³ and does not warrant modification of the method to remove the interference.     

Development of Instrumentation for Continuous On-line Monitoring of Non-Methane Organic Carbon in Air Emissions

ABSTRACT

Non-methane organic carbon (NMOC) is a measure of total organic carbon in an air emission, excluding that from methane. Thus, it measures the total amount of carbon, irrespective of the structure and functional groups in the molecule. The U.S. Environmental Protection Agency (EPA) Method 25 is used for quantification of NMOC in emission sources and in ambient air. This method involves laboratory analysis of collected air samples and cannot be used for real-time measurements. It is prone to interferences from CO₂, CH₄, and CO, as well as moisture. In this paper, a novel method for continuous, on-line monitoring of NMOC in air emissions and ambient air is presented. Detection limits are at ppb levels, and interference of permanent gases have been eliminated.     

Investigation into atmospheric deposition through precipitation studies at New Delhi (India)

Rain water samples were collected during the monsoon of 1994, using automatic wet-only and manual bulk collectors at a height of 30 m above the ground at the National Physical Laboratory, New Delhi. The average pH of the rain water was 5.7 and its chemical composition was dominated by NH₄ and SO₄. The free acidity of the rain water was found to be due to S04 rather than N03 and it was mainly neutralized by NH₄ and Ca. Calculation of sea salt fraction and enrichment factor revealed that this site is free for marine influence. On an average the bulk samples had 13% higher concentration than that of wet-only samples which may be due to the deposition of soil-derived particles during the 24 h period of exposure. The higher neutralization factors of Ca and Mg in bulk samples and highest dry deposition rates for Ca in comparison to other components, indicated the positive interference of dust particles in neutralization.     

Ambient and Source SO2 Detector Based on a Fluorescence Method

The principle of this detector is based on the measurement of the intensity of the ultraviolet fluorescence of SO₂ produced by absorption of the Zn 2138 Å or Cd 2288 Å line. The fluorescence intensity was found to be linear from 0.1 to 500 ppm of SO₂ in air with the Zn lamp and from 0.1 to 1600 ppm with the Cd lamp. The detection limit at present is about 20 ppb. There is no detectable interference from O₃, H₂S, NO₂, CO₂, CO, or H₂, although the presence of a large concentration of CS₂ (500 times as much as SO₂) NO (500 times) or C₂H₄ (4000 times) interferes with the measurement. The presence of 2% H₂0 reduces the signal by 25%, while up to 1 % CH₄ has almost no effect.     

Optimization and validation of enzyme-linked immunosorbent assay for the determination of endosulfan residues in food samples

In this study, an enzyme-linked immunosorbent assay (ELISA) was optimized and applied to the determination of endosulfan residues in 20 different kinds of food commodities including vegetables, dry fruits, tea and meat. The limit of detection (IC₁₅) was 0.8 μg kg^?1 and the sensitivity (IC₅₀) was 5.3 μg kg^?1. Three simple extraction methods were developed, including shaking on the rotary shaker at 250 r min^?1 overnight, shaking on the rotary shaker for 1 h and thoroughly mixing for 2 min. Methanol was used as the extraction solvent in this study. The extracts were diluted in 0.5% fish skin gelatin (FG) in phosphate-buffered saline (PBS) at various dilutions in order to remove the matrix interference. For cabbage (purple and green), asparagus, Japanese green, Chinese cabbage, scallion, garland chrysanthemum, spinach and garlic, the extracts were diluted 10-fold; for carrots and tea, the extracts were diluted 15-fold and 900-fold, respectively. The extracts of celery, adzuki beans and chestnuts, were diluted 20-fold to avoid the matrix interference; ginger, vegetable soybean and peanut extracts were diluted 100-fold; mutton and chicken extracts were diluted 10-fold and for eel, the dilution was 40-fold. Average recoveries were 63.13–125.61%. Validation was conducted by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The results of this study will be useful to the wide application of an ELISA for the rapid determination of pesticides in food samples.     

Laboratory testing of a continuous emissions monitor for hydrochloric acid

Continuous monitoring of exhaust flue gas has become a common practice in power plants in response to Federal Mercury and Air Toxics Standards (MATS) standards. Under the current rules, hydrochloric acid (HCl) is not continuously measured at most plants; however, MATS standards have been proposed for HCl, and tunable diode laser (TDL) absorption spectroscopy is one method that can be used to measure HCl continuously. The focus of this work is on the evaluation and verification of the operation performance of an HCL TDL over a range of real-world operating environments. The testing was conducted at the University of California at Riverside (UCR) spectroscopy evaluation laboratory. Laboratory tests were conducted at three separate temperatures, 25ºC, 100ºC, and 200ºC, and two distinct moisture levels for the enhanced temperatures, 0%, (2 tests) and 4%, over a concentration range from 0 ppmv to 25 ppmv-m at each of the elevated temperatures. The results showed good instrument accuracy as a function of changing temperature and moisture. Data analysis showed that the average percentage difference between the ammonia concentration and the calibration source was 3.33% for varying moisture from 0% to 4% and 2.69% for varying temperature from 25 to 100/200ºC. An HCl absorption line of 1.742 μm was selected for by the manufacturer for this instrument. The Hi Tran database indicated that CO₂ is probably the only major interferent, although the CO₂ absorption is very weak at that wavelength. Interference tests for NO, CO, SO₂, NH₃, and CO₂ for a range of concentrations typical of flue gasses in coal-fired power plants did not show any interference with TDL HCl measurements at 1.742 μm. For these interference tests, CO₂ was tested at a concentration of 11.9% concentration in N₂ for these tests. Average precision over the entire range for all 10 tests is 3.12%.

Implications: The focus of this study was an evaluation of the operation performance of a tunable diode laser (TDL) for the measurement of hydrochloric acid (HCl) over a range of real-world operating environments. The results showed good instrument accuracy as a function of changing temperature from 25ºC to 200ºC and moisture from 0% to 4%. Such as an instrument could be used for continuous monitoring of exhaust flue gas in power plants once the Federal Mercury and Air Toxics Standards (MATS) standards have been fully implemented.     

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.     

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.     

Issues Related to Solution Chemistry in Mercury Sampling Impingers

ABSTRACT

Analysis of Hg speciation in combustion flue gases is often accomplished in standardized sampling trains in which the sample is passed sequentially through a series of aqueous solutions to capture and separate oxidized Hg (Hg²⁺) and elemental Hg (Hg⁰). Such methods include the Ontario Hydro (OH) and the Alkaline Mercury Speciation (AMS) methods, which were investigated in the laboratory to determine whether the presence of Cl₂ and other common flue gas species can bias the partitioning of Hg⁰ to front impingers intended to isolate Hg²⁺ species. Using only a single impinger to represent the front three impingers for each method, it was found that as little as 1-ppm Cl₂ in a simulated flue gas mixture led to a bias of approximately 10-20% of Hg⁰ misreported as Hg²+ for both the OH and the AMS methods. Experiments using 100-ppm Cl₂ led to a similar bias in the OH method, but to a 30-60% bias in the AMS method. These false readings are shown to be due to liquid-phase chemistry in the impinger solutions, and not necessarily to the gas-phase reactions between Cl₂ and Hg as previously proposed. The pertinent solution chemistry causing the interference     

二氧化钛纳米管光催化降解水中百草枯

二氧化钛纳米管被用于光催化氧化水体中的百草枯,对光催化反应条件、常见Fe3+离子的干扰情况和百草枯光催化降解动力学规律进行了研究。结果表明,浓度为25 mg/L的百草枯溶液,在二氧化钛纳米管(TNT)1.0 g/L,H2O20.5 mL/50 mL,pH=5.0的最优光催化氧化条件下,经过30 min反应可以被100%从水体中去除,表现出非常高的光催化降解效率;动力学方程拟合表明,百草枯光催化氧化反应符合拟一级动力学规律,动力学方程为ln(C0/C)=1.0267t-0.1282,反应速率常数K为1.0267 h-1;双氧水存在时常见的Fe3+能够进一步提高百草枯光催化降解率;该光催化反应体系对低浓度百草枯废水有很好的处理效果,预示着光催化氧化技术适合地表或地下水体中百草枯的去除。     

Measurement of Ozone Concentrations in Ambient Air Using a Badge-Type Passive Monitor

Abstract

A badge-type passive monitor was used to evaluate the effectiveness of four ozone trapping reagents for measuring O₃ concentrations in the air. These were sodium nitrite (NaNO₂), 3-methyl-2-benzothiazolinone acetone azine (MBTH), p-acetamidophenol (p-ATP), and indigo carmine. Experiments in an exposure chamber showed that only NaNO₂ and MBTH monitors gave sensitive and linear responses over realistic ranges of O₃ concentrations. When tested in ambient air, NaNO₂ and MBTH monitors with a single-layer diffusion barrier overestimated O₃ concentrations by a significant amount. This was largely canceled out in the NaNO₂ monitor by using a multi-layered diffusion barrier to combat wind turbulence effects. However it had almost no effect on the MBTH monitor, and it was found that NO₂ was a source of serious interference. We concluded that using the NaNO₂ monitor with an effective diffusion barrier can measure O₃ in ambient air with an accuracy of ±16%.     

Nighttime variations in HO2 radical mixing ratios at Rishiri Island observed with elevated monoterpene mixing ratios

HO₂ radical concentrations were measured by a laser-induced fluorescence instrument for three nighttime periods during the intensive field campaign at Rishiri Island, Japan, in June 2000. The HO₂ mixing ratio had temporal variations around its average of 4.2±1.2 (1σ) pptv and showed a positive correlation with the summed mixing ratio of four monoterpene species, α-pinene, β-pinene, camphene, and limonene, that sometimes reached 1 ppbv. Our model calculations suggested that ozonolysis reactions of monoterpenes were the main source of nighttime radicals and they explained 58% of measured HO₂ concentration levels. The model roughly reproduced the dependence of the HO₂ mixing ratio on the square root of the radical production rate due to the ozonolysis reactions of the monoterpenes. However, the absolute HO₂ mixing ratio was significantly underpredicted by the model. We discuss possible reasons in terms of misunderstood RO₂ chemistry, RO₂ interference with HO₂ observations, unknown radical production process associated by high NO₂ mixing ratio, and the contribution of unmeasured olefinic species to radical production via their reactions with ozone.     

A continuous monitor-sulfur chemiluminescence detector (CM-SCD) system for the measurement of total gaseous sulfur species in air

A continuous monitor-sulfur chemiluminescence detector (CM-SCD) system with a flameless, temperature-controlled furnace combustion source was developed for real-time measurement of total sulfur gases in air. This measurement system demonstrated a linear dynamic range exceeding five orders of magnitude and equimolar sensitivity to the most prevalent atmospheric sulfur gases. A detection limit of 10 pptv was obtained using 10 min signal averages. On a real-time basis, detection in the 20–50 pptv range was demonstrated. After modification of its sample inlet system, the CM-SCD showed no appreciable interference effects from the addition of H₂O vapor, NO₂ or O₃ to the sample matrix. In the recent Gas-Phase Sulfur Intercomparison Experiment (GASIE), the CM-SCD compared favorably with SO₂ measurement methods.     

The Effect of Coal Combustion Flue Gas Components on Low-Level Chlorine Speciation Using EPA Method 26A

ABSTRACT

U.S. Environmental Protection Agency (EPA) Method 26A is the recommended procedure for capturing and speci-ating halogen (X₂) and hydrogen halide (HX) stack emissions from combustion sources. Previous evaluation studies of Method 26A have focused primarily on hydrogen chloride (HCl) speciation. Capture efficiency, bias, and the potential interference of Cl₂ at high levels (>20 ppm [u,g/m³]) and NH₄Cl in the flue gas stream have been investigated. It has been suggested that precise Cl₂ measurement and accuracy in quantifying HX or X₂ using Method 26A are difficult to achieve at Cl₂ concentrations <5 ppm; however, no performance data exist to support this. Coal contains low levels of Cl, in the range of 5-2000 ppmw, which results in the presence of HCl and Cl₂ in the products of combustion. HCl is the predominant Cl compound formed in the high-temperature combustion process, and it persists in the gas as the products of combustion cool. Concentrations of Cl₂ in coal combustion flue gas at stack temperatures typically do not exceed 5 ppm. For this research, bench-scale experiments using simulated combustion flue gas were designed to validate the ability of Method 26A to speci-ate low levels of Cl₂ accurately. This paper presents the results of the bench-scale tests. The effect of various flue gas components is discussed. The results indicate that SO₂ is the only component in coal combustion flue gas that has an appreciable effect on Cl₂ distribution in Method 26A impingers, and that Method 26A cannot accurately speciate HCl and Cl₂ in coal combustion flue gas without modification.     

重金属杂质对磷酸铵镁结晶法处理制药废水的影响

磷酸铵镁沉淀法处理废水时常受众多因素影响,其中重金属类杂质对该过程影响较大。实验研究了Mn²⁺、Cu²⁺杂质存在条件下,用该法去除制药废水中PO₄^3-、NH₄⁺污染物,同时,用XRD、XRF、SEM等手段研究了所得鸟粪石晶体特征。结果表明,pH为9.5时,Mn²⁺、Cu²⁺杂质存在条件下,PO₄^3-去除率分别为88%和85%,投加晶种有利于回收特定晶形鸟粪石,Cu²⁺对磷酸铵镁结晶过程的影响较Mn²⁺存在时显著。