A Fabric Denuder for Sampling Semi-Volatile Species

ABSTRACT

A new style of diffusion denuder has been evaluated specifically for sampling HNO₃. A coated fabric is used as the denuder substrate, which can be loaded directly into a standard filter holder. This approach allows direct denuder sampling with no additional capital costs over filter sampling and simplifies the coating and extraction process.

Potential denuder materials and coatings were evaluated in the laboratory to test the removal efficiency. NaCl coatings were used to assess more than 20 materials for HNO₃ collection efficiency. Particle retention, which would cause a denuder to have a positive bias for gas concentration measurements, was evaluated by ambient air sampling using particulate sulfate as the reference aerosol. Particle retention varied from 0 to 15%, depending on the denuder material tested. The best performing material showed an average particle retention of less than 3%.

Denuder efficiency of four fabric materials was tested under ambient conditions to determine removal efficiency. The fabric denuder method was compared with a long path-length Fourier transform infrared (FTIR) spectrometer, a tunable diode laser absorption spectrometer (TDLAS), and a denuder difference sampler to independently measure HNO₃. HNO₃ collection efficiency was typically 90% for the denuders, whether coated with NaCl or not. For 10-L/min sampling rates with the fabric denuder, the square of the correlation coefficient with the FTIR spectrometer was 0.73, compared to 0.24 with the TDLAS.     

Precision and Accuracy in the Determination of Sulfur Oxides,Fluoride, and Spherical Aluminosilicate Fly Ash Particles in Project MOHAVE

The precision and accuracy of the determination of particu-late sulfate and fluoride, and gas phase SO₂ and HF are estimated from the results obtained from collocated replicate samples and from collocated comparison samples for high-and low-volume filter pack and annular diffusion denuder samplers. The results of replicate analysis of collocated samples and replicate analyses of a given sample for the determination of spherical aluminosilicate fly ash particles have also been compared. Each of these species is being used in the chemical mass balance source apportionment of sulfur oxides in the Grand Canyon region as part of Project MOHAVE, and the precision and accuracy analyses given in this paper provide input to that analysis. The precision of the various measurements reported here is ±1.8 nmol/m³ and ±2.5 nmol/m³ for the determination of SO₂ and sulfate, respectively, with an annular denuder. The precision is ±0.5 nmol/m³ and ±2.0 nmol/m³ for the determination of the same species with a high-volume or low-volume filter pack. The precision for the determination of the sum of HF(g) and fine particulate fluoride is ±0.3 nmol/m³. The precision for the determination of aluminosilicate fly ash particles is ±100 particles/m³. At high concentrations of the various species, reproducibility of the various measurements is ±10% to ±14% of the measured concentration. The concentrations of sulfate determined using filter pack samplers are frequently higher than those determined using diffusion denuder sampling systems. The magnitude of the difference (e.g., 2-10 nmol sulfate/m³) is small, but important relative to the precision of the data and the concentrations of particulate sul-fate present (typically 5-20 nmol sulfate/m³). The concentrations of SO₂(g) determined using a high-volume cascade impactor filter pack sampler are correspondingly lower than those obtained with diffusion denuder samplers. The concentrations of SO_x (SO₂(g) plus particulate sulfate) determined using the two samplers during Project MOHAVE at the Spirit Mountain, NV, and Hopi Point, AZ, sampling sites were in agreement. However, for samples collected at Painted Desert, AZ, and Meadview, AZ, the concentrations of SO_x and SO₂(g) determined with a high-volume cascade impactor filter pack sampler were frequently lower than those determined using a diffusion denuder sampling system. These two sites had very low ambient relative humidity, an average of 25%. Possible causes of observed differences in the SO₂(g) and sulfate results obtained from different types of samplers are given.     

Comparing field performances of denuder techniques in the high Arctic

A field evaluation between two annular denuder configurations was conducted during the spring of 2003 in the marine Arctic at Ny-Ålesund, Svalbard. The IIA annular denuder system (ADS) employed a series of five single-channel annular denuders, a cyclone and a filter pack to discriminate between gas and aerosol species, while the EPA-Versatile Air Pollution Sampler (VAPS) configuration used a single multi-channel annular denuder to protect the integrity of PM_2.5 sample filters by collecting acidic gases. We compared the concentrations of gaseous nitric acid (HNO₃), nitrous acid (HONO), sulfur dioxide (SO₂) and hydrochloric acid (HCl) measured by the two systems. Results for HNO₃ and SO₂ suggested losses of gas phase species within the EPA-VAPS inlet surfaces due to low temperatures, high relative humidities, and coarse particle sea-salt deposition to the VAPS inlet during sampling. The difference in HNO₃ concentrations (55%) between the two data sets might also be due to the reaction between HNO₃ and NaCl on inlet surfaces within the EPA-VAPS system. Furthermore, we detected the release of HCl from marine aerosol particles in the EPA-VAPS inlet during sampling contributing to higher observed concentrations. Based on this work we present recommendations on the application of denuder sampling techniques for low-concentration gaseous species in Arctic and remote marine locations to minimize sampling biases. We suggest an annular denuder technique without a large surface area inlet device in order to minimize retention and/or production of gaseous atmospheric pollutants during sampling.     

A Passive Flux Denuder for Evaluating Emissions of Ammonia at a Dairy Farm

Abstract

Passive samplers have been shown to be an inexpensive alternative to direct sampling. Diffusion denuders have been developed to measure the concentration of species such as ammonia (NH₃), which is in equilibrium with particulate ammonium nitrate. Conventional denuder sampling that inherently requires air pumps and, therefore, electrical power. To estimate emissions of NH₃ from a fugitive source would require an array of active samplers and meteorological measurements to estimate the flux. A recently developed fabric denuder was configured in an open tube to passively sample NH₃ flux. Passive and active samplers were collocated at a dairy farm at the California State University, Fresno, Agricultural Research Facility. During this comparison study, NH₃ flux measurements were made at the dairy farm lagoon before and after the lagoon underwent acidification. Comparisons were made of the flux measurements obtained directly from the passive flux denuder and those calculated from an active filter pack sampler and wind velocity. The results show significant correlation between the two methods, although a correction factor needed to be applied to directly compare the two techniques. This passive sampling approach significantly reduces the cost and complexity of sampling and has the potential to economically develop a larger inventory base for ambient NH₃ emissions.     

Simultaneous sampling of NH3, HNO3, HC1, SO2 and H2O2 in ambient air by a wet annular denuder system

A new sampling device is described for the simultaneous collection of NH₃, HNO₃, HCl, SO₂ and H₂O₂ in ambient air. The apparatus is based on air sampling by two parallel annular denuder tubes. The gases are collected by absorption in solutions present in the annulus of the denuder tubes. After a sampling time of 30 min at flow rate of 32 ℓ min⁻¹ the solutions are extracted from the denuders and analyzed off-line. The detection limits of NH₃, HNO₃, HCL and SO₂ are in the order of 0.1–0.5 μm⁻³. For H₂O₂ the detection limit is 0.01 μm⁻³. The reproducibility is 5–10% at the level of ambient air concentrations. Comparison of this novel technique with existing methods gives satisfactory results. The compact set-up offers the possibility of field experiments without the need of extensive equipment.     

A comparison of three methods for measurement of atmospheric nitric acid and aerosol nitrate and ammonium

Three methods for measuring gaseous ambient nitric acid in the low concentration range 0–15 μ m⁻³ were compared under field conditions in Southwestern Ontario during 1–14 June 1982. The methods employed were (1) tunable diode laser absorption, (2) a tungstic acid denuder tube and (3) a filter pack containing a Teflon-nylon-W41 filter combination operated over 3-h sampling periods. In general, the three methods measured nitric acid with adequate sensitivity (sub ppb) and they correlated reasonably over the low ambient concentrations considered here. However, there were some notable differences (up to a factor of 2 at night) by the tungstic acid and filter pack methods. During the field comparison, particulate nitrate and ammonium were also measured (0–6 μg m⁻³ range) by the tungstic acid and filter pack methods. These correlated well but some differences (less than 20%) were observed in the particulate nitrate measurements. In view of the low concentrations measured and the resultant larger experimental error, specific loss or interfering mechanisms could not be unambiguously identified.     

Long-range transport of gaseous and particulate oxidized nitrogen compounds

Measurements of NO₂, PAN, HNO₃ and particulate NO₃⁻ as 24-h mean concentration values have been carried out at a clean air station. The station is situated on the Swedish west coast 40 km south of Gothenburg. Measurements from November 1981 to October 1982 are presented. NO₂ was measured with the Saltzman method after removal of ozone and PAN with a gas Chromatographic method. HNO₃ was sampled in a denuder and particulate NO₃⁻ was sampled on an impregnated filter behind the denuder. The denuder and filter were then analysed by ion chromatography. Episodes of high concentrations occurred several times during the year. On these occasions the air was transported over a long range from the continent. 72-h back trajectories are available for this station at 6-h intervals during the whole period of measurements. Monthly mean concentrations of these four species, together with data on SO₂ and particulate sulphate are presented. Analysis of the data as a function of the direction of the back trajectory and correlations between nitrogen and sulphate compounds are also given.     

A fabric denuder for sampling semi-volatile species

A new style of diffusion denuder has been evaluated specifically for sampling HNO3. A coated fabric is used as the denuder substrate, which can be loaded directly into a standard filter holder. This approach allows direct denuder sampling with no additional capital costs over filter sampling and simplifies the coating and extraction process. Potential denuder materials and coatings were evaluated in the laboratory to test the removal efficiency. NaCl coatings were used to assess more than 20 materials for HNO3 collection efficiency. Particle retention, which would cause a denuder to have a positive bias for gas concentration measurements, was evaluated by ambient air sampling using particulate sulfate as the reference aerosol. Particle retention varied from 0 to 15%, depending on the denuder material tested. The best performing material showed an average particle retention of less than 3%. Denuder efficiency of four fabric materials was tested under ambient conditions to determine removal efficiency. The fabric denuder method was compared with a long path-length Fourier transform infrared (FTIR) spectrometer, a tunable diode laser absorption spectrometer (TDLAS), and a denuder difference sampler to independently measure HNO3. HNO3 collection efficiency was typically 90% for the denuders, whether coated with NaCl or not. For 10-L/min sampling rates with the fabric denuder, the square of the correlation coefficient with the FTIR spectrometer was 0.73, compared to 0.24 with the TDLAS.     

A sodium carbonate coated denuder for determination of nitrous acid in the atmosphere

A denuder technique for sampling and analysing nitrous acid at sub ppb levels in air is described. After sampling, the sodium carbonate coated denuder is leached in water, and the NO⁻₂ concentration is determined spectrophotometrically. Field tests show that PAN is partly sorbed and hydrolized to nitrite in the sodium carbonate layer. It seems as HNO₂ also can be formed by heterogeneous reactions between NO and NO₂ at the denuder wall. These sampling artifacts were overcome by sampling with two or three denuders in series. The presence of PAN deteriorates the detection limit, which during optimal conditions is about 0.5 nmole m⁻³ (0.01 ppb). The method is therefore not recommended for measurements in background air, where HNO₂ concentrations normally are low compared to PAN concentrations.     

Determination of PM2.5 Sulfate and Nitrate with a PC-BOSS Designed for Routine Sampling for Semi-Volatile Particulate Matter

ABSTRACT

Ambient particles contain substantial quantities of material that can be lost from the particles during sample collection on a filter. These include ammonium nitrate and semi-volatile organic compounds. As a result, the concentrations of these species are often significantly in error for results obtained with a filter pack sampler. The accurate measurement of these semi-volatile fine particulate species is essential for a complete understanding of the possible causes of health effects associated with exposure to fine particles. Past organic compound diffusion denuder samplers developed by the authors (e.g., the Brigham Young University Organic Sampling System [BOSS]) are not amenable to routine field use because of the need to independently determine the gas-phase semi-volatile organic material efficiency of the denuder for each sample. This problem has been eliminated using a combined virtual impactor, particle-concentrator inlet to provide a concentrated stream of 0.1-2.5-μm particles. This is followed by a BOSS diffusion denuder and filter packs to collect particles, including any semi-volatile species lost from the particles during sampling. The samp ler (Particle Concentrator-Brigham Young University Organic Sampling System [PC-BOSS]) contains a post-denuder multifilter pack unit to allow for the routine collection of several sequential samples. The PC-BOSS can be used for the determination of both fine particulate nitrate and semi-volatile organic material without significant “positive” or “negative” sampling artifacts. Validation of the sampler for the determination of PM_2.5 sulfate and nitrate based on comparison of results obtained at Riverside, CA with collocated PC-BOSS, annular denuder, and Chem Spec samplers indicates the PC-BOSS gives accurate results for these species with a precision of ±5-8%. An average of 33% of the PM_2.5 nitrate was lost from the particles during sampling for both denuder and single filter samplers.     

An intercomparison of formaldehyde measurement techniques at ambient concentration

A study was conducted to evaluate five techniques for determining ambient formaldehyde concentrations. One technique used a spectroscopic determination, and the other four techniques used derivatization followed by fluorometric analysis or high-performance liquid chromatography with detection by u.v. absorption. Formaldehyde was generated by two techniques. In the first technique, zero air was bubbled through a solution of aqueous formaldehyde to produce gas-phase formaldehyde. Various compounds serving as possible interferences were added singly or in combination to these air mixtures. In the second technique, formaldehyde was generated as a product from controlled irradiations of hydrocarbons and nitrogen oxides in a smog chamber operated in a dynamic mode. The study was conducted as a blind intercomparison with no knowledge by the participants of the HCHO concentrations or the interferences added.The data from each of the techniques were compared against mean values in each sampling period. For formaldehyde in zero air, average deviations for each of the techniques ranged between 15 and 30%. At a formaldehyde concentration of 10 ppb, each technique showed no evidence for interferences by O₃ (190 ppbv), NO₂ (300 ppbv), SO₂ (20 ppbv), and H₂O₂ (7 ppbv). The agreement for formaldehyde concentrations measured for the photochemical mixtures was similar to that of the mixtures in zero air.Ambient measurements were also performed on three evenings and for one 36-h period. Ambient formaldehyde concentrations ranged from 1 to 10 ppbv. Ambient H₂O₂ measurements were also performed. A strong correlation in the diurnal concentration profile for formaldehyde and H₂O₂ was observed over the 36-h period.     

Environmental Issues—Persistent Problems and Emerging Challenges

Abstract

A field study was performed to evaluate indoor air concentrations and vapor intrusion (VI) of tetrachloroethylene (PCE) and other chlorinated solvents at a commercial retail site in Dallas, TX. The building is approximately 40 yr old and once housed a dry cleaning operation. Results from an initial site characterization were used to select sampling locations for the VI study. The general approach for evaluating VI was to collect time-integrated canister samples for off-site U.S. Environmental Protection Agency Method TO-15 analyses. PCE and other chlorinated solvents were measured in shallow soil gas, subslab soil-gas, indoor air, and ambient air. The subslab soil gas exhibited relatively high values: PCE ≤2,600,000 parts per billion by volume (ppbv) and trichloroethylene ≤170 ppbv. The attenuation factor, the ratio of indoor air and subslab soil-gas concentrations, was unusually low: approximately 5 x 10^-6 based on the maximum subslab soil-gas concentration of PCE and 1.4 x 10^-5 based on average values.     

A passive flux denuder for evaluating emissions of ammonia at a dairy farm

Passive samplers have been shown to be an inexpensive alternative to direct sampling. Diffusion denuders have been developed to measure the concentration of species such as ammonia (NH3), which is in equilibrium with particulate ammonium nitrate. Conventional denuder sampling has required active sampling that inherently requires air pumps and, therefore, electrical power. To estimate emissions of NH3 from a fugitive source would require an array of active samplers and meteorological measurements to estimate the flux. A recently developed fabric denuder was configured in an open tube to passively sample NH3 flux. Passive and active samplers were collocated at a dairy farm at the California State University, Fresno, Agricultural Research Facility. During this comparison study, NH3 flux measurements were made at the dairy farm lagoon before and after the lagoon underwent acidification. Comparisons were made of the flux measurements obtained directly from the passive flux denuder and those calculated from an active filter pack sampler and wind velocity. The results show significant correlation between the two methods, although a correction factor needed to be applied to directly compare the two techniques. This passive sampling approach significantly reduces the cost and complexity of sampling and has the potential to economically develop a larger inventory base for ambient NH3 emissions.     

Performance of the Annular Denuder System with Different Arrangements for HNO3 and HNO2 Measurements in Taiwan

ABSTRACT

Experiments on different annular denuder system (ADS)arrangements for sampling nitrous acid (HNO₂) and ni-tric acid (HNO₃) gases were conducted in this study toevaluate their sampling artifacts. The evaluation basis isthe one that employed one sodium chloride denuder forsampling HNO₃ gas and two sodium carbonate (Na₂CO₃)denuders for sampling HNO₂ gas, which is a commonlyemployed ADS arrangement in many field applicationsin the United States. A field study was conducted inHsinchu, Taiwan, and the results indicated that this ADSarrangement may yield over 80% relative errors for HNO₃gas. It also showed that the relative errors for HNO₂ gascan be less than 10% as sampled with only one Na₂CO₃denuder. This is attributed to the fact that the ambientHNO₃ concentration measured in this study was relativelylow while the HNO₂ concentration was high, as comparedto typical concentrations of these two gases measured inthe United States.

The sampling error of HNO₃ gas may be due to highconcentrations of N-containing interfering speciespresent in Taiwan’s atmosphere. Because the relative sam-pling errors of HNO₃ and HNO₂ gases depend mainly ontheir concentrations in the atmosphere as well as con-centrations caused by interfering species, the risk for higherror while measuring low HNO₂ concentrations by onlyone Na₂CO₃ denuder is also possible. As a result, it is sug-gested that pretests are necessary to evaluate possiblesources and degrees of sampling errors before fieldsampling of HNO₂ and HNO₃ gases. The sampling errorsof these two gases can, therefore, be minimized with abetter arrangement of the ADS.     

Use of Two Different Acidic Aerosol Samplers To Measure Acidic Aerosols in Hsinchu,Taiwan

ABSTRACT

Acidic aerosol concentrations measured by an annular denuder system (ADS) and a honeycomb denuder system (HDS) in Hsinchu, Taiwan, were compared. Aerosols were also sampled by a MOUDI (micro-orifice uniform deposit impactor) and analyzed by an ion chromatograph to determine the size distributions of different species. Using the measured aerosol size distribution, theoretical analysis showed that positive HNO₃ artifact due to volatiliza-tion of NH₄NO₃ is generally negligible for both samplers.4 3Comparing two different denuder samplers, the average concentration of HNO₃ measured by the ADS was found3to be lower than that measured by the HDS, while the difference between the two samplers for the average concentration of other species was found to be within ±15%. A possible cause of the difference in HNO₃ con-3centrations is due to a greater loss of HNO₃ in the cyclone3 used by the ADS than in the impactor used by the HDS. The study also showed incomplete absorption of the evaporated HCl and HNO₃ from the particles on the Teflon3filter by the first nylon filter in the filter pack of the ADS. Collection efficiency and capacity of HCl and HNO₃ by3the nylon filters need further investigation.     

Concentration of tetrachloroethylene in indoor air at a former dry cleaner facility as a function of subsurface contamination: a case study

A field study was performed to evaluate indoor air concentrations and vapor intrusion (VI) of tetrachloroethylene (PCE) and other chlorinated solvents at a commercial retail site in Dallas, TX. The building is approximately 40 yr old and once housed a dry cleaning operation. Results from an initial site characterization were used to select sampling locations for the VI study. The general approach for evaluating VI was to collect time-integrated canister samples for off-site U.S. Environmental Protection Agency Method TO-15 analyses. PCE and other chlorinated solvents were measured in shallow soil gas, subslab soil-gas, indoor air, and ambient air. The subslab soil gas exhibited relatively high values: PCE < or =2,600,000 parts per billion by volume (ppbv) and trichloroethylene < or =170 ppbv. The attenuation factor, the ratio of indoor air and subslab soil-gas concentrations, was unusually low: approximately 5 x 10(-6) based on the maximum subslab soil-gas concentration of PCE and 1.4 x 10(-5) based on average values.     

Ammonia concentrations and modeling of inorganic particulate matter in the vicinity of an egg production facility in Southeastern USA

Ammonia (NH₃) is an important base gas and can react with acidic species to form atmospheric aerosols. Due to the rapid growth of poultry and swine production in the North Carolina Coastal Plain, atmospheric NH₃ concentrations across the region have subsequently increased. Ammonia concentrations and inorganic particulate matter (PM) at four ambient stations in the vicinity of an egg production facility were measured for 1 year using PM_2.5 speciation samplers with honeycomb denuders and ion chromatography (IC). Meanwhile, concentrations of NH₃ and inorganic PM in one of the egg production houses were also simultaneously measured using a gas analyzer for NH₃ and the filter pack plus IC method for inorganic PM. An equilibrium model-ISORROPIA II was applied to predict the behavior of inorganic aerosols in response to precursor gas concentrations and environmental parameters. Average ambient NH₃ concentrations varied from 10.0 to 27.0 μg/m³, and they were negatively correlated with the distances from the ambient location to the nearest egg production house exhausts. Ambient NH₃ concentrations were higher in warm seasons than in cold seasons. Measured NH₃ concentrations agreed well with ISORROPIA II model predictions at all sampling stations. For the ambient stations, there was a good agreement in particle phase NH₄ ⁺ between the model simulation and observations. For the in-house station, the model simulation was applied to correct the overestimation of particle phase NH₄ ⁺ due to gas phase NH₃ breaking through the denuders. Changes in SO₄ ^2?, NO₃ ^?, and Cl^? yield proportional changes in inorganic PM mass. Due to the abundance of NH₃ gas in the vicinity area of the monitored farm, changes in NH₃ concentrations had a small effect on inorganic PM mass. Aerosol equilibrium modeling may be used to assess the influence of precursor gas concentrations on inorganic PM formation when the measurements for some species are unavailable.     

Sampling of carbonaceous particles in the atmosphere

A method for measurement of atmospheric carbonaceous particulate matter is proposed and evaluated. A diffusion denuder, placed ahead of a filter, is used to exclude vapor-phase organic compounds which might be retained by adsorption on the filter. The carbonaceous material lost by volatilization from the filter after collection is retained with a fluidized bed of powdered Al₂O₃. The efficiencies of the denuder and Al₂O₃ were evaluated with vapor phase phenanthrene and octadecane. Atmospheric sampling was done at two urban and one background site in California using the proposed technique in parallel with conventional hi-vol and dichotomous samplers. Hi-vol samples appear to have lost up to half of the particle phase carbon during sampling. About half this error could be eliminated by sampling at low face velocity. The effects of face velocity appear to distort the proportions of fine and coarse particulate carbon obtained with a dichotomous sampler.     

Simultaneous Sampling of Gas- and Aerosol-Phase TDI with a Triple Filter System

Abstract

A new triple filter system sampler/model is proposed for the precise and accurate simultaneous sampling and determination of gas- and aerosol-phase 2,4-toluene diisocyanate (TDI). The system consists of two front Teflon filters for sampling aerosol-phase TDI and a final coated glass fiber filter to collect gas-phase TDI. The aerosol-phase TDI is collected on the first Teflon filter, while the second Teflon filter is used to estimate gaseous TDI adsorbed by the first. According to the gas adsorption test of two Teflon filters in series, the TDI gas adsorption fraction of the two filters is almost the same. Results of the evaporation test using pure TDI aerosols collected on the Teflon filter show that significant evaporation of the compound does not occur during sampling. These two findings allow the use of a model to estimate accurate gasand aerosol-phase TDI concentrations. The comparison test with an annular denuder shows that the triple filter system can minimize the TDI sampling bias between the dual filter and the annular denuder systems.