Robert K. Stevens

Evaluation of an annular denuder/filter pack system to collect acidic aerosols and gases

A glass impactor was designed and evaluated along with an annular denude/filter pack system. The glass impactor has a 50% aerodynamic cutoff of 2.1 ..mu..m at a flow of 10 L min/sup /minus/1/ and allows a quantitative transfer of gases and fine particles to the annular denuder and filter pack components. Fine particle and gas concentrations, determined by using the glass impactor along with the annular denuder/filter pack, were in good agreement with those obtained with colocated reference samplers. Measurements of SO/sub 2/, HNO/sub 3/, and HNO/sub 2/ gases showed mean collection efficiencies of 0.993, 0.984, and 0.952, respectively, which compare well with predicted values. Additionally, it was found that artifact formation of nitrate and nitrite ions, representing about 5-10% of the concentrations of HNO/sub 3/ and HNO/sub 2/, occurs in the Na/sub 2/CO/sub 3/-coated annular denuder. Corrections for these artifacts were made with a second Na/sub 2/CO/sub 3/-coated annular denuder. The results of this pilot study suggest that the glass impactor/annular denuder/filter pack sampling system is suitable for measuring acidic aerosols and gases.

Measurements of atmospheric nitrate and nitric acid: The denuder difference experiment

The denuder difference experiment is a relatively simple method of measuring fine particulate nitrate and gaseous nitric acid in the atmosphere while avoiding losses of particulate nitrate caused by sampling artifacts. Results of a field trial in a rural area are presented and show excellent reproducibility and precision even at low ambient concentrations. Values for samples collected over 23 h runs were in the range 0.8–2.1 μm −3for nitric acid and 0.6–1.2 μm−3 for particulate nitrate. During the field test gaseous nitric acid showed strong diurnal changes and particle nitrates did not. Mean day and night-time concentrations for nitric acid were 3.4 and 0.7 μg m−3, respectively; mean day- and night-time concentrations for pariculate nitrate were 1.2 and 0.9 μg m−3, respectively. We also made measurements of nitrate artifacts occurring during collection of aerosols using Teflon and glass fiber filters. In most cases, more than half of the particulate nitrate was lost from Teflon filters. Glass fiber filters collected substantial but varying amounts of nitric acid.

Intercomparison of methods for sampling and analysis of atmospheric mercury species

An intercomparison for sampling and analysis of atmospheric mercury species was held in Tuscany, June 1998. Methods for sampling and analysis of total gaseous mercury (TGM), reactive gaseous mercury (RGM) and total particulate mercury (TPM) were used in parallel sampling over a period of 4 days. The results show that the different methods employed for TGM compared well whereas RGM and TPM showed a somewhat higher variability. Measurement results of RGM and TPM improved over the time period indicating that activities at the sampling site during set-up and initial sampling affected the results. Especially the TPM measurement results were affected. Additional parallel sampling was performed for two of the TPM methods under more controlled conditions which yielded more comparable results.

Particle bounce errors in cascade impactors

Abstract In a series of field measurements on atmospheric aerosols using cascade impactors, it was found that the nature of the impaction surface has a significant effect on the apparent size distribution. For each measurement, two identical cascade impactors were operated simultaneously. Dry impaction surfaces were used in the first and, as a reference, grease-coated foils were used in the second. The types of dry impaction surfaces which were tested include aluminum foil, glass fiber filter material and wax-coated paper. From a comparison of the measured size distributions, it can be concluded that the use of dry impaction surfaces leads to a distorted measurement of the size distribution. The distortion is attributed to particles bouncing off of the dry impaction surfaces and being collected on subsequent stages or on the backup filter.

A refinement of the potassium tracer method for residential wood smoke

Abstract Potassium has been used as a tracer for the mass of fine particles emitted to the air from residential wood burning stoves and fireplaces. The technique involves measurement by x-ray fluorescence of the total K collected on fine particle filters. Since wind blown soil particles also contain K, a correction for this contribution is made based upon soil analysis or an assumed K Fe ratio in local soil. K in excess of this ratio is considered to be from wood smoke. The purpose of this study is to demonstrate an alternative method for determining wood smoke K. The underlying assumption is that wood smoke K is water soluble but that K in crustal particles is in a mineralized form and only slightly water soluble. Results from analyses of particle samples indicate the two methods yield essentially the same amount of wood smoke K.

Intercomparison of concentration results from fine particle sulfur monitors

A one week intercomparison study was carried out to evaluate the ability of selected analytical instruments to measure fine paniculate sulfur concentrations. The instruments compared included five modified flame photometric detection systems and an automated dichotomous sampler that was coupled to an on-line, wavelength dispersive X-ray fluorescence analyzer tuned to measure sulfur on fine particle filters. All instruments were connected to a common duct. Concentrations were obtained in periods ranging from 1 to 30 min, but data were reported and intercompared on an hourly basis. Paniculate sulfur concentrations varied from 1 to 9 μg m −3. Results at all concentration levels from four of the six systems agreed to better than ± 5 % throughout the study; while all six agreed within ± 25 % of their composite-mean concentration values at the higher concentrations measured. Linear regression between the composite means and each sampler systems data set showed that the standard estimate of errors ranged from 0.2 to 0.6μgm−3, correlation coefficients from 0.979 to 0.994, and slopes ranged from 0.88 to 1.22. Sulfate concentrations were also determined every 6 h from the fine particle stage of another dichotomous sampler. They were 26 % lower than expected relative to the aerosol sulfur concentrations; thus it appears that either the aerosol sulfur was not entirely in sulfate form, or sulfate was not completely extractable by conventional procedures.

Field performance of dichotomous sequential PM air samplers

Abstract For over one year, the Environmental Protection Commission of Hillsborough County (EPCHC) in Tampa, Florida, operated two dichotomous sequential particulate matter air samplers collocated with a manual Federal Reference Method (FRM) air sampler at a waterfront site on Tampa Bay. The FRM was alternately configured as a PM 2.5 , then as a PM 10 sampler. For the dichotomous sampler measurements, daily 24-h integrated PM 2.5 and PM 10–2.5 ambient air samples were collected at a total flow rate of 16.7xa0lxa0min −1 . A virtual impactor split the air into flow rates of 1.67 and 15.0xa0lxa0min −1 onto PM 10–2.5 and PM 2.5 47-mm diameter PTFE ® filters, respectively. Between the two dichotomous air samplers, the average concentration, relative bias and relative precision were 13.3xa0μgxa0m −3 , 0.02% and 5.2% for PM 2.5 concentrations ( n =282), and 12.3xa0μgxa0m −3 , 3.9% and 7.7% for PM 10–2.5 concentrations ( n =282). FRM measurements were alternate day 24-h integrated PM 2.5 or PM 10 ambient air samples collected onto 47-mm diameter PTFE ® filters at a flow rate of 16.7xa0lxa0min −1 . Between a dichotomous and a PM 2.5 FRM air sampler, the average concentration, relative bias and relative precision were 12.4xa0μgxa0m −3 , −5.6% and 8.2% ( n =43); and between a dichotomous and a PM 10 FRM air sampler, the average concentration, relative bias and relative precision were 25.7xa0μgxa0m −3 , −4.0% and 5.8% ( n =102). The PM 2.5 concentration measurement standard errors were 0.95, 0.79 and 1.02xa0μgxa0m −3 ; for PM 10 the standard errors were 1.06, 1.59, and 1.70xa0μgxa0m −3 for two dichotomous and one FRM samplers, respectively, which indicate the dichotomous samplers have superior technical merit. These results reveal the potential for the dichotomous sequential air sampler to replace the combination of the PM 2.5 and PM 10 FRM air samplers, offering the capability of making simultaneous, self-consistent determinations of these particulate matter fractions in a routine ambient monitoring mode.

Evaluation of a personal air sampler for twenty-four hour collection of fine particles and semivolatile organics.

The U.S. EPA has conducted an evaluation of a commercially available lightweight fine particle personal sampler for use in human exposure and biomarker studies involving 24-h collections of particulate matter, particle-bound organics such as polycyclic aromatic hydrocarbons (PAHs), and semivolatile organics (PAHs). Independent laboratory evaluation of the prototype design, intended to produce a 2.5-μm aerodynamic diameter cut-point at a flow between 1.5 and 1.7 lpm (liters per minute), indicated that at a challenge flow rate of 1.5 lpm, the sampler provided an aerodynamic cut-point (dp50) of only 1.7 μm. The variance between expected size selection resulted from the prototypes jet diameter dimension being inadvertently based upon a flow rate of 2.0 lpm rather than an intended 1.5–1.7 lpm. Other aerodynamic factors not presently accounted for may also have played an influence. Extrapolated cut-points for flow rates at 1 and 3 lpm for the prototype were determined to be 2.1 and 1.2 μm, respectively. Total losses attributed to unwanted particle retention within the sampler ranged from 10% for 0.91 μm size particles to essentially zero approaching diameters greater than 2.0 μm. The ambient concentration of particles (<1.7 μm) needed for acceptable gravimetric filter measurements involving 24-h sample collection was determined to be 10 μg/m3. Investigation of the sampler to retain and recover PAHs using XAD-2 resin, often of importance in human exposure biomarker studies, indicated that corrected recoveries between 94% and 108% could be obtained for 16 priority pollutant PAH species. Overall evaluation of the personal monitor indicates that it has research utility due to its modular features and size but reconfiguration should be performed that would permit true PM2.5 size selection. The current configuration collects particles less than 2.5 μm at approximately 95% collection efficiency.

Wind Tunnel Evaluation of an Aircraft-Borne Sampling System

The U.S. Environmental Protection Agency (EPA), the Florida Department of Environmental Protection (FLDEP), and Texas A&M University collaborated in the design, construction, and testing of a unique, highly crosslinked, Teflon-coated inlet and manifold gas and aerosol sampling system that is being used in EPA aircraft atmospheric pollution characterization studies. The aircraft-borne ambient sampling system, which consists of a Teflon-coated shrouded probe coupled to a Teflon-coated aluminum manifold, is designed to collect reactive gases (e.g., mercury and halide species) and aerosols for subsequent analysis and characterization. The shrouded inlet probe was tested for particle transmission ratios in a high-speed aerosol wind tunnel. An existing wind tunnel was upgraded from a maximum wind speed of 13.4 m/s (48 km/h or 30 miles/h) to 50.5 m/s (182 km/h or 113 miles/h) to test this probe. The wind tunnel was evaluated for compliance with the criteria of ANSI 13.1 to establish the acceptability of its use in testing probes. The results demonstrated that the velocity and tracer gas concentration profiles were within the specified limits. A wellcharacterized ThermoAndersen Shrouded Probe (Model RF-2-112) was also tested to check tunnel performance and test methodology. The results obtained from these tests are in close agreement with earlier published data. When operated at a sampling flow rate of 90 L/min, the aircraft-borne shrouded probe showed a transmission ratio of about 0.76 at 45 m/s (162 km/h or 100 miles/h) for 10 μ m aerodynamic diameter particles. To improve the transmission ratio of the sampling probe, the sampling flow rate was reduced to 80 L/min and the air speed increased to 50.5 m/s, which increased the transmission ratio to about 0.9 for 10 μ m particles. Further reduction of the flow rate to 60 L/min increased the transmission to 1.2. The Teflon-coated manifold, which is located downstream of the shrouded probe, was statically tested for transmission ratio at flow rates of 90 L/min and 30 L/min. The results were a transmission ratio of about 0.80 for 10 μ m aerodynamic diameter particles. The combination of the shrouded probe operated at 60 L/min with a transmission ratio of 1.2 and the manifold with its transmission of 0.8 will give an overall transmission of about unity for 10 μ m aerodynamic diameter particles at a flight speed of 50.5 m/s. These findings suggest that shrouded probes can be used for low speed (∼ 100 miles/h) aircraft applications. The transmission ratio of these probes is a significant improvement over the conventional aircraft-mounted, sharp-edged isokinetic diffuser-type inlets.

Sampling and analysis methods for use in source apportionment studies to determini impact of wood burning on fine particle mass

Abstract Aerosol sampling and analysis procedures are described which may be used to obtain ambient air measurements for input into source apportionment studies to estimate the impact of wood burning on ambient particulate loadings. Ambient wintertime fine particle compositions in Denver, CO, and Albuquerque, NM, are used as examples for receptor model calculations.