Final Report: Atmospheric Fate and Dry Deposition of Urban Soot to Great Waters Using a Novel, State-of-the-Art Isotopic Particulate TracerEPA Grant Number: R825247
Title: Atmospheric Fate and Dry Deposition of Urban Soot to Great Waters Using a Novel, State-of-the-Art Isotopic Particulate Tracer
Institution: University of Maryland - College Park
EPA Project Officer: Chung, Serena
Project Period: December 6, 1996 through December 5, 1999 (Extended to December 5, 2000)
Project Amount: $454,976
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:Venkataraman and Friedlander (1994) reported that, whereas freshly emitted soot aerosol is confined to 0.12- µm particles, size distributions of aged urban soot particles contain significant mass in 0.5 to 2- or 3- µm particles, which they attributed to growth by accumulation of secondary aerosol products (e.g., sulfates and nitrates) on the timescale of urban residence time. Thus, their results suggested that aged soot, transported from outside an urban area, might be discriminated from fresh soot on the basis of its size distribution and chemical composition, especially if a spatially discrete emission source could be isolated and uniquely tagged with an appropriate intentional tracer. In this project, our work encompassed tagging urban soot aerosol by adding an enriched isotopic tracer (administered as the 2,2-6,6-tetramethyl heptane dione) to diesel fuel burned by the city of Baltimore's sanitation truck fleet to provide an unambiguous source of "tagged" soot aerosol and ambient measurements of the tagged soot and ambient aerosol concentrations/size distributions. The project objectives were to: (1) develop optimum tracer analysis techniques; (2) use tracer methodology to investigate the atmospheric behavior and deposition of tagged soot particles; (3) investigate gradient flux measurement techniques for determining the flux and deposition velocity of carbon soot particles depositing onto the Chesapeake Bay; and, ultimately, (4) determine the amount of soot depositing to the Chesapeake Bay. An additional objective was to determine the feasibility of quantitative surface-layer sampling of freshly deposited soot.
In the first year of the project, mass spectrometry procedures for highly precise and accurate determinations of iridium (Ir) were developed for application to dry deposition flux measurements using difference methods (i.e., relaxed eddy accumulation and the gradient method) and for determining changes in the size distribution and atmospheric concentration of urban diesel soot tagged with an Ir tracer. The work encompassed development of new software for instrument control and primary data reduction (i.e., oxygen correction) for both the Faraday cup and electron multiplier detectors of our thermal-ionization mass spectrometers; error propagation was written into the isotope dilution software, and new software for the isotope dilution mass spectrometry and error propagation was specially developed for the Ir analyses. Measurement precision of ±0.2 percent for ambient aerosol samples containing 1 to 10 pg quantities of Ir (detection limit 150 fg) was achieved. Analysis by instrumental and radiochemical neutron activation analyses (INAA and RNAA, respectively) also were investigated. Precision of ±0.5 percent is achieved by INAA with a detection limit of 90 fg. No additional benefit was achieved by RNAA. A screen sampler for measuring deposition to the surface microlayer was constructed and tested, but it was found to be too irreproducible for our application. Lastly, Ir background concentrations in fine particles (i.e., <2.5 m particle diameter) collected previously (November1995, July 1996, and October 1996) in Baltimore were determined.
During the second project year, we developed and tested a pair of systems for precise volumetric control of our MOI flowrates and a system for sampling at elevated (i.e., 90%) relative humidity (RH). The flow control system limits flow rate fluctuations to ±0.12 percent during sampling with Micro-Orifice impactors. The system for RH control permitted sampling at elevated RH (e.g., 90%) with an average relative precision of ±1.7 percent during field measurements. These systems were deployed in and around Baltimore from July to December, 1998, to determine differences in the size distributions of trace elements and Ir-tagged soot (emitted from Baltimore City sanitation trucks) at high and low RH (two MOIs operated simultaneously at the same site) and after transport of up to 30 km (two MOIs operated simultaneously at different sites). The hypothesis to be tested was that soot particles accumulate hygroscopic material (such as nitrates and sulfates) during transport and, thus, grow in size when exposed to high ambient RH. At 90 percent RH, soot particles (initially 0.2 m) could be expected to grow hygroscopically if they contained about 20 percent ammonium sulfate, by mass, such as might occur after atmospheric aging (Borgoul Maciejczyk, 2000). Additionally, airfoil deposition collectors and dichotomous samplers also were frequently operated. Sampling sites included Lake Clifton Park (center, Figure 1), slightly northeast of downtown Baltimore, and counter clockwise from bottom center in Figure 1, from Baltimore/Washington International Airport (BWI), Ft. Howard, Fallston Airport, Oregon Ridge, and Sykesville. Shown in Figure 1 are Ir concentration "roses" determined in PM2.5 at these sites during the tracer release periods. Tracer emissions tended to be most concentrated in the southwestern quadrant of Baltimore due to the locations of the fuel depots participating in tagging.
During the third project year, all of the samples collected during 1998 (i.e., 73 10-stage impactor sets, 108 dichotomous filter samples, and 14 airfoils) were analyzed for Ir and up to 20 elements, including Ce, Cr, Cs, Fe, Sb, Sc, Se, and Zn, by instrumental neutron activation. Companion meteorological data, wind speed, direction, and temperature and RH data were collected and processed. Also, atmospheric mixing heights were determined for all days of sampling by plotting the T-skew plot from the Aberdeen soundings data downloaded from the Internet.
Particle dry deposition fluxes are determined by gradient measurements by application of Fick's Law (Seinfeld, 1986) or by the eddy accumulation method (Speer, et al., 1985). In the former, the concentration difference is measured at two heights above the surface and the flux is determined as the product of the concentration gradient and the turbulent diffusivity (Kturb) derived from micrometeorological methods. The latter makes use of measurements of the fluctuation (σW) of the vertical wind speed and particle concentrations measured during up and down excursions (Speer, et al., 1985). Calculations were made to estimate concentration differences that could be expected from these approaches. Estimates of critical parameters (friction velocity, σW) were made using published formulae/approximations as described below. Micrometeorological determinations of these were made in collaboration with Pahlow and Parlange in May 2000, at Clifton Park.Iridium Analysis. Ir analyses in tagged aerosol samples must be made with as good a precision as possible to permit differentiate of small (1%) concentration gradients and small differences in the concentrations of tagged soot in various size intervals to be determined from impactor measurements. Our target precision is 0.1 percent. The development of a precise and accurate mass spectrometry analysis technique for picogram quantities of Ir requires high ionization efficiency, low filament blank, and low reagent and column blanks. As a practical matter, ionization efficiency of the analyte is a function of the work function of the element with which the filament is composed and can be improved through the use of an "emitter substance." Eight filament materials (i.e., Re, Mo, Ta, W, Ni, Au, Pt, and Pd) were tested for use in the Ir analyses. Poor or little ionization was achieved with Re, Mo, Ta, and W. An ion yield of 0.5 to 1 percent was achieved with Ni, whereas those for Au, Pt, and Pd all exceeded 10 percent. Unfortunately, Au melts too low (mp 1060°C) to be useful in Ir analyses, and both Pt and Pd had high Ir blanks (3 and 0.2 pg, respectively), making Ni, for which the Ir blank is only 0.03 pg, the material of choice, despite the lower ionization efficiency achieved with this material. A seven-fold reduction in the Ir blank was achieved with a Pt filament after treatment with molten potassium pyro-sulfate. This reduced level (about 0.4 pg) is still ten-fold greater than the blank attained for Ni.
Four emitter compounds were tested: Ba(NO3)2, Ba(OH)2, La(NO3)3, and [La+Gd](NO3)3. Barium is a common emitter used (in form of nitrate or hydroxide) for mass spectrometry analyses of Re, Os, Ru, and Ir. The emitter supplies oxygen for oxidation of the analyte metal and at the same time reduces the electron work function of the filament material. For Re and Os, Ba is used as a sole emitter. However, for Ir, we found that addition of La nitrate to Ba(OH)2 improves ionization efficiency by a factor of 2.
Figure 2 shows a signal profile during the determination of ionization yield. In this particular case, ~100 pg of Ir were loaded onto Pt filament with BaOH)2 as emitter. During the experiment, four independent sets of measurements were made, each of 60 individual ratios. Each set gave result with uncertainty less than 0.05 percent, and all four averages are consistent with each other within error limits. The area shaded represents total electrical charge detected during the experiment and, thus, is proportional to the total number of ions registered. The ionization yield is defined as [total number of ions registered/total number of atoms loaded]. In this case, the ionization yield was 7.5 percent.
For some metals, oxides are formed more easily from the neutral metal than from halogen salt (e.g., Os). In such cases, a preheating of sample on the filament up to ~800°C under high vacuum substantially improves the ionization. Preheating causes the decomposition of metal halides and reduction of metal to zero valency. However, the Ir ionization efficiency was not affected by preheating.
As indicated by the equation below, the total amount of Ir (T) in a sample is calculated from differences between the measured Ir (191Ir/193Ir) and analytical "spike" ratios and the ratio of Ir isotopes in a reference (i.e., natural material).
T = [Gt*Ct]*[(µIr / µ191)*((1+Kn)/Kt)]*[(Kt-Km)/(Km-Kn)],
where Ct is the concentration of 191Ir in spike solution, ng/g; Gt is the weight of spike solution used, g; mIr and m191 are the atomic masses in carbon units of Ir (192.192) and 191Ir (190.961), respectively; and Kn, Kt, and Km are the Ir ratios (191Ir/193) Ir in the natural reference material, analytical spike, and sample, respectively.
Errors in Ct, affect only the accuracy of the tracer determinations. Errors in Kn, and Kt affect precision and accuracy. A series of analyses were undertaken to determine the uncertainties in these parameters for our analyses. Differences in the 191Ir/193Ir ratio are negligible in natural materials, but are subject to small fractionation (of the order of 0.1%) during ionization in the mass spectrometer. As Ir has only two isotopes, fractionation corrections cannot be made directly. However, we can account for any instrument fractionation by using the measured ratio for pure Ir salts rather than the true natural ratio. As long as the same reference standard is used for all analyses, then only the statistical scatter in the fractionation is important. Tests in our laboratory show a 0.1 percent difference between the natural Ir ration measured by Walczyk and Heumann (1993), but the scatter (1 standard deviation of the mean of 8 replicate measurements) is only 0.0075 percent, a value nearly three-fold better than that reported by Walczyk and Heumann. Likewise, the precision (2 standard deviations of 13 replicate measurements) in our analytical spike, Kt, is 0.055 percent. Because of the absence of Ir compounds with fixed stoichiometry, we have calibrated our analytical spike relative to both a standard solution and a well-characterized iron meteorite. Absolute calibration accuracy in our laboratory for the determination of 191Ir is 0.9 percent, which largely represents the accuracy in weighing in this particular set of measurements. Precision of replicate determinations was typically ±0.2 percent (1 standard deviation, Figure 3).
The remaining work conducted involved development of techniques to dissolve Ir in particulate samples and separate the Ir from interfering isobars prior to mass spectrometry analysis. A Carious tube digestion procedure involving heating the sample in a sealed pyrex tube containing aqua regia for 12 to 14 hours at a temperature of 230°C was adapted. Separation of Ir involves oxidation to Ir(IV) prior to a one-step anion-exchange chromatography technique wherein Ir is eluted from the column with 6 N HCl after reduction. Various reducing agents were tested, including H2SO3 (Anbar, et al., 1997), thiourea, and NaNO2. We found thiourea to be more effective in reducing Ir(IV) than H2SO3 while eliminating the suppression of ionization by sulfuric acid that accompanies reduction with this reagent. The Ir blank contribution from the chromatographic separation is currently 500±400 pg, which we deem marginally acceptable.
Surface Layer Sampling. Experiments with a metal screen (Garret, 1965) indicated that segments of a water surface were removed intact when a screen was placed either in horizontal contact with or drawn vertically through the liquid surface. In that matter, surface water could be trapped between the wires and lifted from the bulk water. The screen entrapped liquid is then drained into a collection vessel. Though 100 percent efficiency could not be attained due to initial absorption of a portion of the monolayer onto the screen wires, the screen method is about 70 percent efficient according to Garret. Collection efficiency by Hatcher and Parker (1974) came out with much poorer results; therefore, we initiated our own tests.
We constructed a screen sampler from a rectangular piece (37.4 × 37.6 cm) of stainless steel 8 × 8 mesh with 60.2 percent open area, wire diameter 0.028 inches, attached to a PVC frame. Tests on the reproducibility of the drawn volumes were performed. The sampler was dipped into a rectangular pan, and the first volume was collected. Immediately, the second volume was collected. After second collection, the screen was rinsed with a small amount of water, which would be a part of a sample. Samples containing two volumes and a rinse were 108.4±4.5 ml.
Laboratory tests of the screens for microlayer sampling were performed using an aerosol containing fluorescent particles (3-5 m diameter) dispersed in an 8-m aerosol chamber. The particles were deposited onto the water in a plexiglass pan at the opposite end of the chamber and water samples were collected with the screenbefore exposure and after exposure. In addition, three successive aliquots of pan water were collected. The collection efficiencies of the collected particles were determined by fluorescence. Results indicated that: (1) the screen sampler removes particles from the surface on the first dip with approximately 78 percent efficiency; (2) the second screen sample still contained dye; (3) bulk pan water samples collected before and after screening contains the same amount of dye, indicating that particles stay on the surface and do not sink to the bottom of the pan; and (4) the results between trials are not the same, which means that either the dispersion method or screening technique was not reproducible.
Iridium Background Measurements. Ir background concentrations for fine particles (i.e., <2.5 m particle diameter) collected in Baltimore were determined from samples collected in November 1995, July 1996, and October 1996, and are shown in Table 1. As indicated, the screen sampler was employed to determine an Ir blank for surface water collected in the Baltimore inner harbor.
Impactor Measurements. To assure consistency in our impactor measurements, the cutoff diameters of Micro-Orifice Impactor plates were redetermined and a new nozzle plate (D50 of 0.093µ) was installed to achieve an identical cut point for the final stage of two of our impactors.
System for precise flow control. A numerical model (Borgoul Maciejczyk, et al., 2000) was constructed to calculate uncertainties in the size distribution measured with Micro-Orifice Impactors as a result of fluctuations in the sample volumetric flow rate. The model generates uni- or bimodal aerosol size distributions and processes these with the full set of multipoint impactor calibration curves for the Micro-Orifice Impactor, to calculate the mass that would be collected on each stage at a specified flow rate. The effect of fluctuations in the sampling flow rate was determined by running the model with positive and negative deviations about the mean flow. The sizes of these errors depend on the size distribution of the sampled aerosol and the level of flow rate fluctuation. For a log-normally distributed particle population, mass errors due to flow rate fluctuations were bimodally distributed about stages with cut points near the aerosol MMAD. The largest errors occurred, uniformly, on the stage with the smallest cut point (here, 0.059 m). These errors were asymmetric with respect to sign, which leads to a net error for a randomly fluctuating flow rate. In general, mass errors increased with decreasing geometric standard deviation (σg), and were substantially greater for populations with 0.5- m MMADs than for those with 0.2 m MMADs. The largest net errors for the former were 4, 110, and 560 percent for σg of 1.2 and flow rate fluctuations of ±1, ±5, and ±10 percent, respectively; but they decreased to 0.03, 0.9, and 4 percent, respectively, for a σg of 1.9. Also, the net mass errors were 1 percent when flow rate fluctuations were 1 percent, for all but the most narrow aerosol σg (i.e., that of 1.2). It was apparent from the calculations that flow rate fluctuations would need to be controlled to <0.5 percent to achieve the 1 percent net mass error for the most narrow test aerosol. Such narrow distributions have been observed in-stack and in the ambient plume of municipal incinerators (Ondov and Wexler, 1998), and may be a common feature of high-temperature combustion sources in which particle growth is dominated by condensation (Biswas, et al., 1992). Flow rate fluctuations, therefore, lead to a positive bias in the geometric standard deviation inferred from the measured masses and reduces the user's ability to interpret differences in size distributions.
Therefore, to achieve accuracy in size distributions that are to be measured at different locations and compared, a flow control system was constructed to provide MOI flow rates at 30 LPM with a fluctuation (1 standard deviation) of 0.06 percent (600 ms averaging time, 0.67% for 5 ms averaging; Borgoul Maciejczyk, et al., 2000), a level of precision which allows for accurate relative calibration between flow systems, and for which errors from flow rate fluctuations are negligible. The system was assembled using a Hastings mass flow meter (HFM-201), MKS capacitance manometer (220CD-01000 A2B), and Sierra (model 840-L-VO) proportional control valve. A data logger (14 Bit) was used for data acquisition and control. RH and temperature are monitored downstream of the impactors with a Vaisala HMP45C-L11 sensor (±1% RH at 89% RH; ±0.2°C). Atmospheric pressure is monitored with a Vaisala Barometer (CSZ105), ±0.5 mb precision; ±2mb accuracy). A separate precision linear amplifier, designed and built at the University of Maryland, was used to drive the proportional control valve. Mass errors in field test were reduced from -60 percent for an uncontrolled flow system to <0.22 percent when the new system was used. In two replicate tests of the system, agreements between stage masses collected on MOI stage 7 (D50 = 0.173 m) of two simultaneously operated flow-controlled impactors sampling 0.2 m-diameter monodisperse test particles were 0.997 and 0.996, although differences as large as 4 percent were observed for some stages. The system is suitable for use with standard "Federal Reference Method" samplers.
Figure 7.Average-size spectra for Ir in tagged soot and background aerosol particles, by wind direction. The number of spectra, n, averaged given in parentheses.
System for aerosol humidification. An inlet chamber was designed and constructed to permit humidification of the aerosol during sampling by steam injection. The purpose was to observe changes in the hygroscopic nature of tagged soot particles as they are transported downwind. RH is monitored downstream of the impactor. Inlet air is humidified by metering water into a flash-distillation coil at the top of the plenum with a computer-controlled peristaltic pump (see Borgoul Maciejczyk, 2000). As shown in Figure 5, a 1-L tube (residence time, 2 s) allows sufficient time for particle growth. Air is sampled through a 0.4-cm slit between the chamber body and top plate, to permit sampling of particles with diameters as large as 133 µm. In the laboratory, the RH maintained by the system was 90.0±0.3 percent, while ambient RH varied from 50 to 35 percent. During field sampling, a level of 90.0±1.4 percent RH was maintained for ambient RH changes, typically between 75 and 55 percent.
During field sampling, one MOI was operated at ambient RH conditions. The second MOI was operated at controlled RH of 90 percent to provide particle growth. To compare the results, both MOIs were operated with the precision flow control systems. Application of the Kelvin Equation and Raoult's Law (see Borgoul Maciejczyk, 2000) suggests that 0.2 m soot particles would need to contain about 20 percent ammonium sulfate (by mass) to induce growth at 91.5 percent RH.
Size Distributions for Ir-Tagged Soot. The background concentration versus particle size spectra were determined for Ir at Clifton Park, Ft. Howard, and Fallston Airport during periods when no tracer was released. Subsequently, all samples collected during the tracer releases were corrected for background. Ir-size spectra for Clifton samples (collected during background and tracer-release periods) were sorted into four prevailing wind directions (NW, SW, NNE, SSE) and averaged (Figure 7). Tracer-induced Ir concentrations in submicrometer size intervals were clearly well above background, but were highly variable from day to day. For the interval centered at 2.4 µm (third impactor stage), the Ir concentration observed during release periods often was comparable to the background concentration measured for this interval, presumably owing to resuspension of previously deposited Ir tracer on urban surface dust. The obvious multimodal nature of tagged soot aerosol emissions from the diesel sanitation trucks presented a confounding problem for interpretation of subsequent paired MOI studies, as these were predicated on the existence of a narrow, unimodal soot distribution as reported by Venkataraman and Friedlander (1994), for which changes in size due to atmospheric processing could be observed more readily.
To observe such changes with distance, it was necessary for sites to be aligned so that they experienced the same plume. Only one set of MOI samples (i.e., those collected on August 24 simultaneously at Clifton Park and Fallston) satisfied this condition (Figure 8). At the average wind speed of 1.7 m s-1, the estimated transit time between these sites was 4.9 hours. The individual stage masses of these samples were normalized to the total mass of Ir and are shown in Figure 8. Both size distributions were clearly multimodal and similar in shape. The large fraction of 2.4 mm particles (on stage 3) was attributed to resuspended dust. The fractions of tagged soot mass in 0.2 and 0.8 m particles at Clifton Park were each 18 percent greater than at Fallston (downwind), whereas the fractions in 2.4 µm particles grew by 32 percent. However, neither intermodal coagulation nor hygroscopic/condensational growth could account for such a magnitude of mass transfer. Therefore, it appears that these differences reflect inhomogenieties in the aerosol plume and differences in the levels of resuspended dust between the two sites.
Effect of Humidification on Size Distributions. Size distributions for the following elements were plotted for each day of sampling: Ir, Fe, Zn, Sb, Cr, Se, and Cs. Nonparametric geometric mass mean diameters (gmmad) and geometric standard deviations were calculated when possible for the listed elements. Mass geometric mean diameters for elements concentrated in fine particles collected with ambient and humidified MOIs were examined to determine particle growth at elevated RH. Elements for which growth could be observed were Se (nine samples), Sb (seven samples), Zn (three samples), and Cs (two samples). The results indicate that average growth for Se was 22.4±3.7 percent (average change in RH 29.4±9.8%), 17.8±6.5 percent for Sb (average RH change of 30.0±11%), and 14.0±2.0 percent for Zn (average RH change of 36.4±9.2%). The growth for Cs was 26.9 percent (RH change of 39.6%) for the first sample, and only 3.7 percent when the difference between ambient and humidified MOI was very small (i.e., 7.5%). We found that the gmmads of humidified (typically 90%) and nonhumidified aerosol bearing the various elements were well correlated with linear fit for all elements (i.e., humidified gmmad = 0.044 + 1.054*dry gmmad, with an R2 of 0.91) (Figure 9).
For dilute droplets grown from particles larger than 0.2 m, the cube of particle diameter is related to the inverse of natural logarithm of aw (i.e., RH; Friedlander, 1977). The lower panel of Figure 9 shows gmmad determined from dry/ambient MOI for particles bearing several elements against aw. Interestingly, the data lie along two curves. Ondov, et al. (1995) observed similar behavior for As, Se, Sb, and Zn collected from Washington, DC, ambient aerosol, and suggested that the higher curve represents older and more processed aerosol while the lower curve represents more local sources. Least-square linear regressions for both curves are:
upper curve d3 = 0.0441-(0.0334±0.0069)/ln aw
lower curve d3 = 0.00019-(0.0173±0.0024)/ln aw
where R2=0.74 for the upper curve, and R2=0.65 for the lower curve. It is noteworthy that at 50 percent RH (aw 0.5), the expected gmmad for the upper curve is calculated to be 0.452±0.016 mm, and for the lower curve is 0.293±0.014 mm, confirming that the lower curve represents fresher aerosol emitted from high temperature combustion source (HTCS). The latter diameter agrees well to that for vanadium aerosol emitted from oil-fired power plants (i.e., 0.266±0.021 mm expected at 50% RH) (Divita, 1996). The upper curve is for aged aerosol from HTCSs or sources of an entirely different nature (e.g., particles produced by pouring molten steel). The Fe and Zn points on the upper curve collected at Ft. Howard on October 20 and 21 most likely represent emissions from the local steel mill.
Interestingly, another Zn sample collected at Ft. Howard on August 18 falls onto the lower curve and coincides almost perfectly with Sb from the same sample. This is evidence that these two elements wer
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Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 9 publications||2 publications in selected types||All 2 journal articles|
|| Heller-Zeisler SF, Borgoul PV, Moore RR, Smoliar M, Suarez AE, Ondov JM. Comparison of INAA and RNAA methods with thermal-ionization mass spectrometry for iridium determinations in atmospheric tracer studies. Journal Of Radioanalytical And Nuclear Chemistry 2000;244(1):93-96.
|| Maciejczyk PB, Kidwell C, Ondov JM. System for precise control of volumetric flow rate during sampling with a cascade impactor. Aerosol Science and Technology 2002;36(4):397-406.