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Measurement of Polycyclic Aromatic Hydrocarbons Associated with Size-segregated Atmospheric Aerosols in Massachusetts
J. O. Allen, N. M. Dookeran, K. Taghizadeh, A. L. Lafleur, K. A. Smith, and A. F. Sarofim. Environ. Sci. Technol., 30:1023–1031, 1996. DOI:10.1021/es950517o
Size-segregated atmospheric aerosols were collected from urban and rural locations in Massachusetts using a micro-orifice impactor. The samples were analyzed for polycyclic aromatic hydrocarbons (PAH) with molecular weights between 178 and 302, using gas chromatography/mass spectrometry. Fifteen PAH were quantified in the urban samples and nine in the rural samples. The quantification results are in good agreement with available ambient monitoring data. In the urban samples, PAH were distributed among aerosol size fractions based on molecular weight. PAH with molecular weights between 178 and 202 were approximately evenly distributed between the fine (aerodynamic diameter < 2 µm) and coarse (aerodymanic diameter > 2 µm) aerosols. PAH with molecular weights greater than 228 were associated primarily with the fine aerosol fraction. In the rural samples, low and high molecular weight PAH were associated with both the fine and coarse aerosols. Slow mass transfer by vaporization and condensation is proposed to explain the observed PAH partitioning among aerosol size fractions.
Polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (OPAH) are mutagenic air pollutants formed as by-products of combustion. After formation and emission, these compounds partition between the gas phase and atmospheric aerosols. The environmental fate of PAH and OPAH depends, in part, on their distribution between the gas and particulate phases and among particle size fractions. Particle size effects the removal rate of the associated PAH from the atmosphere by dry and wet deposition. The mechanism and location of deposition of particulate phase compounds in the lung are also affected by particle size. The large particles tend to impact on the upper regions of the lung and small particles diffuse to the surface of the alveoli. The goal of this work is a better understanding of the atmospheric partitioning of PAH and OPAH necessary to determine the environmental fate of, and human exposure to, these pollutants.
Size-segregated atmospheric aerosols were collected from urban and rural locations in Massachusetts using a micro-orifice inertial impactor. The samples were analyzed for PAH and OPAH using gas chromatography/mass spectrometry. In the urban samples, PAH were distributed among aerosol size fractions based on molecular weight. PAH with molecular weights between 178 and 202 were approximately evenly distributed between the fine (aerodynamic diameter, Dp < 2 µm) and coarse (Dp > 2 µm) particles. PAH with molecular weights greater than 228 were associated primarily with the fine aerosol fraction. In the rural samples, low and high molecular weight PAH were associated with both the fine and coarse aerosols. PAH are primarily emitted by combustors with fine particles. Slow mass transfer by vaporization and sorption is proposed to explain the observed PAH partitioning among aerosol size fractions.
OPAH were also generally distributed among aerosol size fractions based on molecular weight in the urban aerosol. Compounds with molecular weights between 168 and 208 were approximately evenly distributed between the fine and coarse particles. OPAH with molecular weights of 248 and greater were associated primarily with the fine aerosol fraction. Most OPAH were distributed with particle size in a broad, unimodal hump similar to the the distributions observed for PAH in the same samples. These results indicate that OPAH were initially associated with fine particles following emission by combustors or formation by gas phase photooxidation. OPAH then redistributed from fine particles to larger particles by vaporization and sorption. Two OPAH were distributed in bimodal distributions with peaks at Dp ~ 0.2 µm and Dp ~ 2.5 µm. The bimodal distributions suggest that these compounds have solution behavior very different from PAH and other OPAH.
Size-segregated atmospheric aerosols were collected on oiled impaction media in this work to prevent particle bounce during sampling. The use of the oiled impaction media, however, may have introduced another sampling artifact — the absorption of species from the gas phase. Such absorption would artificially increase the amount of PAH attributed to the aerosol and possibly distort the measured size distributions. Absorption of pyrene from the gas phase to the oiled impaction media was measured in the laboratory. The amounts absorbed were approximately equal for the impactor stages, indicating that, in the worst case, the absorption artifact resulted in a small increase in the amount of PAH collected and no qualitatively significant distortion in the measured size distribution.
The experimental measurements of the absorption artifact are valid only for the impactor design and impaction media used in this work. A model of the absorption artifact, based on the laminar impinging jet mass transfer literature, was developed to predict the absorption artifact for other impactor designs and impaction media. The model predictions compared well to experimental measurements for the first four impactor stages. The model overpredicted absorption on the last four stages, probably due to the tight packing of impinging jets on these stages. A lower estimate of absorption on these stages was made by considering the mass transfer from a single jet. The experimental results were bracketed by the array of jets and single jet models.
A widely circulated hypothesis holds that PAH-particle associations can be described as adsorption and that the association is in equilibrium. This hypothesis was examined and found to be inconsistent with available atmospheric data. In place of the adsorptive partitioning hypothesis, we propose that PAH partitioning in the atmosphere is best explained as non-equilibrium absorptive partitioning. This explanation is consistent with the observation that the apparent enthalpy of gas-particle partitioning is greater than the enthalpy of vaporization. It is also consistent with the observations that, in urban aerosols, lower molecular weight PAH partition to both fine and coarse particles while higher molecular weight PAH partition mainly to fine particles. This description was implemented as a Lagrangian model of aerosol partitioning in an urban environment. The model results match well the measured distributions of low and intermediate molecular weight PAH (M < 278) with particle size. These results indicate that association of organic compounds with aerosols is by absorption and that, in the urban atmosphere, these compounds do not attain equilibrium partitioning.
Measurement of Oxygenated Polycyclic Aromatic Hydrocarbons Associated with a Size-segregated Urban Aerosol
J. O. Allen, N. M. Dookeran, K. Taghizadeh, A. L. Lafleur, K. A. Smith, and A. F. Sarofim. Environ. Sci. Technol., 31:2064–2070, 1997. DOI: 10.1021/es960894g
Size-segregated atmospheric particles were collected in Boston, MA, using a micro-orifice impactor. The samples were analyzed for oxygenated polycyclic aromatic hydrocarbons (OPAH) using gas chromatography/mass spectrometry. Seven PAH ketones (1-acenaphthenone, 9-fluorenone, 11H-benzo[a]fluoren-11-one, 7H-benzo[c]fluoren-7-one, 11H-benzo[b]fluoren-11-one, benzanthrone, and 6H-benzo[cd]pyrene-6-one), four PAH diones (1,4-naphthoquinone, phenanthrenequinone, 5,12-naphthacenequinone, and benzo[a]pyrene-6,12-dione), and one PAH dicarboxylic acid anhydride (naphthalic anhydride), were identified. Seven additional compounds with mass spectra typical of OPAH were tentatively identified. OPAH were generally distributed among aerosol size fractions based on molecular weight. Compounds with molecular weights between 168 and 208 were approximately evenly distributed between the fine (aerodynamic diameter, Dp, < 2 µm) and coarse (Dp > 2 µm) particles. OPAH with molecular weights of 248 and greater were associated primarily with the fine aerosol fraction. Most OPAH were distributed with particle size in a broad, unimodal hump similar to the the distributions observed for PAH in the same samples. These results suggest that OPAH are initially associated with fine particles after formation by either combustion or gas phase photooxidation, then partition to larger particles by vaporization and sorption. Two OPAH were distributed in bimodal distributions with peaks at Dp approximately 0.2 µm and 2.5 µm. These bimodal distributions may be indicative of sorption behavior different from PAH and other OPAH.
E. E. Gard, M. J. Kleeman, D. S. Gross, L. S. Hughes, J. O. Allen, B. D. Morrical, D. P. Fergenson, T. Deines, M. E. Gälli, R. J. Johnson, G. R. Cass, and K. A. Prather. Science, 279:1184–1187, 1998. DOI: 10.1126/science.279.5354.1184
Heterogeneous replacement of chloride by nitrate in individual sea-salt particles was monitored continuously over time in the troposphere with the use of aerosol time-of-flight mass spectrometry. Modeling calculations show that the observed chloride displacement process is consistent with a heterogeneous chemical reaction between sea-salt particles and gas-phase nitric acid, leading to sodium nitrate production in the particle phase accompanied by liberation of gaseous HCl from the particles. Such single-particle measurements, combined with a single-particle model, make it possible to monitor and explain heterogeneous gas/particle chemistry as it occurs in the atmosphere.
Measurement of C24H14 Polycyclic Aromatic Hydrocarbons Associated with a Size-segregated Urban Aerosol
J. O. Allen, J. L. Durant, N. M. Dookeran, K. Taghizadeh, E. F. Plummer, A. L. Lafleur, A. F. Sarofim, and K. A. Smith. Environ. Sci. Technol., 32:1928–1932, 1998. DOI: 10.1021/es970919r
Six ring C24H14 (MW 302 Da) polycyclic aromatic hydrocarbons (PAH), some of which are potent mutagens, are present in urban aerosols. Size-segregated atmospheric aerosol samples from Boston, MA, were analyzed for C24H14 PAH by gas chromatography/mass spectrometry. Eleven peaks were found with mass to charge ratios of 302; of these, eight were identified using authentic standards. Five of the peaks were quantified. For each of these five, the distributions with respect to particle size were bimodal with the majority of the mass associated with accumulation mode particles (0.3-1.0 µm) and a smaller fraction of the mass associated with ultrafine mode particles (0.09-0.14 µm). These distributions are similar to those observed for PAH of molecular weight 252 to 278 in the same sample, but different from those of benzo[ghi]perylene (MW = 276) and coronene (MW = 300), which were associated to a greater degree with ultrafine particles. The data suggest that C24H14 PAH repartition to larger particles by vaporization and sorption more rapidly than do benzo[ghi]perylene and coronene. The total concentration of C24H14 PAH, 1.5 ng/m3, was comparable to that of benzo[a]pyrene in the same sample. Because their mutagenicities, C24H14 PAH may make a contribution to the genotoxicity of urban aerosols comparable to that of benzo[a]pyrene.
J. O. Allen, A. F. Sarofim, and K. A. Smith. Polycyclic Aromatic Compounds, 13:261–283, 1999. DOI: 10.1080/10406639908020550
A knowledge of the solution behavior of polycyclic aromatic hydrocarbons (PAHs) at ambient temperatures is required for predictions of their fate in the environment. Unsubstituted PAHs are crystalline at ambient temperatures, but their behavior in solution depends on the properties of the hypothetical subcooled liquid state, and these properties must be estimated. Enthalpies, entropies, and Gibbs free energies of fifteen PAHs in the subcooled liquid state are calculated using available sublimation, melting, and heat capacity data. Because PAH melting points are as much as 250 K above ambient temperatures, heat capacity corrections can be important for the accurate extrapolation of thermodynamic data to the subcooled liquid state. For example, subcooled liquid vapor pressures, p(l), at 298 K calculated without heat capacity corrections are up to 6 times higher than those estimated with heat capacity corrections. Calculated values for p(l) are expressed as log p(l) = -A/T + B and used to develop a group contribution method for the calculation of p(l) for other unsubstituted PAHs.
L. S. Hughes, J. O. Allen, M. J. Kleeman, R. J. Johnson, G. R. Cass, E. E. Gard, D. S. Gross, M. E. Gälli, B. D. Morrical, D. P. Fergenson, T. Dienes, C. A. Noble, D.-Y. Liu, P. J. Silva, K. A. Prather. Environ. Sci. Technol.,33:3506–3515, 1999. DOI: 10.1021/es980884a
Continuous measurements of single particle size and chemical composition in the atmosphere are made using aerosol time-of-flight mass spectrometers (ATOFMS) operated alongside more conventional reference air sampling instruments at a network of three urban air monitoring sites in southern California. Electrical aerosol analyzers and optical particle counters are employed to acquire continuous particle size distribution data, and inertial impactor and bulk filter samples with 4-h resolution are taken for determination of particle size and chemical composition. Filter and impactor samples also are taken upwind of the air basin at Santa Catalina Island in order to characterize background air quality. The airborne particle size and composition distribution was measured by the cascade impactors at inland sites differ from that over the ocean principally due to depletion of sea salt particles accompanied by the addition of fine carbon-containing particles and secondary aerosol nitrate. Data from the ATOFMS systems create a continuous time series of sodium-, ammonium-, nitrate-, and carbon-containing particle counts that provide a high-resolution view of differences in particle composition as a function of location in the air basin. Results show that the characteristic peak in the Los Angeles area aerosol mass distribution in the 0.2-0.3 µm size range observed during the 1987 SCAQS experiments has been reduced, consistent with reductions in diesel soot and elemental carbon emissions since that time.
Absorption of Semi-volatile Compounds in Oiled Impaction Substrates: Measurement of Pyrene Absorption
J. O. Allen, J. S. Paschkewitz, E. F. Plummer, A. L. Lafleur, A. F. Sarofim, and K. A. Smith. Aerosol Sci. Technol., 30:16–29, 1999. DOI: 10.1080/027868299304859
Oiled impaction substrates have been used to prevent particle bounce during the collection of size-segregated aerosol samples, which have been analyzed for trace-level airborne organic compounds, including polycyclic aromatic hydrocarbons (PAHs). The use of oiled impaction substrates, however, may introduce another sampling artifact – the absorption of semi-volatile species from the gas phase which could artificially increase the amount of PAHs attributed to the aerosol. In this article, laboratory measurements of the absorption of a particular PAH, pyrene, from the gas phase to impaction substrates of polytetrafluoroethylene membranes impregnated with dibutyl phthalate are reported. Overall mass transfer coefficients are determined from the data. These results are used to calculate the absorption of gas phase PAHs during sampling of size-segregated atmospheric particles. Criteria are developed to determine if the absorption is negligible. The first criterion requires that the analyte be negligibly absorbed in the oil; this criterion is met by none of the impaction oils reported in the literature. The second criterion is that species do not have time to reach an equilibrium distribution between the gas phase and impaction oil; this criterion is met for nonvolatile species, those with vapor pressures equal to or less than that of benzo[a]pyrene (3.5 x 10-6 Pa). We recommend that oiled impaction substrates be used only if the absorption artifact is expected to be negligible on the basis of these criteria.
Source Contributions to the Size and Composition Distribution of Atmospheric Particles: Southern California in September, 1996
M. J. Kleeman, L. S. Hughes, J. O. Allen, G. R. Cass. Environ. Sci. Technol., 33:4331–4341, 1999. DOI:10.1021/es990632p
An air quality model that follows the evolution of single particles in the atmosphere has been combined with new emissions measurements and then used to predict the size distribution and chemical composition of the airborne fine particle mixture observed at Long Beach, Fullerton, and Riverside, CA, during September 1996. Model predictions show good agreement with ambient measurements of particle size and chemical composition at all three air monitoring sites. The air quality model is used to separately track individual particles released from different sources as they evolve over time. Four major classes of particles are observed: (1) large mineral dust and road dust particles that accumulate only small amounts of secondary aerosol products; (2) primary combustion particles (released initially from diesel vehicles, noncatalyst gasoline-powered vehicles, and food processing) that grow by accumulation of secondary reaction products; (3) sea salt particles that are almost completely transformed by conversion from NaCl to NaNO3 during transport across the air basin; and (4) sulfate-containing nonsea salt background particles advected into the air basin from upwind over the ocean. The sulfate-containing nonsea salt background particles have an initial PM2.5 concentration of only 8 µg m-3, but they accumulate significant secondary aerosol reaction products to produce a largely nitrate-containing aerosol having a PM2.5 concentration of 40 µg m-3 by the time that the air masses studied here reach Riverside, CA.
Particle Detection Efficiencies of Aerosol Time of Flight Mass Spectrometers under Ambient Sampling Conditions
J. O. Allen, D. P. Fergenson, E. E. Gard, B. D. Morrical, L. S. Hughes, M. J. Kleeman, D. S. Gross, M. E. Gälli, K. A. Prather, and G. R. Cass. Environ. Sci. Technol., 34:211–217, 2000. DOI: 10.1021/es9904179
Aerosol time-of-flight mass spectrometers (ATOFMS) measure the size and chemical composition of single aerosol particles. To date, these instruments have provided qualitative descriptions of aerosols, in part because the fraction of particles actually present in the atmosphere that is detected by these instruments has not been known. In this work, the particle detection efficiencies of three ATOFMS instruments are determined under ambient sampling conditions from the results of colocated sampling with more conventional reference samplers at three locations in southern California. ATOFMS particle detection efficiencies display a power law dependence on particle aerodynamic diameter (Da) over a calibration range of 0.32 Da < 1.8 µm. Detection efficiencies are determined by comparison of ATOFMS data with inertial impactor data and are compared to detection efficiencies determined independently by the use of laser optical particle counters. Detection efficiencies are highest for the largest particles and decline by approximately 2 orders of magnitude for the smallest particles, depending on the ATOFMS design. Calibration functions are developed here and applied to scale ATOFMS data to yield continuous aerosol mass concentrations as a function of particle size over an extended period of time.
L. S. Hughes, J. O. Allen, P. Bhave, M. J. Kleeman, G. R. Cass, D.-Y. Liu, D. P. Fergenson, B. D. Morrical, and K. A. Prather. Environ. Sci. Technol., 34:3058–3068, 2000. DOI: 10.1021/es9908671
Trajectory analysis shows that the air masses arriving at Riverside, CA, on the afternoons of September 24 and 25, 1996, previously passed near air monitoring sites at Santa Catalina Island, Long Beach, and Fullerton, CA, in succession. At those sites, electrical aerosol analyzers and optical particle counters acquired continuous particle size distribution data, inertial impactor and bulk filter samples were taken with 4-h time resolution for determination of particle size and chemical composition during intensive sampling periods once per day at each site, and aerosol time-of-flight mass spectrometers acquired continuous data on particle size and composition at the single-particle level. These data permit particle evolution to be studied within single air masses as they sequentially pass several monitoring sites over a 2-day period. Air parcels associated with both of the trajectories studied show mineral dust, organic carbon, particulate nitrate and ammonium, and total suspended particulate matter concentrations that increase as transport occurs across the air basin. Large increases in particulate ammonium and nitrate concentrations occur between Fullerton and Riverside due to overnight air stagnation in an area with high gaseous ammonia emissions. The aerosol time-of-flight mass spectrometers show how the externally mixed population of individual particles is modified chemically during transport from Long Beach to Riverside, CA. The coastal aerosol at Long Beach containing sea-salt particles and primary carbon particles is changed substantially as these particles individually accumulate secondary ammonium nitrate and organics during travel across the air basin.
G. R. Cass, L. S. Hughes, P. Bhave, M. J. Kleeman, J. O. Allen, and L. G. Salmon. Phil. Trans. R. Soc. Lond., A 358:2581–2592, 2000. DOI: 10.1098/rsta.2000.0670
Atmospheric ultrafine particles (with diameter less than 0.1 µm) may be responsible for some of the adverse health effects observed due to air-pollutant exposure. To date, little is known about the chemical composition of ultrafine particles in the atmosphere of cities. Ultrafine particle samples collected by inertial separation on the lower stages of cascade impactors can be analysed to determine a material balance on the chemical composition of such samples. Measurements of ultrafine particle mass concentration made in seven Southern California cities show that ultrafine particle concentrations in the size range 0.056-0.1 µm aerodynamic diameter average 0.55-1.16 µg m-3. The chemical composition of these ultrafine particle samples averages 50% organic compounds, 14% trace metal oxides, 8.7% elemental carbon, 8.2% sulphate, 6.8% nitrate, 3.7% ammonium ion (excluding one outlier), 0.6% sodium and 0.5% chloride. The most abundant catalytic metals measured in the ultrafine particles are Fe, Ti, Cr, Zn, with Ce also present. A source emissions inventory constructed for the South Coast Air Basin that surrounds Los Angeles shows a primary ultrafine particle emissions rate of 13 tonnes per day. Those ultrafine particle primary emissions arise principally from mobile and stationary fuel combustion sources and are estimated to consist of 65% organic compounds, 7% elemental carbon, 7% sulphate, 4% trace elements, with very small quantities of sodium, chloride and nitrate. This information should assist the community of inhalation toxicologists in the design of realistic exposure studies involving ultra fine particles.
D. P. Fergenson, X.-H. Song, Z. Ramadan, J. O. Allen, L. S. Hughes, G. R. Cass, P. K. Hopke, and K. A. Prather. Anal. Chem., 73:3535–3541, 2001. DOI: 10.1021/ac010022j
Aerosol time-of-flight mass spectrometry (ATOFMS) is capable of measuring the sizes and chemical compositions of individual polydisperse aerosol particles in real time. A qualitative estimate of the particle composition is acquired in the form of a mass spectrum that must be subsequently interpreted in order to draw conclusions regarding atmospheric relevance. The actual problem involves developing a calibration that allows the mass spectral data to be transformed into estimates of the composition of the atmospheric aerosol. A properly calibrated ATOFMS system should be able to quantitatively determine atmospheric concentrations of various species. Ideally, it would be able to accomplish this more rapidly, accurately, with higher size and time resolution, and at a far lower marginal cost than the manual sampling methods that are currently employed. Attempts have already been made at using ATOFMS and similar techniques to extract the bulk chemical species concentration present in an ensemble of particles. This study represents the use of a multivariate calibration method, two-dimensional partial least-squares analysis, for calibrating single-particle mass spectral data. The method presented here is far less labor-intensive than the univariate methods attempted to date and allows for less observer bias. Because of the labor savings, this is also the most comprehensive calibration performed to date, resulting in the quantification of 44 different chemical species.
J. O. Allen, P. R. Mayo, L. S. Hughes, L. G. Salmon, and G. R. Cass. Environ. Sci. Technol., 35:4189–4197, 2001. DOI: 10.1021/es0015545
Particulate matter emissions from the California in-use vehicle fleet were measured as 37500 vehicles traveled through two bores of the Caldecott Tunnel located in the San Francisco Bay area. Microorifice cascade impactors and filter-based samplers were used to determine the particle chemical composition as a function of particle size. Ammonia emissions from the vehicle fleet were measured as well. Concentrations of aerosol mass, organic carbon, elemental carbon, sulfate ion, nitrate ion, and ammonium ion, as well as 13 elements are reported. The particle mass distribution peaks in the particle size range 0.1-0.18 µm aerodynamic diameter (Da). Elemental carbon and organic matter were the largest components of particle mass in all the size ranges studied. The Caldecott Tunnel bores studied include one which carries light-duty vehicle traffic and one which carries a mixture of light- and heavy-duty vehicle traffic. From experiments conducted in both bores, estimates are made of the size distribution and chemical composition of particulate matter emissions extrapolated to the 100% light-duty and 100% heavy-duty vehicle fleets. The heavy-duty vehicle fleet emitted 1285 ± 237 mg of fine particulate matter (Da < 1.9 µm) per kg C contained in the fuel burned (corresponding to approximately 430 ± 79 mg per km driven). Light-duty vehicles emitted less than 85 ± 6 mg per kg C in the fuel burned (corresponding to less than approximately 5.5 ± 0.4 mg per km driven). Emissions of gas-phase ammonia in the Caldecott Tunnel were measured to be 194 and 267 mg per liter of gasoline-equivalent fuel burned in the tunnel, compared to 400 mg per liter measured in 1993 in the Los Angeles area Van Nuys Tunnel. The ammonia emissions are attributed to automobiles that were equipped with 3-way catalysts and operating fuel rich.
J. O. Allen, Software Reference Manual, Version 2.0, 2005 (also Versions 1.0 in 2001, 1.2 in 2003, 1.3 in 2005). Full Text (0.5 MB)
Researchers are now able to measure the size and composition of single aerosol particles using Single Particle Mass Spectrometry (SPMS) instruments like the Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) instruments developed by Prof. Kimberly Prather and her research group at the University of California. Complete mass spectra are collected on individual particles at a rate greater than one per second. Thus very large data sets can be collected during a multi-day, multi-instrument experiment. These data sets are too large for ad hoc data analysis techniques. YAADA is a package of data management and analysis functions written for Matlab which are designed to process these large data sets. YAADA includes functions to import, query, plot, and quantitatively analyze ATOFMS data. YAADA is available as free software. Users can write Matlab functions to extend YAADA in order to develop novel analyses of ATOFMS data.
P. V. Bhave, M. J. Kleeman, J. O. Allen, L. S. Hughes, K. A. Prather, and G. R. Cass. Environ. Sci. Technol., 36:2154–2163, 2002. DOI: 10.1021/es0112700
Air quality model predictions of the size and composition of atmospheric particle classes are evaluated by comparison with aerosol time-of-flight mass spectrometry (ATOFMS) measurements of single-particle size and composition at Long Beach and Riverside, CA, during September 1996. The air quality model tracks the physical diameter, chemical composition, and atmospheric concentration of thousands of representative particles from different emissions classes as they are transported from sources to receptors while undergoing atmospheric chemical reactions. In the model, each representative particle interacts with a common gas phase but otherwise evolves separately from all other particles. The model calculations yield an aerosol population, in which particles of a given size may exhibit different chemical compositions. ATOFMS data are adjusted according to the known particle detection efficiencies of the ATOFMS instruments, and model predictions are modified to simulate the chemical sensitivities and compositional detection limits of the ATOFMS instruments. This permits a direct, semiquantitative comparison between the air quality model predictions and the single-particle ATOFMS measurements to be made. The air quality model accurately predicts the fraction of atmospheric particles containing sodium, ammonium, nitrate, carbon, and mineral dust, across all particle sizes measured by ATOFMS at the Long Beach site, and in the coarse particle size range ( Da > 1.8 µm) at the Riverside site. Given that this model evaluation is very likely the most stringent test of any aerosol air quality model to date, the model predictions show impressive agreement with the single-particle ATOFMS measurements.
L. S. Hughes, J. O. Allen, L. G. Salmon, P. R. Mayo, R. J. Johnson, and G. R. Cass. Environ. Sci. Technol., 36:3928–3935, 2002. DOI: 10.1021/es0110630
Ambient aerosol sampling was conducted in Diamond Bar, Mira Loma, and Riverside, CA, to observe at close range the effects of ammonia emissions on air quality. These sites are located upwind, within, and downwind, respectively, of the Chino dairy area, the largest single source of ammonia emissions in the Los Angeles area. Inertial impactors and bulk filter samplers provided 4-7-h measurements of aerosol chemical composition and size distribution. Daily average fine particle mass concentrations were in the range 22.4-143.0 µg m-3. On some days the fine particulate matter concentrations were more than two times greater than the proposed 24-h Federal standard of 65 µg m-3. Ammonium nitrate was the largest component of fine particle mass at all three sites; 24-h average fine particulate ammonium plus nitrate concentrations ranged from 11.7 to 75.4 µg m-3. A single air mass was studied as it passed the Diamond Bar air monitoring site in the morning and stagnated near Mira Loma in the evening of the same day. Between these two sites NO was oxidized to NO2, and the ammonia concentration increased by a factor of 5. A second air parcel trajectory, which stagnated near Mira Loma during the early morning and passed near the Riverside site approximately 24 h later, showed a decrease in ammonia concentration over time that is consistent with dilution as the air mass moved downwind from the source of ammonia in the dairy area. Particulate NH4NO3 concentration in that air parcel remained approximately constant over time, consistent with a continued excess of NH3 relative to HNO3 downwind of the dairy area.
P. V. Bhave, J. O. Allen, B. D. Morrical, D. P. Fergenson, G. R. Cass, and K. A. Prather. Environ. Sci. Technol., 36:4868–4879, 2002. DOI: 10.1021/es015823i
Aerosol time-of-flight mass spectrometry (ATOFMS) instruments measure the size and chemical composition of individual particles in real-time. ATOFMS chemical composition measurements are difficult to quantify, largely because the instrument sensitivities to different chemical species in mixed ambient aerosols are unknown. In this paper, we develop a field-based approach for determining ATOFMS instrument sensitivities to ammonium and nitrate in size-segregated atmospheric aerosols, using tandem ATOFMS-impactor sampling. ATOFMS measurements are compared with collocated impactor measurements taken at Riverside, CA, in September 1996, August 1997, and October 1997. This is the first comparison of ion signal intensities from a single-particle instrument with quantitative measurements of atmospheric aerosol chemical composition. The comparison reveals that ATOFMS instrument sensitivities to both and decline with increasing particle aerodynamic diameter over a 0.32-1.8 µm calibration range. The stability of this particle size dependence is tested over the broad range of fine particle concentrations (PM1.8 = 17.6 ± 2.0-127.8 ± 1.8 µg m-3), ambient temperatures (23-35 C), and relative humidity conditions (21-69%), encountered during the field experiments. This paper describes a potentially generalizable methodology for increasing the temporal and size resolution of atmospheric aerosol chemical composition measurements, using tandem ATOFMS-impactor sampling.
Ambient Single Particle Analysis in Riverside, CA, by Aerosol Time-of-Flight Mass Spectrometry during SCOS97-NARSTO
S. H. Pastor, J. O. Allen, L. S. Hughes, P. V. Bhave, G. R. Cass, and K. A. Prather. Atmos. Environ., 37:S226–S258, 2003. DOI: 10.1016/S1352-2310(03)00393-5
Single-particle measurements were made using aerosol time-of-flight mass spectrometry (ATOFMS) instruments in conjunction with the 1997 Southern California Ozone Study-North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO). The size and chemical composition of individual ambient particles in Riverside, CA during the summer of 1997 are described. Data collected using co-located micro-orifice uniform deposit impactors (MOUDI) impactors are used to scale the ATOFMS number counts, providing a unique picture of the particle population which complements information obtained using traditional sizing and composition analysis techniques in this and previous studies. Changes in single particle composition are observed over time, and compared and contrasted with observed changes in visibility, ozone, and PM10 concentrations. Details are provided on changes in the particle size and composition observed during three morning periods with low ozone and elevated PM10 versus three afternoon periods with both elevated ozone and PM10 levels between 21–23 August 1997. The ATOFMS size profiles show afternoon periods dominated by sub-m particles composed of organic carbon coupled with ammonium nitrate, and morning periods with relatively high levels of super-m dust particles. The observed changes in particle size and composition are consistent with differences in air mass trajectories arriving at Riverside during the morning and afternoon time periods.
Temporal variations in single particle types detected over a 40 day sampling period are presented, demonstrating the type of unique information that can be obtained regarding atmospheric particle processes through long term sampling studies. The broader availability of single particle mass spectrometers coupled with recent advances in the field are now providing unique information on the associations of multiple chemical species (i.e. mixing state) within individual particles with high size and temporal resolution.
J. O. Allen. California Air Resources Board Contract 01-338, 2004. Full Text (0.6 MB)
Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) instruments have been used to measure the size and composition of single aerosol particles in California. ATOFMS data are not direct quantitative measurements of aerosol composition because the instruments exhibit non-linear response to aerosol concentration, particle size, and particle composition. The goal of this work is to quantitatively compare ATOFMS and reference sampler data in order to both understand ATOFMS instrument operation, and to develop procedures to transform ATOFMS data to semi-quantitative aerosol compositions. Ambient measurements from the Bakersfield Instrument Intercomparison Study were analyzed. ATOFMS instrument busy time (the time required to save acquired data) was found using a statistical comparison of the number of particles detected to that expected for a Poisson process modified to include busy time. The fraction of busy time was highly variable and in the range 0.05 to 0.95; thus busy time cannot be ignored for accurate quantitative comparison of ATOFMS data with reference sampler data. The sensitivities of the ATOFMS instrument to aerosol mass, organic carbon (OC), and elemental carbon (EC) were determined by a selecting single particle response function for each analyte. ATOFMS response to mass, OC and EC fit well to a power law in particle size; these responses are similar to those found previously for aerosol mass and inorganic ions. Using the recommended response functions, ATOFMS data were scaled for comparison with independent aerosol measurements. Comparison of scaled aerosol mass with that measured using Beta Attenuation Monitors (BAMs) showed semi-quantitative agreement, i.e., scaled ATOFMS data agreed within a factor of three with the BAMs data. ATOFMS data scaled for OC and EC concentrations were systematically lower than from collocated filter-based measurements because the scaled ATOFMS data propagated the systematic difference between short-term impactor measurements and the filter-based measurements. The novel data analysis methods developed here will be included in the next public release of YAADA, a software toolkit to analyze single particle mass spectral data.
C. A. Tyree and J. O. Allen. Aerosol Sci. Technol., 38:1019–1026, 2004. DOI: 10.1080/027868290519201
Isokinetic sampling, in which a subsample is extracted from the center of laminar aerosol flow, is routinely used to collect representative particles for analysis. Isokinetic sampling minimizes wall effects, including particle loss due to Brownian diffusion to the tube wall. This particle diffusion is analogous to the heat transfer problem originally posed by Graetz in 1883. Analytical solutions to the Graetz problem have been applied to calculate particle loss averaged over the entire main flow. However, these solutions overestimate diffusional particle loss near the center of the main flow. In the present solution, confluent hypergeometric functions are used to solve analytically for particle concentration as a function of radius. The solution is integrated near the center of the main flow to determine particle loss for isokinetically sampled aerosols. Sampling efficiencies valid down to nanometer-sized particles are presented in terms of dimensionless parameters. Diffusional particle loss for isokinetically sampled aerosol can be 1.8 times less than that from the main flow aerosol. The present results can be used to design isokinetic sampling systems and to assess particle loss in these systems. For 5 nm diameter particles sampled isokinetically from a laminar flow tube (0.318 cm tube radius, 10 m length) into an ultrafine condensation particle counter, sampling efficiency is strongly affected by main flow Reynolds number, Re; sampling efficiency increases from 4.9% at Re = 100 to 99% at Re = 1500.
J. O. Allen, P. V. Bhave, J. R. Whiteaker, and K. A. Prather. Aerosol Sci. Technol., 40:615–626, 2006. DOI: 10.1080/02786820600754623
Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) instruments have been used widely to measure the size and composition of single ambient aerosol particles. ATOFMS data do not directly and quantitatively represent aerosol composition because the instruments exhibit non-linear response to particle concentration, size, and composition. Our approach is to analyze separately the components of non-linear ATOFMS response using field sampling data in order to understand ATOFMS response to ambient aerosols so that ATOFMS data can be scaled to more closely represent ambient aerosols. In this work we examine the effect of instrument busy time, mainly the time to process and save data, on ATOFMS response to ambient aerosols sampled during the 1999 Bakersfield Instrument Intercomparison Study (BIIS). During this study an ATOFMS instrument was operated alternately in normal and fast scatter data acquisition modes. In fast scatter mode, the instrument does not record mass spectra, minimizing instrument busy time; these data were used to determine particle arrival rates. Busy time in normal mode was found by a comparison of the number of particles detected to that expected for a Poisson process modified to include busy time. During the BIIS experiment, the ATOFMS instrument was busy between 5 and 95% of the nominal sampling time; thus busy time cannot be ignored for accurate quantitative analysis of ATOFMS data. ATOFMS data were scaled for on-line time and transmission efficiency, found by comparison with reference aerosol measurements, in order to estimate fine particle mass concentrations. Fine aerosol mass concentrations from scaled ATOFMS data demonstrate semi-quantitative agreement with independent measurements using Beta Attenuation Monitors. We recommend that ATOFMS instruments be modified to measure busy time directly.
C. A. Tyree, V. M. Hellion, O. A. Alexandrova, and J. O. Allen. J. Geophys. Res., 112:D12204, 2007. DOI: 10.1029/2006JD007729
Submicrometer sea salt aerosol (SSA) particles are routinely observed in the remote marine boundary layer (MBL); these aerosols include cloud condensation nuclei and so affect the earth’s radiative balance. Here foams designed to mimic oceanic whitecaps were generated in the laboratory using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. The number and sizes of dried foam droplets in the particle diameter, Dp, range 15–673 nm were measured. Particle size distributions for natural and artificial seawaters were unimodal with a dN/d logDp mode at Dp ≈ 100 nm (≈200 nm at 80% RH). The foam droplet mode falls within the range of reported mode diameters (Dp = 40–200 nm) for submicrometer SSA particles observed in the remote MBL. The present laboratory results were scaled up to estimate submicrometer SSA particle fluxes; this extrapolation supports the hypothesis that foam droplets are the most important source of SSA particles by number. The foam droplet flux from the oceans was estimated to be 980 cm−2 s−1 for a fractional white cap coverage, W, of 0.2%. These results compared well with foam droplet fluxes reported elsewhere. The origins of variability in foam droplet fluxes were also evaluated. Natural organic matter affected foam droplet flux by a factor of 1.5; this was less than (1) the effect of bubbling flow rate on foam droplet flux (factor of 5) and (2) the uncertainty in W (factor of 3–7).
Impact of Asphalt Rubber Friction Course Overlays on Tire Wear Emissions and Air Quality Models for Phoenix, Arizona, Airshed
O. A. Alexandrova, K. E. Kaloush, and J. O. Allen, Transportation Res. Rec., 2011: 98-106, Transportation Research Board, Washington, D.C., 2007. DOI: 10.3141/2011-11
Tire wear contributes to atmospheric particulate matter (PM) and is regulated by the U.S. Environmental Protection Agency because PM has been shown to affect human health. Vehicle emissions are a significant source of both PM2.5 and PM10. Vehicle fleet emissions per mile traveled have been reduced significantly in the past 30 years as a result of improved engine operation and tailpipe controls. However,
zero emission vehicles will continue to generate PM from tire wear, road wear, brake wear, and resuspended road dust. In this study, aerosol measurement techniques at Arizona State University were applied to evaluate tire wear emissions from the vehicle fleet by using the Deck Park Tunnel in Phoenix, Arizona. The Deck Park Tunnel highway surface was portland cement concrete (PCC) and was resurfaced with an asphalt rubber friction course (ARFC) layer as part of the Arizona Department of Transportation Quiet Pavements Program. This study took advantage of a rare opportunity to sample tire wear emissions at the tunnel before and after the ARFC overlay. The hypothesis was that an ARFC surface results in less tire wear than the existing PCC road surface. This paper reports on the measured PM emissions from the on-road vehicle traffic during typical highway driving conditions for the two different roadway surfaces. It presents the analysis of representative tire tread samples for tire wear marker compounds and a comparison of roughness and fractional surface characteristics as measured before and after the ARFC overlay. The study found that emission rates of tire wear per kilometer driven on PCC road surfaces were 1.4 to 2 times higher than emission rates of tire wear on ARFC road surfaces.
C. A. Tyree and J. O. Allen, J. Nanopart. Res., 10:465-473, 2008. DOI: 10.1007/s11051-007-9280-0.
A novel approach to nanoparticle synthesis was developed whereby foam bubble bursting produced aerosol droplets, an approach patterned after the marine foam aerosol cycle. The droplets were dried to remove solvent, leaving nanometer-sized particles composed of precursor material. Nanoparticles composed of sodium chloride (mean diameter, Dp ~ 100 nm), phosphotungstic acid (Dp ~ 55 nm), and bovine insulin (Dp ~ 5-30 nm) were synthesized. Foam droplet separation can be carried out at ambient temperature and pressure. The “soft” nature of the process makes it compatible with a wide range of materials.
K. A. Lohse, D. Hope, R. Sponseller, J. O. Allen, N. B. Grimm, Sci. Total Environ., 402:95–105, 2008. DOI:10.1016/j.scitotenv.2008.04.044.
Urbanization is increasing rapidly in semi-arid environments and is predicted to alter atmospheric deposition of nutrients and pollutants to cities as well as to ecosystems downwind. We examined patterns of wet and coarse dry deposition chemistry over a five-year period at 7 sites across the Central Arizona-Phoenix (CAP) study area, one of two urban sites within the National Science Foundation’s Long-Term Ecological Research (LTER) program. Wet and dry deposition of organic carbon (oC) were significantly elevated in the urban core; in contrast, mean annual wet and dry fluxes of nitrogen (N) were low (< 6 kg ha-1 yr-1) compared to previous estimates and did not differ significantly among sites. Wet deposition of sulfate (SO42-) was high across CAP (mean 1.39 kg ha-1 yr-1 as S) and represented the dominant anion in rainfall. Dry deposition rates did not show strong seasonal trends with the exception of oC, which was 3-fold higher in winter than in summer; ammonium (NH4+) deposition was high but more variable. Dry deposition of NO3– and oC was strongly correlated with particulate base cations and dust-derived soluble reactive phosphorus (SRP), suggesting that urban-derived dust is scrubbing the atmosphere of acidic gases and entrained particles and increasing local deposition. Differences between measured and predicted rates of dry N deposition to the urban core may be explained by incomplete collection of gas phase N on surrogate deposition surfaces in this hot and and environment. The extent of urban enhancement of cations and oC inputs to desert ecosystems appears to be restricted to the urbanized metropolitan area rather than extending far downwind, although a low number of sites make it difficult to resolve this spatial pattern. Nevertheless, wet and dry inputs may be important for biogeochemical cycles in nutrient and carbon-poor desert ecosystems within and near and cities.
Hygroscopic behavior and liquid-layer composition of aerosol particles generated from natural and artificial seawater
M. E. Wise, E. J. Freney, C. A. Tyree, J. O. Allen, S. T. Martin, L. M. Russell, and P. R. Buseck. J. Geophys. Res., 114, D03201, 2009. DOI: 10.1029/2008JD010449.
Sea-salt aerosol (SSA) particles affect the Earth’s radiative balance and moderate heterogeneous chemistry in the marine boundary layer. Using conventional and environmental transmission electron microscopes (ETEM), we investigated the hygroscopic growth and liquid-layer compositions of particles generated from three types of aqueous salt solutions: sodium chloride, laboratory-synthesized seawater (S-SSA particles), and natural seawater (N-SSA particles). Three levels of morphological change were observed with the ETEM as the laboratory-generated particles were exposed to increasing relative humidity (RH). The first level, onset of observable morphological changes, occurred on average at 70, 48, and 35% RH for the NaCl, S-SSA, and N-SSA particles, respectively. The second level, rounding, occurred at 74, 66, and 57% RH for NaCl, S-SSA, and N-SSA particles, respectively. The third level, complete deliquescence, occurred at 75% RH for all particles. Collected ambient SSA particles were also examined. With the exception of deliquescence, they did not exhibit the same hygroscopic characteristics as the NaCl particles. The ambient particles, however, behaved most similarly to the synthesized and natural SSA particles, although the onset of morphological change was slightly higher for the S-SSA particles. We used energy-dispersive X-ray spectrometry to study the composition of the liquid layer formed on the S-SSA and N-SSA particles. The layer was enriched in Mg, S, and O relative to the solid particle core. An important implication of these results is that MgSO4-enriched solutions on the surface of SSA particles may be the solvents of many heterogeneous reactions.
Source apportionment of lead-containing aerosol particles in Shanghai using single particle mass spectrometry
Y. Zhang, X. Wang, H. Chen, X. Yang, J. Chen, J. O. Allen, Chemosphere, 74:501–507, 2009. DOI: 10.1016/j.chemosphere.2008.10.004.
Lead (Pb) in individual aerosol particles was measured using single particle aerosol mass spectrometer (ATOFMS) in the summer of 2007 in Shanghai, China. Pb was found in 3% of particles with diameters in the range 0.1-2.0 µm. Single particle data were analyzed focusing on the particles with high Pb content which were mostly submicron. Using the ART-2a neural network algorithm, these fine Pb-rich particles were classified into eight main classes by their mass spectral patterns. Based on the size distribution, temporal variation of number density, chemical composition and the correlation between different chemical species for each class, three major emission sources were identified. About 45% of the Pb-rich particles contained organic or elemental carbon and were attributed to the emission from coal combustion; particles with good correlation between Cl and Pb content were mostly attributed to waste incineration. One unique class of particles was identified by strong phosphate and Pb signals, which were assigned to emissions from phosphate industry. Other Pb-rich particles included aged sea salt and particles from metallurgical processes.
Quantitative extraction of organic tracer compounds from ambient particulate matter collected on polymer substrates
Q. Sun, O. A. Alexandrova, P. Herckes, and J. O. Allen. Talanta, 78:1115-1121, 2009. DOI: 10.1016/j.talanta.2009.01.039.
Organic compounds in ambient particulate matter (PM) samples are used as tracers for PM source apportionment. These PM samples are collected using high volume samplers; one such sampler is an impactor in which polyurethane foam (PUF) and polypropylene foam (PPF) are used as the substrates. The polymer substrates have the advantage of limiting particle bounce artifacts during sampling; however these substrates may contain background organic additives. A protocol of two extractions with isopropanol followed by three extractions with dichloromethane (DCM) was developed for both substrate precleaning and analyte extraction. Some residual organic contaminants were present after precleaning; expressed as concentrations in a 24-h ambient PM sample, the residual amounts were 1 µg m-3 for plasticizers and antioxidants, and 10 ng m-3 for n-alkanes with carbon number lower than 26. The quantification limit for all other organic tracer compounds was approximate to 0.1 ng m-3 in a 24-h ambient PM sample. Recovery experiments were done using NIST Standard Reference Material (SRM) Urban Dust (1649a); the average recoveries for polycyclic aromatic hydrocarbons (PAHs) from PPF and PUF substrates were 117 +/- 8% and 107 +/- 11%, respectively, Replicate extractions were also done using the ambient samples collected in Nogales, Arizona. The relative differences between repeat analyses were less than 10% for 47 organic tracer compounds quantified. After the first extraction of ambient samples, less than 7% of organic tracer compounds remained in the extracted substrates. This method can be used to quantify a suite of semi-and non-polar organic tracer compounds suitable for source apportionment studies in 24-h ambient PM samples.
D. Lowenthal, B. Zielinska, B. Mason, S. Samy, V. Samburova, D. Collins, C. Spencer, N. Taylor, J. O. Allen, N. Kumar, J. Geophys. Res., 114, D08206, 2009. DOI:10.1029/2008JD011274.
A study was conducted during summer 2006 at Great Smoky Mountains (GRSM) National Park (NP), TN, to address issues related to estimating aerosol light extinction in the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The revised IMPROVE equation calculates PM2.5 light scattering (Bsp) from ammonium sulfate, ammonium nitrate, organic carbon mass, and fine soil concentrations; dry scattering efficiencies; and factors that account for hygroscopic growth. Organics are assumed to be nonhygroscopic. The organic compound mass (OCM)/organic carbon (OC) ratio is assumed to be 1.8. Experiments involving in situ and laboratory measurements were conducted to address issues related to (1) concentration-varying scattering efficiencies; (2) aerosol hydration state; (3) the OCM/OC ratio; and (4) the organic hygroscopicity. Filter-based measurements indicated that sulfate was acidic, with an average NH4+/SO4= molar ratio of 1.16. Ambient State Hygroscopic Tandem Differential Mobility Analyzer measurements of the ambient hydration state rarely indicated deliquescence. The frequency of hysteresis ranged from 29% to 46% for 0.05 and 0.2 mu m particles, respectively. There was a clear relationship between dry particle mean diameter and volume at GRSM, supporting the assumption that an increase in particle size during transport increases both the scattering efficiency and concentration. Water-soluble organic carbon (WSOC) was isolated from water extracts of high-volume filter samples using XAD solid-phase absorbents. The average ratios of OCM measured gravimetrically to OC measured by thermal optical reflectance in residues of isolated WSOC and dichloromethane (DCM) extracts were 2.4 +/- 0.3 and 1.9 +/- 0.2, respectively. Hygroscopic growth factors (GF) of aerosols generated from WSOC extracts averaged 1.10 +/- 0.02, 1.13 +/- 0.03, and 1.19 +/- 0.04 at 80%, 85%, and 90% RH, respectively. These results indicate that, at GRSM during summer, at least some of the organic aerosol was hygroscopic.
C. A. Tyree and J. O. Allen, Power Magazine, 154(7):26-32, 2010.
The U.S. Environmental Protection Agency is expected to propose an emissions standard for mercury and other hazardous air pollutants emitted by coal- and oil-fired electric generating units in March of 2011. The anticipated rule would require emission control to meet the various standards using maximum achievable control technology, as determined by the prescriptive requirements of the Clean Air Act. In response to the expected rule-making, utilities will be required to make technology decisions in order to ensure compliance. One cost-effective approach to compliance may be the use of “co-benefits” from air quality control systems (AQCSs) already in service that are designed to remove other pollutants.
J. O. Allen, R. Chang, and C. A. Tyree, in MEGA Symposium, Paper 92. Air & Waste Management Association, 2010.
Production coal-fired power plant data were analyzed in order to evaluate the effectiveness
of existing pollution control equipment (SCR-ESP-wFGD) for the control mercury emissions.
More than forty plant-months of process and emissions data were collected into a large data
set. Mercury emissions were calculated in lb/TBtu using EPA methods. Mercury control efficiencies
were 90% or greater on approximately 50% of the study days. Lower collection
efficiencies were observed when 1) load was near capacity, 2) SCR units were off-line, and 3)
mercury was re-emitted from FGD liquor. These results are consistent with mercury oxidation
in SCR units and elsewhere, followed by collection and sequestration of oxidized mercury
in FGD liquor. Consistent collection efficiencies of 90% or greater will likely require additional
process controls based on improved understanding of mercury oxidation and sequestration
M. B. Looney, D. Woodson, R. Merritt, and J. O. Allen, in MEGA Symposium, Paper 99. Air & Waste Management Association, 2010.
Georgia Power Company recently retrofitted four coal-fired units with TOXECON technology for mercury control. TOXECON utilizes a baghouse with sorbent injection to control the emission of toxic air pollutants. Within the first year of operation it was discovered that the units were not operating as expected with regard to mercury removal and baghouse pressure drop. A technical investigation was conducted to determine the cause of the poor performance and assess possible modifications to the hardware, component configurations, or process parameters to improve the performance. The low ash loading at the inlet to the baghouse was determined to be a key contributor to the poor performance. This paper will detail the technical investigation and present the operating enhancements achieved through implementation of the process modifications as well as longer term operational findings from the TOXECON installation.
N. Upadhyay, Q. Sun, J. O. Allen, P. Westerhoffd and P. Herckes, J. Water Res., 45:1071-1078, 2011. DOI: 10.1016/j.watres.2010.10.024.
Wastewater aeration basins at publicly owned treatment works (POTWs) can be emission sources for gaseous or aerosolized sewage material. In the present study, particle and gas phase emissions of synthetic musks from covered and uncovered aeration basins were measured. Galaxolide (HHCB), tonalide (AHTN), and celestolide (ADBI) were the most abundant, ranging from 6704 to 344,306 ng m-3, 45–3816 ng m-3, and 2–148 ng -3 in the gas phase with particle phase concentrations 3 orders of magnitude lower. The musk species were not significantly removed from the exhaust air by an odor control system, yielding substantial daily emission fluxes (~200 g d-1 for HHCB) into the atmosphere. However, simple dispersion modeling showed that the treatment plants are unlikely to be a major contributor to ambient air concentrations of these species. Emission of synthetic musk species during wastewater treatment is a substantial fate process; more than 14% of the influent HHCB is emitted to the atmosphere in a POTW as opposed to the <1% predicted by an octanol–water partition coefficient and fugacity-based US EPA fate model. The substantial atmospheric emission of these compounds is most likely due to active stripping that occurs in the aeration basins by bubbling air through the sludge.
J. O. Allen, M. B. Looney, and C. A. Tyree, Paper 2011-A-636-AWMA, Air & Waste Management Association Conference, 2011.
Reference mercury emissions measurements are based on short-term stack test results, while regulatory compliance standards are usually based on long-term average emissions. A number of statistical methods have been used to develop long-term compliance standards based on short-term stack tests; however, if emissions data do not fit the assumed parametric distribution, the calculated compliance standards may not be achievable by the best performing units. We have compiled over 200 plant-months of operational and emissions data from ten plants which control mercury using co-benefit or activated carbon injection technologies. Monthly and annual averaged emission rates were calculated using block and rolling averages. A large number of stack test results were estimated from continuous mercury emissions data for periods when operating conditions were like those used during stack tests. Statistical methods were used to estimate upper emission limits from these short-term measurements. The statistical methods were based on normal and log-normal distributions. These statistical methods were then evaluated by comparing the calculated upper emission limits with the actual 99th percentile long-term emissions for the same plant and control technology.