Intercomparison of measurement methods for black carbon aerosols

In this study, two method intercomparisons were performed. One thermal and two optical methods for the measurement of black carbon (BC) were applied to laboratory generated aerosols containing only BC. For the optical measurements, an aethalometer (Hansen et al., 1984. Science of Total Environment 36, 191–196) and an integrating sphere technique (Hitzenberger et al., 1996b. Journal of Geophysical Research 101, D14, 19 601–19 606) were used. The thermal method was described by Cachier et al. (1989a. Tellus 41B, 379–390). In an additional comparison, the integrating sphere was compared to a thermal optical technique (Birch and Cary, 1996. Aerosol Science Technology 25, 221–241) on ambient aerosol samples. The absorption coefficients were obtained from transmission measurements on filter samples for both the aethalometer and the integrating sphere. The BC mass concentration for the aethalometer was derived from this absorption measurement. The BC mass concentration for the integrating sphere, however, was obtained using an independent calibration curve. The agreement between the absorption coefficient σ_a obtained for the BC test aerosol on parallel filters with the aethalometer and the integrating sphere was satisfactory. The slope of the regression lines depended on filter type. A comparison between BC mass concentrations, however, showed that the aethalometer values were only 23% of those obtained by the integrating sphere technique indicating that for pure BC aerosols, the standard aethalometer calibration should not be used. Compared to the thermal method, the integrating sphere gave an overestimation of the BC mass concentrations by 21%. For the ambient samples, the integrating sphere and the thermal optical methods for BC mass concentration determination showed agreement within 5% of the 1 : 1 line, although the data were not so well correlated.     

On the relation between the size and chemical composition of aerosol particles and their optical properties

The light scattering and absorption coefficients of fine atmospheric aerosol particles were recorded in Hungary under rural conditions in 1998–1999 by an integrating nephelometer and particle soot absorption photometer, respectively. In some cases optical properties in the fine size range were compared to those in the coarse particles. Results obtained indicate, as expected, that fine particles control the scattering and absorption caused by the aerosol. In 1999 the size distribution of aerosol particles was also monitored by means of an electric low pressure impactor (ELPI). This makes it possible the study of the relationship between the number, surface and mass concentration in the size range of 0.1–1.0 μm and the optical characteristics by also considering the chemical composition of the particles.     

Comparison of continuous and filter-based carbon measurements at the Fresno supersite

Results from six continuous and semicontinuous black carbon (BC) and elemental carbon (EC) measurement methods are compared for ambient samples collected from December 2003 through November 2004 at the Fresno Supersite in California. Instruments included a multi-angle absorption photometer (MAAP; lambda = 670 nm); a dual-wavelength (lambda = 370 and 880 nm) aethalometer; seven-color (lambda = 370, 470, 520, 590, 660, 880, and 950 nm) aethalometers; the Sunset Laboratory carbon aerosol analysis field instrument; a photoacoustic light absorption analyzer (lambda = 1047 nm); and the R&P 5400 ambient carbon particulate monitor. All of these acquired BC or EC measurements over periods of 1 min to 1 hr. Twenty-four-hour integrated filter samples were also acquired and analyzed by the Interagency Monitoring of Protected Visual Environments (IMPROVE) thermal/optical reflectance carbon analysis protocol. Site-specific mass absorption efficiencies estimated by comparing light absorption with IMPROVE EC concentrations were 5.5 m2/g for the MAAP, 10 m2/g for the aethalometer at a wavelength of 880 nm, and 2.3 m2/g for the photoacoustic analyzer; these differed from the default efficiencies of 6.5, 16.6, and 5 m2/g, respectively. Scaling absorption by inverse wavelength did not provide equivalent light absorption coefficients among the instruments for the Fresno aerosol measurements. Ratios of light absorption at 370 nm to those at 880 nm from the aethalometer were nearly twice as high in winter as in summer. This is consistent with wintertime contributions from vehicle exhaust and from residential wood combustion, which is believed to absorb more shorter-wavelength light. To reconcile BC and EC measurements obtained by different methods, a better understanding is needed of the wavelength dependence of light-absorption and mass-absorption efficiencies and how they vary with different aerosol composition.     

On the contribution of black carbon to the composite aerosol radiative forcing over an urban environment

This paper discusses the extent of Black Carbon (BC) radiative forcing in the total aerosol atmospheric radiative forcing over Pune, an urban site in India. Collocated measurements of aerosol optical properties, chemical composition and BC were carried out for a period of six months (during October 2004 to May 2005) over the site. Observed aerosol chemical composition in terms of water soluble, insoluble and BC components were used in Optical Properties of Aerosols and Clouds (OPAC) to derive aerosol optical properties of composite aerosols. The BC fraction alone was used in OPAC to derive optical properties of BC aerosols. The aerosol optical properties for composite and BC aerosols were separately used in SBDART model to derive direct aerosol radiative forcing due to composite and BC aerosols. The atmospheric radiative forcing for composite aerosols were found to be +35.5, +32.9 and +47.6 Wm^?2 during post-monsoon, winter and pre-monsoon seasons, respectively. The average BC mass fraction found to be 4.83, 6.33 and 4 μg m^?3 during the above seasons contributing around 2.2 to 5.8% to the total aerosol load. The atmospheric radiative forcing estimated due to BC aerosols was +18.8, +23.4 and +17.2 Wm^?2, respectively during the above seasons. The study suggests that even though BC contributes only 2.2–6% to the total aerosol load; it is contributing an average of around 55% to the total lower atmospheric aerosol forcing due to strong radiative absorption, and thus enhancing greenhouse warming.     

Use of two different acidic aerosol samplers to measure acidic aerosols in Hsinchu, Taiwan

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

Spectroscopic measurement of horizontal atmospheric extinction and its practical application

At a suburban location in southern Korea, the optical properties of the atmosphere were characterized over a horizontal light path of 1.5 km (in two ways) from 22 October to 19 November 2002. This was achieved by measuring light extinction with a long path differential optical absorption spectrometer system in the ultraviolet and visible wavelength region. The extinction coefficients were obtained relatively as a ratio of a target air spectrum to a defined reference spectrum measured over the same light path (290–760 nm). To assess the measured extinction coefficients, the extinction coefficients at 550 nm were compared to those measured with a commercial long-path transmissometer.To avoid the absorption of known gases (H₂O, NO₂, and O₃), extinction coefficients at the spectral bands of 325, 394, 472, 550, 580, 680, and 753 nm, with 3 nm window widths, were selected and analyzed for the purpose of the study of the aerosol properties. Importantly, the atmospheric physical properties during the episodes (such as a biomass burning or a dust storm) were investigated by means of the Angstrom parameters and the mass ratio of fine-to-coarse particles. In addition, it was found that the Angstrom exponents decrease monotonously for relative humidity above 50%.     

Optical and chemical properties of marine boundary-layer aerosol around Japan determined from shipboard measurements in 2002

Shipboard measurements of the optical and chemical properties of marine boundary-layer aerosol were made around Japan over the period from 28 August to 25 September 2002. Measurements were conducted aboard the Research Vessel (R/V) Shirase along cruise tracks beginning from Yokosuka, and proceeding on to Hakodate, Sakata, Sasebo, Naha, Kure, and Yokkaichi. This paper describes the results of optical measurements using an Optical Particle Counter (OPC), an Integrating Nephelometer (IN), and a Particle Soot/Absorption Photometer (PSAP), as well as chemical analyses of water-soluble aerosol particles collected by impactor and filter systems. Coulter Multisizer measurements were used for water-insoluble aerosol particles. The complex refractive index (CRI), scattering and absorption coefficients, and size distribution of aerosols were estimated from combined measurements made using OPC, IN, and PSAP. Contrasting aerosol characteristics were observed during different stages of the cruise. Discussion on these differences focuses mainly on two legs: Leg-1 from Yokosuka to Hakodate and Leg-4 from Sasebo to Naha. Backward trajectory analyses indicate that the air sampled during Leg-1 originated from the Pacific Ocean, whereas the air sampled during Leg-4 originated from the Chinese Continent via the Korean Peninsula. For the first half of Leg-1, the number concentration was low and larger particles were relatively predominant. The real and imaginary parts of the CRI were estimated to be 1.38–1.40 and close to zero, respectively. This estimation is consistent with the results of chemical analyses, which show that the sea salt is rich in aerosols sourced from remote ocean areas. In contrast, small particles were predominant during Leg-4, and the real and imaginary parts of the CRI were estimated to be 1.52–1.59 and approximately −0.002, respectively. These findings are also consistent with chemical analyses that reveal a mixture of mineral dust and sulfate aerosol likely transported from China and Korea. The Coulter Multisizer measurements conducted during Leg-4 reveal abundant water-insoluble particles with a mode radius of 1.35 μm.     

Use of diffuse reflectance spectrometry in spot test reactions for quantitative determination of cations in water

Diffuse reflectance spectroscopy can be successfully used for the quantitative determination of small amounts of pollutants like metals. The remission function was found to be linearly proportional to the concentration, when we applied the Kubelka–Munk equation. The color reactions of Cu(II), Co(II), and Ni(II) with dithiooxamide, were realised on filter paper. Reaction between Fe(III) and ammonium thiocyanate was realized on filter paper and gelatine matrix. All measurements were accomplished with a laboratory-constructed reflectometer. We have obtained a calibration curve by plotting the optical density of reflectance A_R vs log of the mol l⁻¹ concentration. Limits of detection at the 10⁻⁴ M level were estimated for all the compounds. Linear dynamic range extend over one order of magnitude and shows the potential of device for the quantitative analysis of environmental pollutants.     

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

ABSTRACT

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

Comparisons of aerosol properties measured by impactors and light scattering from individual particles: refractive index,number and volume concentrations,and size distributions

The southeastern aerosol and visibility study (SEAVS) was conducted in Great Smoky Mountains National Park in summer 1995 to investigate variations in ambient aerosol size distributions and their effect on visibility. In this paper, we compare dry aerosol size distribution parameters from a MOUDI impactor and two different optical particle counters (OPCs). Size distributions from the various instruments are expressed in a common measure of size, specifically, optical and aerodynamic diameters are converted to a dry, geometric diameter basis. Comparisons of the real part of particle refractive index obtained directly from light scattering measurements and inferred from aerosol composition measurements are also shown. Real refractive indices from direct measurements and those computed from measured fine aerosol chemical composition were generally within ±0.02. Maximum differences in estimated accumulation mode integrated volume concentrations from all instruments were within ∼22%. Accumulation mode integrated number concentrations and geometric standard deviations from the two OPCs agreed within ∼30% and ∼3%, respectively. Differences between MOUDI- and OPC-derived accumulation mode number concentrations and geometric standard deviations were ∼20% and ∼8%, respectively. The average geometric volume mean diameters derived from the three instruments agreed within 15% or less. The volume median diameters obtained by fitting the CSU number concentration data to a lognormal function were typically the smallest. We show that these discrepancies can be related to the differences and biases in the measurement and data analysis techniques.     

The Relationship between “Soiling Index” and Suspended Particulate Matter Concentrations

Experimental relationships between the mass concentration of suspended particulate matter and the optical density of particulates collected on paper tape have been determined for the atmospheric aerosol and for aerosols of constant optical properties. Simultaneous samples were obtained on membrane filters (for gravimetric analysis) and on Whatman No. 4 paper tape (for optical evaluation). Sampling procedures were adopted which ensured that the efficiency of sampling was the same in both cases.

Consistent relationships between mass concentrations of suspended particulate matter and optical density expressed in terms of per cent transmittance or per cent reflectance were found for dispersions of coal, limestone, fly ash, and a coal-limestone mixture.

For atmospheric aerosol samples collected on the roof of the Graduate School of Public Health, University of Pittsburgh, the relationship between mass concentration and per cent transmittance or per cent reflectance was found to be linear over the range of values observed. The correlation coefficients were ?0.93 (for concentration versus per cent transmittance) and ?0.89 (for concentration versus per cent reflectance).     

Comparisons of the physical and optical properties of atmospheric aerosol from airborne and surface measurements on the east coast of England

Results are presented of a three-day joint surface and aircraft experiment during July 1984 measuring aerosol properties at a coastal site in Lincolnshire. Scattering and absorption coefficients were measured and are in good agreement with coefficients modelled by Mie theory using measured size spectra. The dependence of aerosol size and concentration on relative humidity and altitude is also considered.     

Chemical composition and complex refractive index of Saharan Mineral Dust at Izaña,Tenerife (Spain) derived by electron microscopy

Samples from two strong homogeneous dust plumes from the Saharan desert reaching Izaña (Tenerife, Spain) in July and August 2005 were taken with a miniature impactor system and filter samplers. Size, aspect ratio and chemical composition of more than 22,000 individual particles were studied by scanning electron microscopy. The mineralogical phase composition of about 200 particles was investigated by transmission electron microscopy. In addition, the aerosol size distribution was measured with an optical particle spectrometer. In all samples, the aerosol was dominated by mineral dust with an average composition (by volume) of 64% silicates, 6% quartz, 5% calcium-rich particles, 14% sulfates, 1% hematite, 1% soot and 9% other carbonaceous material. Sulfate was found predominantly as coating on other particles with an average thickness of approximately 60 nm. The aerosol calcium content is correlated with the calcite concentrations of soils in the source region, highest values were observed for northern and central Algeria and Morocco. The average aspect ratio of the particles was 1.64. The distributions of the aspect ratios are parameterized by log-normal functions for modeling purpose. Single-scattering albedo (0.95) and asymmetry factor (0.74–0.81) was measured by polar aerosol photometry on filter samples using a light source resembling the solar spectrum. The apparent soot content of the sample (1 vol%) was determined by the same technique. From the mineralogical data, an average complex refractive index of 1.59–9×10⁻³i for visible light was derived. The imaginary part of the complex refractive index decreases with increasing particle size from −2.5×10⁻²i to <−10⁻³i, reflecting the decreasing hematite and soot contents. The imaginary part derived from optical measurements was −7×10⁻³i.     

Measurement of fine particulate matter nonvolatile and semi-volatile organic material with the Sunset Laboratory Carbon Aerosol Monitor

Semi-volatile organic material (SVOM) in fine particles is not reliably measured with conventional semicontinuous carbon monitors because SVOM is lost from the collection media during sample collection. We have modified a Sunset Laboratory Carbon Aerosol Monitor to allow for the determination of SVOM. In a conventional Sunset monitor, gas-phase organic compounds are removed in the sampled airstream by a diffusion denuder employing charcoal-impregnated cellulose filter (CIF) surfaces. Subsequently, particles are collected on a quartz filter and the instrument then determines both the organic carbon and elemental carbon fractions of the aerosol using a thermal/optical method. However, some of the SVOM is lost from the filter during collection, and therefore is not determined. Because the interfering gas-phase organic compounds are removed before aerosol collection, the SVOM can be determined by filtering the particles at the instrument inlet and then replacing the quartz filter in the monitor with a charcoal-impregnated glass fiber filter (CIG), which retains the SVOM lost from particles collected on the inlet filter. The resulting collected SVOM is then determined in the analysis step by measurement of the carbonaceous material thermally evolved from the CIG filter. This concept was tested during field studies in February 2003 in Lindon, UT, and in July 2003 in Rubidoux, CA. The results obtained were validated by comparison with Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) results. The sum of nonvolatile organic material determined with a conventional Sunset monitor and SVOM determined with the modified Sunset monitor agree with the PC-BOSS results. Linear regression analysis of total carbon concentrations determined by the PC-BOSS and the Sunset resulted in a zero-intercept slope of 0.99 +/- 0.02 (R2 = 0.92) and a precision of sigma = +/- 1.5 microg C/m3 (8%).     

The effect of temperature on the gas–particle partitioning of reactive mercury in atmospheric aerosols

Measurements of gas–particle-partitioning coefficients for reactive mercury in dry urban and laboratory aerosol were found to strongly depend on ambient temperature. Samples of atmospheric and laboratory aerosols (defined as both the gas and particle phases) were collected using filter and absorbent methods and analyzed for reactive mercury using thermal desorption combined with cold vapor atomic fluorescence spectroscopy. Synthetic ambient aerosols were generated in the laboratory from ammonium sulfate and adipic acid mixed with mercuric chloride in a purpose-built aerosol reactor. The aerosol reactor was operated in a temperature-controlled laboratory. Linear relationships between the logarithm of inverse gas–particle partitioning and inverse temperature were observed and parameterized for use in the atmospheric modeling of reactive mercury. Reactive mercury was observed to partition from the particle to the gas phase as ambient temperature increased. Good agreement between measurements made using urban and laboratory aerosols was seen after gas–particle-partitioning coefficients were normalized for surface area instead of mass. Thermodynamic analyses of the urban and laboratory gas–particle-partitioning measurements revealed that the strength of interaction between reactive mercury and particle surfaces was suggestive of chemisorption. Gas–particle-partitioning coefficients made with the Tekran ambient mercury analyzer (AMA) also showed a dependence on temperature. However, the Tekran AMA partitioning coefficients did not agree well with partitioning coefficients measured using the filter-based methods. The disagreement is consistent with the 50 °C operational temperature of the Tekran AMA.     

Urban impacts on regional carbonaceous aerosols: Case study in central Texas

Rural and background sites provide valuable information on the concentration and optical properties of organic, elemental, and water-soluble organic carbon (OC, EC, and WSOC), which are relevant for understanding the climate forcing potential of regional atmospheric aerosols. To quantify climate- and air quality-relevant characteristics of carbonaceous aerosol in the central United States, a regional background site in central Texas was chosen for long-term measurement. Back trajectory (BT) analysis, ambient OC, EC, and WSOC concentrations and absorption parameters are reported for the first 15 months of a long-term campaign (May 2011–August 2012). BT analysis indicates consistent north–south airflow connecting central Texas to the Central Plains. Central Texas aerosols exhibited seasonal trends with increased fine particulate matter (<2.5 μm aerodynamic diameter, PM_2.5) and OC during the summer (PM_2.5 = 10.9 μg m^?3 and OC = 3.0 μg m^?3) and elevated EC during the winter (0.22 μg m^?3). When compared to measurements in Dallas and Houston, TX, central Texas OC appears to have mixed urban and rural sources. However, central Texas EC appears to be dominated by transport of urban emissions. WSOC averaged 63% of the annual OC, with little seasonal variability in this ratio. To monitor brown carbon (BrC), absorption was measured for the aqueous WSOC extracts. Light absorption coefficients for EC and BrC were highest during summer (EC MAC = 11 m² g^?1 and BRC MAE₃₆₅ = 0.15 m² g^?1). Results from optical analysis indicate that regional aerosol absorption is mostly due to EC with summertime peaks in BrC attenuation. This study represents the first reported values of WSOC absorption, MAE₃₆₅, for the central United States.

Implications:Background concentration and absorption measurements are essential in determining regional potential radiative forcing due to atmospheric aerosols. Back trajectory, chemical, and optical analysis of PM_2.5 was used to determine climatic and air quality implications of urban outflow to a regional receptor site, representative of the central United States. Results indicate that central Texas organic carbon has mixed urban and rural sources, while elemental carbon is controlled by the transport of urban emissions. Analysis of aerosol absorption showed black carbon as the dominant absorber, with less brown carbon absorption than regional studies in California and the southeastern United States.     

The Analysis of Airborne Sulfate

This report reviews the current state of analytical methodology for sulfate in airborne particles. Methods for determination of total aerosol sulfate and total soluble sulfate are assessed. A more detailed review of the relatively new techniques for quantitative speciation of airborne sulfate then follows. Size distribution methodology and filter sampling difficulties relating to the collection of airborne sulfate-containing particles are enumerated. Experiments are suggested which use new, improved, and potentially applicable sulfate speciation techniques to obtain a better understanding of the generation, transport, transformation and removal processes that, in turn, determine the identity and concentrations of sulfate species in ambient air.     

Removal of Toxic Gases in Smog with Metal Filters

Highly sensitive radiotracer techniques were used to see if the H₂SO₄ mist from SOt oxidation allows the incorporation of organic matter. Tagged organic compounds were irradiated with and without SO₂. When the aerosol was filtered off, no radioactivity was found on the filter paper showing the absence of organic matter and the lack of SO₂ synergism.     

Size-resolved mass and chemical properties of dust aerosols from Australia’s Lake Eyre Basin

Australia is the dominant mineral dust source in the southern hemisphere, yet the physical, chemical and optical properties of Australian dust aerosol are presently poorly understood. We have investigated the properties of Australian aerosol at a site near Lake Eyre in central Australia, which is strongly influenced by mineral dust. During a field campaign in November 2007 we collected eight sets of size-resolved aerosol samples for laboratory analysis: six during quiescent conditions, and two during dust storms. Ion Beam Analysis was used to determine the elemental composition of all filter samples. Scatter plots showed that Fe, Al and Ti were well correlated with Si, and hence soil-derived. The Fe/Si ratio was consistently higher than the global crustal average, confirming that Australian dusts are comparatively rich in Fe. Scatter plots for Na and Cl against Si showed clear evidence of a second aerosol population, associated with maritime advection. Profiles of water soluble ions for two sample sets, showed the importance of marine influences on both the fine and coarse modes, as well as the presence of organic acids. Estimates of the mass fraction of NaCl in our samples suggest that, for quiescent days, roughly 0.5% of the sample mass was NaCl.     

Validation of three new methods for determination of metal emissions using a modified Environmental Protection Agency Method 301

Three new methods applicable to the determination of hazardous metal concentrations in stationary source emissions were developed and evaluated for use in U.S. Environmental Protection Agency (EPA) compliance applications. Two of the three independent methods, a continuous emissions monitor-based method (Xact) and an X-ray-based filter method (XFM), are used to measure metal emissions. The third method involves a quantitative aerosol generator (QAG), which produces a reference aerosol used to evaluate the measurement methods. A modification of EPA Method 301 was used to validate the three methods for As, Cd, Cr, Pb, and Hg, representing three hazardous waste combustor Maximum Achievable Control Technology (MACT) metal categories (low volatile, semivolatile, and volatile). The modified procedure tested the methods using more stringent criteria than EPA Method 301; these criteria included accuracy, precision, and linearity. The aerosol generation method was evaluated in the laboratory by comparing actual with theoretical aerosol concentrations. The measurement methods were evaluated at a hazardous waste combustor (HWC) by comparing measured with reference aerosol concentrations. The QAG, Xact, and XFM met the modified Method 301 validation criteria. All three of the methods demonstrated precisions and accuracies on the order of 5%. In addition, correlation coefficients for each method were on the order of 0.99, confirming the methods' linear response and high precision over a wide range of concentrations. The measurement methods should be applicable to emissions from a wide range of sources, and the reference aerosol generator should be applicable to additional analytes. EPA recently approved an alternative monitoring petition for an HWC at Eli Lilly's Tippecanoe site in Lafayette, IN, in which the Xact is used for demonstrating compliance with the HWC MACT metal emissions (low volatile, semivolatile, and volatile). The QAG reference aerosol generator was approved as a method for providing a quantitative reference aerosol, which is required for certification and continuing quality assurance of the Xact.