Fourier transform infrared spectroscopy of size - OSA Publishing

Fourier transform infrared spectroscopy of sizesegregated aerosol deposits on foil substrates Judith A. Hopey,1,2 Kirk A. Fuller,1,* Venkataramanan Krishnaswamy,1,3 David Bowdle,1 and Michael J. Newchurch4 1

Earth System Science Center, University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, Alabama 35899, USA 2

Now with the Mathematics Department, Middlesex County College, 2600 Woodbridge Avenue, Edison, New Jersey 08818-3050, USA

3

Now with the Near Infrared Imaging Group, Thayer School of Engineering, Dartmouth College, 8000 Cummings Hall, Hanover, New Hampshire 03755-8000, USA

4

Atmospheric Science Department, University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, Alabama 35899, USA *Corresponding author: [email protected] Received 11 September 2007; revised 3 March 2008; accepted 4 March 2008; posted 4 March 2008 (Doc. ID 87376); published 25 April 2008

A method based on Fourier transform infrared (FTIR) double-pass transmittance spectroscopy was developed for determining functional group loading in size-segregated ambient aerosol deposits. The impactor employed for sample collection utilized rotating stages, which produced uniform particulate matter (PM) deposits on standard Al foil substrates. Each sample was analyzed without extraction using an FTIR spectrometer equipped with a reflectometer accessory. The use of the reflectometer obviated the need for infrared window materials as substrates. ðNH4 Þ2 SO4 aerosol generated under laboratory conditions were used to calibrate deposit mass to the band strength of the relatively isolated ν4 bending mode −1 of SO2− 4 centered near 620 cm . Atmospheric PM was sampled during the summer of 2004 in Huntsville, Ala. Sulfate concentrations determined in this initial study correlated well with measurements made by collocated EPA air samplers. © 2008 Optical Society of America OCIS codes: 300.6300, 010.1280, 120.6200.

1. Introduction

The atmospheric aerosol has a significant impact on the transfer of radiant energy, and thus plays a major role in climatology and remote sensing. The aerosol– climate interaction comes about through effects that are categorized as direct (scattering and absorption), indirect (cloud nucleation), and semidirect (alteration of thermodynamics in cloud fields). Scattering and absorption by airborne particulate matter (PM) also plays a role in both active and passive remote sensing. Collected atmospheric PM samples typically consist of very small quantities of material containing many different compounds with a wide range of 0003-6935/08/132266-09$15.00/0 © 2008 Optical Society of America 2266

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properties, all of which play a role in understanding the in situ radiative properties of the aerosol. Methods currently applied to the chemical assay of PM samples include gas and ion chromatography, mass spectrometry, thermo/optic analysis (TOA), x ray emission, and optical spectroscopy. Many analytical techniques require separation, heating, derivatization, or extraction, and are often limited in their ability to quantify large portions of the organic fraction in typical PM samples. Fourier transform infrared (FTIR) analysis for characterization of both organic and inorganic aerosol species complements these measurements by providing functional group and bond information for the entire aerosol without sample manipulation beyond collection. Allen et al. [1] developed a method for FTIR spectroscopy using a low pressure impactor (LPI). This

method was useful, but collection with the LPI led to volatilization of some compounds at low pressure. In addition, each stage of the LPI has one hole, which produces a pile of particulates on the substrate under it. Analysis of the deposited particulates was based on spectroscopic transmittance through both the substrate and the deposit, and so the substrate of choice was ZnSe. Blando et al. [2–4] also employed the LPI/FTIR analysis to data collected by the Southeastern Aerosol and Visibility Study (SEAVS). One focus [2] was on identification of the constituents of the spectra and used the LPI/FTIR method to discuss the SEAVS data, while [3,4] incorporated another aspect of the SEAVS program which included concurrent measurements by the University of Minnesota using a multiorifice uniform deposit (MOUDI) sampler [5] and ion chromatography. The work reported herein relates to the analysis of PM collected with a size-segregating 10-stage MOUDI. While the first stage has one hole, this number increases with decreasing size cut-point (stages nine and 10 have 2000). A larger number of holes produces smaller deposit spots with less piling. In addition, this study employed a rotating-stage MOUDI that distributed the PM over the substrate, providing a more uniform sample for spectroscopic analysis. The MOUDI stages do not readily accommodate ZnSe substrates, which are expensive, brittle, and would have to be made impractically thin. This necessitated finding a new substrate. Teflon film was tried at the beginning of this study, but thin film interference and spectral saturation by the fluoroethylene polymers at wave bands critical to PM analysis rendered it unsatisfactory. One viable solution to substrate interference is attenuated total reflection (ATR)-FTIR spectroscopy, as recently demonstrated by Coury and Dillner [6]. In this approach, the evanescent field of the internally reflected beam only slightly penetrates the Teflon, eliminating thin film interference and minimizing absorption by the substrate. Although this technique may damage that part of the sample that is pressed against the ATR prism, it allows spectroscopic interrogation of deposits on standard filter media. Our work involves the insertion of a reflectance accessory into the sample compartment of the spectrometer and measuring the spectrum of the beam that is reflected from deposits on aluminum foil substrates. Such substrates have proven satisfactory, all the more so since they are also the standard substrate for the MOUDI impaction plates. 2. Experimental

This work describes the use of FTIR spectroscopy to analyze aerosol samples collected in a Model 110, rotating-stage MOUDI. The MOUDI 110 has an inlet and 10 stages. Ratios of the absolute pressure at the exit of each stage relative to the inlet pressure range from essentially 1.00 for the inlet

and first four stages to 0.53 at the exit of stage 10. The flow rate through the MOUDI is maintained at 30 L= min. An impaction plate is located on top of the first stage, which along with the inlet tube, serves as a coarse particle precut to establish an upper size limit for particles collected on the first stage. The substrates were made from common aluminum foil by using a 47 mm Arch punch. The diameter of the deposit is 26 mm. Each sample was analyzed on the foil, using a ThermoElectron Nexus 870 FTIR spectrometer with a Seagull reflectance accessory (Harrick Scientific) in the sample compartment, with automatic atmospheric correction applied. Spectra were taken over the 4000–400 cm−1 region at a resolution of 4 cm−1 (determined to be sufficient for the condensed phase samples considered here). The signal was averaged over 200 scans, requiring about four minutes net acquisition time. Spectra are analyzed with the Seagull reflectance accessory set at a 75° (from normal) incident angle. The incident angle was chosen to be fairly high in order to spread the IR beam more uniformly over the sample. Before the spectra were taken, a background spectrum was recorded with a clean piece of Al foil. The Environmental Protection Agency (EPA) supports a speciated trends network (STN) monitoring station in Huntsville, Ala. in cooperation with the City of Huntsville’s Air Quality Division. As part of the STN operation, PM samples are collected on the roof of a small building about 5 m above ground. PM10 and PM2:5 samples (PM with aerodynamic diameters less than 10 and 2:5 μm, respectively) are collected at this site every sixth day, correlating to the EPA’s monitoring schedule. The EPA’s STN PM2:5 measurements include ion chromatography for determination of the mass of inorganic moieties such as sulfates, nitrates, and ammonium [7]. The MOUDI was collocated with the EPA samplers from May through September of 2004, and data from the EPA’s PM2:5 measurements are used for comparison. Samples for this study were taken during the same 24 h period each day as the EPA particulate collection. The collection of each sample began at 2300 CDT on the previous day and ended at 2300 CDT on the nominal date. Samples for this study were collected from 15 May to 30 September 2004, with only three exceptions (due to mechanical failures and severe weather). The MOUDI shelter was about 0:5 m from the PM2:5 monitor. Polyvinyl chloride (PVC) pipe with an inner diameter of 3:4 cm was used as an inlet. The PVC pipe had an inverted U-shape to prevent rain from entering the MOUDI inlet. A vacuum pump and a timer to turn the pump and the rotating stage of the MOUDI on and off were located inside the building. The vacuum pump drew air through the MOUDI at the prescribed nominal rate of 30 L= min. On the business day before each run clean foil substrates were placed on the impaction stages, and the 1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS

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Standards for major components of the ambient aerosol spectra were created in the laboratory, and examples are shown in Fig. 1. Aerosols of ammonium sulfate, ammonium bisulfate, ammonium nitrate, and soil were generated with a medical nebulizer. The soil sample was taken from a construction site about 30 m away from the EPA monitoring station. The soil was mixed with distilled water then allowed to settle for two minutes. About 5:0 ml of the remaining suspension was decanted and run through the nebulizer. In all cases the aerosol was passed through the MOUDI, and FTIR spectra were taken from the collected substrate deposits. O-ring grease, which is used to reduce friction between the stages of the MOUDI, is a frequently encountered interferant, and may have been misinterpreted in the past as organonitrates in native samples [8]. Because the MOUDI was exposed to fairly high daytime temperatures while sampling, it is probable that volatile components of the O-ring grease evaporated and recondensed onto the substrates. A spectrum was taken of a very light smear of the O-ring grease and is included with the spectral standards in Fig. 1. (A modification of the MOUDI 110 is now available with impaction plates that

are rotated by individual motors. This eliminates the need for O-ring grease, and thus eliminates a potential source of substrate contamination.) Gasoline exhaust was collected with the MOUDI from a vehicle that was well tuned and from one that was poorly tuned. (The spectrum from the tuned engine was rather featureless, and is not considered further.) Spectra were also taken of MOUDI deposits of diesel exhaust that were provided by Michael Kleeman (University of California at Davis). The strong peak in the spectrum of diesel exhaust could be reproduced in the laboratory with aluminum foil that had been baked at 200 °C for several hours and is therefore attributed to oxidation of the substrate rather than to material native to the deposit itself. The present work explores the possibility of quantifying sulfate loadings, and for such, the relatively isolated ν4 fundamental of SO2− 4 , which peaks near 620 cm−1, is utilized. (The prominent higherfrequency sulfate ion absorption produced by the ν3 fundamental was not used for quantification due to difficulty in determination of spectral baselines and to overlapping silicate absorptions.) A calibration curve was developed for stage six of the MOUDI, beginning with the preparation of a stock solution of 1:0117 g ammonium sulfate in 200:0 ml of distilled water. A series of dilutions was made from the stock solution, 5:0 ml from each dilution was nebulized, and the resulting PM collected by the MOUDI. The deposits were dried under a heat lamp, weighed, and analyzed with the FTIR. An important consideration here is whether or not the extinction of the reflected beam is dominated by absorption. If not, then the effect of scattering would have to be calibrated out in order for the method to remain quantitative. To explore this issue, we performed Lorenz–Mie calculations of the extinction and absorption cross sections for a lognormal size distribution of ðNH4 Þ2 SO4 spheres. Figure 2 compares extinction and absorption for a population of dry ammonium sulfate spheres having

Fig. 1. Representative reference spectra used to help interpret peak positions in aerosol spectra. Deposit masses for the top four panels were not determined and so the band intensities for these should not be intercompared, nor should they be compared with those of the ammonium salts.

Fig. 2. Lorenz-Mie calculation comparing extinction and absorption by a lognormally distributed ammonium sulfate aerosol. In the upper panel, the cross sections are averaged over the entire distribution. In the lower panel, the average is taken over the size ranges that would be collected by stage 6 of the MOUDI.

assembled MOUDI was brought to the collection site, placed in the shelter, and attached to the pump and timer. On the business day after sample collection, it was removed from the shelter and brought to the laboratory. The substrates were removed from the MOUDI and placed in sealed Petri dishes. 3. Analysis

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a median radius of 0:10 μm and a standard deviation of 2.24—characteristic of ambient sulfate aerosols. Two cases are considered: In the first, the attenuation coefficients for absorption and extinction, babs and bext , respectively, are integrated over the entire distribution. In the second case the integral is truncated at the cutpoints of stage six of the MOUDI. As is seen in Fig. 2, scattering by the larger particles in the distribution contributes significantly to extinction, especially at shorter wavelengths. Most of the sulfate particles collected by the MOUDI, however, occur on stages 6–10. The lower panel in Fig. 2 shows that in this case, absorption dominates scattering over the entire spectral range, and especially below 1000 cm−1. As indicated in Fig. 3, scattering within and reflection from the deposit effects the intensity of the emergent beam. In cases where the ammonium sulfate concentration was too high, the spectra were dominated by reflectance from the deposit itself and strongly resembled specular reflectance spectra of the bulk sample. When the concentration is low enough, then reflectance and scattering from the sample is negligible. In this case, the IR spectra represent folded path transmission spectra through a thin layer of analyte. These two extremes are illustrated in Fig. 3 and their corresponding spectra are shown in Fig. 4. In both cases the deposits were produced in the laboratory as described above. The sample in Fig. 3 is depicted as homogeneous, rather than granular, because the PM sizes that reached stages 5–10 of the MOUDI were in or near the Rayleigh regime at 620 cm−1 . This depiction is also consistent with the results shown in Fig. 2 as it applies to the spectra of the samples considered

Fig. 3. (Color online) Illustration of different sample loadings encountered in this work. The ideal condition is labeled a and represents a folded path transmission measurement (the sample is drawn separated from the substrate for ease of visualization). Case b depicts a sample thick enough to block the direct beam from the reflecting substrate, and having the effect of producing a reflectance spectrum of the bulk material.

Fig. 4. Spectra typical of those that would be encountered for folded path transmittance (upper curve) and bulk reflectance (lower curve).

below. Such independence from scattering cannot be assumed for quantitative analysis of substrates that host larger particles, nor could it be expected to apply if the deposits contained micrometer-scale clumping. The close resemblance of measured spectra to those calculated for the fine particle fraction of the aerosol indicate that the latter case did not occur. Although the spectra from the atmospheric samples were usually dominated by double-pass absorption, there were instances where atmospheric loading was high enough to produce deposit spectra that constituted a hybridization of reflectance and absorbance spectra. The area of the ν4 sulfate band was determined over the interval 570–640 cm−1 using a peak area tool native to the spectrometer. The calibration data that relate the ðNH4 Þ2 SO4 mass to band strength are displayed in Fig. 5. The best-fit calibration curve is described by sulfate mass M sulfate ðμgÞ ¼ 3:7x2 þ 35x, where x is the area under the curve in absorbance units (defined in terms of transmittance T as logð1=TÞ). The R2 value for this equation is 0.98. Sulfate masses for small sample loadings were estimated by extrapolating the calibration curve toward 0.

Fig. 5. Calibration curve for 612 cm−1 band of ammonium sulfate spectrum for stage 6. The error bars represent the range in the microbalance after measurements as well as the range among three repetitions of each measurement. 1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS

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The quadratic form of the calibration curve may be a manifestation of the loading effect discussed above. Possibly, it might be well represented by two lines, one in the area where the spectrum is dominated by double-pass absorption and the other where the spectrum is dominated by reflectance at the heavier loadings. For the analysis below, inferred substrate masses ranged from 0 to no more than about 300 μg. After determining ν4 band strengths for a sample, the inferred mass was converted to concentration based on the standard flow rate of 30 liters= min through the MOUDI. Most of the sulfate particles collected for the calibration, as well as the ambient samples, were submicrometer, and for this work, it was assumed that one calibration curve could be used for all the stages in which sulfate was collected. 4. Results

Three representative infrared spectra of the Huntsville area aerosol are shown in Fig. 6. The stage number, the number of nozzles, the nominal aerodynamic diameter of the cutpoints, the physical diameter, and the number of days during this study on which sulfate (as indicated by the presence of the ν4 signature) was found on a given impactor stage is shown in Table 1. In none of the atmospheric samples could sulfate be clearly identified on stages 1–3 of the MOUDI. In the present work, ammonium sulfate absorptions were found to dominate the spectra of all fine ðPM2:5 Þ aerosol, as would be expected from the known contribution of this species to total aerosol fine mass in the South Eastern United States [9].

Table 1.

Fig. 6. Sample spectra of the Huntsville urban aerosol from stages 5, 7, and 10 of the MOUDI collected on 13 August 2004. Peaks associated with O-ring grease from the rotational stages of the MOUDI are labeled G.

The heaviest loadings consistently occurred on stages 6, 7, and 8. Absorptions due to silicates (soil), ammonium nitrate, and sometimes water are also present, although they are not as strong as the ammonium sulfate absorptions. Present in many of the spectra is contamination by trace amounts of O-ring grease which was used to lubricate the MOUDI. Absorptions due to sulfite, nitrite, carbonate, organic sulfur, and chlorides were not observed. A summary of peak positions for inorganic absorptions, along with those for O-ring grease, observed in this study is given in Table 2. Discussions of the peak assignments are provided below.

Characteristics of Nozzle Plates, Cutpoints, and Occurrence of Sulfate for MOUDI Stagesa

Stage

No. of Holes

Cutpoint

Physical Diameter

Days SO4 Detected

inlet 1 2 3 4 5 6 7 8 9 10

1 1 10 10 20 40 80 900 900 2000 2000

> 18 10:–18 5:6–10: 3:2–5:6 1:8–3:2 1:0–1:8 0:56–1:0 0:32–0:56 0:18–0:32 0:10–0:18 0:056–0:10

> 14 7:5–14 4:2–7:5 2:4–4:2 1:3–2:4 0:73–1:3 0:40–0:73 0:22–0:40 0:12–0:22 0:063–0:12 0:032–0:063

0 0 0 0 7 20 22 21 22 22 10

a

Particle diameters are given in μm. Table 2.

Moiety

Characteristic Peak Locations (in cm−1 ) for Various Species Observed in This Studya

Stage 10

Stage 7

Stage 5

SO2− 4

620, 1120

622, 1131

620

NHþ 4

1427, 2800–3350

1427, 2800–3350

1427, 2800–3350

Soil

—

—

547, 1045, 3629

821, 1000–1150, 1266, 2970

821, 1000–1150, 1268, 2970

822, 1000–1150, 1275, 2968

O-ring grease a

The stages are selected to represent the large, median and small components of the PM2:5 distribution.

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The infrared optical properties of solid ðNH4 Þ2 SO4 have been investigated by Toon et al.[10] and those of aqueous solutions by Downing et al. [11]. This latter reference, along with the data for water provided by Downing and Williams [12], is useful in addressing the degree of hydration of the deposits studied here. Jarzembski et al. [13] determined the optical constants for solid NH4 NO3 and also revisited those for solid ðNH4 Þ2 SO4 . Absorptions due to sulfate ions were found at 612–615 cm−1 and 1103–1135 cm−1 . Although Allen [1] and Blando [3] distinguished bisulfate from sulfate, this study did not distinguish them. If there were ammonium bisulfate in the ambient aerosol samples, it may have been converted to ammonium sulfate during sample handling and storage between collection and analysis. Ammonium ion absorptions were observed in infrared spectra for all ambient aerosol size ranges (0:056–10 μm), but were most distinct for PM2:5. Submicron ammonium ion absorptions were associated primarily with ðNH4 Þ2 SO4 . On some days, supermicron ammonium absorptions also included NH4 NO3 contributions. Ammonium ion absorptions in these salts are strongest at 1410–1435 cm−1 ; this study consistently found a strong peak at 1427 cm−1 , which is associated with the ν4 bending mode of NHþ 4 . Absorptions associated with the ammonium ion were also present in the 2800–3350 cm−1 region. These are attributed to symmetric and asymmetric N─H stretching modes (ν3 and ν1 fundamentals with band centers near 3230 cm−1 and 3075 cm−1 , respectively) and the first overtone (2ν4 ) of the symmetric bending mode. In all the laboratory-generated spectra containing ammonium ions, these three distinct peaks were present in this region. The band center of the ν4 mode occurs at about 1425 cm−1 . In the spectra of the field samples, some had three distinct peaks. However, in some cases the ammonium loading was light causing one or two of the peaks to be difficult to detect. Another factor in the field samples is the presence of anions other than sulfate. This study did not identify them, but data from the collocation with the EPA instrument suggest the presence of phosphate. The presence of this ion could cause displacement and apparent overlap of ammonium peaks. The spectra of the field samples clearly indicate the presence of soil in the larger size fractions, with a prominent peak associated with silicates occurring at 1045cm−1 . For stages 1–5, this feature overlapped the sulfate feature peaking at 1153 cm−1 . There were also soil features in the spectra of field samples in the 475–550 and the 3620–3630 cm−1 ranges. The only nitrate absorptions believed to be strong enough to appear in our spectra are the ν2 and ν3 fundamentals with band centers at 831 and 1390 cm−1 , respectively. Allen et al. [1] found these absorptions at 825–835 cm−1 and 1318–35 cm−1 . Blando et al. [2,3] list these absorption frequencies in SEAVS in the Smokey Mountains (15 July–25 August 1995) at 830 and 1340–1412 cm−1 . The standards created

in the laboratory for this study from pure ammonium nitrate and distilled water had peaks at 827 and 1394 cm−1 . Likely nitrate peaks were occasionally identified at 830 and 1395 cm−1 in the field samples. The primary spectral features of liquid water appear at around 3400, 1640 and 650 cm−1 . The first of these is due to a combination of the ν1 and ν3 O─H stretching modes and the 2ν2 overtone of the symmetric bending mode. The ν2 mode itself gives rise to the band centered near 1640 cm−1 , and the feature at 650 cm−1 is a libration (hindered rotation) band. Water strongly bound to aerosol salts absorbs at 1600–1700 cm−1 , peaking at around 1620– 1655 cm−1 . Salt hydrates also absorb broadly at 3100–3600 cm−1 . These absorptions were seen in only a few aerosol spectra as a weak shoulder on the high frequency side of the broad, strong ammonium ion absorptions. In those cases, water absorption near 1645 cm−1 would also have been present. For the most part, the dry conditions in the laboratory where the deposits were stored resulted in deposit spectra that did not display features attributable to hydration. This study found peaks at 2847 and 2915 cm−1 , which are associated with symmetric and asymmetric stretching in CH2 bonds. The 2915 and 2847 cm−1 absorptions are readily identified in many of the PM spectra. The aliphatic carbon absorptions were superimposed on ammonium ion absorptions. The higher-frequency absorptions with peaks at 2847–2915 cm−1 were distinct and easily separated from the three relatively broad ammonium ion absorptions between 2800 and 3350 cm−1 . Absorption peaking at 1442 cm−1 , which is attributable to CH3 and CH2 aliphatic bending modes, may also have been present. This feature, however, was not easily distinguishable from the 1427 cm−1 peak of ammonium. Allen [1] identifies this peak at 1452–1455 cm−1 . The carbonyl complex absorbs over the range of 1650–1850 cm−1 . This C═O stretching mode may include contributions of aldehydes (─CHO), ketones (─CO─), carboxylic acids (─COOH), esters (─CO─O─), amides (─CO─N), acid anhydrides (─CO─O─CO), and acyl halides (─CO─X). These different molecular environments, when present in the same sample, merge into a broadened absorption band that makes further interpretation difficult [1]. Features peaking at 1631 and 1278–1280 cm−1 that may arise from organonitrates were observed in many of the spectra. The 1631 cm−1 peak was stronger, but the 1278–1280 cm−1 peak was sharper. It should be borne in mind, however, that O-ring grease was also found to have a sharp peak near 1275 cm−1 , and in some cases water absorption might overlap with the 1631 cm−1 peak. Useful references on FTIR analysis of organic carbon in aerosols include Garnes and Allen [14], Kiss et al. [15], and Maria et al. [16]. Table 3 shows the band strength, as determined by the spectrometer’s peak area tool, of the ν4 1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS

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Table 3.

Stage 1 2 3 4 5 6 7 8 9 10

Table 4.

Stage

Areas Under the ν4 Peak for Substrate Deposits Collected During August, 2004 of This Study

1 Aug

7 Aug

13 Aug

19 Aug

25 Aug

31 Aug

— — — — — 1.12 2.27 1.60 0.38 0.36

— — — — 0.05 1.46 3.83 2.86 1.26 0.13

— — — — 0.47 0.46 3.32 2.73 1.22 —

— — — — 0.27 2.80 4.65 1.09 0.13 —

— — — — 0.07 0.57 1.44 2.38 1.40 —

— — — 0.20 1.79 2.65 4.83 1.81 1.50 0.56

Size-Segregated and Total Mass (μg) Determined from MOUDI Samples During August 2004; Also Shown Are Total PM2:5 Masses and Concentrationsa for MOUDIb and EPAc Measurements

First

Seventh

Thirteenth

Nineteenth

Twenty-fifth

Thirty-first

1 2 3 4 5 6 7 8 9 10

— — — — — 43.7 98.4 65.6 13.7 13.0

— — — — 1.90 59.1 188. 130. 50.1 4.56

— — — — 17.4 16.8 157. 123. 48.1

— — — – 9.60 127. 243. 42.6 4.65

— — — — 2.61 21.2 57.9 104. 56.1

— — — 7.16 74.4 119. 255. 75.6 60.9 20.7

Sum

234

434

363

427

242

613

CMOUDI CEPA

3.96 5.08

7.34 7.54

5.13 5.43

7.21 10.6

4.09 6.21

10.4 14.2

Given in units of μg=m3 .bCMOUDI in table.cCEPA in table.

a

SO2− 4 mode for deposits on all stages of the MOUDI for August 2004, and is representative of the other months included in the study. Values for the spectroscopically determined masses and concentrations of PM2:5 ðNH4 Þ2 SO4 from this study are shown in Table 4. Even though the pump drew over four times as much air through the MOUDI as the EPA sampler had, no MOUDI substrate appears to have accumulated more than about 300 μg of sulfate. EPA values for SO2− 4 concentration were converted to ammonium sulfate mass by multiplying by the molar ratio of ðNH4 Þ2 SO4 to NHþ 4 and adjusted to account for differences in flow rates. The resulting EPA-determined ðNH4 Þ2 SO4 concentrations are included in Table 4. All the stages in the MOUDI whose spectra have detectable sulfate have cutpoints of less than 2:5 μm, and can, therefore, be summed for comparison with the EPA’s PM2:5 measurements. Figure 7 shows the relationship between the concentrations that were determined spectroscopically and EPA concentrations for the corresponding date. The 1∶1 and the best-fit to the data points are included. As noted in Section 3, the deposits were occasionally heavy enough to produce spectra intermediate between those illustrated in Fig. 4. Interestingly, exclusion of those masses determined from the hybridized 2272


Fig. 7. (Color online) Sulfate mass comparison from data collected during the summer of 2004. The linear correlation indicated below the 1∶1 line has a slope of 0.81 and an R2 of 0.74.

spectra did not decrease the correlation with EPA measurements. 5.

Conclusions and Recommendations

Infrared double-pass transmittance spectroscopy can be used to characterize size segregated atmospheric aerosol deposits, providing qualitative information on ammonium, sulfates, nitrates, silicates, and

organics. This method avoids the need for the use of infrared window materials such as ZnSe. Quantitative assays, reported herein, of sulfate loadings derived from the infrared data correlate well with the more established methodology used by the EPA STN. On the other hand, the current analysis tended to underestimate the actual (as determined by the EPA) sulfate concentrations. Probably the most significant source of error in the present study is the lack of a microbalance that is sufficiently accurate. This prevented collection of calibration data of the quality desired since extrapolation had to be made for deposit masses of less than about 25 μg. It is believed that the results could be improved upon if calibration data were obtained for each stage of the MOUDI on which measurable amounts of sulfate were found. At the outset, it was assumed that a calibration curve for a single stage (stage 6) would be sufficient. This was because the ν4 fundamental that was used in the quantitation of sulfate appears at a wavelength of 16 μm, and the particle sizes involved (usually less than 2 μm in diameter) would behave as weak Rayleigh scatterers as long as the substrate was not too heavily loaded. In other words, over the ν4 band, extinction would be dominated by absorption and particle size effects could be neglected. In any event, calibration for anions or cations, which present absorption bands at shorter wavelengths and/or which would be associated with large particles, such as silicates, would require calibration curves for each stage. A useful reference in this regard is Weis and Ewing [17], wherein they present both experimental and theoretical analysis of scattering and absorption by ammonium sulfate aerosol in the infrared. Another reason to provide calibration curves for each stage is that, although the rotating-stage MOUDI produces more uniform deposits, those deposits may present themselves to the spectrometer beam as sets of concentric rings. Thus the area of the deposit sampled by the beam likely varies from stage to stage. Such additional calibration data were simply beyond the scope of this initial study, and are planned to be taken for future studies. Another potential source of error is the collection efficiency of the MOUDI. An attempt at analysis of the Teflo afterfilters was made with the FTIR. A sulfate signature was found on the afterfilters, but this could not be quantified because of spectral saturation by the filter material itself. There is also a loss of sulfate in the plumbing of the MOUDI that is hard to quantify [18]. Despite these sources of error, the estimates of sulfate loadings based on the infrared spectra agree sufficiently well with the more established methodology used by the EPA to warrant further development of the method, with the aim of extending it to the quantitation of other analytes. The spectra in this study were found to be dominated by double-pass absorption in most cases. It

is recommended, however, that the effect of heavier loadings be investigated since the air sampled for this research was typically much cleaner than that encountered in areas where there is significantly greater industrial activity. We acknowledge Debra Hobson, with the City of Huntsville’s Air Quality Division, who provided the data from the EPA for the Huntsville site as well as allowing the authors access for set up and retrieval of the MOUDI. This work was supported by grants from the National Science Foundation (ATM-0220465), National Oceanic and Atmospheric Administration (NA06NES4400008), and the Department of Energy Experimental Program to Stimulate Competitive Research (DE-FG0205ER45187). The FTIR, MOUDI and reflectometer were acquired with funds from Army (Defense Universities Research Instrumentation Program) grant DAAD19–01–1–0461. References 1. D. T. Allen, E. J. Palen, M. I. Haimov, S. V. Herring, and J. R. Young, “Fourier transform infrared spectroscopy of aerosol collected in a low pressure impactor (LPI/FTIR): method development and field calibration,” Aerosol Sci. Technol. 21, 325– 342 (1994). 2. J. D. Blando, R. Porcja, T.-H. Li, D. Bowman, P. Lioy, and B. Turpin, “Secondary formation and the smoky mountain organic aerosol: an examination of aerosol polarity and functional group composition during SEAVS,” Environ. Sci. Technol. 32, 604–613 (1998). 3. J. D. Blando, “Secondary formation of organic particulate matter in the Smokey Mountains,” Ph.D. dissertation (Rutgers, the State University of New Jersey, 1999). 4. J. D. Blando, R. J. Porcja, and B. J. Turpin, “Issues in the quantitation of functional groups by FTIR spectroscopic analysis of impactor-collected aerosol samples,” Aerosol Sci. Technol. 35, 899–908 (2001). 5. V. A. Marple, K. L. Rubow, and S. Behm, “A microorifice uniform deposit impactor (MOUDI): description, and use,” Aerosol Sci. Technol. 14, 434–446 (1991). 6. C. Coury and A. Dillner, “Quantitative determination of ambient aerosols using attenuated total reflectance Fourier transform infrared spectroscopy and multivariate chemometric techniques,” in “24th Annual AAAR Conference,” (American Association for Aerosol Reseach, 2005). 7. J. C. Chow and J. G. Watson, Guideline on Speciated Particulate Monitoring. Desert Research Institute (Available from Desert Research Institute P.O. Box 60220, Reno, NV 89506, 1998). 8. W. Weathers and , “Comments on “Size distribution of organonitrates in ambient aerosol collected in Houston, Texas, Aerosol science and technology 36:983-992 (2002),” Aerosol Sci. Technol. 38, 782–786 (2004). 9. W. C. Malm, B. A. Schichtel, M. L. Pitchford, L. L. Ashbaugh, and R. A. Eldred, “Spatial and monthly trends in speciated fine particle concentration in the United States,” J. Geophys. Res. 109, D03306 (2004). 10. O. B. Toon, J. B. Pollack, and B. N. Khare, “The optical constants of several atmospheric aerosol species: ammonium sulfate, aluminum oxide, and sodium chloride,” J. Geophys. Res. 81, 5733–5748 (1976). 11. H. D. Downing, L. W. Pinkley, P. P. Sethna, and D. Williams, “Optical constants of ammonium sulfate in the infrared,” J. Opt. Soc. Am. 67, 186–190 (1977). 1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS

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12. H. D. Downing and D. Williams, “Optical constants of water in the infrared,” J. Geophys. Res. 80, 1656–1661 (1975). 13. M. A. Jarzemski, M. L. Norman, K. A. Fuller, V. Srivastava, and D. R. Cutten, “Complex refractive index of ammonium nitrate in the 2–20 μm spectral range,” Appl. Opt. 42, 922– 930 (2003). 14. L. A. Garnes and D. T. Allen, “Size distributions of organonitrates in ambient aerosol collected in Houston, Texas,” Aerosol Sci. Technol. 36, 983–992 (2002). 15. K. Varga, I. Galambos, and I. Ganszky, “Characterization of water-soluble organic matter isolated from atmospheric fine aerosol,” J. Geophys. Res. D 107, (2002). 16. S. F. Maria, L. M. Russell, B. J. Turpin, R. J. Porcja, T. L. Campos, R. J. Weber, and B. J. Huebert, “Source signatures

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of carbon monoxide and organic functional groups in Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) submicron aerosol types,” J. Geophys. Res. D 108, 8637–8650 (2003). 17. D. D. Weis and G. E. Ewing, “Infrared spectroscopic signatures of ðNH4 Þ2 SO4 ;,” J. Geophys. Res. D 101, 18, 709–720 (1996). 18. B. J. Huebert, T. Bates, P. B. Russell, G. Shi, Y. J. Kim, K. Kawamura, G. Carmichael, and T. Nakajima, “An overview of ACE-Asia: Strategies for quantifying the relationships between Asian aerosols and their climatic impacts,” J. Geophys. Res. D 108, 8633 (2003).