Infrared extinction spectroscopy and Raman microspectroscopy of selected components of. mineral dust with organic compounds.

Transcription

1 Infrared extinction spectroscopy and Raman microspectroscopy of selected components of mineral dust with organic compounds. 1. Introduction Mineral dust is one of the major constituents of particulate matter in the atmosphere. 1 Atmospheric aerosols determine the energy balance of the atmosphere, 2 yet there are many uncertainties associated with the role of mineral dust aerosol on climate. 3 Satellite spectral data in the infrared (IR) region is extensively used in determination of important properties of both the atmosphere and hydrosphere. 4 In order to properly process data from satellites, the radiative effect of atmospheric aerosols must be included in interpretation. 5 Mid-IR region may be used to indicate both the presence of dust, and its specific mineral characterization. 4 Algorithms that are used in remote sensing data processing and climate modeling calculations often use Mie theory. 5 However, Mie theory is derived for homogeneous spherical particles. 6 Yet, it is well known, that authentic atmospheric dust particles have morphology that can greatly deviate from spherical shape. 7 Shape effects influence optical properties and inaccuracies associated with the Mie approximation are a significant source of error in calculations of aerosol radiative impact. 8 Thus, this can restrict the application of Mie theory for dust particles. Earlier studies showed that for authentic, multicomponent mineral dust samples such as Iowa loess and Saharan sand, Mie theory poorly simulates the measured IR extinction spectra, while the Rayleigh model analytic solutions based on characteristic particle shapes have been shown to give much better agreement. 9 As aerosols are transported in the atmosphere they can undergo atmospheric processing. Since mineral dust aerosol surfaces can be highly reactive, aerosol particles can undergo 1

2 heterogeneous reactions during transport. Besides heterogeneous chemistry, atmospheric processing can also produce coated particles or externally mixed samples. Atmospheric aging of aerosols alter their chemical and physical properties, including optical properties. Mineral dust in the atmosphere is often associated with organic acids. 10,11 Acetic acid was observed to be associated with mineral dust aerosol. 12 It is also well known that oxalic acid is often internally mixed with atmospheric dust. 11 In addition to small organic acids, macromolecular polyacidic compounds are also well known to be associated with mineral dust. 11 In this study, we focus on several important components of mineral dust, which were processed through interactions with aqueous solutions of organic acids in the laboratory. Calcite is used as an example of a mineral that is highly reactive with acids, kaolinite as a clay mineral with a highly eccentric shape, and quartz as an example of a less reactive mineral with a noneccentric shape. Acetic and oxalic acid are used as the most abundant examples of small organic acids, and higher molecular weight humic acid sodium salt was also used to coat particles. We investigate the spectral characterization of the resulting aerosol particles using IR and Raman spectroscopy, and then we examine spatial distribution of chemical species in the processed particles using micro-raman mapping. Finally we investigate the accuracy of modeling extinction of these aerosols using Mie theory and analytic solutions for characteristic particle shapes. 2. Methodology 2.1. Sources of materials and sample preparation Calcium carbonate (CaCO 3, 98%) was purchased from OMYACARB, quartz (SiO 2, 100%, product # ) was purchased from Strem Chemicals and kaolinite (KGa-1b, low-defect, Washington County, Georgia, USA) was purchased from The Source Clays Repository. Calcium 2

3 oxalate monohydrate (CaC 2 O 4 H 2 O, %, product #13007) and glacial acetic acid (C 2 H 4 O 2, 99.7%, product #36289) were purchased from Alfa Aesar. Calcium acetate monohydrate (Ca(CH 3 CO 2 ) 2 H 2 O, 99.0%, product #402850), oxalic acid (H 2 C 2 O 4, %, product #658537) and humic acid sodium salt (technical grade, product H16752) were purchased from Sigma Aldrich. Calcium carbonate, calcium oxalate monohydrate, calcium acetate monohydrate and kaolinite were used as received; quartz was ground with mortar and pestle prior to use. For analysis of pure materials suspensions or solutions were prepared in Optima water (Fisher Scientific, W7-4, Lot ). For analysis of minerals with acids mineral (calcite, quartz or kaolinite) was suspended in acid or salt solution. Resulting mixtures were allowed to undergo physical and/or chemical transformations for 1.5 hours under ultrasonication and then were left overnight. Aerosols were formed by atomizing a sample using the aerosol generator (TSI Inc., Model 3076). The aerosol flow then passed through diffusion dryers (TSI Inc., Model 3062) to achieve relative humidity <5%. The size range of resulting aerosol particles is approximately 10 nm 2.5 µm in diameter FTIR extinction spectra Fourier transform infrared (FTIR) extinction spectra were measured directly from the flow of aerosol after it passed through diffusion dryers using an FTIR spectrometer (Thermo Nicolet, Nexus Model 670) with an external MCT-A detector. Spectra were acquired in the range of cm -1 with 8 cm -1 resolution by averaging 465 scans. Although relative humidity was maintained below 5% during the experiment, weak signals from gas-phase water remained in the spectra and have been subtracted. 3

4 2.3. Raman microspectroscopy Particles were collected on substrates for Raman analysis from the aerosol flow. A polished quartz disc (1" x 1/16", Ted Pella product # ) or 1/4"x 1/4" square of titanium foil (0.002" thick, ESPI Metals) was placed in the particle flow at the exit of the IR cell for 15 minutes. In situ Raman spectroscopy was performed using a high-performance dispersive Raman spectrometer (Thermo Fisher Scientific, Nicolet Almega XR). The spectrometer is equipped with an Olympus optical microscope with 10x, 20x, 50x and 100x magnification lenses. In the experiments described in this study the objective lens with 100x magnification was used. Raman spectra were generally recorded in the range of cm -1, or in narrower range with high (4.5 cm -1 ) or low (15 cm -1 ) resolution, depending on the sample. Raman scattering was performed using a laser operating at 532 nm; the laser spot size focused on the sample was 0.6 µm in diameter. Two exposures of 15 s each were averaged to obtain the resulting spectrum. The laser power was adjusted to avoid sample damage, as was verified by comparing particle images before and after spectrum collection. Raman maps of individual particles were collected by scanning the specified area of the particle in XY direction and gathering point-by-point spectral data with a step size of 0.6 µm Model simulations Infrared extinction spectra were simulated using models based on Mie and Rayleigh theories. These theories require the particle size distribution and optical constants as input in order to simulate extinction spectra. In this study, extinction spectra were simulated for pure minerals (calcite, quartz, kaolinite) utilizing the particle size distribution experimentally measured in the laboratory and published optical constants. Size distributions were measured using a scanning mobility particle sizer (SMPS, TSI, Inc., Model 3936) and an aerodynamic particle sizer (APS, 4

5 TSI, Inc., Model 3321), operating in tandem. These sizing methods were combined to yield a single size distribution as a function of volume-equivalent diameter. 13 Simulated IR extinction spectra of minerals were then compared with experimentally measured IR spectra of samples that had been processed with different organic compounds. In the Rayleigh regime, analytic solutions have been used for characteristic particles shapes. Here we apply the same particle shape models that have earlier shown to most accurately simulate extinction spectra of the dust samples used in this study (a continuous distribution of ellipsoids model for quartz and calcite, and a disc model for kaolinite), to determine if heterogeneous chemical interactions can significantly affect the quality of the spectral fits previously determined for these minerals. 3. Results and Discussion 3.1. Raman microspectroscopy Raman spectra shown in Figure 1a give evidence of a reaction between calcite and oxalic acid as, in addition to strong CO 2-3 stretching vibration at 1087 cm -1 and weaker CO 2-3 bending mode at 713 cm -1 of calcite, new bands appear at 1491, 1464 and 897 cm -1 that can be assigned as the C=O stretching band of calcium oxalate monohydrate (1491 cm -1 ), and C-O (1464 cm -1 ) and C-C (897 cm -1 ) stretching modes, that are associated with calcium oxalate formation. Similarly, for calcite added to acetic acid we can see the spectral signature of calcium acetate in the resulting spectra (Figure 1b), namely a strong band at 2933 cm -1 due to the symmetric stretching vibration of C-H bonds of the methyl group in acetic acid. Interesting results were obtained for quartz particles, aerosolized from the solution of oxalic acid. As can be seen in Figure 1c, even at the highest concentration of oxalic acid used in this study (2 wt%) there are no spectral signatures of oxalic acid in the Raman spectra, nor other vibrational bands that give evidence for any chemical changes taking place in quartz upon 5

6 interaction with oxalic acid. It is important to note that Raman spectroscopy is performed on single particles and the fact that there are no spectral signatures of oxalic acid in the spectra of the processed particle suggests that there is no oxalic acid on the surface of quartz. These data suggests that quartz and oxalic acid form a simple external mixture. Raman spectra of calcite (Figure 1d), quartz (Figure 1e) and kaolinite (Figure 1f) with humic acid sodium salt show that humic material does not cause chemical transformations in these minerals. Spectral signatures of humic acid sodium salt are present at 1578 cm -1 and 1383 cm -1 that are related to polycondensed aromatic moieties present in humic material. This suggests that humic material is present as a coating on these particles since spectral signatures of both the mineral and humic acid are present within one particle. Raman mapping is a very valuable tool for non-destructive physicochemical characterization of aerosol particles that can be used to show how chemicals are distributed within a particle. The two-dimensional intensity profile of one of the resonance lines in Raman spectra can be used to demonstrate spatial distribution of corresponding chemical species. Additional information about particle homogeneity (or heterogeneity) can be gained from comparison of the chemical and optical images. Raman images of calcium carbonate particles that were aerosolized from a 1.5 wt% solution of oxalic acid are shown in Figure 1a. The spectral map of the 1087 cm -1 peak, attributed to CO 2-3 stretching vibration illustrates the distribution of calcium carbonate within a particle and is shown in red. The intensity of the 1464 cm -1 peak, which is due to C-O stretch of calcium oxalate, and thus illustrates its distribution, is highlighted in blue. Note that distribution of calcium oxalate on the carbonate core is uneven. Areas of carbonate and oxalate do not overlap, which means that carbonate and oxalate phases are separated within a particle. 6

7 A similar result was obtained for calcite reacted with acetic acid. Figure 1b shows that the area covered with calcium acetate, represented by a spectral map of the C-H stretching mode of CH 3 at 2933 cm -1 and shown in blue, does not overlap with the map of carbonate peak at 1087 cm -1 (shown in red). This suggests that calcium acetate is segregated from the carbonaceous part of the particle. However, it is unclear if segregation occurred originally during the reaction in the solution, or upon aerosol drying. In contrast to these observations, humic material does not show evidence for such segregation. As can be seen on Figures 1d, 1e and 1f, the spectral map of humic acid sodium salt (shown in blue) overlaps with spectral map of the mineral (shown in red), suggesting that humic material forms a uniform coating over the entire particle. Raman map of quartz aerosolized from oxalic acid solution was not performed since Raman spectra of these mixed particles do not show the presence of oxalic acid associated with the quartz FTIR extinction spectroscopy As it was shown by the results of Raman spectroscopy calcite undergoes reactions when mixed with acetic or oxalic acid. Heterogeneous reactions that take place on a particle surface may lead to significant changes in the particle s physical and chemical properties, in particular these transformations may lead to changes in the aerosol optical properties. Investigation of the carbonate region ( cm -1 ) in the IR spectra shows that the addition of oxalic acid to calcite causes a clear red shift of the CO 2-3 vibrational mode at 1461 cm -1. The red line in Figure 2a shows a gradual shift in this band peak position as a function of the concentration of oxalic acid added to calcite. At the maximum concentration of oxalic acid used in this study (2 wt%) the carbonate peak is red-shifted by 9 cm -1. 7

8 Similar results were obtained for calcite atomized from a solution of acetic acid. As indicated by the blue line in Figure 2a, the CO 2-3 peak gradually shifts to lower wavenumbers as more acetic acid is added to calcite. For the interactions associated with the highest concentration of acetic acid used (2 wt%) this shift is 8 cm -1. When calcite was added to the solution of humic acid sodium salt the chemical composition of the core is not altered as shown by Raman spectroscopy. However, the formation of a coating still leads to changes in physical properties of the resulting particles. IR extinction spectra of aerosols that were obtained from calcite in solution of humic material show a red shift of the CO 2-3 peak. As can be seen in Figure 2a (green line), this shift is smaller than was found in the case of reactions between calcite and small organic acids, only 4 cm -1 compared to 8 and 9 cm -1 for acetic and oxalic acid respectively. IR spectra of quartz aerosol in oxalic acid solution also show that Si-O stretching is redshifted as illustrated on Figure 2b (red line). However the maximum concentration of oxalic acid used in this study (2 wt%) causes a slight red shift (3 cm -1 ) that is near the limit of the experimental uncertainty (±2 cm -1 ). The addition of humic acid sodium salt to quartz and kaolinite gives similar effects to those observed for calcite. Figures 2b and 2c (green lines) indicate that Si-O peak is red-shifted as a result of the interaction with humic acid. As can be seen on Figure 2b (green line) this shift is quite significant for quartz, 10 cm -1 ; however for kaolinite (Figure 2c) the shift is lower, only 3 cm -1, which, again, is close to the limit of the experimental uncertainty in the peak position Model simulations Extinction spectra for calcite, quartz and kaolinite were simulated using Mie theory and analytic solutions derived in the Rayleigh regime for characteristic particle shapes to model the 8

9 extinction spectra of these mineral aerosols. For a simple analysis the simulated IR resonance peak position of the pure mineral was compared with the experimentally determined peak position before and after processing with acids. The results of model simulations are summarized in Table 1. As shown in Table 1, peak positions for the unprocessed minerals predicted using Mie theory are routinely blue-shifted from the experimentally observed by values ranging from 8 to 30 cm -1. Following the interaction with the organic acids, the observed spectral resonances are found to be red-shifted from bare material thus worsen the agreement with Mie theory. Therefore, Mie theory continues to perform poorly for aerosol particles that have undergone chemical or physical processing, including the incorporation of coatings of reactive or unreactive materials. As can be seen in Table 1 analytic solutions that are based on Raleigh theory for characteristic particle shapes give better agreement for the IR resonance peak positions than Mie theory. As it was noted above, simple Mie theory assumes that particles are spherical and homogeneous, and its application may not be appropriate for particles that have undergone atmospheric processing. 4. Conclusions and future work In this study we incorporated and integrated the use of multiple techniques to study the effect of multiphase reactions involving components of mineral dust with organic acids in laboratory. FTIR and Raman spectroscopy were used to gain chemical information about the resulting particles. Raman mapping was used to investigate the spatial distribution of chemical species within a particle. Finally, spectral simulations were performed to predict the extinction spectra of minerals and then they were compared with experimentally measured IR data of processed 9

10 aerosols. Several important conclusions were drawn upon the examination of the obtained data and are summarized in Figure 3. Interactions of components of mineral dust with acids can lead to chemical reactions that will form heterogeneous particles with phase segregation (calcite with small organic acids). Other acids (humic) can form a uniform coating around the mineral core. Finally, a simple external mixture may be formed, as was observed for quartz with oxalic acid. Processing of mineral dust components in the presence of acids causes a red shift of the resonance IR peak (CO 3 2- or Si-O). These changes associated with atmospheric aging can exacerbate the errors associated with Mie theory modeling of the optical properties and adversely affect the processing of data received from satellites and for predicting optical properties of mineral dust aerosols. However, the spectral line shifts associated with processing are less significant than the line shifts associated with particle shape effects. As the result, the analytic solutions for the resonance spectral line profiles derived in the Rayleigh regime for different characteristic particle shapes continue to fit the data for the processed particles much better than models based on Mie theory, as it was earlier shown for common mineral particles. We also expect other properties (e.g. reactivity, hygroscopicity, cloud condensation nuclei (CCN) activity) to be altered as the result of interactions of mineral dust with organic acids. Particularly, the change in hygroscopicity can be linked to changes in size, CCN activity and optical properties. Thus it is particularly important to investigate properties of these processed particles as a function of relative humidity, which we are planning to accomplish in the future. 10

11 Relative Raman intensity a) 1 µm Calcium oxalate +1.5 wt% OA Calcium carbonate Raman shift (cm -1 ) Relative Raman intensity b) 2933 Calcium acetate +2.0 wt% AA Calcium carbonate 1 µm Raman shift (cm -1 ) Relative Raman intensity c) Oxalic acid +2.0 wt% OA Quartz Raman shift (cm -1 ) Relative Raman intensity d) NaHA +1.5 wt% NaHA Calcium carbonate 1 µm 1 µm 1 e) Raman shift (cm -1 ) Relative Raman intensity NaHA +0.1 wt% NaHA Quartz Raman shift (cm -1 ) Relative Raman intensity NaHA wt% NaHA Kaolinite Raman shift (cm -1 ) Figure 1. Raman spectra and maps of calcite (red) atomized from (a) 1.5 wt% solution of oxalic acid (OA), (b) 2.0 wt% acetic acid (AA) (both blue); (c) quartz (red) atomized from 2.0 wt% solution of OA (blue); and (d) calcite, (e) quartz and (f) kaolinite (all red) atomized from (d) 1.5 wt%, (e) 0.1 wt% and (f) 0.25 wt% solution of humic acid sodium salt (NaHA) (blue). f) 1 µm Wavenumber (cm -1 ) 1462 a) Calcite + OA Calcite + AA Calcite + NaHA Acid concentration (wt%) Wavenumber (cm -1 ) b) Quartz + OA Quartz + NaHA Acid concentration (wt%) 3 10 Wavenumber (cm -1 ) 1044 c) Kaolinite + NaHA NaHA concentration (wt%) 3 Figure 2. (a) Position of CO 2-3 mode of calcite as a function of oxalic acid (OA) (red line), acetic acid (AA) (blue line) and humic acid sodium salt (NaHA) (green line) concentration; (b) position of Si-O mode of quartz as a function of OA (red line) and NaHA (green line) concentration; (c) position of Si-O mode of kaolinite as a function of NaHA concentration. 11

12 Table 1. Shifts in peak positions for minerals and coated/reacted minerals comparing to theoretical calculations (positive : simulation blue shifted from experiment; negative: red shift). Sample Peak position (cm -1 ) Mie Analytic solution Calcite 1461±2 8 5 Calcite+2 wt% oxalic acid 1452± Calcite+2 wt% acetic acid 1453± Calcite+1.5 wt% NaHA 1457± Quartz 1125± Quartz+2 wt% oxalic acid 1122± Quartz+1.5 wt% NaHA 1115± Kaolinite 1044± Kaolinite+1.5 wt% NaHA 1041± Figure 3. Atmospheric processing of mineral dust may lead to chemical reactions and formations of heterogeneous particles, to uniformly coated particles or to simple external mixtures. Mie theory simulation results in resonance peaks that are typically blue shifted relative to the experiment. IR extinction spectra of dust that have undergone processing show distinct red shifts of the prominent IR peaks, so deviation from Mie theory becomes even more pronounced for these samples. Analytic solutions for characteristic particle shapes give better agreement for the IR resonance peak positions of aerosol particles that have undergone processing. 12

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