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Laboratory study of the effect of oxalic acid on the cloud condensation nuclei activity of mineral dust aerosol
Atmospheric Environment 46 (2012) 125e130
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Laboratory study of the effect of oxalic acid on the cloud condensation nuclei activity of mineral dust aerosol Kelly M. Gierlus, Olga Laskina, Tricia L. Abernathy, Vicki H. Grassian* Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2010 Received in revised form 4 October 2011 Accepted 4 October 2011
Dicarboxylic acids, which make up a significant portion of the atmospheric organic aerosol, are emitted directly through biomass burning as well as produced through the oxidation of volatile organic compounds. Oxalic acid, the most abundant of the dicarboxylic acids, has been shown by recent field studies to be present in mineral dust aerosol particles. The presence of these internally mixed organic compounds can alter the water absorption and cloud condensation nuclei (CCN) abilities of mineral particles in the Earth’s atmosphere. The University of Iowa’s Multi-Analysis Aerosol Reactor System (MAARS) was used to measure the CCN activity of internally mixed particles that were generated from a mixture of either calcite or polystyrene latex spheres (PSLs) in an aqueous solution of oxalic acid. Although PSL is not a mineral dust component, it is used here as a non-reactive, insoluble particle. CCN measurements indicate that the internally mixed oxalate/calcite particles showed nearly identical CCN activity compared to the original calcite particles whereas oxalic acid/PSL internally mixed particles showed much greater CCN activity compared to PSL particles alone. This difference is due to the reaction of calcite with oxalic acid, which produces a relatively insoluble calcium oxalate coating on the particle surface and not a soluble coating as it does on the PSL particle. Our results suggest that atmospheric processing of mineral dust aerosol through heterogeneous processes will likely depend on the mineralogy and the specific chemistry involved. Increase in the CCN activity by incorporation of oxalic acid are only expected for unreactive insoluble dust particles that form a soluble coating. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cloud chemistry Mineral dust Oxalic acid CCN activity
1. Introduction Dicarboxylic acids (DCAs) are a significant portion of the organic aerosol mass concentration, with oxalic acid being the most abundant (Legrand et al., 2005; Sorooshian et al., 2006; Martinelango et al., 2007; Hsieh et al., 2009; Hsu and Ding, 2009; Miyazaki et al., 2009; Kundu et al., 2010) having concentrations measured as high as 1.43 mg m3 (Miyazaki et al., 2009). This accounts for as much as 84% of the total DCAs in the atmosphere (Hsieh et al., 2009). Formation pathways for oxalic acid are still being investigated, but one primary source is known to be biomass burning (Li et al., 2009; Yang et al., 2009). Secondary production of oxalic acid occurs through oxidation of gaseous glyoxal (Rinaldi et al., 2011) and photooxidation of longer-chain DCAs (Yang et al., 2008a, 2008b; Kundu et al., 2010) which is enhanced in aqueous-phase reactions in cloud droplets (Carlton et al., 2007; Chen et al., 2007; Ervens et al., 2008). Field measurements by Hsieh et al. of suburban aerosol in
* Corresponding author. Tel.: þ1 319 335 392; fax: þ1 319 335 1270. E-mail address: [email protected] (V.H. Grassian). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.10.027
southern Taiwan reported that peak concentrations of oxalic acid occurred in the droplet mode, indicating an association with cloud processing (Hsieh et al., 2009). Model simulations (Ervens et al., 2004; Chen et al., 2007) and field studies (Tan et al., 2009) and have shown the production of secondary organic aerosols (SOAs), including oxalic acid, to be enhanced in the presence of clouds. Additionally, measurements of the contribution of oxalic acid to the total water soluble organic concentrations in New Delhi showed higher mass fractions of oxalate during nighttime compared to daytime, suggesting the production of oxalate via the oxidation of longer-chain DCAs in aqueous aerosols which were more abundant under higher relative humidity conditions present during the nighttime (Miyazaki et al., 2009). Similarly, sulfate which has been long recognized to form during cloud processing has been shown to have concentrations highly correlated with those of oxalate, suggesting oxalate is also produced via cloud processing (Yu et al., 2005; Sorooshian et al., 2006; Yang et al., 2009). Along with heterogeneous gas-phase reactions (Usher et al., 2003; Krueger et al., 2004; Sullivan and Prather, 2007; Sullivan et al., 2009b), cloud processing has also been shown to be an important pathway for the formation of coated or internally mixed
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mineral dust aerosol (Liu et al., 2005; Matsuki et al., 2010). Generated from windblown soil, mineral dust can be transported globally with atmospheric lifetimes of days to weeks, during which time it can impact the Earth’s climate as well as the chemical balance of the atmosphere (Bauer et al., 2004). Coating of the mineral dust particles can occur through cloud processing as the particles are collected by existing cloud droplets which were nucleated by soluble particles and that dissolved soluble material is redistributed until evaporation leaves internally mixed particles. On a global average it has been estimated that any particular atmospheric aerosol has undergone the evaporation/condensation cycle three times, (Pruppacher and Jaenicke, 1995) and repetition of this cycle has been shown to significantly increase the mass of soluble material incorporated into dust particles (Wurzler et al., 2000). Freshly emitted dust and mineral aerosols can have a significant hygroscopicity and CCN activity (Kumar et al., 2011) which has been shown to increase due to the cloud processing of dust aerosols (Crumeyrolle et al., 2008) as well as impact the deposition of dust (Fan et al., 2004). Conversely, the association of hygroscopic and CCN active substances like diacids can be decreased due to their incorporation into mineral dust (Sullivan and Prather, 2007). Carbonates, a significant and reactive component of mineral dust aerosol, have been found enriched in some cloud residual samples (Matsuki et al., 2010) providing evidence that there is potential for these coatings or aqueous reactions to occur. In a study of Asian aerosol by Sullivan et al. 10.2% of the mineral dust particles contained oxalic acid which was only detected in the dust particles (Sullivan and Prather, 2007). They hypothesized that DCAs may be found in increased amounts in mineral dust due to their ability to chelate divalent cations such as Ca2þ and Mg2þ (Sullivan and Prather, 2007). The presence of internally mixed oxalate has also been measured in another Asian dust aerosol, as well as a Saharan dust aerosol (Falkovich et al., 2004; Yang et al., 2009). It is also possible for calcite to react with oxalic acid to form calcium oxalate, according to:
CaCO3 þ H2 C2 O4 4 CaC2 O4 þ CO2 þ H2 O
(1)
Although the hygroscopicity (Prenni et al., 2001; Fan et al., 2004; Svenningsson et al., 2006) and CCN activity (Sullivan et al., 2009a; Falkovich et al., 2004; Kumar et al., 2003; Giebl et al., 2002; Hori et al., 2003; Sun and Ariya, 2006) of dicarboxylic acids such as oxalic acid has been measured previously and oxalic acid has been shown to act as heterogeneous ice nucleus (Zobrist et al., 2006), it is still not clear how internally mixed DCAs may impact the climate forcing properties of mineral dust. Xue et al. reported increased light scattering and absorption of soot aggregates with thin coatings of dicarboxylic acids, which was further enhanced when the coated soot was exposed to high relative humidity (Xue et al., 2009) and increases in hygroscopicity and CCN activity have been measured when calcite was internally mixed with organic humic substances (Hatch et al., 2008, 2009). In this study, measurements of the CCN activity of internally mixed particles generated from mixtures of either calcite (CaCO3) or PSLs in an aqueous solution of oxalic acid were made. CaCO3 and PSLs were used as models for mineral dust aerosol with one being a reactive component and the other an unreactive, insoluble component. PSLs were used widely in earlier studies to investigate atmospheric processes. Internally mixed aerosol particles consisting of PSLs and soluble components were used before to study the impact of heterogeneous processing of mineral dust aerosol on climate (Gibson et al., 2007). The CCN activity of coated PSLs was also used to explore the effectiveness of humic coatings on insoluble particles (Hatch et al., 2008). In addition, coated PSL spheres were used to study products and mechanisms of ozone reactions with oleic acid for aerosol particles (Katrib et al., 2004). Besides CCN measurements, Fourier transform infrared
(FT-IR) extinction measurements were performed to confirm speciation of oxalate in the internally mixed calcite particles. 2. Experimental methods 2.1. Materials All of the chemicals used in this study were commercially available and used as received. Oxalic acid (99.999%) was purchased from Aldrich, and aerosols were generated from a 0.2-wt% solution using Optima water (Fisher Scientific). Calcite (OMYA) and PSLs (Polysciences, Inc., Cat. #07304) were atomized from suspensions in Optima water. Internally mixed particles were generated by atomizing a suspension of insoluble powder in an aqueous solution of soluble material. Calcium oxalate monohydrate (99.9985%) was purchased from Alfa Aesar. 2.2. Size distribution and cloud condensation nuclei activity measurements and Fourier transform infrared extinction spectra Size distributions and cloud droplet activation measurements were obtained using the University of Iowa’s Multi-Analysis Aerosol Reactor System (MAARS), which has been described in detail elsewhere (Gibson et al., 2006). Aerosols were generated by a constant output atomizer (Model 3076, TSI, Inc.) from an aqueous solution or suspension as described above. The aerosol was then passed through two diffusion dryers (Model 3062, TSI, Inc.) and dried to a low relative humidity (RH 10%). The particles follow a flow stream from the aerosol generator through the path of the IR beam by a combination of conductive tubing and glass flow tubes. The glass flow tube system consists of an initial conditioning tube and an observation cell that is collinear with the IR beam. For the CCN measurements the aerosol was passed through a differential mobility analyzer (DMA1; Model 3080, TSI, Inc.) in order to size select 200 nm particles. The sheath and aerosol flows of the DMA1 were set to be 3 lpm and 0.3 lpm, respectively. This monodisperse aerosol was then split between a continuous flow streamwise thermal-gradient cloud condensation nuclei counter (Droplet Measurement Technologies; Model CCN-2) (Roberts and Nenes, 2005) and a condensation particle counter (CPC; Model 3025A, TSI, Inc.). For a given diameter and supersaturation, the CCN activity is reported as the ratio of particles that act as a cloud condensation nuclei (#CCN) to the number of particles counted by the CPC (#CN). The ammonium sulfate activity data presented by Tang and Munkelwitz was used to calibrate the supersaturation generated in the CCN counter to the thermal gradient of the diffusion chamber (Tang and Munkelwitz, 1994). This is useful so as to compare to other measurements made previously in our laboratory. Full size distributions were measured by bypassing DMA1 and directing the dry polydisperse aerosol into a scanning mobility particle sizer (SMPS; Model 3936, TSI, Inc.), which consists of a second DMA (DMA2) (flow rates are 3.0 (sheath) and 0.3 (sample) lpm) coupled to the CPC. The FT-IR extinction spectra were measured using a Fourier-transform infrared (FT-IR) spectrometer (Thermo Nicolet Nexus Model 670) with a liquid nitrogen cooled external MCT-A detector. The IR spectra were acquired from 740 to 4000 cm1 using 8 cm1 resolution by co-adding 256 scans. All FT-IR spectra were referenced to the background signal collected under dry conditions. 2.3. Theoretical methods A single hygroscopicity parameter, l, has recently been developed using “l-Köhler theory” (Petters and Kreidenweis, 2007, 2008) in order to determine a single parameter which can be
K.M. Gierlus et al. / Atmospheric Environment 46 (2012) 125e130
used to describe both hygroscopic growth and CCN activity. In l-Köhler theory, the saturation ratio, S, is expressed in the following equation:
SðDÞ ¼
D3 D3d
!
D3 D3d ð1 kÞ
4Mw ss=a RT rw D
(2)
exp
where D and Dd are the aqueous droplet diameter and the dry diameter, respectively, ss/a is the surface tension of the solution/air interface, R is the ideal gas constant, T is the temperature, and Mw and rw are the molecular weight and density of water, respectively. Value of l can may be determined by fitting observed values of critical saturation to the maximum of the l-Köhler curve (equation (2)), where the critical saturation, Sc, is the minimum saturation needed for particle to nucleate cloud droplet, and is related to the critical supersaturation, sc, by sc ¼ Sc 1. For multicomponent particles simple mixing rule can be applied for overall value of l:
k ¼
X
εi ki
(3)
i
To fit l to CCN data values of the aerosol particles studied a temperature of 298.15 K and a surface tension for pure water of 0.072 Jm2 (Lide, 2004) were assumed.
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The amount of externally mixed oxalic acid or ammonium sulfate particles can be predicted to a first approximation by dividing the number concentration of soluble particles at 200 nm by the total number concentration of particles at 200 nm (soluble plus insoluble component). From the size distributions of the individual components, it is estimated that for CaCO3 there would be less than 1.0% externally mixed oxalic acid and for PSLs there would be less than 0.7% externally mixed particles. Fig. 2 shows the full size distributions of the particles generated by atomizing calcite in an aqueous solution of oxalic acid. Size distributions were measured continuously over intervals of three and a half minutes for about 1.5 h. The size distributions have been normalized such that the maximum concentration of the peak centered at approximately 200 nm is one. The arrow demonstrates the decrease in concentration with time of the peak at smaller diameters. When calcite was atomized in a solution of 0.2-wt% oxalic acid initially the size distribution was bimodal demonstrating the presence of externally mixed oxalic acid particles at approximately 60 nm, however, this peak is found to decrease significantly as a function of time and is near baseline after ca. 30 min. This change is contributed to the reaction of oxalic acid with calcite to yield calcium oxalate. In comparison, when PSLs were atomized in a solution of 0.2-wt% oxalic acid the externally mixed oxalic acid particles showed little decrease in concentration and peak diameter.
3. Results and discussion 3.2. CCN activity of 200 nm particles 3.1. Size distributions Full size distributions for the individual components studied are shown in Fig. 1 by lognormal curve fits to the average of multiple measurements. Peak diameters are 45 nm for 0.2-wt% oxalic acid (long dashed curve), 195 nm for calcite (solid curve), and 196 nm for PSLs (short dashed curve). When generating internally mixed particles the insoluble component (calcite or PSLs) was atomized in a 0.2-wt% solution of the soluble component (oxalic acid), and size selected at 200 nm. The use of a particle size near 200 nm was necessary to minimize the amount of externally mixed oxalic acid.
In order to study the effect of CCN activity, internally mixed or reacted particles were generated from a mixture of either PSLs or CaCO3 in an aqueous solution of 0.2-wt% oxalic acid. Fig. 3a shows normalized CCN activity of PSLs atomized in oxalic acid and size selected at 200 nm. The single hygroscopicity parameter of the PSLs increased from l ¼ 0.0010 0.0007 (% sc ¼ 0.74 0.09) for uncoated to l ¼ 0.0249 0.0061 (% sc ¼ 0.26 0.03) for PSLs with
1 0.2 wt% H2C2O4
dN/dlogDp (#/cm3)
PSL CaCO3
1 10 6
5 10 5
Normalized dN/dlogDp (#/cm3)
1.5 10 6
0.8
0.6
0.4
0.2
0 10
100
1000
Diameter (nm)
0 10
100
1000
Diameter (nm) Fig. 1. Lognormal curve fits to the average of multiple measurements of the full size distributions of 0.2-wt% H2C2O4 (dashed), PSL (dotted), and CaCO3 (solid).
Fig. 2. Full size distributions of calcite atomized in a solution of 0.2-wt% oxalic acid. The size distributions have been normalized such that the maximum concentration of the peak centered at approximately 200 nm is one. The arrow demonstrates the decrease in the peak at approximately 45 nm over time with the peak at approximately 200 nm staying constant.
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a 1
#CCN/#CN
0.8
0.6
0.4
0.2 PSL in H2 C2O4 PSL 0 0
0.5
1
1.5
2
% Supersaturation
b
1
#CCN/#CN
0.8
0.6
Fig. 3b shows the CCN activity of calcite and oxalate/calcite particles. The l of the 200 nm calcite particles was measured to be 0.0070 0.0017 (% sc ¼ 0.44 0.04) which is in agreement with previous measurements made in our laboratory (% sc ¼ 0.37 0.02), (Gibson et al., 2007) however, is much higher than those reported for calcite aerosolized using a dry technique (l ¼ 0.0011) (Sullivan et al., 2009a). A similar difference was also observed by Koehler et al. In the case of using aqueous suspension to generate an aerosol of Arizona test dust a single hygroscopicity parameter was much higher (l ¼ 0.35) than when the aerosol was generated dry (l ¼ 0.025) (Koehler et al., 2009). Such differences can be due to the presence of soluble material in the water (Koehler et al., 2009) or generation of soluble species from the dust. This assumption can be also supported by the work of Bilde and Svenningsson, they noticed that trace amounts of impurities that can be present in water facilitate the cloud droplet activation of pure particles (Bilde and Svenningsson, 2004). When calcite is atomized in aqueous oxalic acid the resulting particles had l ¼ 0.0090 0.0019 (% sc ¼ 0.40 0.03), which is only slightly more CCN active within error bars than the pure calcite particles. This is a much smaller enhancement, essentially no enhancement, in CCN activity as compared to the internally mixed oxalic acid-PSL particles which exhibit a dramatic increase in CCN activity compared to uncoated PSLs. This difference in adding of oxalic acid to calcite versus PSLs is due to the reaction of calcite with oxalic acid to produce a coating of calcium oxalate. Furthermore, assuming the similar fraction of oxalic acid incorporated into calcium carbonate as into PSLs (4.8%) and using a mixing rule for single hygroscopicity parameter it can be estimated that l should increase up to the value of 0.031, which is significantly lower than the experimental value of 0.009. Low hygroscopicity of calcium oxalate particles (l ¼ 0.05) compared to that of oxalic acid (l ¼ 0.5) (Sullivan et al., 2009a) can further support the formation of calcium oxalate on the surface of calcium carbonate. These calculated values are summarized in Table 1. As described in the next section, spectroscopic measurements provide definitive proof of the reaction product. 3.3. FT-IR extinction spectra
0.4
0.2 CaCO3 in H2C2O4 CaCO3 0 0
0.5
1
1.5
2
% Supersaturation Fig. 3. CCN activity of 200 nm (a) PSLs atomized in the presence (circles) and absence (squares) of oxalic acid and (b) CaCO3 atomized in the presence (circles) and absence (squares) of oxalic acid. Each data point is the average of 3e10 measurements and the error bars represent the standard deviation of these multiple measurements. The lines represent sigmoidal fits to the data. The dashed line represents the 50% activation point.
internally mixed oxalic acid. Using a mixing rule (equation (3)) for l it can be estimated that the fraction of oxalic acid acquired is 4.8%. Such a large increase in CCN activity demonstrates that the incorporation of small amounts of soluble material can significantly alter the climate properties of insoluble aerosols. The single hygroscopicity parameter of pure 200 nm oxalic acid was measured to be higher than 0.17 (sc less than 0.1%). This result corresponds to l ¼ 0.5 reported by Sullivan et al. (2009a). The ability of oxalic acid to be CCN active was also shown in other earlier studies (Kumar et al., 2003; Giebl et al., 2002; Hori et al., 2003; Sun and Ariya, 2006).
The FT-IR extinction measurements were performed in order to confirm the presence of calcium oxalate in the reacted calcite particles. Spectra of calcium oxalate and calcite were also collected for comparison and can be seen in Fig. 4. The reference spectra for calcium oxalate shows absorption bands at 1631 and 1323 cm1 due to the asymmetric carbonyl stretching mode and the symmetric carbonyl stretching mode, respectively. This is consistent with calcium oxalate monohydrate which has been shown to have bands at 1618 and 1317 cm1, and the dihydrate or trihydrate which have bands at 1646 and 1327 cm1 or 1670 and 1325 cm1, respectively (Ouyang et al., 2005). FT-IR extinction spectrum of pure calcite does not have any of these peaks, as seen in the reference spectrum. Peaks characteristic of calcite are seen at 879 and 1460 cm1. However, calcite after reaction with oxalic acid shows additional peaks close in frequency to that of calcium oxalate. In particular, the spectrum of oxalic acidreacted calcite shows absorptions at 1325, and 1618 cm1 suggesting the presence of calcium oxalate monohydrate and Table 1 Measured values of % sc and corresponding single hygroscopicity parameters for 200 nm particles. Measured % sc PSL Oxalic acid/PSL CaCO3 Oxalic acid/CaCO3
0.74 0.26 0.44 0.40
0.09 0.03 0.04 0.03
Hygroscopicity Parameter 0.0010 0.0249 0.0070 0.0090
0.0007 0.0061 0.0017 0.0019
K.M. Gierlus et al. / Atmospheric Environment 46 (2012) 125e130
Extinction
1460
1458
CaCO3 2515
1797
2513
17951618
879
879
Reacted CaCO3
1325
1631 1323
CaC2O4
4000
3500
3000
2500 2000 Wavenumber (cm-1)
1500
1000
Fig. 4. FT-IR extinction spectra collected for CaC2O4, CaCO3, and CaCO3 reacted with oxalic acid.
trihydrate species. These data confirm the reaction of calcium carbonate and oxalic acid which leads to the formation of insoluble coating on calcium carbonate particles. This result can be further used to explain a negligible change in the CCN activity of calcite in the presence of aqueous oxalic acid. 4. Summary Internally mixed particles were generated to model processed dust in the atmosphere. PSL particles which had been generated from a suspension in oxalic acid were significantly more CCN active than PSL particles alone. These mixed composition particles represent a model for an unreactive mineral dust core with a soluble coating. However, in some cases, components of mineral dust aerosol will likely react such as seen in the formation of calcium oxalate from and oxalic acid. CCN measurements indicated that the internally mixed oxalate/calcite particles were similar to the unreacted calcite particles and showed no measurable enhancement in CCN activity compared to the internally mixed PSL particles, which is reflective of the decreased hygroscopicity and CCN activity of the less soluble oxalate salt that forms as compared to an oxalic acid coating. FT-IR extinction measurements were used to confirm the presence of a calcium oxalate coating on the calcite particle. This study demonstrates the link between interfacial chemistry and climate, the specificity of mineral dust aerosol chemistry and the importance of mineralogy. Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. CHE-0952605. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. References Bauer, S.E., Balkanski, Y., Schulz, M., Hauglustaine, D.A., Dentener, F., 2004. Global modeling of heterogeneous chemistry on mineral aerosol surfaces: influence on tropospheric ozone chemistry and comparison to observations. Journal of Geophysical Research 109, D02304. doi:10.1029/2003JD003868. Bilde, M., Svenningsson, B., 2004. CCN activation of slightly soluble organics: the importance of small amounts of inorganic salt and particle phase. Tellus B 56, 128e134.
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Matsuki, A., Schwarzenboeck, A., Venzac, H., Laj, P., Crumeyrolle, S., Gomes, L., 2010. Cloud processing of mineral dust: direct comparison of cloud residual and clear sky particles during AMMA aircraft campaign in summer 2006. Atmospheric Chemistry and Physics 10, 1057e1069. Miyazaki, Y., Aggarwal, S.G., Singh, K., Gupta, P.K., Kawamura, K., 2009. Dicarboxylic acids and water-soluble organic carbon in aerosols in New Delhi, India, in winter: characteristics and formation processes. Journal of Geophysical Research 114, D19206. doi:10.1029/2009JD011790. Ouyang, J.-M., Deng, S.-P., Zhou, N., Tieke, B., 2005. Effect of tartrates with various counterions on the precipitation of calcium oxalate in vesicle solutions. Colloids and Surfaces A 256, 21e27. Petters, M.D., Kreidenweis, S.M., 2007. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmospheric Chemistry and Physics 7, 1961e1971. Petters, M.D., Kreidenweis, S.M., 2008. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity - Part 2: including solubility. Atmospheric Chemistry and Physics 8, 6273e6279. Prenni, A.J., DeMott, P.J., Kreidenweis, S.M., Eli Sherman, D., Russell, L.M., Ming, Y., 2001. The effects of low molecular weight dicarboxylic acids on cloud formation. Journal of Physical Chemistry A 105, 11240e11248. Pruppacher, H.R., Jaenicke, R., 1995. The processing of water vapor and aerosols by atmospheric clouds, a global estimate. Atmospheric Research 38, 283e295. Rinaldi, M., Decesari, S., Carbone, C., Finessi, E., Fuzzi, S., Ceburnis, D., O’Dowd, C.D., Sciare, J., Burrows, J.P., Vrekoussis, M., Ervens, B., Tsigaridis, K., Facchini, M.C., 2011. Evidence of a natural marine source of oxalic acid and a possible link to glyoxal. Journal of Geophysical Research 116, D16204. doi:10.1029/2011JD015659. Roberts, G., Nenes, A., 2005. A continuous-flow streamwise thermal-gradient CCN chamber for atmospheric measurements. Aerosol Science and Technology 39, 206e221. Sorooshian, A., Varutbangkul, V., Brechtel, F.J., Ervens, B., Feingold, G., Bahreini, R., Murphy, S.M., Holloway, J.S., Atlas, E.L., Buzorius, G., Jonsson, H., Flagan, R.C., Seinfeld, J.H., 2006. Oxalic acid in clear and cloudy atmospheres: analysis of data from International Consortium for atmospheric Research on Transport and Transformation 2004. Journal of Geophysical Research 111, D23S45. doi:10.1029/2005JD006880. Sullivan, R.C., Moore, M.J.K., Petters, M.D., Kreidenweis, S.M., Roberts, G.C., Prather, K.A., 2009a. Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles. Atmospheric Chemistry and Physics 9, 3303e3316. Sullivan, R.C., Moore, M.J.K., Petters, M.D., Kreidenweis, S.M., Roberts, G.C., Prather, K.A., 2009b. Timescale for hygroscopic conversion of calcite mineral particles through heterogeneous reaction with nitric acid. Physical Chemistry Chemical Physics 11, 7826e7837.
Sullivan, R.C., Prather, K.A., 2007. Investigations of the diurnal cycle and mixing state of oxalic acid in individual particles in Asian aerosol outflow. Environmental Science and Technology 41, 8062e8069. Sun, J., Ariya, P.A., 2006. Atmospheric organic and bio-aerosols as cloud condensation nuclei (CCN): a review. Atmospheric Environment 40, 795e820. Svenningsson, B., Rissler, J., Swietlicki, E., Mircea, M., Bilde, M., Facchini, M.C., Decesari, S., Fuzzi, S., Zhou, J., Mønster, J., Rosenørn, T., 2006. Hygroscopic growth and critical supersaturations for mixed aerosol particles of inorganic and organic compounds of atmospheric relevance. Atmospheric Chemistry and Physics 6, 1937e1952. Tan, Y., Perri, M.J., Seitzinger, S.P., Turpin, B.J., 2009. Effects of precursor concentration and acidic sulfate in aqueous glyoxal-OH radical oxidation and implications for secondary organic aerosol. Environmental Science and Technology 43, 8105e8112. Tang, I.N., Munkelwitz, H.R., 1994. Water activities, densities, and refractive indices of aqueous sulfates and sodium nitrate droplets of atmospheric importance. Journal of Geophysical Research 99, D9. doi:10.1029/94JD01345. Usher, C.R., Michel, A.E., Grassian, V.H., 2003. Reactions on mineral dust. Chemical Reviews 103, 4883e4939. Wurzler, S., Reisin, T.G., Levin, Z., 2000. Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions. Journal of Geophysical Research 105, 4501e4512. Xue, H., Khalizov, A.F., Wang, L., Zheng, J., Zhang, R., 2009. Effects of dicarboxylic acid coating on the optical properties of soot. Physical Chemistry Chemical Physics 11, 7869e7875. Yang, F., Chen, H., Wang, X., Yang, X., Du, J., Chen, J., 2009. Single particle mass spectrometry of oxalic acid in ambient aerosols in Shanghai: mixing state and formation mechanism. Atmospheric Environment 43, 3876e3882. Yang, L., Ray, M.B., Yu, L.E., 2008a. Photooxidation of dicarboxylic acids - Part I: effects of inorganic ions on degradation of azelaic acid. Atmospheric Environment 42, 856e867. Yang, L., Ray, M.B., Yu, L.E., 2008b. Photooxidation of dicarboxylic acids - Part II: kinetics, intermediates and field observations. Atmospheric Environment 42, 868e880. Yu, J.Z., Huang, X.-F., Xu, J., Hu, M., 2005. When aerosol sulfate Goes up, so does oxalate: implications for the formation mechanisms of oxalate. Environmental Science Technology 39, 128e133. Zobrist, B., Marcolli, C., Koop, T., Luo, B.P., Murphy, D.M., Lohmann, U., Zardini, A.A., Krieger, U.K., Corti, T., Cziczo, D.J., Fueglistaler, S., Hudson, P.K., Thomson, D.S., Peter, T., 2006. Oxalic acid as a heterogeneous ice nucleus in the upper troposphere and its indirect aerosol effect. Atmospheric Chemistry and Physics 6, 3115e3129.
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Laboratory study of the effect of oxalic acid on the cloud condensation nuclei activity of mineral dust aerosol Kelly M. Gierlus, Olga Laskina, Tricia L. Abernathy, Vicki H. Grassian* Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2010 Received in revised form 4 October 2011 Accepted 4 October 2011
Dicarboxylic acids, which make up a significant portion of the atmospheric organic aerosol, are emitted directly through biomass burning as well as produced through the oxidation of volatile organic compounds. Oxalic acid, the most abundant of the dicarboxylic acids, has been shown by recent field studies to be present in mineral dust aerosol particles. The presence of these internally mixed organic compounds can alter the water absorption and cloud condensation nuclei (CCN) abilities of mineral particles in the Earth’s atmosphere. The University of Iowa’s Multi-Analysis Aerosol Reactor System (MAARS) was used to measure the CCN activity of internally mixed particles that were generated from a mixture of either calcite or polystyrene latex spheres (PSLs) in an aqueous solution of oxalic acid. Although PSL is not a mineral dust component, it is used here as a non-reactive, insoluble particle. CCN measurements indicate that the internally mixed oxalate/calcite particles showed nearly identical CCN activity compared to the original calcite particles whereas oxalic acid/PSL internally mixed particles showed much greater CCN activity compared to PSL particles alone. This difference is due to the reaction of calcite with oxalic acid, which produces a relatively insoluble calcium oxalate coating on the particle surface and not a soluble coating as it does on the PSL particle. Our results suggest that atmospheric processing of mineral dust aerosol through heterogeneous processes will likely depend on the mineralogy and the specific chemistry involved. Increase in the CCN activity by incorporation of oxalic acid are only expected for unreactive insoluble dust particles that form a soluble coating. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cloud chemistry Mineral dust Oxalic acid CCN activity
1. Introduction Dicarboxylic acids (DCAs) are a significant portion of the organic aerosol mass concentration, with oxalic acid being the most abundant (Legrand et al., 2005; Sorooshian et al., 2006; Martinelango et al., 2007; Hsieh et al., 2009; Hsu and Ding, 2009; Miyazaki et al., 2009; Kundu et al., 2010) having concentrations measured as high as 1.43 mg m3 (Miyazaki et al., 2009). This accounts for as much as 84% of the total DCAs in the atmosphere (Hsieh et al., 2009). Formation pathways for oxalic acid are still being investigated, but one primary source is known to be biomass burning (Li et al., 2009; Yang et al., 2009). Secondary production of oxalic acid occurs through oxidation of gaseous glyoxal (Rinaldi et al., 2011) and photooxidation of longer-chain DCAs (Yang et al., 2008a, 2008b; Kundu et al., 2010) which is enhanced in aqueous-phase reactions in cloud droplets (Carlton et al., 2007; Chen et al., 2007; Ervens et al., 2008). Field measurements by Hsieh et al. of suburban aerosol in
* Corresponding author. Tel.: þ1 319 335 392; fax: þ1 319 335 1270. E-mail address: [email protected] (V.H. Grassian). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.10.027
southern Taiwan reported that peak concentrations of oxalic acid occurred in the droplet mode, indicating an association with cloud processing (Hsieh et al., 2009). Model simulations (Ervens et al., 2004; Chen et al., 2007) and field studies (Tan et al., 2009) and have shown the production of secondary organic aerosols (SOAs), including oxalic acid, to be enhanced in the presence of clouds. Additionally, measurements of the contribution of oxalic acid to the total water soluble organic concentrations in New Delhi showed higher mass fractions of oxalate during nighttime compared to daytime, suggesting the production of oxalate via the oxidation of longer-chain DCAs in aqueous aerosols which were more abundant under higher relative humidity conditions present during the nighttime (Miyazaki et al., 2009). Similarly, sulfate which has been long recognized to form during cloud processing has been shown to have concentrations highly correlated with those of oxalate, suggesting oxalate is also produced via cloud processing (Yu et al., 2005; Sorooshian et al., 2006; Yang et al., 2009). Along with heterogeneous gas-phase reactions (Usher et al., 2003; Krueger et al., 2004; Sullivan and Prather, 2007; Sullivan et al., 2009b), cloud processing has also been shown to be an important pathway for the formation of coated or internally mixed
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mineral dust aerosol (Liu et al., 2005; Matsuki et al., 2010). Generated from windblown soil, mineral dust can be transported globally with atmospheric lifetimes of days to weeks, during which time it can impact the Earth’s climate as well as the chemical balance of the atmosphere (Bauer et al., 2004). Coating of the mineral dust particles can occur through cloud processing as the particles are collected by existing cloud droplets which were nucleated by soluble particles and that dissolved soluble material is redistributed until evaporation leaves internally mixed particles. On a global average it has been estimated that any particular atmospheric aerosol has undergone the evaporation/condensation cycle three times, (Pruppacher and Jaenicke, 1995) and repetition of this cycle has been shown to significantly increase the mass of soluble material incorporated into dust particles (Wurzler et al., 2000). Freshly emitted dust and mineral aerosols can have a significant hygroscopicity and CCN activity (Kumar et al., 2011) which has been shown to increase due to the cloud processing of dust aerosols (Crumeyrolle et al., 2008) as well as impact the deposition of dust (Fan et al., 2004). Conversely, the association of hygroscopic and CCN active substances like diacids can be decreased due to their incorporation into mineral dust (Sullivan and Prather, 2007). Carbonates, a significant and reactive component of mineral dust aerosol, have been found enriched in some cloud residual samples (Matsuki et al., 2010) providing evidence that there is potential for these coatings or aqueous reactions to occur. In a study of Asian aerosol by Sullivan et al. 10.2% of the mineral dust particles contained oxalic acid which was only detected in the dust particles (Sullivan and Prather, 2007). They hypothesized that DCAs may be found in increased amounts in mineral dust due to their ability to chelate divalent cations such as Ca2þ and Mg2þ (Sullivan and Prather, 2007). The presence of internally mixed oxalate has also been measured in another Asian dust aerosol, as well as a Saharan dust aerosol (Falkovich et al., 2004; Yang et al., 2009). It is also possible for calcite to react with oxalic acid to form calcium oxalate, according to:
CaCO3 þ H2 C2 O4 4 CaC2 O4 þ CO2 þ H2 O
(1)
Although the hygroscopicity (Prenni et al., 2001; Fan et al., 2004; Svenningsson et al., 2006) and CCN activity (Sullivan et al., 2009a; Falkovich et al., 2004; Kumar et al., 2003; Giebl et al., 2002; Hori et al., 2003; Sun and Ariya, 2006) of dicarboxylic acids such as oxalic acid has been measured previously and oxalic acid has been shown to act as heterogeneous ice nucleus (Zobrist et al., 2006), it is still not clear how internally mixed DCAs may impact the climate forcing properties of mineral dust. Xue et al. reported increased light scattering and absorption of soot aggregates with thin coatings of dicarboxylic acids, which was further enhanced when the coated soot was exposed to high relative humidity (Xue et al., 2009) and increases in hygroscopicity and CCN activity have been measured when calcite was internally mixed with organic humic substances (Hatch et al., 2008, 2009). In this study, measurements of the CCN activity of internally mixed particles generated from mixtures of either calcite (CaCO3) or PSLs in an aqueous solution of oxalic acid were made. CaCO3 and PSLs were used as models for mineral dust aerosol with one being a reactive component and the other an unreactive, insoluble component. PSLs were used widely in earlier studies to investigate atmospheric processes. Internally mixed aerosol particles consisting of PSLs and soluble components were used before to study the impact of heterogeneous processing of mineral dust aerosol on climate (Gibson et al., 2007). The CCN activity of coated PSLs was also used to explore the effectiveness of humic coatings on insoluble particles (Hatch et al., 2008). In addition, coated PSL spheres were used to study products and mechanisms of ozone reactions with oleic acid for aerosol particles (Katrib et al., 2004). Besides CCN measurements, Fourier transform infrared
(FT-IR) extinction measurements were performed to confirm speciation of oxalate in the internally mixed calcite particles. 2. Experimental methods 2.1. Materials All of the chemicals used in this study were commercially available and used as received. Oxalic acid (99.999%) was purchased from Aldrich, and aerosols were generated from a 0.2-wt% solution using Optima water (Fisher Scientific). Calcite (OMYA) and PSLs (Polysciences, Inc., Cat. #07304) were atomized from suspensions in Optima water. Internally mixed particles were generated by atomizing a suspension of insoluble powder in an aqueous solution of soluble material. Calcium oxalate monohydrate (99.9985%) was purchased from Alfa Aesar. 2.2. Size distribution and cloud condensation nuclei activity measurements and Fourier transform infrared extinction spectra Size distributions and cloud droplet activation measurements were obtained using the University of Iowa’s Multi-Analysis Aerosol Reactor System (MAARS), which has been described in detail elsewhere (Gibson et al., 2006). Aerosols were generated by a constant output atomizer (Model 3076, TSI, Inc.) from an aqueous solution or suspension as described above. The aerosol was then passed through two diffusion dryers (Model 3062, TSI, Inc.) and dried to a low relative humidity (RH 10%). The particles follow a flow stream from the aerosol generator through the path of the IR beam by a combination of conductive tubing and glass flow tubes. The glass flow tube system consists of an initial conditioning tube and an observation cell that is collinear with the IR beam. For the CCN measurements the aerosol was passed through a differential mobility analyzer (DMA1; Model 3080, TSI, Inc.) in order to size select 200 nm particles. The sheath and aerosol flows of the DMA1 were set to be 3 lpm and 0.3 lpm, respectively. This monodisperse aerosol was then split between a continuous flow streamwise thermal-gradient cloud condensation nuclei counter (Droplet Measurement Technologies; Model CCN-2) (Roberts and Nenes, 2005) and a condensation particle counter (CPC; Model 3025A, TSI, Inc.). For a given diameter and supersaturation, the CCN activity is reported as the ratio of particles that act as a cloud condensation nuclei (#CCN) to the number of particles counted by the CPC (#CN). The ammonium sulfate activity data presented by Tang and Munkelwitz was used to calibrate the supersaturation generated in the CCN counter to the thermal gradient of the diffusion chamber (Tang and Munkelwitz, 1994). This is useful so as to compare to other measurements made previously in our laboratory. Full size distributions were measured by bypassing DMA1 and directing the dry polydisperse aerosol into a scanning mobility particle sizer (SMPS; Model 3936, TSI, Inc.), which consists of a second DMA (DMA2) (flow rates are 3.0 (sheath) and 0.3 (sample) lpm) coupled to the CPC. The FT-IR extinction spectra were measured using a Fourier-transform infrared (FT-IR) spectrometer (Thermo Nicolet Nexus Model 670) with a liquid nitrogen cooled external MCT-A detector. The IR spectra were acquired from 740 to 4000 cm1 using 8 cm1 resolution by co-adding 256 scans. All FT-IR spectra were referenced to the background signal collected under dry conditions. 2.3. Theoretical methods A single hygroscopicity parameter, l, has recently been developed using “l-Köhler theory” (Petters and Kreidenweis, 2007, 2008) in order to determine a single parameter which can be
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used to describe both hygroscopic growth and CCN activity. In l-Köhler theory, the saturation ratio, S, is expressed in the following equation:
SðDÞ ¼
D3 D3d
!
D3 D3d ð1 kÞ
4Mw ss=a RT rw D
(2)
exp
where D and Dd are the aqueous droplet diameter and the dry diameter, respectively, ss/a is the surface tension of the solution/air interface, R is the ideal gas constant, T is the temperature, and Mw and rw are the molecular weight and density of water, respectively. Value of l can may be determined by fitting observed values of critical saturation to the maximum of the l-Köhler curve (equation (2)), where the critical saturation, Sc, is the minimum saturation needed for particle to nucleate cloud droplet, and is related to the critical supersaturation, sc, by sc ¼ Sc 1. For multicomponent particles simple mixing rule can be applied for overall value of l:
k ¼
X
εi ki
(3)
i
To fit l to CCN data values of the aerosol particles studied a temperature of 298.15 K and a surface tension for pure water of 0.072 Jm2 (Lide, 2004) were assumed.
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The amount of externally mixed oxalic acid or ammonium sulfate particles can be predicted to a first approximation by dividing the number concentration of soluble particles at 200 nm by the total number concentration of particles at 200 nm (soluble plus insoluble component). From the size distributions of the individual components, it is estimated that for CaCO3 there would be less than 1.0% externally mixed oxalic acid and for PSLs there would be less than 0.7% externally mixed particles. Fig. 2 shows the full size distributions of the particles generated by atomizing calcite in an aqueous solution of oxalic acid. Size distributions were measured continuously over intervals of three and a half minutes for about 1.5 h. The size distributions have been normalized such that the maximum concentration of the peak centered at approximately 200 nm is one. The arrow demonstrates the decrease in concentration with time of the peak at smaller diameters. When calcite was atomized in a solution of 0.2-wt% oxalic acid initially the size distribution was bimodal demonstrating the presence of externally mixed oxalic acid particles at approximately 60 nm, however, this peak is found to decrease significantly as a function of time and is near baseline after ca. 30 min. This change is contributed to the reaction of oxalic acid with calcite to yield calcium oxalate. In comparison, when PSLs were atomized in a solution of 0.2-wt% oxalic acid the externally mixed oxalic acid particles showed little decrease in concentration and peak diameter.
3. Results and discussion 3.2. CCN activity of 200 nm particles 3.1. Size distributions Full size distributions for the individual components studied are shown in Fig. 1 by lognormal curve fits to the average of multiple measurements. Peak diameters are 45 nm for 0.2-wt% oxalic acid (long dashed curve), 195 nm for calcite (solid curve), and 196 nm for PSLs (short dashed curve). When generating internally mixed particles the insoluble component (calcite or PSLs) was atomized in a 0.2-wt% solution of the soluble component (oxalic acid), and size selected at 200 nm. The use of a particle size near 200 nm was necessary to minimize the amount of externally mixed oxalic acid.
In order to study the effect of CCN activity, internally mixed or reacted particles were generated from a mixture of either PSLs or CaCO3 in an aqueous solution of 0.2-wt% oxalic acid. Fig. 3a shows normalized CCN activity of PSLs atomized in oxalic acid and size selected at 200 nm. The single hygroscopicity parameter of the PSLs increased from l ¼ 0.0010 0.0007 (% sc ¼ 0.74 0.09) for uncoated to l ¼ 0.0249 0.0061 (% sc ¼ 0.26 0.03) for PSLs with
1 0.2 wt% H2C2O4
dN/dlogDp (#/cm3)
PSL CaCO3
1 10 6
5 10 5
Normalized dN/dlogDp (#/cm3)
1.5 10 6
0.8
0.6
0.4
0.2
0 10
100
1000
Diameter (nm)
0 10
100
1000
Diameter (nm) Fig. 1. Lognormal curve fits to the average of multiple measurements of the full size distributions of 0.2-wt% H2C2O4 (dashed), PSL (dotted), and CaCO3 (solid).
Fig. 2. Full size distributions of calcite atomized in a solution of 0.2-wt% oxalic acid. The size distributions have been normalized such that the maximum concentration of the peak centered at approximately 200 nm is one. The arrow demonstrates the decrease in the peak at approximately 45 nm over time with the peak at approximately 200 nm staying constant.
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a 1
#CCN/#CN
0.8
0.6
0.4
0.2 PSL in H2 C2O4 PSL 0 0
0.5
1
1.5
2
% Supersaturation
b
1
#CCN/#CN
0.8
0.6
Fig. 3b shows the CCN activity of calcite and oxalate/calcite particles. The l of the 200 nm calcite particles was measured to be 0.0070 0.0017 (% sc ¼ 0.44 0.04) which is in agreement with previous measurements made in our laboratory (% sc ¼ 0.37 0.02), (Gibson et al., 2007) however, is much higher than those reported for calcite aerosolized using a dry technique (l ¼ 0.0011) (Sullivan et al., 2009a). A similar difference was also observed by Koehler et al. In the case of using aqueous suspension to generate an aerosol of Arizona test dust a single hygroscopicity parameter was much higher (l ¼ 0.35) than when the aerosol was generated dry (l ¼ 0.025) (Koehler et al., 2009). Such differences can be due to the presence of soluble material in the water (Koehler et al., 2009) or generation of soluble species from the dust. This assumption can be also supported by the work of Bilde and Svenningsson, they noticed that trace amounts of impurities that can be present in water facilitate the cloud droplet activation of pure particles (Bilde and Svenningsson, 2004). When calcite is atomized in aqueous oxalic acid the resulting particles had l ¼ 0.0090 0.0019 (% sc ¼ 0.40 0.03), which is only slightly more CCN active within error bars than the pure calcite particles. This is a much smaller enhancement, essentially no enhancement, in CCN activity as compared to the internally mixed oxalic acid-PSL particles which exhibit a dramatic increase in CCN activity compared to uncoated PSLs. This difference in adding of oxalic acid to calcite versus PSLs is due to the reaction of calcite with oxalic acid to produce a coating of calcium oxalate. Furthermore, assuming the similar fraction of oxalic acid incorporated into calcium carbonate as into PSLs (4.8%) and using a mixing rule for single hygroscopicity parameter it can be estimated that l should increase up to the value of 0.031, which is significantly lower than the experimental value of 0.009. Low hygroscopicity of calcium oxalate particles (l ¼ 0.05) compared to that of oxalic acid (l ¼ 0.5) (Sullivan et al., 2009a) can further support the formation of calcium oxalate on the surface of calcium carbonate. These calculated values are summarized in Table 1. As described in the next section, spectroscopic measurements provide definitive proof of the reaction product. 3.3. FT-IR extinction spectra
0.4
0.2 CaCO3 in H2C2O4 CaCO3 0 0
0.5
1
1.5
2
% Supersaturation Fig. 3. CCN activity of 200 nm (a) PSLs atomized in the presence (circles) and absence (squares) of oxalic acid and (b) CaCO3 atomized in the presence (circles) and absence (squares) of oxalic acid. Each data point is the average of 3e10 measurements and the error bars represent the standard deviation of these multiple measurements. The lines represent sigmoidal fits to the data. The dashed line represents the 50% activation point.
internally mixed oxalic acid. Using a mixing rule (equation (3)) for l it can be estimated that the fraction of oxalic acid acquired is 4.8%. Such a large increase in CCN activity demonstrates that the incorporation of small amounts of soluble material can significantly alter the climate properties of insoluble aerosols. The single hygroscopicity parameter of pure 200 nm oxalic acid was measured to be higher than 0.17 (sc less than 0.1%). This result corresponds to l ¼ 0.5 reported by Sullivan et al. (2009a). The ability of oxalic acid to be CCN active was also shown in other earlier studies (Kumar et al., 2003; Giebl et al., 2002; Hori et al., 2003; Sun and Ariya, 2006).
The FT-IR extinction measurements were performed in order to confirm the presence of calcium oxalate in the reacted calcite particles. Spectra of calcium oxalate and calcite were also collected for comparison and can be seen in Fig. 4. The reference spectra for calcium oxalate shows absorption bands at 1631 and 1323 cm1 due to the asymmetric carbonyl stretching mode and the symmetric carbonyl stretching mode, respectively. This is consistent with calcium oxalate monohydrate which has been shown to have bands at 1618 and 1317 cm1, and the dihydrate or trihydrate which have bands at 1646 and 1327 cm1 or 1670 and 1325 cm1, respectively (Ouyang et al., 2005). FT-IR extinction spectrum of pure calcite does not have any of these peaks, as seen in the reference spectrum. Peaks characteristic of calcite are seen at 879 and 1460 cm1. However, calcite after reaction with oxalic acid shows additional peaks close in frequency to that of calcium oxalate. In particular, the spectrum of oxalic acidreacted calcite shows absorptions at 1325, and 1618 cm1 suggesting the presence of calcium oxalate monohydrate and Table 1 Measured values of % sc and corresponding single hygroscopicity parameters for 200 nm particles. Measured % sc PSL Oxalic acid/PSL CaCO3 Oxalic acid/CaCO3
0.74 0.26 0.44 0.40
0.09 0.03 0.04 0.03
Hygroscopicity Parameter 0.0010 0.0249 0.0070 0.0090
0.0007 0.0061 0.0017 0.0019
K.M. Gierlus et al. / Atmospheric Environment 46 (2012) 125e130
Extinction
1460
1458
CaCO3 2515
1797
2513
17951618
879
879
Reacted CaCO3
1325
1631 1323
CaC2O4
4000
3500
3000
2500 2000 Wavenumber (cm-1)
1500
1000
Fig. 4. FT-IR extinction spectra collected for CaC2O4, CaCO3, and CaCO3 reacted with oxalic acid.
trihydrate species. These data confirm the reaction of calcium carbonate and oxalic acid which leads to the formation of insoluble coating on calcium carbonate particles. This result can be further used to explain a negligible change in the CCN activity of calcite in the presence of aqueous oxalic acid. 4. Summary Internally mixed particles were generated to model processed dust in the atmosphere. PSL particles which had been generated from a suspension in oxalic acid were significantly more CCN active than PSL particles alone. These mixed composition particles represent a model for an unreactive mineral dust core with a soluble coating. However, in some cases, components of mineral dust aerosol will likely react such as seen in the formation of calcium oxalate from and oxalic acid. CCN measurements indicated that the internally mixed oxalate/calcite particles were similar to the unreacted calcite particles and showed no measurable enhancement in CCN activity compared to the internally mixed PSL particles, which is reflective of the decreased hygroscopicity and CCN activity of the less soluble oxalate salt that forms as compared to an oxalic acid coating. FT-IR extinction measurements were used to confirm the presence of a calcium oxalate coating on the calcite particle. This study demonstrates the link between interfacial chemistry and climate, the specificity of mineral dust aerosol chemistry and the importance of mineralogy. Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. CHE-0952605. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. References Bauer, S.E., Balkanski, Y., Schulz, M., Hauglustaine, D.A., Dentener, F., 2004. Global modeling of heterogeneous chemistry on mineral aerosol surfaces: influence on tropospheric ozone chemistry and comparison to observations. Journal of Geophysical Research 109, D02304. doi:10.1029/2003JD003868. Bilde, M., Svenningsson, B., 2004. CCN activation of slightly soluble organics: the importance of small amounts of inorganic salt and particle phase. Tellus B 56, 128e134.
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