Probing Heterogeneous Chemistry of Individual ... - ACS Publications

Aug 28, 2003 - M. J. Iedema, J. P. Cowin, and A. Laskin*. William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National ...
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Anal. Chem. 2003, 75, 5170-5179

Probing Heterogeneous Chemistry of Individual Atmospheric Particles Using Scanning Electron Microscopy and Energy-Dispersive X-ray Analysis B. J. Krueger and V. H. Grassian*

Department of Chemistry and the Center for Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa 52242 M. J. Iedema, J. P. Cowin, and A. Laskin*

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352

In this paper, we demonstrate the utility of single-particle analysis to investigate the chemistry of isolated, individual particles of atmospheric relevance such as NaCl, sea salt, CaCO3, and SiO2. A variety of state-of-the-art scanning electron microscopy techniques, including environmental scanning electron microscopy and computer-controlled scanning electron microscopy/energy-dispersive X-ray analysis, were utilized for monitoring and quantifying phase transitions of individual particles, morphology, and compositional changes of individual particles as they react with nitric acid. Clear differences in reaction mechanisms were observed between SiO2, CaCO3, NaCl, and sea salt particles. SiO2 particles exposed to HNO3 showed no change, indicating that the reaction of SiO2 particles is limited to the particle surface and would not involve bulk atoms in its reactivity. Calcium carbonate was seen to convert to aqueous calcium nitrate droplets while sodium chloride formed microcrystallites of sodium nitrate on top of the particle. Sea salt particles showed morphology changes that could be described as a combination of these as both spherical droplets and microcrystallites were observed. This is consistent with the multicomponent composition of sea salt. Further differences were found in the reaction rates for sea salt and sodium chloride with nitric acid. Sea salt yielded a significant increase in reactivity when compared to the NaCl particles under similar conditions. The reaction of nitric acid with calcium carbonate was found to be strongly enhanced at higher relative humidity. In recent years the study of aerosol particles and their impact on the environment and climate has intensified. The impact of mineral dust aerosol and sea salt on atmospheric processes such as climate forcing1,2 and heterogeneous chemistry3,4 is just * To whom correspondence should be addressed. E-mail: (V.H.G.) [email protected]; (A.L.) [email protected]. (1) Sokolik, I. N.; Toon, O. B. J. Geophys. Res., [Atmos.] 1999, 104, 94239444. (2) Myhre, G.; Stordal, F. J. Geophys. Res., [Atmos.] 2001, 106, 18193-18204.

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beginning to be understood. Soil dust and sea salt are known to play an important role in the partitioning of nitric acid throughout most of the troposphere.5 Nitric acid is a prevalent species in the atmosphere in both gas and aqueous phases. Nitric acid is produced through reactions of NO2, abstraction of hydrogen by NO3 from organics, and hydrolysis of N2O5 in water droplets and sprays as well as in water adsorbed on aerosol surfaces.6 It is estimated that 1000-3000 Tg of mineral aerosols are emitted into the atmosphere annually.7 Using a simple kinetic model, Song and Carmichael predicted that >70% of the gaseous nitric acid is partitioned onto dust particles over the Gobi Desert.5 During the Indian Ocean Experiment (INDOEX), the average chemical composition of aerosols by mass was found to contain 10% nitrates.8 A large correlation between the amount of nitrate and the amount of crustal elements, particularly calcium, has been reported,9,10 and for this reason, calcite, CaCO3, is thought to be a particularly reactive component of the aerosol present in the Earth’s atmosphere.9,11 An important heterogeneous reaction involving mineral dust that contains calcium carbonate can be written as

CaCO3(s) + 2HNO3(g) f Ca(NO3)2 + H2CO3 f Ca(NO3)2 + CO2 + H2O (R1)

Sea salt aerosol, originating from sea spray or wind-induced wave capping, is another important sink for nitric acid. Erickson (3) Grassian, V. H. Int. Rev. Phys. Chem. 2001, 20, 467-548. (4) Finlayson-Pitts, B. J.; Hemminger, J. C. J. Phys. Chem. A 2000, 104, 1146311477. (5) Song, C. H.; Charmichael, G. R. J. Geophys. Res. 2001, 106, 18131-18154. (6) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics from Air Pollution to Climate Change; John Wiley & Sons: New York, 1997. (7) Jonas, P.; Charlson, R.; Rodhe, H. Aerosols in Climate Change; Cambridge University Press: New York, 1995. (8) Ramanathan, V.; Crutzen, P. J.; Kiehl, J. T.; Rosenfeld, D. Science 2001, 294, 2119-2124. (9) Song, C. H.; Charmichael, G. R. J. Atmos. Chem. 2001, 40, 1-22. (10) Wolff, G. T. Atmos. Environ. 1984, 18, 977-981. (11) Hoornaert, S.; Malderen, H. V.; Van Greiken, R. E. Environ. Sci. Technol. 1996, 30, 1515-1520. 10.1021/ac034455t CCC: $25.00

© 2003 American Chemical Society Published on Web 08/28/2003

et al. estimated an annual integrated flux of sea salt to be ∼2800 Tg.12 The heterogeneous replacement of chloride by nitrate in sea salt has also been observed in many places around the world.13-31 Sea salt, due to its multicomponent nature, has several potential reactions involving nitric acid including those with the NaCl and MgCl2 components:32

NaCl(s,aq) + HNO3(g) f NaNO3(s,aq) + HCl(g) (R2) MgCl2(s,aq) + 2HNO3(g) f Mg(NO3)2(s,aq) + 2HCl(g) (R3) The phase of an aerosol can impact both the heterogeneous chemistry of the aerosol and the effect the aerosol has on reflecting and scattering solar radiation.2,32,33 For example, the rate of the sodium chloride reaction is much faster in the aqueous phase compared to the solid phase.34 Chemical, morphological (size and shape), and phase data at the single-particle level are of crucial importance for purposes of tracing and understanding the formation and reaction mechanisms of aerosols as well as their possible history and source apportionment. An effective technique for such measurements is singleparticle mass spectrometry, which is the only method that provides real-time in situ analysis of the size and composition of individual particles.35,36 However, mass spectrometry techniques do not provide morphological information and also completely (12) Erickson, D. J., III; Seuzaret, C.; Keene, W. C.; Gong, S. L. J. Geophys. Res. 1999, 104, 8347-8372. (13) Laskin, A.; Iedema, M. J.; Cowin, J. P. Environ. Sci. Technol. 2002, 36, 4948-4955. (14) Martens, C. S.; Wesolowski, J. J.; Harriss, R. C.; Kaifer, R. J. Geophys. Res. 1973, 78, 8778-8792. (15) Moyers, J. L.; Duce, R. A. J. Geophys. Res. 1972, 77, 5330-5338. (16) Raemdonck, H.; Maenhaut, W.; Andreae, M. O. J. Geophys. Res. 1986, 91, 8623-8636. (17) Keene, W. C.; Pszenny, A. A. P.; Jacob, D. J.; Duce, R. A.; Galloway, J. N.; Schultz-Tokos, J. J.; Sievering, H.; Boatman, J. F. Global Biogeochem. Cycles 1990, 4, 407-430. (18) Shaw, G. E. J. Geophys. Res. 1991, 96, 22/369-372. (19) Pio, C. A.; Cerqueira, M. A.; Castro, L. M.; Salgueiro, M. L. Atmos. Environ. 1996, 30, 3115-3127. (20) Kerminen, V.-M.; Pakkanen, T. A.; Hillamo, R. E. Atmos. Environ. 1997, 31, 2753-2765. (21) Kerminen, V.-M.; Teinila, K.; Hillamo, R.; Pakkanen, T. J. Aerosol Sci. 1998, 29, 929-942. (22) Zhuang, H.; Chan, C. K.; Fang, M.; Wexler, A. S. Atmos. Environ. 1999, 33, 4223-4233. (23) Pakkanen, T. A. Atmos. Environ. 1996, 30, 2475-2482. (24) McInnes, L. M.; Covert, D. S.; Quinn, P. K.; Germani, M. S. J. Geophys. Res. 1994, 99, 8257-8268. (25) Posfai, M.; Anderson, J. R.; Buseck, P. R.; Sievering, H. J. Geophys. Res. 1995, 100, 23063-23074. (26) Mouri, H.; Okada, K. Geophys. Res. Lett. 1993, 20, 49-52. (27) Posfai, M.; Anderson, J. R.; Buseck, P. R.; Shattuck, T. W.; Tindale, N. W. Atmos. Environ. 1994, 28, 1747-1756. (28) De Bock, L. A.; Van Malderen, H.; Van Grieken, R. E. Environ. Sci. Technol. 1994, 28, 1513-1520. (29) Ebert, M.; Weinbruch, S.; Hoffmann, P.; Ortner, H. M. J. Aerosol Sci. 2000, 31, 613-632. (30) De Bock, L. A.; Joos, P. E.; Noone, K. J.; Pockalny, R. A.; Van Grieken, R. E. J. Atmos. Chem. 2000, 37, 299-329. (31) Ro, C. U.; Oh, K. Y.; Kim, H. K.; Kim, Y. P.; Lee, C. B.; Kim, K. H.; Kang, C. H.; Osan, J.; de Hoog, J.; Worobiec, A.; Van Grieken, R. E. Environ. Sci. Technol. 2001, 35, 4487-4494. (32) Slanina, J.; Ten Brink, H. M.; Khlystov, A. Chemosphere 1999, 38, 14291444. (33) Marsh, N.; Svensmark, H. Space Sci. Rev. 2000, 94, 215-230. (34) Ten Brink, H. M. J. Aerosol Sci. 1998, 29, 57-64.

destroy particles in the analysis process. Atomic force microscopy can provide morphological information but no elemental data.37,38 Off-line electron beam microscopy techniques offer another single-particle analysis approach, which involves deposition of particles onto a substrate that can be later probed by the electron beam. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with energy-dispersed X-ray (EDX) spectrometers have been extensively used to characterize the size, morphology, and elemental composition of individual particles.39-41 Very recently, it has also been demonstrated that environmental SEM (ESEM), originally developed to eliminate charging problems and to enhance the imaging of nonconductive samples,42 can be successfully used to investigate the hydration reactivity of atmospheric particles as a function of relative humidity (RH).43 Electron beam techniques, when used carefully, are sufficiently nondestructive for many chemical substances and therefore, in principle, can be used to follow the physical and chemical transformations of individually selected particles. In this study, we demonstrate the effectiveness of the combined use of SEM, computer-controlled SEM (CCSEM), ESEM, and EDX to probe morphological and compositional changes of individual particles as they undergo phase transitions and chemical reactions. Atmospherically relevant particles, including CaCO3, SiO2, NaCl, and sea salt, are investigated in this study in order to demonstrate the utility of the aforementioned techniques and to provide qualitative and quantitative information on the chemistry of aerosol particles with nitric acid as well as their hydration reactivity. EXPERIMENTAL METHODS Materials and Sample Preparation. Laboratory-generated particles of atmospheric relevance were deposited onto TEM grids for analysis and chemical processing. TEM grids, coated with a thin (∼50 nm) carbon film, were purchased from Ted Pella, Inc. (carbon type-B on Au 200-mesh grid) and used as substrates in this work. Powders of calcium carbonate (calcite) purchased from Alfa Aesar (99.95% purity) and silicon dioxide (quartz) purchased from Strem Chemicals (99.5% purity) were first ground between two glass slides and then dispersed onto TEM grids. The mean diameter of the dispersed particles was on the order of ∼1 µm. The particle density was estimated as ∼18 000 particles mm-2 of the substrate area. The separation distance between single (35) Wexler, A. S.; Johnstron, M. V. In Aerosol Measurement, 2nd ed.; Willike, K., Baron, P. A., Eds.; John Wiley & Sons: New York, pp 365-385, and references therein. (36) Noble, C. A.; Prather, K. A. Mass Spectrom. Rev. 2000, 19, 248-274. (37) Ramirez-Aguilar, K. A.; Lehmpuhl, D. W.; Michel, A. E.; Birks, J. W.; Rowlen, K. L. Ultramicroscopy 1999, 77, 187-194. (38) Lehmpuhl, D. W.; Ramirez-Aguilar, K. A.; Michel, A. E.; Rowlen, K. L.; Birks, J. W. Anal. Chem. 1999, 71, 379-383. (39) Buseck, P. R.; Anderson, J. R. In Advanced Mineralogy; Marfunun, A. S., Ed.; Springer-Verlag: Berlin, 1998; Vol. 3, pp 292-312, and references therein. (40) De Bock, L. A.; Van Grieken, R. E. In Analytical Chemistry of Aerosols; Spurny, K. R., Ed.; Lewis Publishers: Boca Raton, FL, 1999; pp 243-275, and references therein. (41) Fletcher, R. A.; Small, J. A.; Scott, J. H. J. In Aerosol Measurement, 2nd ed.; Wilike, K., Baron, P. A., Eds.; John Wiley & Sons: New York, 2001; pp 295-365, and references therein. (42) Donilatos, G. D. Microsc. Res. Tech. 1993, 25, 354-361. (43) Ebert, M.; Inerle-Hof, M.; Weinbruch, S. Atmos. Environ. 2002, 36, 59095916.

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particles on the TEM substrate was on the order of ∼1-2 µm. Samples of calcium nitrate tetrahydrate (Aldrich Inc., 99% purity) and sodium nitrate (Aldrich Inc., 98% purity) particles were prepared in the same way and were used for reference analysis. Samples of NaCl and sea salt particles were obtained by drying liquid droplets onto TEM grids. The liquid droplets were nebulized from a 0.5 M solution of NaCl and seawater, respectively. Crystalline sodium chloride (Aldrich Inc, 99.99% purity) was used to prepare aqueous NaCl solution. Seawater was taken from Sequim Bay (Pacific Ocean), ∼0.5 miles from the coast near Sequim, WA. Sea salt and NaCl samples had particle densities of ∼2000 mm-2 and separation distances of ∼10-30 µm. Calcite and quartz phases of CaCO3 and SiO2 powders were confirmed by X-ray diffraction analysis. The phase of NaCl (halite) was confirmed by transmission electron microscopy of individual particles assisted with selected area electron diffraction analysis. All samples were initially analyzed using SEM/EDX and CCSEM/EDX prior to the experiments. The samples were then transferred from the microscope to a flow reactor where they were exposed to nitric acid vapor at different relative humidities. After the exposure, the samples were transferred back to the microscope where the same individual particles were analyzed again in order to detect consequential changes in their morphology and chemical composition. Nitric acid exposure and consecutive SEM/ EDX and CCSEM/EDX analyses were repeated several times in order to follow the changes of individual particles as a function of time. CCSEM/EDX Single-Particle Analysis. A FEI XL30 digital field emission gun ESEM was used in this work. The EDX spectrometer is an EDAX 136-10 spectrometer with a Si(Li) detector of an active area of 30 mm2 and ATW2 window, which allows X-ray detection from elements higher than beryllium (Z > 4). The system is equipped with Genesis hardware and software (EDAX, Inc.) for computer-controlled SEM/EDX particle analysis. Using the CCSEM/EDX setup, a matrix of fields of view is set over the sample area, and then the area is automatically inspected on a field-by-field basis. In each field of view particles are recognized by an increase of the detector signal above a preselected threshold level. After recognizing the particles in a field of view, the program acquires an X-ray spectrum from each detected particle. In this work, a magnification of 2000× was used. Imaging of particles was done by acquiring the mixed signal of backscattered and secondary electrons. Features with equivalent circle diameter larger than 0.2 µm were considered as particles by the software. During the X-ray acquisition, the electron beam rastered over the particle projection area. The X-ray spectra were acquired for 5 s of clock time, at a beam current of 400-500 pA and an accelerating voltage of 20 kV. The relative dead times of the EDX spectrometer did not exceed 30%. For quantification of the EDX results, a simple normalization method44 was used, which does not include corrections for the effects of particle size and shape. These corrections, discussed later, are not necessary for submicrometer particles. The apparent particle composition was determined from the measured intensities of the X-ray peaks relative to the theoretically calculated intensities of corresponding elements.

Flow Reactor. A schematic diagram of the gas flow reactor used to expose particles to nitric acid vapor is shown in Figure 1. The flow reactor was built from glass and Teflon parts; both materials do not react with nitric acid vapor. The apparatus consists of the following: a reaction chamber (1), high-vacuum PTFE stopcocks (2), needle valves (3), straight-bore stopcocks (4), two bubblers (5, 6), several pressure transducers (7, 9), and two flow controllers (8). The TEM grids containing particles were loaded into the reaction chamber (1) where they were exposed to nitric acid vapor at different relative humidities and for various exposure times. Nitrogen was used as a carrier gas and allowed to flow through the bubbler (5), which contained a solution composed of 6.1 mL of concentrated nitric acid (ACS Certified, Fisher Scientific) and 100 mL of H2O. To vary the relative humidity in the reaction chamber, nitrogen was allowed to flow through the second bubbler (6), which contained pure water. Nitrogen flow through the bubbler with water was restrained with a flow controller (8a). The pressures in the bubblers and in the reaction chamber were monitored and recorded using the pressure transducers (7). A flow controller (8b) was used to monitor the total flow through the system (0.5 L min-1). Partial pressures of nitric acid and water in the reaction chamber were estimated via the equation of state by assuming thermodynamic vapor-liquid equilibrium in the bubblers and using measured values of the pressure drop between the bubblers and the reaction chamber. Equilibrium partial pressures of water and HNO3 vapors in bubblers were calculated using an on-line aerosol inorganic model.45 For the nitric acid solution used in this work, the equilibrium partial pressures of water and nitric acid at 295 K are pH2O ) 0.249 × 10-1 atm (18.9 Torr) and pHNO3 ) 0.151 × 10-6 atm (1.14 × 10-4 Torr), respectively. Partial pressure of carrier nitrogen was then calculated as a difference between total pressure (measured) and partial pressures of water and nitric acid. Needle valves (3) were used to adjust the pressure drop to set the desired conditions inside the reaction chamber with respect to nitric acid pressure and relative humidity. Two sets of the

(44) Laskin, A.; Iedema, M. J.; Cowin, J. P. Aerosol Sci. Technol. 2003, 37, 246260.

(45) Carslaw, K. S.; Clegg, S. L.; Brimblecombe, P. J. Phys. Chem. 1995, 99, 11557-11574.

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Figure 1. Schematic diagram of the gas flow reactor: (1) reaction chamber, (2) high vacuum PTFE stopcocks, (3) needle valves, (4) straight-bore stopcocks, (5) and (6) bubblers, (7) and (9) pressure transducers, and (8) flow controllers.

exposure conditions were used in this work: (1) low relative humidity set, RH ) 17 ( 1%, pHNO3 ) 20 ( 1 µTorr, and ptotal ) 49.7 ( 0.7 Torr; and (2) high relative humidity set, RH ) 41 ( 1%, pHNO3 ) 19 ( 1 µTorr, and ptotal ) 103 ( 4 Torr. The partial pressures for nitric acid at both the high and low relative humidity sets are statistically similar and hence will be referred to as 20 µTorr (6.5 × 1011 molecules/cm3 or 26 ppb relative to atmospheric pressures) hereafter. The nitric acid pressure used here is slightly higher than those found in polluted environments, which have been measured to be between 10 and 20 ppb.46 RESULTS AND DISCUSSION Monitoring of Phase Transitions of Individual Particles Using ESEM. ESEM42 is a relatively new form of electron-based probe instrumentation, which only recently has been successfully utilized to study hygroscopic behavior of individual particles.43 Unlike conventional high-vacuum SEM where the electron gun and specimen chamber share the same or similar vacuum, the ESEM vacuum system is divided into several regions of increasing vacuum, separated by pressure-limiting apertures. This setup allows retaining up to 20 Torr of residual gas pressure inside of the chamber during the imaging process. Water vapor can be used to provide a wet microenvironment for specimens, which is relevant to real atmospheric conditions. The temperature of the specimen can be varied in the range of -10 to 60 °C using a Peltier stage. The entire range of relative humidity (RH from 1 to 100%) can be established for the specimen by varying both water vapor pressure inside of the chamber and the temperature of the Peltier stage. Therefore, the ESEM technique allows real-time visual observation of the hygroscopic transformations of individual particles (e.g., deliquescence, efflorescence, and hygroscopic growth) with a lateral resolution of 5-10 nm. A series of gaseous secondary electron (GSE) images shown in Figure 2 demonstrates the deliquescence process, i.e., the phase transition from solid to liquid as a function of relative humidity, of a NaCl particle at T ) 288 ( 1 K. In this experiment, a NaCl particle was imaged starting from 15% RH. The relative humidity was then increased. The image shown in (a) was taken at 70.3% RH and is a reference image of the particle prior to its deliquescence. The relative humidity was then gradually increased to 75.0 ( 0.8% RH (pH2O ) 9.6 Torr) where the changes in the morphology of the particle became evident, as seen in the “softening” of the edges of the particle (image b). This morphology change is an indication of bulk water uptake on the particle. Further hygroscopic growth of the particle was monitored over a 35-min time period, and the changes observed can be seen in images c-f. It is evident from these images that as the particle takes up more water and begins to deliquesce, the particle morphology changes. In particular, the particle becomes more spherical in shape as it undergoes deliquescence to form an aqueous droplet. However, even after 35 min at 75.0 ( 0.8% RH a solid NaCl core is still present. Increasing the water pressure by another 0.1 Torr (pH2O (46) Hering, S. V.; Lawson, D. R.; Allegrini, I.; Febo, A.; Perrino, C.; Possanzini, M.; J. E. Sickles, I.; Anlauf, K. G.; Wiebe, A.; Appel, B. R.; John, W.; Ondo, J.; Wall, S.; Braman, R. S.; Sutton, R.; Cass, G. R.; Solomon, P. A.; Eatough, D. J.; Eatough, N. L.; Ellis, E. C.; Grosjean, D.; Hicks, B. B.; Womack, J. D.; Horrocks, J.; Knapp, K. T.; Ellestad, T. G.; Paur, R. J.; Mitchell, W. J.; Pleasant, M.; Peake, E.; MacLean, A.; Pierson, W. R.; Brachaczek, W.; Schiff, H. I.; Mackay, G. I.; Spicer, C. W.; Stedman, D. H.; Winer, A. M.; Biermann, H. W.; Tuazon, E. C. Atmos. Environ. 1988, 22, 1519-1539.

Figure 2. GSE images of a NaCl particle undergoing deliquescence: (a) NaCl particle before deliquescence at 70.3% RH and at 75.0 ( 0.8% RH (T ) 288 ( 1 K), for (b) 9, (c) 20, (d) 25, (e) 30, and (f) 35 min.

) 9.7 Torr; 75.8 ( 0.8% RH) leads to instant droplet formation followed by its very fast hygroscopic growth. By convention, the relative humidity of this fast transition is taken as the deliquescence relative humidity (DRH). It is interesting to see in Figure 2 that over longer times the spherical droplet character changes as the aqueous droplet begins to wet the TEM substrate as shown in image f. A recent study43 has shown that although the DRH determined with ESEM are consistent with other measurements using aerosolized particles, the efflorescence relative humidity (liquid-to-solid phase transition as a function of decreasing relative humidity) can be significantly higher than that found for suspended particles. Wetting of the substrate and subsequent interaction of the liquid with the substrate that supports the particles can alter the relative humidity at which efflorescence occurs. Phase Transitions and Morphology Changes of Individual Particles of Atmospheric Relevance Upon Exposure to Nitric Acid. Comparison of particle images as they undergo physical and chemical transformations due to exposure to trace atmospheric gases can potentially assist in elucidating reaction mechanisms for different types of aerosols present in the atmosphere. Secondary electron (SE) images shown in Figure 3 demonstrate morphology changes observed for SiO2, CaCO3, NaCl, and sea salt following reaction with nitric acid vapor. Images of each of these particles prior to reaction are shown on the left, and images of the particles after exposure to nitric acid are shown on the right. Image a shows a SiO2 particle before and after exposure to high nitric acid pressures (∼1 Torr) for 20 min at 25% RH. For SiO2 particles, no change in particle morphology is observed even after the unrealistically high pressure, a millionfold higher than that found in the atmosphere, used here. In contrast, substantial changes in particle morphology are clearly seen for CaCO3 particles after they have been exposed to trace amounts of gas-phase nitric acid. Image b in Figure 3 shows these changes. It can be seen that the crystalline solid particles Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

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Figure 3. SE images of single particles before (left) and after (right) reaction with gaseous nitric acid and water vapor: (a) SiO2 before and after exposure to 1 Torr HNO3 and ∼25% RH for 20 min, (b) CaCO3 before and after exposure to 20 ( 1 µTorr HNO3 and 41 ( 1% RH for 4 h, (c) NaCl before and after exposure to 20 ( 1 µTorr HNO3 and 17 ( 1% RH for 5 h, and (d) sea salt before and after exposure to 20 ( 1 µTorr HNO3 and 17 ( 1% RH for 5 h.

of calcium carbonate become enlarged and spherical in shape after exposure to nitric acid at 41% RH for 2 h; the coalescence of particles in the lower right-hand corner of the image is also observed. These changes in particle morphology are indicative of the formation of the deliquesced calcium nitrate product according to reaction R1. The mechanism of this reaction has been discussed in detail in a previous publication.47 The process occurs through a two-step mechanism. In the first step, calcium carbonate reacts with nitric acid to form calcium nitrate on the particle surface. In the second step, calcium nitrate deliquesces (DRH ∼12%) and thus creates a liquid coating of the calcium carbonate core. The liquid coating enhances further uptake of HNO3 and also provides unrestricted transport of dissolved NO3- ions toward the unreacted core of CaCO3 together with efficient removal of the reaction products H2O and CO2 from the reaction zone. As a result, even at low relative humidity conditions (17% RH), the reaction is not surface limited and continues until there is full consumption of the available CaCO3 and quantitative formation of Ca(NO3)2.47 It is important to note that in the experiments48 (47) Krueger, B. J.; Grassian, V. H.; Laskin, A.; Cowin, J. P. Geophys. Res. Lett. 2003, 30; doi:10.1029/2002GL016563.

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conducted under very low relative humidity conditions (RH