Real-Time Monitoring of the Surface and Total Composition of Aerosol Particles

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Real-Time Monitoring of the Surface and Total Composition of Aerosol Particles Peter G. Carson, Murray V. Johnston & Anthony S. Wexler To cite this article: Peter G. Carson, Murray V. Johnston & Anthony S. Wexler (1997) Real- Time Monitoring of the Surface and Total Composition of Aerosol Particles, Aerosol Science and Technology, 26:4, , DOI: / To link to this article: Published online: 13 Jun Submit your article to this journal Article views: 151 View related articles Citing articles: 28 View citing articles Full Terms & Conditions of access and use can be found at

2 Real-Time Monitoring of the Surface and Total Composition of Aerosol Particles Peter G. Carson, Murray I/. Johnston*, and Anthony S. Wexler DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY, UNIVFRSITY OF DELAWARE, NEWARK, DE (P.G.c.; M.v.J.) DEPARTMENT OF MECHANICAL ENGINEERING, UhIVERSITY OP DELAWARE, NEWARK, DE (A.s.w.) ABSTRACT. Laser desorption ionization is used to characterize the products of an aerosol reaction in real time. Sodium chloride aerosols are mixed with vapor phase ammonia and nitric acid to produce an ammonium nitrate surface layer. The product aerosols are sampled directly into a mass spectrometer where the surface and total compositions are distinguished by laser desorption ionization. With a low laser irradiance, only material near the particle surface is desorbed and ionized. In this case, the mass spectra are independent of the amount of ammonium nitrate deposited, and simply indicate a nitrate-enriched surface layer. With a higher laser irradiance, both the surface layer and core are ablated. The relative intensities of ions originating from the surface layer and core reflect the total composition. That is, the relative intensities of ions produced from the ammonium nitrate surface layer increase with increasing surface layer thickness. The laser irradiance dependences of the mass spectra allow the surface and total composition to be qualitatively distinguished, but particle-to-particle variations of the relative signal intensities preclude a quantitative measurement of the amount of ammonium nitrate deposited. AEROSOL SCIENCE AND TECHNOLOGY 26: (1997) American Association for Aerosol Research INTRODUCTION Approximately one-two thirds of the mass of urban and regional particulates is of secondary origin. This secondary fraction can appear in particulate form either by condensation on preexisting particles or by nucleation of supersaturated vapors to form new particles. Some of these secondary compounds may also reevaporate in response to composition, temperature, or rel- *To whom correspondence should bc addressed ative humidity changes of the surrounding gas. The chemical composition of an aerosol particle reflects these processes (Graedel et al., 1985). In particular, the core composition can indicate the origin of a particle and the subsequent transformations it has undergone, while the surface composition can indicate recent chemical transformations and future reactivity. Several methods have been used to perform chemical analysis of individual aerosol particles (Jambers, 1995; Spurny, 1986). Some, for example electron probe X-ray Aerosol Science and Tcchnology 26: (1997) O 1997 American Association for Aerosol Research Published by Elsevier Science Inc.

3 292 P. G. Carson et al. Aerosol Science and Technology microanalysis (EPXMA), scanning electron microscopy with wavelength or energy dispersive X-ray analysis (SEM-WDX, SEM-EDX), and particle-induced X-ray emission (PIXE), give the total composition of a particle, while others, for example secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS), give the surface composition. These methods can be used to study particle-to-particle variations in composition, but they are also subject to poor temporal resolution and sampling artifacts since they must be performed off-line. The development of a method which can provide either surface or total composition of individual particles in real time would greatly aid our understanding of the source and fate of atmospheric aerosols. Recently, laser desorption ionization has been adapted for on-line analysis of single aerosol particles (Johnston and Wexler, 1995). The aerosol is drawn directly into the source region of a mass spectrometer where individual particles are ablated and ionized by an ultraviolet laser pulse. The resulting ions are mass analyzed, and a complete mass spectrum is recorded for each particle. This method has been used to characterize ambient aerosols (Noble and Prather, 1996; Hinz et al., 1996; Murphy and Thompson, 1995) to study multicomponent crystallization in aerosols (Ge et al., 1996), to quantitate inorganic ions (Mansoori et al., 1994) and peptides (Mansoori et al., 1996) in single microdroplets, and to speciate chromium (Neubauer et al., 1995) and sulfur (Neubauer et al., 1996) in single particles. In these experiments, the desorption ionization process is similar to that in laser microprobe mass spectrometry (LAMMS) which has been used extensively for off-line analysis of bulk and particulate matter. An important feature of LAMMS is the ability to distinguish surface-enriched species from the bulk composition. Wouters et al. (1988) showed that surface adsorbed lead could be distinguished from homogeneously coprecipitated lead in calcite particles. Bruynseels and Van Grieken (1986) showed that car- bon bilayers having a C enriched surface layer gave enhanced levels of in the mass spectra. In each case, a low laser irradiance enhanced the signal intensity of surface-enriched species in the mass spectra, while a high laser irradiance gave mass spectra that reflected the bulk composition. Presumably, a low laser irradiance desorbs only a small amount of material near the surface, while a high laser irradiance ablates a large fraction of both the surface and core. Other results consistent with enhanced detection of surface-enriched species by LAMMS include organic and inorganic coatings on asbestos fibers, minerals, and sodium chloride particles (De Waele et al., 1983a; De Waele et al., 1983b; Bruynseels and Van Grieken, 1985; De Waele et al., 1987). In this work, we use on-line laser desorption ionization to characterize the products of an aerosol reaction in real time. Sodium chloride aerosols are mixed with vapor phase ammonia and nitric acid to produce an ammonium nitrate surface layer. The product aerosols are sampled directly into a mass spectrometer where the surface and total compositions are distinguished by laser desorption ionization. EXPERIMENTAL SECTION Aerosol Reactor Monodisperse aerosols were produced with a vibrating orifice aerosol generator (Model 2450, TSI, St. Paul, MN). Primary aerosol droplets were formed from a solution of 0.02 M sodium chloride in 1 : 1 ethanol: water. A compressed dry air flow of 60 L/min suspended and transferred the droplets to a drying tube where 3.5 pm diameter dry aerosoi particles were produced. The dry aerosol was periodically sent into an aerodynamic particle sizer (Model 3310, TSI, Inc., St. Paul, MN) to monitor the particle size distribution. The aerosol reactor for this work, shown in Fig. 1, consisted of a mixing cell, a tygon tube of variable length connecting the mixing cell to the mass spectrometer, and a 13

4 Aerosol Science and Technology Composition of Individual Aerosol Particles 293 NaCl Particles To Mass Spectrometer (1.4 Llmin) FIGURE 1. Experimental setup for aerosol reaction studies. fixed volume stainless steel tube attached to the inlet. Due to the high volume of airflow from the aerosol generator, a diverter was inserted between the mixing cell and tygon tube so that the airflow through the tube was equivalent to the 1.4 L/min pumping capacity of the mass spectrometer inlet. The total volume of the tygon and stainless steel tubes was 2.8 L, which corresponded to a reaction time of approximately 2 min. The reaction time can be compared to the calculated diffusionlimited time constant for equilibration, 7, (Wexler and Seinfeld, 1990): where d, is the particle diameter (3.5 pm), D is the average gas phase diffusivi '7 ammonia and nitric acid (ca. 0.1 cm /s), and N is the number density of particles (80 ~m-~). Assuming the values given in parentheses, the equilibration time is estimated to be 56 s, or roughly one half of the total reaction time. Thus, the ammonia and nitric acid vapors were near equilibrium with solid ammonium nitrate by the time the particles were analyzed. Nitric acid vapor was generated by bubbling 6 L/min (regulated to +lo%) of dry air through a 7.0 M HNO, solution to yield a partial pressure of 30 ppm. After mixing with the 60 L/min airflow from the aerosol generator, the nitric acid partial pressure in the reactor was 3.0 ppm. Ammonia was generated by diluting the gas from either a 30 ppm or a 300 ppm ammonia cylinder with the 60 L/min air flow from the aerosol generator. The flow rate from the ammonia cylinder was varied between and 0.6 L/min to obtain partial pressures between and 3.0 ppm. The ammonia flow was regulated to IfI 50% for L/min for ppm and + 15% for all other partial pressures. No attempt was made to regulate the relative humidity in the reactor. Based upon the liquid flow rate through the aerosol generator, the relative humidity of the surrounding air was estimated to be less than 4%. The air flowing through the nitric acid solution was assumed to be saturated. When these two air flows were combined in the reactor, the final relative humidity was 13% or less. Mass Spectrometer The mass spectrometer used in this work has already been described in detail (Carson et al., 1995). The main components

5 294 P. G. Carson et al. Aerosol Science and Technology are an aerosol inlet, a particle detection system based upon light scattering, an excimer laser for laser ablation, and a reflectron time-of-flight mass analyzer. The aerosol inlet consisted of three coaligned skimmers and two differentially pumped regions. Mallina et al. (1996) have shown that water condensation is the main artifact associated with sampling inlets that transmit particles into a low-pressure region. Water condensation can be substantial even at low relative humidities-for example, Mallina's calculations show that 100 nm particles can more than double in size when sampled through a capillary inlet at 5% relative humidity. For surface composition measurements, even a 1% growth of the particle diameter during sampling of a micronsize particle would be detrimental since the condensation layer would be several tens of nanometers deep. Fortunately, the combination of an orifice inlet design and a low relative humidity were sufficient to keep the (calculated) condensation laver to a fraction of a nanometer in the experiments described here. Aerosol particles transmitted through the inlet passed through a continuous heliumcadmium laser beam (Model M, Omnichrome, Chino, CA) where they were detected by light scattering. The scatter pulse from each particle triggered an excimer laser (MPB Technologies, Dorval, Quebec) operating at 248 nm which ablated the particle in-flight in the source region of a reflectron time-of-flight mass spectrometer (R. M. Jordan Co., Grass Valley, CA). Each mass spectrum corresponded to a single particle ablated by a single laser pulse. A variable beam attenuator (Reynard Corporation, San Clemente, CA) was used to adjust the excimer laser pulse energy. The laser pulse energy was monitored periodically by inserting a joulemeter (Model DGX-3A-P-RP, Ophir Optronics Ltd., Peabody, MA) into the laser beam path. Although shot-to-shot variations of the laser pulse energy are normally less than 10% when the excimer laser is fired at a constant rate, the sporadic triggers initiated by particle detection in the experiments de- scribed here produced shot-to-shot variations up to If20%. Since the excitation laser was focused to a beam diameter of 330 pm in the mass spectrometer, a laser pulse energy of 1 mj corresponded to an irradiance of 1.2 J/cm2. The laser irradiances cited below were calculated from the mean laser pulse energy of 20 laser shots. RlESULTS AND DISCUSSION When vapor phase ammonia and nitric acid are exposed to dry sodium chloride particles, a surface layer of ammonium nitrate forms. Changes in the mass spectra that accompany this process are shown in Fig. 2. Each spectrum in Fig. 2 is the average of 20 single-particle spectra. Averaging reduces digitization noise and the effects of shotto-shot variations in the laser pulse energy and particle-to-particle variations in morphology (Mansoori et al., 1994). Figure 2a shows the negative ion spectrum of 3.5 pm diameter sodium chloride particles that have not been exposed to the reactive gases. The composition of these particles is indicated by intense C1-, NaCI-, and (NaC1)CI- ions. Figure 2b shows the change that occurs when these particles are exposed to ppm ammonia and 3.0 ppm nitric acid. Growth of an ammonium nitrate coating is indicated by the presence of ions such as 0-, OH-, NO;, and NO;. These ions are observed in the mass spectra of pure ammonium nitrate particles. Figure 2b also shows ions from the sodium chloride core. In addition, "mixed" ions such as NaCI.NO;, Na(N02);, NaCl. NO;, and NaNO,.NO; are observed. These ions may arise directly from material that was located near the surface-core interface in the particle or they may be produced by ion-molecule reactions of ablated surface and core material in the plume (Neubauer et a]., 1995). Figure 2c shows the spectrum that is obtained when the sodium chloride particles are exposed to 1.0 ppm ammonia and 3.0 ppm nitric acid. Condensation theory predicts that the higher ammonia partial pressure in Fig. 2c

6 Aerosol Science and Technology Composition of Individual Aerosol Particles 295 FIGURE 2. Negative ion spectra of 3.5 pm diameter NaCI particles: a) not exposed to ammonia and nitric acid, b) exposed to ppm ammonia and 3.0 ppm nitric acid, c) exposed to 1.0 ppm ammonia and 2.7 ppm nitric acid. Laser irradiance 0.3 ~/cm'. Each spectrum is the average of 20 single-particle spectra. should permit a thicker surface layer to be formed, and this is suggested by the larger intensities of ions associated with the nitrate layer relative to ions associated with the chloride core. Previous LAMMS studies have shown that the ablation laser irradiance can be used to differentiate between the surface and bulk composition of a sample (Wouters et ai., 1988; Bruynseels and Van Grieken, 1986). A low laser irradiance is thought to desorb and ionize material only near the surface, while a high laser irradiance is thought to ablate a greater fraction of the sample. Figure 3 shows the laser irradiance dependence of the mass spectra of sodium chloride particles exposed to ppm ammonia and 3.0 ppm nitric acid. The spectrum in Fig. 3a was taken with an irradiance just above the threshold for ion formation, 0.2 J/cm2. The dominant ions correspond to the ammonium nitrate surface layer (0-, NO;, and NO;). Ions corresponding just to the sodium chloride core (Cl-, NaCl-, and (NaC1)ClV) are very weak, while " mixed ions7' are fairly intense. When the laser irradiance is increased to 0.5 ~/cm~ and above (Figs. 3b-d), ions originating from the sodium chloride core (C1-, NaCl-, and (NaC1)CI-) become dominant, while "mixed ions" and ions corresponding to the ammonium nitrate surface layer become weak. The high relative intensities of ions from the sodium chloride core are

7 296 P. G. Carson et al, Aerosol Science and Technology FIGURE 3. Negative ion spectra of 3.5 km diameter NaCl particles exposed to ppm ammonia and 3.0 pprn nitric acid. The laser irradiance was a) 0.2, b) 0.5, c) 1.0, and d) 1.6 ~ /cm~. Each spectrum is the average of 20 single-particle spectra. consistent with a thin surface layer of ammonium nitrate. Figure 4 shows the laser irradiance dependence of the mass spectra of sodium chloride particles exposed to 3.0 ppm ammonia and 3.0 ppm nitric acid. Relative to the ppm ammonia experiments, these particles should have a much thicker ammonium nitrate coating, and this is indicated in the mass spectra. In Fig. 4a, a near threshold irradiance of 0.2 J/cm2 gives a similar spectrum to Fig. 3a. The dominant ions correspond to the ammonium nitrate surface layer, while ions corresponding to the sodium chloride core are very weak. The similarity of Figs. 3a and 4a is not surprising since the surface composition is essentially the same. However, differences between the two types of particles are observed as the laser irradiance is increased. In Figs. 4b-d, the signal intensities of ions from the sodium chloride core increase with increasing laser irradiance, but the effect is not as pronounced as in Fig. 3. Even at an irradiance of 1.5 J/cm2, ions corresponding to the ammonium nitrate surface layer remain intense. The larger signal intensities of these ions reflect the larger amount of nitrate relative to chioride in these particles.

8 Aerosol Science and Technology Composition of Individual Aerosol Particles 297 NO; FIGURE 4. Negative ion spectra of 3.5 +m diameter NaCl particles exposed to 3.0 ppm ammonia and 3.0 ppm nitric acid. The laser irradiance was a) 0.2. b) 0.5. c) 1.0, and d) 1.6 ~Icm'. Each spectrum is the average of 20 single-particle spectra. The trends in Figs. 3 and 4 are summarized in Fig. 5 where the peak area ratio of the NO; ion (m/z 46) to the C1- ion (m/z 35) is plotted versus laser irradiance for particles exposed to 3.0 ppm nitric acid and various partial pressures of ammonia. With a low laser irradiance, primarily the surface layer is sampled, and the NO;/Clpeak area ratio is relatively invariant with ammonia partial pressure. With a high laser irradiance, a greater fraction of the particle is ablated, and the NO;/Cl- peak area ratio increases with increasing ammonia partial pressure. These dependences allow the surface and core composition to be qualitatively distinguished-nitrate on the surface, chloride in the core, and an increasing thickness of the nitrate layer with an increasing partial pressure of ammonia. How thick is the ammonium nitrate surface layer in these particles? Quantitative determination of the relative amounts of nitrate and chloride is not possible by laser desorption ionization owing to large particle-to-particle variations of the relative peak areas. This variation is indicated by the error bars in Fig. 5. The data points and error bars correspond to the mean relative peak area and standard deviation determined from the spectra of 20 individual

9 298 P. G. Carson et a!. Aerosol Science and Technology Laser lrradiance(~/cm*). FIGURE 5. Laser irradiance dependence of the NO, (m li 46) to C1 (rn li 35) peak area ratio for several ammonia partial pressures ( r = 3.0 ppm, A = 1.0 ppm, = 0.5 ppm, = ppm). The data points and error bars correspond to the mean peak area ratio and standard deviation of 20 single-particle spectra. Data points at each laser irradiance are offset slightly along the abscissa for clarity. particles. For each ammonia partial pressure, the aerosol size distribution measured with an aerodynamic particle sizer is the same before and after exposure to the reactive gases. This confirms that the increase in particle size due to growth of an ammonium nitrate surface layer is less than the resolution of the particle sizer, or a 200 nm increase in the diameter of a 3.5 pm particle. Therefore, the trends in Figs. 3-5 arise from changes in particle composition on this scale or smaller. The amount of ammonium nitrate pro- duced per particle can be estimated from the ammonia partial pressure. If we assume that all of the ammonia reacts and that a stoichiometric amount of ammonium nitrate is formed on the particle surface, then the average thickness of the ammonium nitrate surface layer on a 3.5 pm diameter particle should range from 2 nm for an ammonia partial pressure of ppm to 1.0 pm for an ammonia partial pressure of 3.0 ppm. Since no change in the particle size is observed with the aerodynamic particle sizer, not all of the ammonium nitrate is

10 Aerosol Science and Technology 264 April 1997 Composition of Individual Aerosol Particles 299 forming on particles, at least with high ammonia partial pressures. The time constant for ammonium nitrate condensation on the apparatus walls (7- R ~/D) is about 20 s or about three times smaller than that for condensation on the particles. Thus, only about a quarter of the condensation occurs on the particles. The negative ion mode of laser desorption ionization only gives information on the relative amount of nitrate to chloride in a particle. It is possible that some of the excess nitric acid vapor reacts to convert sodium chloride to sodium nitrate. In this case, the amount of nitrate in the particle increases, but the particle size remains constant. Although this process cannot be ruled out, it is not expected. At a relative humidity of 13%, the sodium chloride particles exist as a solid, and the conversion of sodium chloride to sodium nitrate is limited by the low diffusion rate of nitric acid through the solid. In principle, the positive ion mode can overcome this ambiguity by giving the surface/core composition based upon the relative peak areas of NH; and Naf in the mass spectra. However, the poorer detection sensitivity of NH,f relative to Na' makes it difficult to detect trace amounts of the ammonium ion in a particle by laser desorption ionization. CONCLUSION The surface and total composition of particles can be monitored in real time by laser desorption ionization. A low laser irradiance enhances the signal intensity of surface-enriched species, while a high laser irradiance indicates the total composition of a particle. Particle-to-particle variations in the relative and absolute ion signal intensities are substantial. Although these variations preclude quantitative measurements, changes in the mass spectra with laser irradiance are large enough to infer qualitative differences between surface and core compositions. Recently, there has been interest in classifying ambient particles based upon the total composition as determined by on-line laser desorption ionization (Noble and Prather, 1996; Hinz et al., 1996; Murphy and Thomson, 1995). By manipulating the laser irradiance, it may be possible to classify particles based upon the surface composition as well. This research was supported by grants from the National Science Foundation MTM ) and the Encironmental Protection Agency (RH ). References Bruynseels, F., and Van Grieken, R. (1985). Atmos. En~iron. 19: Bruynseels, F., and Van Grieken, R. (1986). Int. J. Mass Spectrom. Ion Proc. 74: Carson, P. G., Neubauer, K. R., Johnston, M. V., and Wexler, A. S. (1995). J. Aerosol Sci De Waele, J. K., Vansant, E. F., Van Espen, P., and Adams, F. C. (1983a). Anal. Chem. 55: De Waele, J. K., Gybels, J. J., Vansant, E. F., and Adams, F. C. (1983b). Anal. Chem. 55: De Waele, J. K., Wouters, H., Van Vaeck, L., Adams, F., and Ruiz-Hitzky, E. (1987). Anal. Cfzinz. Acta 195: Ge, Z., Wexler, A. S., and Johnston, M. V. (1996). Colloid Interface Sci. in press. Graedel, T. E., Hawkins, D. T., and Claxton, L. D. (1985). Atmospheric Chemical Compounds: Sources, Occurrence and Bioassay. Academic, New York. Hinz, K.-P., Kaufmann, R., and Spengler, B. (1996). Aerosol Sci. Technol. 24: Jambers, W., De Bok, L., and Van Grieken, R. (1995). Analyst 120: Johnston, M. V., and Wexler, A. S. (1995). Anal. Chem. 67:72l A-726A. Mallina, R. V., Wexler, A. S., and Johnston, M. V. (1996). J. Aerosol Sci. in press. Mansoori, B. A., Johnston, M. V., and Wexler, A. S. (1994). Anal. Chem. 66: Mansoori, B. A., Johnston, M. V., and Wexler, A. S. (1996). Anal. Chem. 68:

11 300 P. G. Carson et al. Aerosol Science and Technology Murphy, D. M., and Thompson, D. S. (1995). Aerosol Sci. Technol. 22: Neubauer, K. R., Johnston, M. V., and Wexler, A. S. (1995). Int..I. Mass Spectrom. Ion Proc. 151: Neubauer, K. R., Sum, S. T., Johnston, M. V., and Wexler, A. S. (1996). J. Geophys. Res. 101:18,701-18,707. Noble, C. A., and Prather, K. A. (1996). Enuiron. Sci. Technol. 30: Spurny, K. R., ed. (1986). Physical and Chemical Characterization of Individual Airborne Particles. Ellis Honvood Limited, Chichester, West Sussex, England. Wexler, A. S., and Seinfeld, J. H. (1990). Atmos. Emiron. 24A: Wouters, L. C., Van Grieken, R. E., Linton, R. W., and Bauer, C. F. (1988). Anal. Chem. 60: Received June 10, 1996; revised September 11, 1996