humidity effects on the mass spectra of single aerosol

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Atmospheric Environment Vol. 32, No. 14/15, pp. 2521—2529, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00005–3 1352—2310/98 $19.00#0.00

HUMIDITY EFFECTS ON THE MASS SPECTRA OF SINGLE AEROSOL PARTICLES KENNETH R. NEUBAUER,s MURRAY V. JOHNSTON*- and ANTHONY S. WEXLER‡ s Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, U.S.A. and ‡ Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, U.S.A. (First received 22 July 1997; accepted 15 November 1997. Published June 1998) Abstract—On-line laser desorption ionization mass spectrometry has developed into a widely used method for chemical characterization of individual aerosol particles. In the present study, the spectra of laboratorygenerated particles were obtained as a function of relative humidity to elucidate potential artifacts associated with ambient measurements. Several anionic electrolytes typically found in marine aerosols were studied, including chloride, nitrate, sulfate and methanesulfonate. In most experiments, negative ion mass spectra were assessed. Particles were conditioned at humidities both above and below the respective deliquescence-relative humidities (DRH). The mass spectra of the conditioned particles could be affected by relative humidity in several ways: an abrupt transition between a ‘‘wet’’- and ‘‘dry’’-particle spectrum over a narrow humidity range, a gradual transition over a broad humidity range, or virtually no change with humidity. Particulate phase water influenced both the distribution of ions observed and their relative intensities. In most instances, water remained associated with the particle well below the DRH, yielding spectra similar to those of aqueous droplets. Particulate phase water below the DRH appeared to be the result of surface adsorption under ambient conditions rather than condensational growth in the mass spectrometer inlet. Signal quenching in multicomponent aerosols was more prevalent in the spectra of aqueous droplets than dry particles. For mixed composition droplets, only nitrate and chloride could be unambiguously identified from intense peaks in the spectra, while signals due to ions from methanesulfonate and sulfate were suppressed. In contrast, all components could be detected at low relative humidities. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Single-particle analysis, laser ablation mass spectrometry, marine aerosols, deliquescence.

INTRODUCTION

Since its inception several years ago (Sinha, 1984; McKeown et al., 1991), on-line laser desorption/ionization (LDI) mass spectrometry has developed into a widely used method for chemical characterization of individual aerosol particles. (For a review, see Johnston and Wexler, 1995). In particular, several groups have combined the LDI experiment with an optical measurement of particle size to determine the chemical composition of size-selected and/or size-resolved ambient particles (Murphy and Thomson, 1995; Hinz et al., 1996; Noble and Prather, 1996). In ambient measurements the chemical composition of a particle is inferred from the distribution of ions in its LDI mass spectrum. Spectral interpretation is based upon comparisons with spectra of laborat-

* Author to whom correspondence should be addressed. Tel.: 302 831 8014; Fax: 302 831 6335; E-mail: [email protected].

ory-generated particles of known composition that have been acquired under similar experimental conditions (laser fluence and wavelength) as the ambient measurements. However, because the laboratorygenerated particles are usually prepared in a low humidity environment (relative humidity (10%), the resulting spectra may not reflect those of ambient particles with similar compositions sampled from humid air. Since the ambient relative humidity determines the water content of atmospheric particles, it is reasonable to expect that the corresponding LDI mass spectra obtained in field measurements may also be affected. In this study the spectra of laboratorygenerated particles are obtained as a function of relative humidity to elucidate potential artifacts associated with ambient measurements of marine aerosols. Several researchers have observed peaks associated with water in the LDI spectra of individual particles (Murphy and Thomson, 1995; Kievet et al., 1992). In a previous study, we compared the LDI spectra of aqueous and dry aerosol particles (Neubauer et al., 1997). Aqueous particles exhibited higher threshold

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K. R. NEUBAUER et al.

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laser fluences, lower total ion currents, and different ion distributions relative to their dry particle counterparts. In the work reported here, we expand these studies to include the effects of relative humidity and laser wavelength on the LDI spectra. While our earlier work was performed entirely with a desorption ionization wavelength of 248 nm, the work described here is performed primarily with 193 nm radiation. The advantage of using this shorter wavelength is that the threshold laser fluences necessary for ion formation from different compounds vary much less than for wavelengths in the mid ultraviolet between 248 and 308 nm (Thomson and Murphy, 1993; Thomson et al., 1997). The significance of this observation is that if longer wavelengths are used to analyze ambient particles, a sampling bias may be introduced since particles containing certain compounds may not be detected at a given laser fluence. In contrast, 193 nm radiation allows most compounds typically present in ambient aerosols to be detected with lower laser fluences and with significantly less variation in sensitivities than 248 or 308 nm radiation (Thomson et al., 1997). The only exception is that pure sulfuric acid droplets are difficult to ablate and ionize at 193 nm. However, pure sulfuric acid droplets are not expected to exist in polluted air (Kerminen et al., 1997, Middlebrook et al., 1997) and, therefore, should not pose a severe restriction.

humidity system used in this work is given elsewhere (Neubauer et al., 1997). After sampling into the mass spectrometer, the particles were exposed to a vacuum for about one millisecond before undergoing laser desorption/ ionization with an excimer laser operating at 193 nm. Each figure shows the average of ten single-particle spectra. Averaging was performed to minimize the effects of shot-to-shot variations of the laser pulse and potential particle-to-particle variations of the water content (see below). It is possible that individual particles could have obtained higher water contents owing to minor temperature and flow gradients in the humidity system. For example, transient exposure of the aerosol to a high-humidity environment (for example, an anomalously low temperature zone or a region where the humid and dry air flows are not yet completely mixed) could cause particles to gain water by deliquescence, and hysterisis associated with the reverse process, efflorescence, could inhibit re-equilibration as the particles flow through the (lower) steady-state humidity environment. However, transient variations of the relative humidity with the apparatus used in this study were expected to be on the order of 5% or less which was much smaller than range over which anomalous variations of the particulate water content were observed, typically 10% or more. To minimize the impact of transient variations of the relative humidity on the results, experiments were performed at 5% increments of the relative humidity and the mass spectra were averaged over several particles. Furthermore, the results were reproducible from day-to-day suggesting that random variations of the laboratory environment were insignificant. All solutes were used as received without further purification: methanesulfonic acid (Lancaster Synthesis), sodium chloride (Mallinckrodt), ammonium chloride (Fisher), ammonium nitrate (Aldrich) and ammonium sulfate (Aldrich).

RESULTS AND DISCUSSION EXPERIMENTAL

The experimental setup and procedure have been previously described in detail (Carson et al., 1995; Neubauer et al., 1997), so only a brief description follows. Dry particles of a known size and composition were generated from solutions containing the desired analyte in a 1 : 1 ethanol : water mixture. These solutions were then passed through a vibrating orifice aerosol generator (Model 3450, TSI Inc., St. Paul MN) which produced monodisperse droplets composed of solvent and the dissolved solute. The droplets were exposed to a stream of dry air (relative humidity less than 7%) for approximately four seconds which evaporated the solvent leaving behind dry, monodisperse solute particles. The aerosol was then introduced to a humidity chamber comprised of a length of clear PVC tubing and a length of stainless-steel tubing. Saturated air, produced by bubbling dry air through warm water, was combined with the particle-laden air. The desired humidity was achieved by adjusting the relative flow rates of the two air streams. In this fashion, the relative humidity could be varied between less than 7% and 83%, as monitored with a precision humidity meter (PHM-1A, WMS 1/98, Ohmic Instrument Co., Easton, MD). The particles remained in contact with the humid air for approximately one minute before entering the vacuum. This time was more than sufficient for particles to come into equilibrium with the water vapor (e.g. Tang et al., 1977; El Golli et al., 1973). It is important to note that the water content of the aerosol was manipulated by exposing initially dry particles to humid air so that the effect of hysterisis associated with the deliquescence—efflorescence cycle was minimized. Near the inlet to the mass spectrometer, the total flow from the humidity system was split to the mass spectrometer and an aerodynamic particle sizer (Model 3310/33B, TSI, Inc., St. Paul, MN). More information on the

Several electrolytes present in marine aerosols were studied including chloride, nitrate, sulfate and methanesulfonate. Particles were conditioned at several different humidities, both above and below the respective deliquescent relative humidities (DRH). The DRH is the humidity where bulk equilibrium thermodynamics predicts that a dry particle undergoes a spontaneous transformation to an aqueous droplet. The onset of deliquescence can be monitored by observing the change in particle diameter with relative humidity since, at the DRH, the aerosol diameter increases dramatically. For example, Table 1 shows Table 1. Change in sodium chloride particle diameter with relative humidity Relative humidity! (%) 80 75 70 65 60 50 '7

Mean particle diameter! (km) 5.4 5.1 3.8 3.9 3.8 3.6 3.5

! Measured with a precision humidity meter. Estimated uncertainty of the relative humidity in the experimental apparatus was $2%. " Measured to $0.3 km with an aerodynamic particle sizer.

Humidity effects on single aerosol particles

the mean diameter of sodium chloride particles conditioned at different relative humidities in our system. The particle diameter changes abruptly between 70 and 75% relative humidity and is consistent with the known DRH of 75% (Young, 1967; Winston and Bates, 1960). Similar changes at the DRH were found for other salt particles. The mass spectra of single aerosol particles were found to vary with humidity in one of three ways, depending upon the salt being analyzed: (1) a sharp transition between a ‘‘wet’’-particle spectrum and a ‘‘dry’’-particle spectrum over a narrow humidity range; (2) a gradual spectral transformation over a broad humidity range; and (3) no spectral change with humidity. Although both positive and negative ion spectra were considered, most studies were directed toward the characterization of negative ion spectra since anionic electrolytes are readily identified. Predominant negative ions include Cl~ (m/z 35, 37) for chloride, NO~ (m/z 46) and NO~ (m/z 60) for nitrate, SO~ (m/z 2 3 4 96) and HSO~ 4 (m/z 97) for sulfate, and CH3SO~ 3 (m/z 95) for methanesulfonate. In contrast, positive ion spectra are usually dominated by alkali cations such as Na` (m/z 23) from sodium salts and K` (m/z 39, 41) from potassium contamination, and by lower-inten-

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sity ions NH` 3 (m/z 17) and NH` 4 (m/z 18) from ammonium salts. In many cases, anionic electrolytes can be inferred from positive ions such as Na2Cl` from sodium chloride, NO` from nitrate salts, and SO` from sulfate and methanesulfonate salts, but these ions are often weak and may not be as reliable indicators as the negative ions. Sodium chloride Figure 1 shows the negative ion spectra of sodium chloride particles at various humidities acquired with a laser wavelength of 193 nm and a near-threshold laser fluence of 0.09 J cm~2. Sodium chloride has a DRH of 75%, thus implying that the spectra in Fig. 1a and b (RH"80%, 75%, respectively) represent aqueous droplets. These spectra are characterized by large peaks due to Cl~ (m/z 35, 37), as well as smaller peaks corresponding to NaCl~ (m/z 58, 60) and (NaCl)Cl~ (m/z 93—97). In Fig. 1c the humidity is 70%, meaning that a dry particle should be present. However, this spectrum is similar to those of aqueous droplets. When the humidity is 65% (Fig. 1d), the spectrum changes dramatically, with the (NaCl)Cl~ peaks becoming more intense than the Cl~ peaks, and new peaks due to Na~ 2 (m/z 46) and (NaCl)2Cl~ (m/z 151—157) appear. When the humid air is removed

Fig. 1. Negative ion spectra of sodium chloride particles taken at different relative humidities: (a) 80%; (b) 75%; (c) 70%; (d) 65%; (e) (7%. Each spectrum is the average of ten particles. The laser wavelength was 193 nm and the laser fluence was 0.09 J cm~2.

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(Fig. 1e), the spectra are essentially the same as at a humidity of 65%. Thus, the mass spectra show an abrupt water content transition between 65 and 70% relative humidity, or 5—10% below the DRH. In contrast, particle-size measurements (Table 1) show a sharp change in particle diameter, which indicates the onset of deliquescence, between 70% and 75% relative humidity which corresponds to the DRH of sodium chloride. Although the spectra suggest the presence of a ‘‘wet’’ particle below the DRH, particle size measurements indicate that a ‘‘dry’’ particle is present. Sodium chloride aerosols were also studied at 248 nm for comparison. The laser fluence used in these experiments, 1.7 J cm~2, was higher than that used with 193 nm radiation and reflects the lower ablation efficiency at 248 nm. As with 193 nm radiation, the spectra taken at 248 nm exhibit a sharp transition between ‘‘wet’’ and ‘‘dry’’ particle spectra at a humidity below the DRH. With relative humidities at or above 70%, the spectra resemble those of aqueous sodium chloride droplets: narrow peak widths, the absence of a (NaCl)Cl~ peak, and the presence of O~ and OH~ peaks. At humidities below 70% the (NaCl)Cl~ peak is observed and the peak widths are broader. As discussed in our previous work (Neubauer et al., 1997), particulate phase water causes translational cooling in the desorption plume, thus leading to narrower peaks in the mass spectra. This effect is especially pronounced in the 248 nm spectra. The peak broadening in 248 nm spectra obtained with a humidity of 65% or lower suggests that significant amounts of water are no longer associated with the particles. Thus, both the 193 and 248 nm spectra give evidence of particulate phase water below the DRH. Positive ion spectra of sodium chloride aerosols were also obtained with 248 nm radiation. Spectra acquired at relative humidities near or above the DRH clearly indicate the presence of water by peaks corresponding to Na(H2O)` / (n"1—4). However, the spectra change abruptly between 70 and 75% relative humidity. At a humidity of 70% or lower, the spectra are characteristic of ‘‘dry’’ sodium chloride particles in that they show a strong peak corresponding to Na(NaCl)` (m/z 81, 83) which does not appear in the spectra of aqueous droplets. However, peak broadening is not as severe for particles conditioned between 65 and 70% relative humidity as in the spectra of ‘‘dry’’ particles conditioned at much lower humidities. When particles are conditioned at a humidity of 60% or below, both the distribution of ions and peak broadening in the spectra are identical to those obtained from particles conditioned at a very low humidities. Thus, positive ion spectra of sodium chloride particles also indicate particulate phase water well below the DRH. The results also show that information on anionic electrolytes may be lost from positive ion spectra. Chloride is indicated by the ion Na2Cl`, but this ion is observed only in the spectra of dry particles, not aqueous droplets. Additional experi-

ments involving the effect of particulate-phase water on positive ion spectra are described elsewhere (Neubauer et al., 1997; Ge et al., 1998). The presence of water in particles conditioned below the DRH may arise from two sources: condensational growth during sampling into the mass spectrometer vacuum or adsorption of water on the particle surface at ambient pressure. Condensational growth in the mass spectrometer inlet has been studied by Mallina et al. (1997). The amount of water deposited depends upon the inlet shape, relative humidity, and initial particle size. For the sodium chloride particles discussed above, these calculations suggest that at most a 10 nm coating of water forms as the particle travels from ambient pressure into the vacuum. This coating is probably too small to cause the large spectral changes that are observed (Carson et al., 1997). Furthermore, the coating thickness is expected to be virtually the same at 65 and 70% relative humidity and cannot explain the dramatic spectral differences across this region. For these reasons, it is unlikely that condensational growth is a main source of particulate phase water below the DRH. Adsorbed water on particles below the DRH has been observed by infrared spectroscopy under conditions where condensational growth does not occur (Vogt and Finlayson-Pitts, 1994; Barraclough and Hall, 1974), and the LDI results discussed above support this finding. The origin of this water may be linked to defects such as cracks on the particle surface that produce small air capillaries in which condensation of water vapor proceeds at a relative humidity lower than the DRH (Pruppacher and Klett, 1980). Since the meniscus of water in the capillary is concave, the Kelvin equation can be written as

A

B

2M p RH"a exp ! 8 8@! 8 R¹o r 8

where RH is the relative humidity, a8 is the water activity in the capillary, M8 is the molecular weight of water, p8@! is the surface tension, R is the universal gas constant, ¹ is the absolute temperature, o8 is the density of water, and r is the radius of curvature of the water surface in the capillary. This equation suggests that when the radius of curvature is small, water can adsorb inside a crack at an ambient relative humidity significantly below than the DRH. For the sodium chloride particles considered above, defects on the order of 30 nm diameter would be sufficient to induce adsorbed water at a relative humidity of 5—10% below the DRH. A feature of some individual particle spectra that is not reflected by the averaged spectrum in Fig. 1 is the presence of spurious peaks which also appear occasionally in the spectra of other compounds. The most prominent of these peaks correspond to C6H~ 5 (m/z 77), C6H5CO~ (m/z 121) and C H (COH)CO~ 2 6 4 2 (m/z 149) and can be attributed to plasticizers in the PVC


transfer line. In addition, several ions of the form C9H` : are observed in the positive ion spectra. Although organic contaminants are detected with high sensitivity in 193 nm spectra, they are generally not observed in 248 nm spectra. Ammonium chloride Ammonium chloride aerosols show trends similar to those of sodium chloride. Although ammonium chloride has a DRH of 78%, the transition between a ‘‘wet’’- and ‘‘dry’’- particle spectrum (193 nm, 0.09 J cm~2) occurs abruptly between 70 and 65%. At a relative humidity of 70% or above, the spectra are dominated by Cl~ and Cl(H O)~ (n"1—5) clusters. 2 n However, at a relative humidity of 65% and below, the spectra are dominated by Cl~, Cl~, HCl~, HCl~, 2 2 3 and (HCl) Cl~ ions. 2 Ammonium nitrate While sodium chloride and ammonium chloride aerosols show abrupt changes between ‘‘wet’’- and ‘‘dry’’-particle spectra, ammonium nitrate aerosols (DRH 62%) undergo a gradual transformation as shown in Fig. 2. These spectra were acquired with a near-threshold laser fluence of 0.09 J/cm2 while the humidity was varied between 70% (Fig. 2a) and

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(7% (Fig. 2e). Several humidity-dependent changes are observed. First, the total ion current decreases as the relative humidity increases. Second, some peaks decrease in intensity as the relative humidity increases (NO~ at m/z 46 and HNO ) NO~ at m/z 109) while 2 3 2 other peaks increase (NO~ at m/z 62 and 3 HNO ) NO~ at m/z 125). Third, peaks resulting from 3 3 water adducts (NO ) H O~ at m/z 64, NO ) H O~ at 2 2 3 2 m/z 80 and NO ) (H O)~ at m/z 82) gradually de2 2 2 crease in intensity as the humidity is lowered, yet are still present at a humidity of 20% (Fig. 2d). These spectra indicate the presence of adsorbed water even at very low humidities and are consistent with the work of Tang (1980) which suggests hygroscopic rather than deliquescent behavior for this compound. Methanesulfonic acid Another significant component of marine aerosols is methanesulfonic acid. The spectra of this compound (193 nm, 0.09 J cm~2), like those of ammonium nitrate, also change gradually change over a broad humidity range. Above 60% relative humidity, ions characteristic of methanesulfonic acid in dry-particle spectra reported by Neubauer et al. (1996), CH SO~ 3 3 at m/z 95, CH SO H ) HSO~ at m/z 177, and 3 3 3 CH SO H ) CH SO~ at m/z 191, are weak or 3 3 3 3

Fig. 2. Negative ion spectra of ammonium nitrate particles taken at different relative humidities: (a) 70%; (b) 50%; (c) 30%; (d) 20%; (e) (7%. Each spectrum is the average of ten particles. The laser wavelength was 193 nm and the laser fluence was 0.09 J cm~2.


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non-existent. Below 60% relative humidity, these ions become more prominent as the humidity is lowered. The total ion current also increases as the humidity is lowered, suggesting that particulate-phase water decreases the ablation efficiency. Ammonium sulfate Ammonium sulfate (DRH 80%) represents a third type of aerosol whose spectra change little with relative humidity as shown in Fig. 3. These spectra were obtained with a near-threshold laser fluence of 0.23 J cm~2. The only changes observed as the relative humidity is raised are the emergence of a HSO ) H O~ peak at m/z 115 and the disappearance 4 2 of a small SO~ peak at m/z 64. The total ion current is 2 relatively invariant with relative humidity. Ions derived from organic contaminants (CH~, C H~, 9 2 9 C H~, C H~, etc.) are especially intense at most rela3 9 4 9 tive humidities. Organic contaminants are also prevalent in the 193 nm positive and negative ion spectra of sulfuric acid particles (Middlebrook et al., 1997). An advantage of using 193 nm radiation for LDI is that the threshold fluences of ‘‘wet’’ particles are similar, ranging from 0.09 to 0.23 J cm~2 for the compounds studied here. In contrast, the threshold laser fluences with 248 nm radiation vary over a much

wider range, 0.9 to 4.4 J cm~2 for the compounds studied here. These results are consistent with the work of Thompson et al. (1997) for ‘‘dry’’ aerosol particles, and they provide even stronger impetus for using 193 nm rather than 248 or 308 nm in ambient measurements so that composition-dependent biases in the mass spectra of single-component particles are minimized. Mixed composition aerosols The ability to distinguish multiple components in a single particle of mixed composition was explored by producing aerosols containing equimolar amounts of methanesulfonic acid, sodium chloride, ammonium nitrate, and ammonium sulfate. The DRH of this mixture is not known but is most likely less than the DRH’s of the individual components, i.e. below 60% (Wexler and Seinfeld, 1991). The negative ion spectra of these aerosols conditioned at different relative humidities are shown in Fig. 4. The laser fluence used in these experiments, 0.68 J cm~2, is typical of what might be used to study ambient aerosols. The fluence is high enough to produce strong signals from each of the compounds in single-component particles but not so high as to cause excessive peak broadening or fragmentation.

Fig. 3. Negative ion spectra of ammonium sulfate particles taken at different relative humidities: (a) 82%; (b) 75%; (c) 70%; (d) 65%; (e) (7%. Each spectrum is the average of ten particles. The laser wavelength was 193 nm and the laser fluence was 0.23 J cm~2.


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Fig. 4. Negative ion spectra of particles containing equimolar amounts of sodium chloride, ammonium nitrate, ammonium sulfate, and methanesulfonic acid at different relative humidities: (a) 83%; (b) 60%; (c) 40%; (d) 20%; (e) (7%. Each spectrum is the average of ten particles. The laser wavelength was 193 nm and the laser fluence was 0.68 J cm~2 (*"‘‘high’’ mass organic contaminant ions).

At a relative humidity of 83% (Fig. 4a), identification of all components in the particle is difficult. Peaks corresponding to Cl~ (m/z 35, 37) and NO~ (m/z 46) 2 are observed and indicate the presence of chloride and nitrate. However, identification of methanesulfonate and sulfate is tenuous due to the very weak peak intensities of CH SO~ (m/z 95) and HSO~ (m/z 97). In 3 3 4 one sense, this behavior is consistent with the slightly lower ionization threshold of chloride and nitrate relative to methanesulfonate and sulfate in single composition particles. However, it is also perplexing since the laser fluence used for analysis of the mixed composition aerosol is well above the thresholds for ion formation from methanesulfonate and sulfate in single component particles. Therefore, it appears that a quenching effect occurs during LDI of ‘‘wet’’ particles which inhibits detection of certain ions in the mass spectrum. Other prominent ions in the spectra of ‘‘wet’’ mixed component particles include carbon clusters of the form (H)C~ (n"3—6), which can arise n from organic contaminants or methanesulfonate, and water adducts such as Cl ) H O~, NO ) H O~, and 2 2 2 NO ) 2H O~. 2 2 At lower humidities, many spectral differences are observed. The presence of methanesulfonate can be confirmed at a relative humidity of 60% since the

CH SO~ peak appears conclusively in the spectrum. 3 3 As with higher humidities, nitrate and chloride signals still dominate the spectra and sulfate remains undetected. At a relative humidity of 40% (Fig. 4c) sulfate is indicated by the presence of a strong HSO~ peak. 4 At relative humidities below 40%, cluster peaks containing water decrease in intensity and eventually disappear at very low humidities, while cluster ions corresponding to H(HSO )~ (m/z 195) CH SO H ) 42 3 3 HSO~ (m/z 177) and Na(HSO )~ (m/z 217) emerge. 3 42 Previous work in our group has shown that when a multicomponent particle dries, the chemical composition of the outermost layer of the particle is given by the eutonic composition derived from the mutual deliquescence relative humidity (Ge et al., 1996). Since the dry particle spectra show peaks from all electrolytes in the solid, the eutonic composition reached during drying must contain a significant amount of each electrolyte. This is an important result since the laser irradiances typically used in aerosol mass spectrometry are not sufficient to ablate an entire particle. If all chemical components in an aerosol particle are represented in the eutonic composition, then they will also reside at or near the surface when the particle dries and the mass spectra will include peaks assignable to each.


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The spectra in Fig. 4 show the potential difficulty of detecting multiple components in aqueous aerosols. Wet particles yield spectra very different from their dry-particle counterparts and signal quenching in the wet particle spectra can be severe. The origin of this quenching is not known. Particle-to-particle variations of the relative peak intensities in the mass spectra of ‘‘dry’’ and ‘‘wet’’ particles are similar (Neubauer et al., 1997). Although the spectra of nonvolatile liquid droplets can be highly reproducible (Mansoori et al., 1994), aqueous particles may freeze by expansion cooling in the aerosol inlet and thereby exhibit significant particle-to-particle variations. Finally, the multicomponent spectra suggest that dry particles are more amenable to identification of all components present since the presence of water inhibits detection of some components.

CONCLUSIONS

Water clearly influences the laser desorption ionization process. The negative ion mass spectra of anionic electrolytes typically found in marine aerosols can be affected by the relative humidity of the surrounding air in several ways: an abrupt transition between a ‘‘wet’’- and ‘‘dry’’-particle spectrum over a narrow humidity range, a gradual transition over a broad humidity range, or virtually no change with humidity. Particulate-phase water can influence both the distribution of ions observed and their relative intensities. In most instances, water remains associated with the particle below the DRH, thus yielding spectra similar to those of aqueous droplets. Particulate phase water below the DRH appears to result from surface adsorption under ambient conditions rather than condensation in the mass spectrometer inlet. For single-component wet and dry particles, laser desorption/ionization with 193 nm radiation has an advantage over 248 nm radiation in that the threshold fluence is less dependent upon composition. Signal quenching in the mass spectra of multicomponent aerosols is more prevalent in the spectra of aqueous droplets than dry particles. For mixed composition droplets, only nitrate and chloride could be unambiguously identified from intense peaks in the spectra, while signals due to ions from methanesulfonate and sulfate were suppressed. In contrast, all components could be detected at low relative humidities. Changing the laser fluence did not eliminate this effect. Acknowledgement—This work was supported by grants from the National Science Foundation (ATM-9422993) and the Environmental Protection Agency (R82-2980-010).

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