J3

FIGURE 11.72 Negative ion laser ionization mass spectrometry of particles generated in the laboratory that contain equimolar amounts of NaCl, NH4N03, (NH4)2S04, and CH3SO,H at (a) 7%, (b) 40%, and (c) 83% relative humidity (adapted from Neubauer et al., 1998).

negative ion mass spectra of particles containing equimolar amounts of NaCl, NH4N03, (NH4)2S04, and methanesulfonic acid at relative humidities of < 7, 40, and 83%, respectively. At the lowest relative humidity where the particle is a solid, peaks due to all of the salts appear, and the peak due to the anion of methanesulfonic acid is clear. At 83% RH, the latter has almost completely disappeared, as has that due to the sulfate.

However, relative peak intensities can be useful if the nature of the particles has not changed significantly. For example, Gard et al. (1998) followed peaks characteristic of chloride and nitrate in a coastal region in southern California and showed that they had an inverse correlation in single particles. Such behavior is expected from the well-known reaction of gaseous HN03 (and perhaps other gaseous oxides of nitrogen) with NaCl to give gaseous HC1 and solid NaN03 (e.g., see De Haan et al., 1999, and references therein).

A second issue is that while organics can be identified from C„Hm peaks, speciation is generally not possible. For example, although negative ions corresponding to organic acid anions were observed in particles in rural Colorado, Murphy and Thomson (1997b) indicate that these could be due to fragmentation from larger organics. Fragmentation patterns of specific organics can, however, provide clues to the presence of certain classes of compounds. For example, Fig. lf.71d shows the positive ion mass spectrum of a particle that Murphy and Thomson (1997a) attribute to the presence of amines or amides in the particle.

A third issue is illustrated by the mass spectrum in Fig. lf.7fa, which, as described by Murphy and Thomson (1997a), appears to have a peak at almost every mass. Thus, in many instances in the atmosphere, particularly in polluted urban areas, the spectra may be so complex that only major classes of compounds may be discernible.

Finally, the approaches to sizing rely to date on light scattering. As discussed in Chapter 9.A.4, visible light scattering peaks in the 0.1- to l-/j,m range so that particles smaller than this cannot be detected and hence measured using this approach. Although particles down to ~ 10 nm in size can be detected by free firing of the laser (e.g., Reents et al., 1995; Carson et al., 1997b), this clearly gives a rather random selection of particles detected and may not work at low particle concentrations typical of the atmosphere. New approaches for detecting smaller particles in such systems are needed.

b. Alternate Potential Mass Spectrometric Methods for Sizing and Chemical Composition

Figure If.73 is a schematic diagram of another approach to single-particle size and composition measurements that is applicable to volatile and semivolatile species over the size range from ~0.05 to 1 ¡xm (Jayne et al., f 999). This approach is similar in principle to that described by Allen and Gould (1981) and Sinha et al. (1982). Particles are sampled through an aerodynamic inlet that provides a narrow beam of particles with near unit efficiency (Liu et al., f995a, 1995b; Schreiner et al., f998). As the air containing the particle beam expands into the vacuum at the end of the inlet, the particles are accelerated, with smaller particles attaining higher speeds and vice versa. The beam of particles entering the sizing chamber is chopped to provide a time-of-flight

Sampling Sizing Detection

Positive ion QMS

'|| II1 Particle —J vaporization

I IT/ filament

Turbo pump

FIGURE 11.73 Schematic diagram of aerosol mass spectrometer for volatile and semivolatile particle sizing and composition measurement (graciously provided by J. Jayne, D. Worsnop, and C. Kolb, 1999).

Diy Negative Ion Detector

FIGURE 11.72 Negative ion laser ionization mass spectrometry of particles generated in the laboratory that contain equimolar amounts of NaCl, NH4N03, (NH4)2S04, and CH3SO,H at (a) 7%, (b) 40%, and (c) 83% relative humidity (adapted from Neubauer et al., 1998).

Aerodynamic aerosol sampling inlet

TOF chopper motor

Turbo pump

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