Current Research

Over the past decade there have been rapid advances in the use of mass spectrometry to monitor the chemical composition of atmospheric aerosols. Thus, it is now possible to determine the size and chemical composition of individual particles in real time. A number of different instruments, capable of characterising individual particles, have been described in a review by Suess and Prather [1].

A schematic of the instrument used (March 2001 - Dec 2001) at Edinburgh (TSI, Model 3800) is shown in Figure 1. It consisted of three main sections: first, a differentially pumped nozzle/twin skimmer arragement acted as a transition for the paticles to pass from atmospheric pressure to 10-7 Torr in the MS region. Along this passage the particles passed through the second section where two laser beams were used to measure the time-of-flight of the particles and thus their aerodynamic size. In the third section, a high power pulsed ultraviolet laser was used to ablate and ionise the individual particles. Both positive and negative ions were then mass analysed using time-of-flight techniques.

Schematic of the Aerosol Time-of-Flight Mass Spectrometer

Figure 1. Schematic of the Aerosol Time-of-Flight Mass Spectrometer.

The data obtained from a typical run are illustrated below. Figure 2 shows the particle size distribution in the atmosphere close to our laboratory in Edinburgh on 5th of April, 2001. Three modes in the range 0.2-4 microns are clearly observed. Note, however, that the relative size abundance has not been corrected for instrument sensitivity, which varies significantly over this range.

Particle size distribution of ambient air collected

Figure 2. Particle size distribution of ambient air collected
in Edinburgh on the 5th of April 2001.

The mass spectra of two individual particles are shown in figures 3 and 4. The first particle mass spectrum (Figure 3) is evidently that of elemental carbon and it is interesting that this particle, and others in the sample, contained essentially no hydrocarbons.

Positive and negative mass spectra of an Elemental Carbon particle

Figure 3. Positive and negative mass spectra of an Elemental Carbon particle.

Figure 4 shows the positive and negative ion mass spectra from a salt particle. This is clearly more complex and requires careful analysis. An important question which remains to be addressed is the relative sensitivity of the ATOFMS to the various ionic species observed.

Positive and negative mass spectra of a salt particle

Figure 4. Positive and negative mass spectra of a salt particle.

[1] D.T. Seuss and K.A. Prather, Chem. Rev. 99, (1999) 3007.