Online Aerosol Mass Spectrometry
In the contrast to offline methods, the online instruments do not sample the aerosol particles on an impactor or on a filter, but analyze the particles directly, with the advantage that sampling artifacts (e.g., evaporation, reaction with other chemical compounds) can be widely avoided. Also, these instruments offer in general a good time resolution and deliver chemically resolved size distributions.
Generally, two different approaches are used for online aerosol mass spectrometry: the laser ablation method and the thermal desorption method.
This method used a triggered laser pulse to evaporate (ablate) and ionize single particles. The laser pulse is usually triggered by the light scatter signal that are produced when the particles cross two continuous laser beams. This signal is also used to infer the the size of the particles.
The advantage is single particle chemical composition, the disadvantage is the non-quantitative information. Examples for this method are the LAMPAS (Hinz et al., 1994), the ATOFMS (Prather et al., 1994, commercially available from TSI, Inc.), the PALMS (Murphy and Thomson, 1995), the RSMS (Johnston and Wexler, 1995), and SPLAT and , which were developed in our department.
Here, the particles are vaporized upon impact onto a hot surface and the resulting gas molecules are subsequently ionized (e.g., by electron impact or by VUV ionization). Size information is obtained by chopping the particle beam and measuring the particle velocity. The great advantage is the ability to obtain quantitative, size resolved mass concentrations, the main disadvantage is the insensitivity to refractory compounds that do not evaporate at the given temperature (e.g. soot, mineral dust).
Examples for this method are the Aerodyne instruments (, ), which are widely used in our research groups. This technique is described in detail in Jayne et al. (2000), Drewnick et al. (2005), and Canagaratna et al (2007).