Extending the capabilities of single particle mass spectrometry: I. Measurements of aerosol number concentration, size distribution, and asphericity

Single particle mass spectrometers have traditionally been deployed to measure the size and composition of individual particles. The relatively slow sampling rates of these instruments are determined by the rate at which the ionization lasers can fire and/or mass spectra can be recorded. Under most conditions, our single particle mass spectrometer, SPLAT, can detect and size particles at much higher rates than it can record mass spectra. We therefore developed a dual data acquisition mode, in which particle number concentrations, size distributions, and asphericity are measured at a rate determined by particle concentration and the particle detection efficiency, all while the instrument generates and records individual particle sizes and mass spectra at an operator-set rate. Particle number concentrations are calculated from the particle detection rate at the first optical stage and the measured sampling flow rate. We show that SPLAT measured particle number concentrations are in very good agreement with independent measurements by the passive cavity aerosol spectrometer probe (PCASP). Particle asphericity is based on the ratio of the particle detection rates at the first and second optical stages. Particle size is based on the measurement of particle time of flight between the two detection stages. We illustrate the artifact in the measured size distributions that can be introduced by high particle concentrations and present a method to remove it and correct the size distributions. Particle number concentration and asphericity are measured with 1 s resolution and particle vacuum aerodynamic size distributions are measured with 4 to 60 s resolution.