Extending the capabilities of single particle mass spectrometry: II. Measurements of aerosol particle density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below ∼ 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.