Size and Composition Biases on the Detection of Individual Ultrafine Particles by Aerosol Mass Spectrometry

Aerosol mass spectrometers allow particles to be counted on the basis of size and chemical composition. In most instruments, individual particles are ablated with a pulsed laser to obtain a mass spectrum. Using this method to characterize ambient aerosols requires an understanding of biases induced by the measurement process. For particles less than 200 nm diameter, the efficiency of detection is shown to be dependent on both size and composition. These dependencies arise from the transmission characteristics of the particle inlet and the intrinsic ability of a particle to be vaporized and ionized. The relative contributions of each are determined for a series of atmospherically relevant test aerosols. Small particles are generally more difficult to detect and analyze than large particles because they are more difficult to focus through the inlet into a tight beam and because they are more difficult to ablate. Particles composed of polycyclic aromatic hydrocarbons, ammonium nitrate, and alkali metal ions are efficiently ablated by laser ablation. Aliphatic organics are less efficiently ablated, and ammonium sulfate is very difficult to detect in a positive ion spectrum. The mass spectra of ultrafine particles show extensive fragmentation, making it difficult to distinguish aliphatic and aromatic components. However, organic particles may be classified on the basis of inorganic impurities. Initial field measurements of ambient ultrafine particles are consistent with these conclusions.