Organic particle types by single-particle measurements using a time-of-flight aerosol mass spectrometer coupled with a light scattering module

摘要

Chemical and physical properties of individual ambient aerosol particles canvary greatly, so measuring the chemical composition at the single-particlelevel is essential for understanding atmospheric sources and transformations.Here we describe 46 days of single-particle measurements of atmosphericparticles using a time-of-flight aerosol mass spectrometer coupled with alight scattering module (LS-ToF-AMS). The light scattering module opticallydetects particles larger than 180 nm vacuum aerodynamic diameter (130 nmgeometric diameter) before they arrive at the chemical mass spectrometer andthen triggers the saving of single-particle mass spectra. 271 641 particleswere detected and sampled during 237 h of sampling in single-particle mode.By comparing timing of the predicted chemical ion signals from the lightscattering measurement with the measured chemical ion signals by the massspectrometer for each particle, particle types were classified and theirnumber fractions determined as follows: prompt vaporization (46%), delayed vaporization(6%), and null (48%), where null was operationallydefined as less than 6 ions per particle. Prompt and delayed vaporizationparticles with sufficient chemical information (i.e., more than 40 ions perparticle) were clustered based on similarity of organic mass spectra (using-means algorithm) to result in three major clusters: highly oxidizedparticles (dominated by 44), relatively less oxidized particles(dominated by 43), and particles associated with fresh urbanemissions. Each of the three organic clusters had limited chemical propertiesof other clusters, suggesting that all of the sampled organic particle typeswere internally mixed to some degree; however, the internal mixing was neveruniform and distinct particle types existed throughout the study.Furthermore, the single-particle mass spectra and time series of theseclusters agreed well with mass-based components identified (using factoranalysis) from simultaneous ensemble-averaged measurements, supporting theconnection between ensemble-based factors and atmospheric particle sourcesand processes. Measurements in this study illustrate that LS-ToF-AMS providesunique information about organic particle types by number as well as mass.