Sulfate aerosols are ubiquitous in the atmosphere. Ammonium sulfate (AS) is generally the dominant form of sulfate aerosols in urban air, whereas potassium sulfate (PS), sodium sulfate (SS), magnesium sulfate (MS), and calcium sulfate (CS) may be large contributors in remote areas. Currently available methods for online measurements of sulfate aerosols, which are based on ion-chromatography or thermal desorption aerosol mass spectrometry, cannot separately quantify non-refractory and refractory sulfate compounds. We have developed a new particle mass spectrometer to measure non-refractory and refractory sulfate compounds: a refractory aerosol thermal desorption mass spectrometer (rTDMS). Aerosol particles are introduced into a vacuum chamber via an aerodynamic lens unit, and are collected on a cup-shaped graphite target (graphite collector). A focused CO2 laser coupled with the graphite collector enables a high desorption temperature (blackbody equivalent temperature of similar to 1200 K) for detecting AS, PS, SS, and MS aerosols. The gas molecules evolved from these compounds are detected by using an electron ionization quadrupole mass spectrometer. The temporal profiles of ion signals associated with increases in the temperature of the graphite collector are analyzed. The rTDMS sensitivities to various types of sulfate particles have been tested in the laboratory. The temporal profiles of ion signals at m/z 48 and 64 originating from multi-component sulfate particles exhibited bimodal peaks, which can be attributed to non-refractory and refractory sulfate particles. Preliminary data suggested that the ion signals for multi-component sulfate particles could be approximated as the linear combination of ion signals originating from single-component sulfate particles.
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