Reactive nitrogen and total organic carbon calibration techniques for the Aerodyne aerosol mass spectrometer

Abstract

The Aerodyne aerosol mass spectrometer (AMS) is used extensively to measure the chemical composition of non-refractory submicron aerosol particles for laboratory and atmospheric field studies. Typical AMS calibration methods are mass-based, involving generating pure or mixtures of particles with a known size and number concentration. Here we present a new calibration method using a heated platinum/molybdenum catalyst-based conversion technique that provides an independent measurement of reactive nitrogen (Nr) and total organic carbon (Cy) traceable to gas-phase standards of nitric oxide (NO) and carbon dioxide (CO2), respectively. The calibration method was tested using dried particles composed of pure ammonium nitrate (AN), other ammonium salts, and nitrogen-containing organic species. The nitrate ionization efficiencies determined with the AMS single particle or Nr calibration methods matched within experimental uncertainties (±15%). By measuring the AMS lens transmission efficiency and incorporating light-scattering measurements of the AMS collection efficiency, the catalyst method independently showed that the AMS relative ionization efficiency (RIE) for ammonium was essentially the same among different ammonium containing compounds (±9%), regardless of the corresponding anion and stoichiometry, quantifying support of a major assumption inherent in AMS calibrations. The Nr and Cy measurements were used to calibrate the RIE for 4-nitrocatechol (1.1 ± 0.4), isosorbide mononitrate (1.2 ± 0.1) and triammonium citrate (2.1 ± 0.2), which were within the range of organic RIEs used in AMS quantitation. The combined catalyst system provided more accurate measurements of the elemental carbon-to-nitrogen ratios than the high-resolution AMS elemental analysis using the Improved-Ambient method plus the inorganic nitrogen.

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