Measuring the Organic Carbon to Organic Matter Multiplier with Thermal/Optical Carbon-Quadrupole Mass Spectrometer Analyses

Abstract

A thermal/optical carbon analyzer (TOA) was adapted to direct thermally-evolved gases to an electron ionization quadrupole mass spectrometer (QMS), creating a TOA-QMS. While this approach produces spectra similar to those obtained by the Aerodyne aerosol mass spectrometer (AMS), and can quantify sulfate (SO42−), nitrate (NO3−), ammonium (NH4+), and organic carbon (OC) fractions from ambient particle laden quartz-fiber filters, there remains a need to further understand the composition of the organic aerosol fraction. Elemental analysis (EA) of standard organic mixtures and ambient samples demonstrates the feasibility of the TOA-QMS for measuring the ratios of oxygen-to-carbon (O/C), hydrogen-to-carbon (H/C), nitrogen-to-carbon (N/C), sulfur-to-carbon (S/C), and organic matter-to-organic carbon (OM/OC). For ambient samples from Central California, the TOA-QMS returned average ratios for O/C of 1.03 ± 0.27 and H/C of 1.95 ± 0.69, respectively. Higher H/C ratios were observed during clean air episodes, while lower ratios were observed during hazy conditions. A relatively constant level of aerosol oxidation was observed throughout the study. The average OM/OC multiplier was 2.55 ± 0.4, which is higher than the conventionally used values of 1.4 and 1.8, indicating higher contributions from biomass burning and aged aerosols.