title: Gas-phase CO2 subtraction for improved measurements of the organic aerosol mass concentration and oxidation degree by an aerosol mass spectrometer.
abstract: The Aerodyne aerosol mass spectrometer (AMS) has been widely used for real-time characterization of the size-resolved chemical composition of sub-micrometer aerosol particles. The first step in AMS sampling is the pre-concentration of aerosols while stripping away the gas-phase components, which contributes to the high sensitivity of this instrument. The strength of the instrument lies in particle phase measurement; however, ion signals generated from gas-phase species can influence the interpretation of the particle-phase chemistry data. Here, we present methods for subtracting the varying contributions of gas-phase carbon dioxide (CO2) in the AMS spectra of aerosol particles, which is critical for determining the mass concentration and oxygen-to-carbon (O/C) ratio of organic aerosol. This report gives details on the gaseous CO2 subtraction analysis performed on a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) data set acquired from sampling of fresh and diluted vehicle emissions. Three different methods were used: (1) collocated continuous gas-phase CO2 measurement coupled with periodic filter tests consisting of sampling the same particle-free air by the AMS and the CO2 analyzer, (2) positive matrix factorization (PMF) analysis to separate the gas- and particle-phase signals of CO2(+) at m/z 44, and (3) use of the particle time-of-flight (PTOF) size-resolved chemical information for separation of gas- and particle-phase signals at m/z 44. Our results indicate that these three different approaches yield internally consistent values for the gas/particle apportionment of m/z 44, but methods 2 and 3 require certain conditions to be met to yield reliable results. The methods presented are applicable to any situation where gas-phase components may influence the PM signal of interest.
authors.affiliation: Department of Environmental Toxicology, University of California , 1 Shields Avenue, Davis, California 95616, United States.
authors.forename: S
authors.initials: S
authors.lastname: Collier
authors.forename: Q
authors.initials: Q
authors.lastname: Zhang
mesh_terms: D000336:Aerosols / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis*; D013058:Mass Spectrometry / Q000295:instrumentation* / Q000379:methods*; D008970:Molecular Weight; D009584:Nitrogen / Q000032:analysis; D009930:Organic Chemicals / Q000032:analysis*; D010084:Oxidation-Reduction; D010316:Particle Size; D052638:Particulate Matter; D012815:Signal Processing, Computer-Assisted; D013997:Time Factors; D001335:Vehicle Emissions / Q000032:analysis
chemical_list: D000336:Aerosols; D009930:Organic Chemicals; D052638:Particulate Matter; D001335:Vehicle Emissions; D002245:Carbon Dioxide; D009584:Nitrogen