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Gas-Phase CO2 Subtraction for Improved Measurements of the Organic Aerosol Mass Concentration and Oxidation Degree by an Aerosol Mass Spectrometer
journal contribution
posted on 2013-12-17, 00:00 authored by S. Collier, Q. ZhangThe 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.
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Organic Aerosol Mass Concentrationmass concentrationparticle phase measurementvehicle emissionsion signalsPTOFAMS samplingAerosol Mass SpectrometerThe Aerodyne aerosol mass spectrometermatrix factorizationPMFCO 2 analyzeraerosol particlesfilter testsOxidation Degreemethods 2PM signalCO 2 subtraction analysisAMS spectra
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