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Characterization of Organic Aerosol Relative Ionization Efficiency for the Aerosol Mass Spectrometer and Aerosol Chemical Speciation Monitor
BENJAMIN A. NAULT, Philip Croteau, Leah Williams, Manjula Canagaratna, Aerodyne Research, Inc.
Abstract Number: 472
Working Group: Instrumentation and Methods
Abstract
The Aerosol Mass Spectrometer (AMS) and the Aerosol Chemical Speciation Monitor (ACSM) are widely used to measure the chemical composition and mass concentration of submicron, non-refractory aerosol particles. An important component of the non-refractory aerosol is organic aerosol (OA). Unlike non-refractory inorganic aerosol, which can easily be calibrated, OA is composed of 1000s of compounds. The response of the AMS/ACSM to non-NH4NO3 particle mass is determined with species-dependent relative ionization efficiencies (RIE) and a primary ionization efficiency that is obtained from calibrations with an instrument standard (NH4NO3). Although a value of 1.4 has been found to explain the ambient OA observed across various campaigns, recent laboratory and ambient measurements have suggested the RIE of OA may have a dependence on OA oxidation state (OSc), with more reduced OA having higher RIE values than more oxidized OA.
Here, we develop a simple method for calibrating OA_RIE across the range of OSc relevant for ambient OA measurements. We use laboratory measurements of atomized OA surrogates to characterize the OA_RIE for both the AMS/ACSM. These standards have been sampled in a variety of solvents and mixtures to evaluate potential matrix effects and to create a standard procedure for AMS/ACSM users to calibrate their instruments. We identify several binary mixtures of alcohols and NH4NO3 that can be used to produce consistent OA_RIE measurements for organics within the OSc range of –1.5 to 0.5. High OA_RIE values that are observed for laboratory OA species in the OSc range of –2.5 to –1.5 are compared with those calculated for realistic, reduced OA species extracted from ambient filters collected at urban sites and from emission sources. The use of ambient filters allows for deconvolution of potential species-dependent lens transmission and RIE effects in on-line, ambient AMS measurements.