Abstract
Abstract. Chemical characterization of organic coatings is important to advance our understanding of the physio-chemical properties and environmental fate of black carbon (BC) particles. The soot-particle aerosol mass spectrometer (SP-AMS) has been utilized for this purpose in recent field studies. The laser vaporization (LV) scheme of the SP-AMS can heat BC cores gradually until they are completely vaporized, during which organic coatings can be vaporized at temperatures lower than that of the thermal vaporizer (TV) used in a standard high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) that employs flash vaporization. This work investigates the effects of vaporization schemes on fragmentation and elemental analysis of known oxygenated organic species using three SP-AMS instruments. We show that LV can reduce fragmentation of organic molecules. Substantial enhancement of C2H3O+/CO2+ and C2H4O2+ signals was observed for most of the tested species when the LV scheme was used, suggesting that the observational frameworks based on the use of HR-ToF-AMS field data may not be directly applicable for evaluating the chemical evolution of oxygenated organic aerosol (OOA) components coated on ambient BC particles. The uncertainties of H:C and O:C determined using the improved-ambient (I-A) method for both LV and TV approaches were similar, and scaling factors of 1.10 for H:C and 0.89 for O:C were determined to facilitate more direct comparisons between observations from the two vaporization schemes. Furthermore, the I-A method was updated based on the multilinear regression model for the LV scheme measurements. The updated parameters can reduce the relative errors of O:C from −26.3 % to 5.8 %, whereas the relative errors of H:C remain roughly the same. Applying the scaling factors and the updated parameters for the I-A method to ambient data, we found that even though the time series of OOA components determined using the LV and TV schemes are strongly correlated at the same location, OOA coatings were likely less oxygenated compared to those externally mixed with BC.
Highlights
Atmospheric black carbon (BC) particles have significant impacts on climate and human health
The IA method has been widely utilized to quantify H : C and O : C ratios of organic aerosols (OAs) measured by a standard HR-ToF-AMS (i.e., thermal vaporizer (TV) scheme), the applicability of the I-A method for the elemental analysis of organic coatings that are measured using the laser vaporization (LV) scheme of the soot-particle aerosol mass spectrometer (SP-AMS) remains uncertain, especially for ambient oxygenated organic aerosol (OOA) components that are always referred to as freshly formed and aged secondary OA (SOA) materials based on their degree of oxygenation
To address this knowledge gap, this work examined 30 oxygenated organic species with different functional moieties, which were characterized by both TV and LV schemes of three SP-AMS instruments operated in different laboratories
Summary
Atmospheric black carbon (BC) particles have significant impacts on climate and human health. The elemental ratios obtained from HR-ToF-AMS mass spectra can be potentially biased by vaporization and ion fragmentation processes as described in detail by Canagaratna et al (2015a) To account for such measurement uncertainties, the calibration factors between experimentally measured and theoretical elemental compositions of known organic compounds were reported by Aiken et al (2007, 2008). Canagaratna et al (2015a) further established the “improved-ambient” (IA) method that uses specific ion fragments as markers to reduce composition-dependent systematic biases Both A-A and I-A methods have been fully integrated into the standard procedure for analyzing ambient OAs measured by HR-ToFAMSs. the two methods were developed based on OA mass spectra generated by the TV approach; their direct applications for determining elemental compositions of OAs vaporized using the LV scheme (i.e., organic coating on BC particles) may not be appropriate (Canagaratna et al, 2015b).
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