Abstract. Online characterization of aerosol composition at the
near-molecular level is key to understanding chemical reaction mechanisms,
kinetics, and sources under various atmospheric conditions. The recently
developed extractive electrospray ionization time-of-flight mass
spectrometer (EESI-TOF) is capable of detecting a wide range of organic
oxidation products in the particle phase in real time with minimal
fragmentation. Quantification can sometimes be hindered by a lack of
available commercial standards for aerosol constituents, however. Good
correlations between the EESI-TOF and other aerosol speciation techniques
have been reported, though no attempts have yet been made to parameterize
the EESI-TOF response factor for different chemical species. Here, we report
the first parameterization of the EESI-TOF response factor for secondary
organic aerosol (SOA) at the near-molecular level based on its elemental
composition. SOA was formed by ozonolysis of monoterpene or OH oxidation of
aromatics inside an oxidation flow reactor (OFR) using ammonium nitrate as
seed particles. A Vocus proton-transfer reaction mass spectrometer
(Vocus-PTR) and a high-resolution aerosol mass spectrometer (AMS) were used
to determine the gas-phase molecular composition and the particle-phase bulk
chemical composition, respectively. The EESI response factors towards bulk
SOA coating and the inorganic seed particle core were constrained by
intercomparison with the AMS. The highest bulk EESI response factor was
observed for SOA produced from 1,3,5-trimethylbenzene, followed by those
produced from d-limonene and o-cresol, consistent with previous findings. The
near-molecular EESI response factors were derived from intercomparisons with
Vocus-PTR measurements and were found to vary from 103 to 106 ion counts s−1 ppb−1, mostly within ±1 order of magnitude of
their geometric mean of 104.6 ion counts s−1 ppb−1. For aromatic
SOA components, the EESI response factors correlated with molecular weight
and oxygen content and inversely correlated with volatility. The
near-molecular response factors mostly agreed within a factor of 20 for
isomers observed across the aromatics and biogenic systems. Parameterization
of the near-molecular response factors based on the measured elemental
formulae could reproduce the empirically determined response factor for a
single volatile organic compound (VOC) system to within a factor of 5 for the configuration of our mass
spectrometers. The results demonstrate that standard-free quantification
using the EESI-TOF is possible.
Observation of charged droplets from electrospray ionization (ESI) plumes in API mass spectrometers
