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. Online characterization of aerosol composition at thenear-molecular level is key to understanding chemical reaction mechanisms,kinetics, and sources under various atmospheric conditions. The recentlydeveloped extractive electrospray ionization time-of-flight massspectrometer (EESI-TOF) is capable of detecting a wide range of organicoxidation products in the particle phase in real time with minimalfragmentation. Quantification can sometimes be hindered by a lack ofavailable commercial standards for aerosol constituents, however. Goodcorrelations between the EESI-TOF and other aerosol speciation techniqueshave been reported, though no attempts have yet been made to parameterizethe EESI-TOF response factor for different chemical species. Here, we reportthe first parameterization of the EESI-TOF response factor for secondaryorganic aerosol (SOA) at the near-molecular level based on its elementalcomposition. SOA was formed by ozonolysis of monoterpene or OH oxidation ofaromatics inside an oxidation flow reactor (OFR) using ammonium nitrate asseed particles. A Vocus proton-transfer reaction mass spectrometer(Vocus-PTR) and a high-resolution aerosol mass spectrometer (AMS) were usedto determine the gas-phase molecular composition and the particle-phase bulkchemical composition, respectively. The EESI response factors towards bulkSOA coating and the inorganic seed particle core were constrained byintercomparison with the AMS. The highest bulk EESI response factor wasobserved for SOA produced from 1,3,5-trimethylbenzene, followed by thoseproduced from d-limonene and o-cresol, consistent with previous findings. Thenear-molecular EESI response factors were derived from intercomparisons withVocus-PTR measurements and were found to vary from 103 to 106 ion counts s−1 ppb−1, mostly within ±1 order of magnitude oftheir geometric mean of 104.6 ion counts s−1 ppb−1. For aromaticSOA components, the EESI response factors correlated with molecular weightand oxygen content and inversely correlated with volatility. Thenear-molecular response factors mostly agreed within a factor of 20 forisomers observed across the aromatics and biogenic systems. Parameterizationof the near-molecular response factors based on the measured elementalformulae could reproduce the empirically determined response factor for asingle volatile organic compound (VOC) system to within a factor of 5 for the configuration of our massspectrometers. The results demonstrate that standard-free quantificationusing the EESI-TOF is possible.

Constraining the response factors of an extractive electrospray ionization mass spectrometer for near-molecular aerosol speciation