A Bipolar Multi-Reagent Chemical Ionization Mass Spectrometer for Versatile Measurements of Gas and Particle Phase Organics

Organic species in the atmosphere originate from a wide range of sources and processes. While real time chemical ionization mass spectrometry (CIMS) has improved our capability to characterize individual organic species in the atmosphere, the selectivity of CIMS reagent ions can limit the range of species that can be measured.  In this work the need to detect a broader range of species with a single CIMS instrument is addressed. A fast-switching bipolar time-of-flight CIMS that switches between four different reagent ions, including positive and negative ions, is demonstrated.  The performance and utility of this instrument is demonstrated by measurements obtained on board a ship in Antarctica during the PolarChange field campaign and from New York City during the AEROMMA campaign.  During both campaigns the instrument cycled through iodide (I-), benzene (C6H6+), and acetone dimer ((C3H6O)2H+) reagent ions at a 2 second data acquisition rate per cycle.  In the case of PolarChange, this combination of ions enabled simultaneous detection of trends in primary marine biological emissions such as dimethyl sulfide, nucleating species such as ammonia and methyl amine, and acids, such as nitric acid.  During AEROMMA, the fast bipolar switching capability enabled Eddy Correlation measurements of primary biogenic and urban emissions (i.e. monoterpenes and aromatics), secondary products of atmospheric oxidation (i.e. highly oxidized organics and organic nitrates), and reduced nitrogen species.  Preliminary results from this dataset, including positive matrix analyses of the combined multi-reagent ion datasets, are discussed.  Simultaneous gas and aerosol composition measurements obtained by coupling this mass spectrometer with aerosol inlets are also described.