Remote Marine Boundary Layer Observations - Evaluating a New Atmospheric Pressure Ion-Molecule Reaction Region

Often of major concern with in-situ gas-phase mass spectrometry measurements is the ability of the Ion-Molecule Reaction Region (IMR) to effectively facilitate the clustering of ions with sample gases and transmit these charged molecules to be measured, while keeping relatively small residence times and reducing losses. Various surfaces inherent to an IMR may result in sample molecules being lost entirely as well as being temporarily deposited, contributing to artificial background signals at a later time.

Here we show improvements in gas-phase mass spectrometry measurement capabilities using a novel atmospheric pressure IMR (henceforth referred to as the crossflow IMR) in conjunction with an Aerodyne Time-of-Flight Chemical Ionization Mass Spectrometer (ToF-CIMS). The crossflow IMR optimizes low sample residence time with high ion transmission, while maintaining laminar flow typically at 10 standard liters per minute through a 7/8" ID Teflon flow tube. We show improved signal response time by orders of magnitude for relatively lower volatility or "sticky" compounds, often deposited on the walls of an IMR, on the order of 1 second. An electric potential is applied between the ionizer and ToF-CIMS entrance to enhance transmission of charged sample molecules. This improves the signal-to-noise ratio of resultant spectra and shows higher sensitivity for certain measured compounds compared to other IMRs.

Also presented are results from the first field deployment of the crossflow IMR as part of the Bermuda boundary Layer Experiment on the Atmospheric Chemistry of Halogens (BLEACH) campaign in June 2022. The ground-based Tudor Hill Marine Atmospheric Observatory (THMAO) is positioned to sample air masses from the remote Atlantic ocean, making it possible to test the capabilities of the crossflow IMR to detect key indicators of marine boundary layer halogen chemistry.