Abstract
Abstract. We present a chemical ionization quadrupole mass spectrometer (CI-QMS) with a radio-frequency (RF) discharge ion source through N2∕CH3I as a source of primary ions. In addition to the expected detection of PAN, peracetic acid (PAA) and ClNO2 through well-established ion–molecule reactions with I− and its water cluster, the instrument is also sensitive to SO2, HCl and acetic acid (CH3C(O)OH) through additional ion chemistry unique to our ion source. We present ionization schemes for detection of SO2, HCl and acetic acid along with illustrative datasets from three different field campaigns underlining the potential of the CI-QMS with an RF discharge ion source as an alternative to 210Po. The additional sensitivity to SO2 and HCl makes the CI-QMS suitable for investigating the role of sulfur and chlorine chemistry in the polluted marine and coastal boundary layer.
Highlights
Chemical ionization mass spectrometry using iodide anions is a widely used technique to measure various atmospheric trace gases with a high temporal resolution and low detection limit
The potential of I-CIMS for atmospheric trace gas measurement was established in laboratory studies (Huey et al, 1995) on chlorine nitrate (ClONO2), which plays a central role in polar stratospheric O3 depletion (Molina et al, 1987), and dinitrogen pentoxide (N2O5), which, through its heterogeneous hydrolysis on cloud droplets and aerosols, acts as a sink of gas-phase NOx (NO + NO2) (Lelieveld and Crutzen, 1990; Dentener and Crutzen, 1993)
The first applications of I-CIMS for monitoring atmospheric composition were for measurement of N2O5, peroxyacyl nitric anhydride (PAN, CH3C(O)O2NO2) and other peroxycarboxylic nitric anhydrides (Huey, 2007)
Summary
Chemical ionization mass spectrometry using iodide anions (commonly referred to as I-CIMS) is a widely used technique to measure various atmospheric trace gases with a high temporal resolution and low detection limit. Our instrument has previously been deployed with a radioactive ion source (210Po) to investigate the atmospheric chemistry of nitryl chloride (ClNO2), PAN and peracetic acid (PAA, CH3C(O)OOH) (Phillips et al, 2012, 2013, 2016; Crowley et al, 2018). We have developed a CI-QMS instrument (chemical ionization quadrupole mass spectrometer) with an electrical discharge ion source that generates iodide ions without the use of a radioactive ionizer This instrument was originally intended for measurement of PAN, PAA and ClNO2, we discovered that a wider variety of gas-phase species, including SO2, HCl and acetic acid could be detected. Its deployment as a PAN detector is limited to environments where PAN mixing ratios regularly exceed 100 pptv or when high temporal resolution is not necessary
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