Development of a new corona discharge based ion source for high resolution time-of-flight chemical ionization mass spectrometer to measure gaseous H2SO4 and aerosol sulfate

Abstract A new corona discharge (CD) based ion source was developed for a commercial high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) (Aerodyne Research Inc.) to measure both gaseous sulfuric acid (H 2 SO 4 ) and aerosol sulfate after thermal desorption. Nitrate core ions (NO 3 − ) were used as reagent ions and were generated by a negative discharge in zero air followed by addition of excess nitrogen dioxide (NO 2 ) to convert primary ions and hydroxyl radicals (OH) into NO 3 − ions and nitric acid (HNO 3 ). The CD-HRToF-CIMS showed no detectable interference from hundreds parts per billion by volume (ppbv) of sulfur dioxide (SO 2 ). Unlike the atmospheric pressure ionization (API) ToF-CIMS, the CD ion source was integrated onto the ion–molecule reaction (IMR) chamber and which made it possible to measure aerosol sulfate by coupling to a filter inlet for gases and aerosols (FIGAERO). Moreover, compared with a quadrupole-based mass spectrometer, the desired HSO 4 − signal was detected by its exact mass of m / z 96.960, which was well resolved from the potential interferences of HCO 3 − ⋅(H 2 O) 2 ( m / z 97.014) and O − ⋅H 2 O⋅HNO 3 ( m / z 97.002). In this work, using laboratory-generated standards the CD-HRToF-CIMS was demonstrated to be able to detect as low as 3.1 × 10 5 molecules cm −3 gaseous H 2 SO 4 and 0.5 μg m −3 ammonium sulfate based on 10-s integration time and two times of the baseline noise. The CD ion source had the advantages of low cost and a simple but robust structure. Since the system was non-radioactive and did not require corrosive HNO 3 gas, it can be readily field deployed. The CD-HRToF-CIMS can be a powerful tool for both field and laboratory studies of aerosol formation mechanism and the chemical processes that were critical to understand the evolution of aerosols in the atmosphere.