An analysis of the evolution of negative ions produced by a corona ionizer in air

Mass spectrometric measurements of negative ions produced by a dc corona ionizer in air were conducted at different reaction times (ca. 1.0 ms and 10 ms) to observe the evolution of negative ions in air. One of the characteristic features was the rapid dominance of NO3− ions in the mass spectra, which is indicative of the strong influence of O3 and NO2 produced by discharge in the early stages of negative ion evolution. Water vapor was also found to play a significant role in NO3− ion formation. The production of OH radicals by ion–molecule reactions involving H2O leads to the formation of HNO3 which accelerates the conversion of primary negative ions to NO3− ions. As a result, NO3− and NO3−HNO3 ions were observed as terminal ions. Another terminal ion observed was assigned to HCO3−HNO3. The reaction of OH− with CO2 is probably responsible for the formation of HCO3−. From considerations of ion reactivity, NO3− ions likely represent the most long-lived ion species in negative ion evolution in indoor air.